Recent Advances Towards Syntheses of Diterpenoid Alkaloids · Wagner–Meerwein rearrangement, affording the ent-kau-rane 23 and ent-atisane 24 diterpene skeletons. These scaf-folds

3915

C. Dank et al. ReviewSyn thesis

SYNTHESIS0 0 3 9 - 7 8 8 1 1 4 3 7 - 2 1 0 XGeorg Thieme Verlag Stuttgart · New York2019, 51, 3915–3946reviewen

Recent Advances Towards Syntheses of Diterpenoid AlkaloidsChristian Dank 0000-0001-7440-4153 Randy Sanichar 0000-0003-4373-5317 Ken-Loon Choo 0000-0001-7745-5458 Madeline Olsen Mark Lautens* 0000-0002-0179-2914

Davenport Research Laboratories, Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, [email protected]

Dedicated to the 50th anniversary of SYNTHESIS

Received: 20.06.2019 While researchers worldwide are still discovering new
Accepted: 28.06.2019Published online: 05.08.2019DOI: 10.1055/s-0037-1611897; Art ID: ss-2019-z0341-r
License terms:

Abstract The diterpenoid alkaloids serve as a rich source of synthetictargets for organic chemists, due to the intriguing structure of the over-lapping ring systems, along with biological activities commonly associ-ated with compounds of this group. Fifteen total syntheses and numer-ous synthetic studies towards construction of ring fragments have beenreported since 2010. This review article gives a brief overview of diter-penoid alkaloids and summarizes the recent synthetic efforts.1 Introduction1.1 Structural Classification and Biosynthetic Origin1.2 Structure Elucidation of the Aconitum Alkaloids2 Total Syntheses2.1 C18-Diterpenoid Alkaloids2.2 C19-Diterpenoid Alkaloids2.3 C20-Diterpenoid Alkaloids3 Strategies To Synthesize Ring Systems3.1 Radical-Based Cyclizations3.2 Ruthenium-Mediated Enyne Cycloisomerization3.3 Reductive Coupling3.4 Diels–Alder Reactions3.5 Oxidative Dearomatization/Diels–Alder Sequence3.6 Transannular Aziridation3.7 Intramolecular [5+2] Cycloaddition3.8 Miscellaneous4 Conclusion

Key words diterpenoids, alkaloids, total synthesis, aconitine, naturalproducts

1 Introduction

The diterpenoid alkaloids (C18, C19, and C20) are a groupof chemically and structurally complex natural productsthat possess a long list of pharmaceutical properties.1 Thislist includes, but is not limited to, anti-inflammatory, anal-gesic, and antiarrhythmic properties.2 Most of these prop-erties are reported to be derived from interactions betweenthe natural products and voltage-gated ion channels, par-ticularly potassium and sodium channels.3

compounds within this class from numerous plant species,these pseudoalkaloids have been traditionally linked to theAconitum, Consolidum, and Delphinium genus of plants,from which they were extracted and used in traditionalmedicine. However, research into broader applications stillcontinues.4 The most common uses included the treatmentof cardiovascular diseases and as an analgesic.

1.1 Structural Classification and Biosynthetic Origin

The diterpenoid alkaloids family is generally grouped bykey conserved structural features and size of the carbonscaffolds. The smallest of the three main groups are the C18-diterpenoid alkaloids, with over 50 discrete compounds al-ready known.2b These C18-diterpenoid alkaloids can be fur-ther subdivided into two distinct types; the lappaconitine-type 1, and the ranaconitine-type 2 (Figure 1) with the ma-jor difference being the presence of the additional oxygen-ation at the C7 position in the ranaconitine core.

Figure 1 C18-type diterpenoid alkaloid skeletons

The C19-diterpenoid alkaloids are the largest of the threegroups and represent over 600 fully characterized mole-cules.1c They are structurally closer to the C18-diterpenoidcore, with the variation being an additional carbon at thepiperidine ring junction. This group is divided into 6 uniquetypes, with the aconitine- 3 and lycoctonine-type 4seemingly the most prevalent in Nature (Figure 2). The other

lappaconitine skeleton1

H

NR1

R2O

H

H

H

NR1

R2O

H

H OR3

ranaconitine skeleton2

© 2019. Thieme. All rights reserved. — Synthesis 2019, 51, 3915–3946Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

http://orcid.org/0000-0001-7440-4153

http://orcid.org/0000-0003-4373-5317

http://orcid.org/0000-0001-7745-5458

http://orcid.org/0000-0002-0179-2914

3916


Biographical Sketches

Christian Dank (second from right) stud-ied chemistry at the University of Vienna.After receiving the degree Master of Science(M.Sc.) in the group of Prof. Johann Mulzer,during his Ph.D. he worked on syntheses ofnovel antimalarials, supervised by Dr. Hu-bert Gstach and Prof. Walther Schmid. In2016, he joined the medicinal chemistry de-partment of Boehringer Ingelheim in Vien-na, working on oncology projects. Duringhis postdoctoral fellowship in the Lautensgroup, he contributes in his fields of exper-tise, total synthesis and medicinal chemis-try, and explores the field of metal catalysis.

Randy Sanichar (rightmost) completed hisBachelor of Science (B.Sc.) degree in chem-istry at the University of Guyana in 2011,graduating with distinction. After workingat a local rum distillery as a process chemistfor two years, he travelled to Canada to fur-ther his education at the University of Alber-ta. In 2018, he completed his Ph.D. underthe supervision of Professor John C. Vederaswhere he worked on the design and synthe-ses of probes for the elucidation of biosyn-thetic pathways of pharmaceuticallyrelevant polyketides; all to facilitate a che-mobiosynthetic production platform. Hejoined the Lautens group in November2018, as a postdoctoral researcher, wherehis current research interests include the to-tal synthesis of novel natural products andthe development of new catalytic asymmet-ric reactions.

Ken-Loon Choo (leftmost) received hisB.Sc. degree with honors from the Universi-ty of Toronto. He then ventured into theoleochemical industry as a research chemistdeveloping a process to synthesize high pu-rity sulfonated fatty acid esters. In 2016, hereturned to pursue a graduate degree in thelaboratories of Prof. Mark Lautens at theUniversity of Toronto with interest in cata-lytic asymmetric reactions and total synthe-sis.

Madeline Olsen (center) enjoyed a thirty-year career in electrical engineering, havingbeen employed at Toronto’s Hospital forSick Children (1970–1974), Bell NorthernResearch Ottawa (1975–1978), The Nation-al Synchrotron Light Source, Brookhaven Na-tional Laboratory (1979–1994), and SLACNational Accelerator Laboratory (1995–1998). A specialist in digital logic design andsignal processing, she nonetheless designedand constructed many of the magnet powersupplies at the NSLS, ranging from 25 KW to2.2 MW. She was awarded two United Statespatents for her work at Bell Northern Re-search and in 1995, she was invited to CERNGeneva where she gave a lecture to the LEPpower supply group outlining the high pre-cision digital power control system she haddesigned for the NSLS. She enrolled as anundergraduate and having taken the spe-cialist courses offered in organic chemistry,worked in the Lautens lab as a research as-sistant from 2001–2006. During this time,she worked on the synthesis and ring-open-

ing of novel azabenzonorbornadienes anddeveloped a reaction for the synthesis of di-hydronaphthalenes via an aryne Diels–Alderreaction. Additionally, she devised and ranlarge-scale syntheses for early stage materialof a total synthesis project.

Mark Lautens (second from left) was bornin Hamilton, Ontario, Canada. He obtainedhis B.Sc. degree from the University ofGuelph followed by doctoral studies at theUniversity of Wisconsin–Madison under thedirection of Professor Barry M. Trost wherehe discovered Mo-catalyzed allylic alkylationand the Pd-catalyzed enyne cycloisomeriza-tion. In 1985, he moved to Harvard Universi-ty where he conducted his NSERC PDF withProfessor David A. Evans on studies directedtoward the synthesis of bryostatin. Hejoined the University of Toronto in 1987 asan NSERC University Research Fellow andAssistant Professor. Since 1998 he has heldan Endowed Chair, the AstraZeneca Profes-sor of Organic Synthesis, and from 2003–2013 he was named an NSERC/Merck FrosstIndustrial Research Chair in New MedicinalAgents via Catalytic Reactions. In 2012, hewas appointed as University Professor, thehighest rank at the University of Toronto.Most recently he was awarded Officer of theOrder of Canada, the highest civilian honorin Canada. His current research interests arein multicatalyst reactions, isomerization re-actions, and applications of catalysis in thesynthesis of bioactive compounds.

© 2019. Thieme. All rights reserved. — Synthesis 2019, 51, 3915–3946

3917


four subgroups are less frequent in occurrence and arecomprised of the pyro-type, 7,17-seco-type 5, lactone-type6, and rearranged-type alkaloids.

Figure 2 C19-type diterpenoid alkaloid skeletons

The C20-diterpenoid alkaloids (Figure 3) are a moderate-ly large group, with over 300 known compounds.2b Thisgroup shows the broadest structural diversity of the maingroups, and have been categorized into 16 distinct types ofditerpenoid alkaloids: atisine 7, denudatine 8, hetidine 9,hetisine 10, cardionidine 11, albovionitine 12, vakognavine13, veatchine 14, napelline 15, anopterine 16, delnudine 17,

kusnezoline 18, actaline 19, racemulosine 20, arcutine 21,and tricalysiamide 22.2b Interestingly, while almost all ofthe C18- and C19-diterpenoid alkaloids possess a [3.2.1]bicy-cle in the right side of their scaffolds, the majority of theC20-diterpenoid alkaloids contain a [2.2.2]bicycle.

Despite this vast structural diversity amongst the threegroups, these natural products are all derived through bio-synthetic insertion of the nitrogen into the mature diter-pene scaffolds, which then undergoes further elaborationsleading to the final product, as shown in Scheme 1.5 An en-zyme-catalyzed cyclization of geranyl-geranyl pyrophos-phate affords the ent-copalyl pyrophosphate, which thenundergoes a series of transformations, including a criticalWagner–Meerwein rearrangement, affording the ent-kau-rane 23 and ent-atisane 24 diterpene skeletons. These scaf-folds then undergo amination to form the two mainbranching points, the veatchines 14 and the atisines 7, no-tably these can interconvert through a [1,2]-sigmatropic re-arrangement.1,2b,5 Through a series of elegant manipula-tions by Nature, these cores are later transformed into thenumerous natural products observed. Some examples ofthese are the napellines 15 and denudatines 8, which areaccessed through C7–C20 bond formation from the veatch-ines 14 and atisines 7, respectively. In other cases, after therequisite transformations, the scaffold may fragment to ac-cess the smaller core, as seen with the denudatines en routeto the C19 scaffolds 25 and C18 scaffolds 26.

aconitine skeleton3

lycoctonine skeleton4

H

NR1H

H OR3

H

NR1H

H

OR2 OR2

7,17-seco type skeleton5

H

NR1H

H

OR2

lactone skeleton6

H

NR1H

H

OR2

O

O

7 7

77

17

17

1

6

8

Figure 3 The 16 categories of the C20-diterpenoid alkaloids

NR

atisine-type7

NR

denudatine-type8

NRhetidine-type

9

N

hetisine-type10

NR

cardionidine-type11

NR1

albovionitine-type12

RN

vakognavine-type13

NR

veatchine-type14

R2O

NRnapelline-type

15

NR

anopterine-type16

N

delnudine-type17

N

kusnezoline-type18

O

O

NRactaline-type

19

NR

racemulosine-type20

NR

arcutine-type21

RN

tricalysiamide-type22


3918


1.2 Structure Elucidation of the Aconitum Alkaloids

The C18- and C19-diterpenoid alkaloids have been ofgreat interest to civilizations since antiquity. Preparationsfrom members of the genera Aconitum and Delphiniumhave been used for such diverse applications as traditionalChinese medicine to surreptitious homicide, aconitine (27)being the most toxic; LD50 in mice (mg/kg: 0.166, i.v., 0.328i.p.).6 However, due to the complexity of these molecules,very little progress beyond isolation was achieved for al-most a century. The first published studies into the chemis-try of aconitine did not appear until 18947 with additionaldata published in 1895.8 Following a 29 year hiatus, threeadditional papers were published in the German literature(1924–1956).9 British researchers were also immersed inthe aconitine problem, between 1894–1937, twelve papersrelated to structure determination were published by theRoyal Society of Chemistry.10 In this time period, only onepaper was published in the North American literature.11

Despite much information being obtained about occurrenceand activities, little was known about the detailed structuredue to the limited capabilities of the classical methods ofanalysis.

This situation changed with the introduction of UV andIR spectroscopy12 and later, NMR.13 Due to their less com-plex structure, the C20 alkaloids were the initial focus of at-

tention. S. W. Pelletier and W. A. Jacobs made many contri-butions to the structural elucidation, both by chemical andspectroscopic means.14 A partial synthesis of atisine (28),isoatisine (29), and dihydroatisine (30) was achieved in1956.15

The pioneer in the field of detailed structure determina-tion and total synthesis of the Delphinium alkaloids wasKarel Wiesner (1919–1986). He arrived at the University ofNew Brunswick in 1948 determined to clear up the ‘hope-less confusion’ surrounding the structure. However, a re-view indicated that this had been tried many times in thepast to no avail. He thus concluded that the best way to ap-proach the problem would be to solve the structure of themore straightforward C20 members of the class.16 In the pe-riod 1948–1975, several total syntheses were achieved, em-ploying 1st through 4th generation methods (Table 1, Figure4). Some more recent syntheses are included in Table 1. Wi-esner published two reviews of his work.17 Notably, eventoday, the total synthesis of the fully oxygenated, mostcomplex members of the family, aconitine (27), and beiwu-tine (31) remain elusive. This article covers advances in to-tal synthesis towards diterpenoid alkaloids from 2010 on-wards after excellent coverage of the field by other reviewarticles.5,18 Figure 5 depicts structurally unique diterpenoidalkaloids that have been synthesized since 2010, such assalviamine E (32) and F (33);19 vitepyrroloid A (34) and B

Scheme 1 Biosynthetic origins of diterpenoid alkaloids

NR

H

H

ent-atisane skeleton24

veatchine skeleton14

H

H

H

H

ent-kaurane skeleton23

amination NR

H

napelline skeleton15

C7–C20bond formation

20

7

atisine skeleton7

NR

H

Hamination

denudatine skeleton8

NR

H


[1,2]-sigmatropic rearrangement

hetidine skeleton9

NR

H

HN–C6 bond formation

20 14

hetisine skeleton10

N

H

H

6


R'

NR

H

25 (C19, R' = C)

20

7

26 (C18, R' = H or O)




3919


(35);20 paspaline (36);21 paspalinine (37);22 ileabethoxazole(38), pseudopteroxazole (39), and seco-pseudopteroxazole(40);23 (–)-anominine (41), (±)-terpendole E (42);24 afla-vazole (43) and 14-hydroxyaflavinine (44);25 chamobtusinA (45);26 and emindole PB (46).21b However, these diter-penoids are not from the Aconitum/Delphinium family.Thus, the total syntheses of these natural products are notcovered in detail in this review article.

2 Total Syntheses

2.1 C18-Diterpenoid Alkaloids

2.1.1 Total Synthesis of Neofinaconitine

This convergent total synthesis of neofinaconitine (47)by Shi, Tan, and co-workers in 2013 was designed to beginwith an intermolecular Diels–Alder cycloaddition betweenthe in situ generated diene from enone 69 and cyclo-propene 70 (Scheme 2).27 To allow for this strategy, thecyclopentenone intermediate 69 was prepared fromcommercially available furfuryl alcohol,58 while the cyclo-

Table 1 Examples of Significant Total Syntheses by Compound and Year (see Figure 4)

Compound 1960s 1970s 1980s 1990s 2000s 2010s

C18 neofinaconitine (47) 201327

weisaconitine D (48) 201528

C19 talatisamine (49) 197429

197530

chasmanine (50) 197731

197832

197933

13-desoxydelphonine (51) 197832

197933

liljestrandinine (52) 201528

(–)-cardiopetaline (53) 201734

C20 atisine (28) 196335

196436

196637

196738

198839 199040 201241

garryine (54) 196442

196743

veatchine (55) 196442

196743

196844

napelline (56) 197445 198046

nominine (57) 200447

200648

200849

(–)-isoatisine (29) 201450

lepenine (58) 201451

cochlearenine (59) 201652

dihydroajaconine (60) 201653,54

N-ethyl-1-hydroxy-17-veratroyldictyzine (61) 201652

gymnandine (62) 201653

paniculamine (63) 201652

(±)-spiramine C (64) 201654

(±)-spiramine D (65) 201654

azitine (66) 201855

cossonidine (67) 201856

septedine (68) 201857


3920


propene starting material 70 was prepared in 6 steps frommethyl acrylate.59 Notably, due to the rapid Alder–ene di-merization of the cyclopropene intermediate, this com-pound was isolated and was used immediately in the cyc-loaddition reaction. The Diels–Alder cycloaddition was con-ducted by addition of intermediate 70 directly to the in situprepared diene from intermediate 69. Importantly, it wasreported that attempts at isolating the enolization–silyla-tion60 product of 69 led to a difficult-to-control mixture ofdimerized cyclopentadiene. Nonetheless, this direct addi-tion to the in situ prepared diene allowed access to the de-sired product 71 as an inseparable mixture of diastereo-

mers (1:1.6). Given that the major diastereomer was the de-sired product, this mixture was taken through the next foursteps to access intermediate 73 (39% over 6 steps). A seriesof functional group manipulations on intermediate 73 af-forded access to diene 75 and set up the second intermolec-ular Diels–Alder cycloaddition between 75 and 76 catalyzedby SnCl4.27 This is notably an essential step as it afforded akey intermediate 77 as a single diastereomer in excellentyield (87%). This was then converted into the precursor forthe Mannich-type N-acyliminium cyclization by the selec-tive oxidation of the exocyclic olefin to afford a carbonyland followed by elimination of the -bromide to form the

Figure 4 Structures of aconitine, beiwutine, and targets of total syntheses, listed in Table 1

aconitine(27)

H

NEtH

HHO

OMe

MeO

OAc

OH

OMeOH

OBz

beiwutine(31)

OH

NMeH

HHO

OMe

MeO

OAc

OH

OMeOH

OBz

neofinaconitine(47) Ar = 2-aminophenyl

weisaconitine D(48)

talatisamine R = H (49)chasamanine R = OMe (50)

H

NEtH

H

OMe

OH

OMe

OH

R

13-desoxydelphonine(51)

H

NMeH

H

OMe

OH

OMe

OH

MeO

liljestrandinine (52)

cardiopetaline (53)

atisine(28)

garryine(54)

veatchine(55)

H

H

OH

N

H

H

OHO

Et

napelline(56)

N

H

H

OH

OH

OH

nominine(57)

lepenine (58)

cochlearenine (59)

dihydroajaconine (60)

gymnandine (62)

paniculamine (63)

(±)-spiramine C, 19-(S) (64)(±)-spiramine D, 19-(R) (65)

azitine(66)

cossonidine(67)

septedine(68)

H

NEtH

H

OMe

OH

OMe

OHH

NEt

H

H

H

OMe

OEt

OMe

OH

H

NEtH

H

OMe

OH

OH

isoatisine(29)

N

H

H

O

OH

N

H

H

OH

O

N

O

N

H

H

OHEt

HOHO

N

H

HOH

N

H

H

OHEt

HO OH

OH

OC(O)ArMeOMeO

MeO MeOMeO

N

H

H

OH

OHHO

N-ethyl-1α-hydroxy-17-veratroyldictyzine (61) Ar = 3,4-dimethoxyphenyl

Et N

H

H

OH

OH OH

OC(O)Ar

Et N

H

H

OHEt

N

H

H

OHO

OH OH

OH

19N

H

H

O

OHO

H

OH

H

N

H

HOH

OH

H

NEtH

H

OH

OH

OH

N

H

H

OH

ON


3921


enone 78. The Mannich-type N-acyliminium cyclizationwas achieved by treatment of intermediate 78 with Tf2NH,thereby forming the C11–C17 bond, but was unfortunatelyalso followed by the formation of the cyclic enol ether toform 79 (75%). To overcome this undesired, but unavoid-able, formation of the enol ether, a rather nifty trick wasemployed, a leaving group was introduced at the C3 posi-tion by treatment of 79 with CAN, followed by MsCl/Et3N intwo steps, which was fortunately accompanied by the elim-ination to afford the dienone 80 (66%, two steps).27 The lastof the keys steps was completed by an intramolecular radi-cal cyclization of dienone 80 in excellent yield to afford thecompleted neofinacotine scaffold 81 (99%). Finally, installa-tion of the C8 hydroxyl functionality was achieved via ahighly strained enone, allowing for a spontaneous nucleop-hilic attack by water to give 82 and complete the structuralrequirements. Over an additional 8 steps, the remainingmodifications and tailoring were accomplished to affordthe total synthesis of racemic neofinaconitine (47).27

This divergent approach by Shi, Tan, and co-workerswas achieved in 30 longest linear steps and afforded race-mic neofinaconitine (47) from readily available materials.27

It should be noted that this strategy offers numerous op-portunities to access other types of the diterpenoid alka-loids and as such should be exploited for the preparation ofderivatives of such natural products.

2.1.2 Total Synthesis of Weisaconitine D

The Sarpong group completed the total synthesis ofweisaconitine D (48), a ranaconitine-type diterpenoid alka-loid, in 2015 in 30 steps overall (Scheme 3).28 Starting witha Diels–Alder cycloaddition of diene 8561 and a cyclopente-none derivative 86,62 the resulting alkene was reduced togive the basic bicyclic ketone intermediate 87. Preparationof the vinyl triflate by treatment of 87 with LHMDS andphenyl triflimide, followed by Pd(0)-catalyzed cross-cou-pling with cyanide63 afforded the ,-unsaturated nitrile88. This approach allowed the assembly of the A and Frings, and serves as the substrate for their Rh-catalyzedconjugate addition with the lithium boronate 89 to access90 in 60% yield. Notably, this guaiacol derivative was in-stalled with high diastereoselectivity. This transformationshould be well noted as the aryl ring of 90 after severaltransformations will eventually be fashioned into the C/Dring and, as such, this is an excellent point to diverge/modi-fy the oxidation patterns around these rings in the final di-terpenoid alkaloid simply by use of the appropriately sub-stituted arene. The ester group of 90 was selectively re-duced over the nitrile functionality, followed by Dess–Martin oxidation of the resulting primary alcohol to give al-dehyde 91. The newly formed aldehyde 91 underwent aWittig olefination, followed by hydration of the nitrilegroup64 to provide the carboxamide 92. This carboxamide92 was then rearranged via a Hofmann rearrangement us-ing (diacetoxyiodo)benzene [PhI(OAc)2, PIDA], and the

Figure 5 Recently synthesized structurally unique diterpenoid alkaloids

O

N

O

HO

O

N

O

HO

salviamine E (32) salviamine F (33)

NH

CN

OH

H

vitepyrroloid A (34)

N

CN

OH

H

vitepyrroloid B (35)

(CH2)3CO2Et

HNOH

HO

aflavazole (43)

HNOH

HO

14-hydroxyaflavinine (44)

NO

H

OH

NO

H

NO

H

seco-pseudopteroxazole (40)pseudopteroxazole (39)ileabethoxazole (38)

NH

OOH

H

H

H Hpaspaline (36)

NH

OOH

H

H

H H

OH

terpendole E (42)

N

OOH

H

H

H H

emindole PB (46)

NH

O

H

OH

paspalinine (37)

O

O

chamobtusin A (45)

N

H

O

OH

OHNH

anominine (41)


3922



Scheme 2 Total synthesis of lappaconitine-type diterpenoid alkaloid neofinaconitine

OTIPSO

OMe

TBSOTf, Et3N

TBSO

OMeOTIPS

1:1.6 (desired)

O

OMeOTIPS

NaOH, THF/H2O

OMeOTIPS

3 steps

N

O

OMe

(39% over 6 steps)

1)H2C=CHMgBr2) TBSOTf, KHMDS

(80% over 2 steps)

EtN

O

MeO2C

BrSnCl4

87% yield

OMe

O

EtN

O

Br

CO2Me

H

H

OMe

Tf2NH, DCM

75% yieldO

CO2Me

HEtN

O

OMe

1) CAN, MeCN, H2O, 60 °C2) MsCl, Et3N

(66% over 2 steps)

NEtO

MeO2C

O

Br

OMe

NEtO

MeO2C

O

O O

H

n-Bu3SnH, AIBN

99% yield

1) TMSOTf, Et3N2) PhSeCl (86%, 2 steps)3) NaIO4, THF, H2O (59%)

OMe

NEtO

MeO2C

O

O

H

HO

OMe

1) TBAF, THF2) HBr/AcOH, PhF

OTBS

(63% over 2 steps)

Br

MeO

N

O

OMe

Br

OMe

O

EtN

O

Br

CO2Me

H

H

O

1) OsO4, NMO, Pb(OAc)4 (65%)2) DBU, PhMe (87%)

Br

O

Br

1) Pd/C, H22) NaBH4 (89%, 2 steps)3) MeI, t-BuOK (34%)

OMe

NEtMeO2C

H

HO

OMe

O

OMe OMe

NEtHO

H

HO

OMe

O

OMe

1) LiBH42) CrO3, H2SO4

(40% over 2 steps)

OMe

NEtO

H

HO

OMe

OMe

1) LiAlH42) o-NO2BzCl, DMAP3) Zn, HCl, MeOH

(13% over 3 steps)

ONH2

69 70

neofinaconitine (47)848382

818079

7877

76

7574

73

7271

8

1117

Me

Me

Scheme 3 Total synthesis of ranaconitine-type diterpenoid alkaloid weisaconitine D

N

OMOM

OH

OMeH

MeO

OMe

TBSO

MeO2CO 1) 110 °C

2) cat. Pd/C, H2

(70% over 2 steps)

CO2Me

H

MeO O

OTBS

1) LHMDS, PhNTf2, 2) cat. Pd(PPh3)4, CuI, NaCN

(70% over 2 steps)

CO2Me

H

MeO

OTBS

CN89, cat. [RhCOD(OH)]2

H2O, 100 °C

CO2Me

H

MeO

OTBS

CN OMe OMOM

1) Red-Al2) DMP

(75% over 2 steps)

CHO

H

MeO

OTBS

CN OMe OMOM 1) LHMDS, Ph3PCH3Br2) cat. RhCl(PPh3)3, MeCHNOH, 110 °C

(76% over 2 steps)

H

MeO

OTBS

OMe OMOMO NH2

H

MeO

OH

NH OMe OMOM1) PIDA, KOH, MeOH2) TBAF

(96% over 2 steps)

O OMe

1) MsCl, Et3N2) KOt-Bu, t-BuOH

(73% over 2 steps)N

OMeH

MeO O

OMe

OMOM

1) 2 M HCl2) PIDA, NaHCO3

(98% over 2 steps)N

OMeH

MeO O

OMe

OOMe

150 °C

77% yield N

OMeH

MeO O

OMeOMe

O

1) NaBH42) TFA, H2O

(87% over 2 steps) N

OMeH

MeO O

O

OH

H1) MOMCl, i-Pr2NEt2) NaBH4


OMeH

MeO O

OH

OMOM

1) Tf2O, pyridine2) DBU, DMSO, 120 °C

(55% over 2 steps)

O

1) m-CPBA2) NaH, EtI

N

OMOM

OEt

OMeH

MeO O

O(76% over 2 steps)

1) Cp2TiCl2, Mn2) NaH, Me2SO4


OMOM

OEt

OMeH

MeO O

OMe

1) KOH2) Ac2O3) LiAlH44) HCl


OH

OEt

OMeH

OMe

85 90888786

91 9392

969594

weisaconitine D(48)101100

999897

+

OMe

OMOMLi(MeO)3B

89

19

19

60% yield

3923


intermediate isocyanate was trapped with methanol, fol-lowed by deprotection to the tert-butyldimethylsilyl groupwith TBAF to give 93. The primary hydroxyl group was thenactivated by mesylation and treated with KOt-Bu to formthe C19–N bond, thereby fashioning the piperidinyl ring of94. This key step allowed completion of the A, E, and F ringsfor the C18-diterpenoid alkaloids. In order to set construc-tion of the B, C, and D rings, the MOM protecting group of94 was cleaved, and the phenol was then subjected to anoxidative dearomatization to afford the dienone 95. Bymerely heating intermediate 95, it underwent an intramo-lecular Diels–Alder cycloaddition forming 96. This step ef-fectively allowed access to the C20 core framework of thedenudatine-type diterpenoid alkaloids. As such, this path-way can be viewed as an equally effective route to the de-nudatine-type diterpenoid alkaloids.

To effect the transformation of the bicyclo[2.2.2] struc-tural motif to the bicyclo[3.2.1] core, the ketone functional-ity of 96 was stereoselectively reduced. It was postulatedthat this selectivity is as a result of torsional strain from the-disposed methoxy group of the dimethyl ketal. The ketalprotecting group was then hydrolyzed to afford 97. Inter-mediate 97 was then treated with MOMCl to protect thefree secondary hydroxyl group, followed by the diastereose-lective reduction of the ketone group to provide alcohol 98.The triflation of alcohol 98, followed by treatment withDBU and DMSO at 120 °C led to a Wagner–Meerwein-typerearrangement and thus completed the B, C, and D rings in99.6 While several strategies were explored for a formal hy-dro-methoxylation of the C15–C16 double bond of 99, itwas found that this was best achieved via an epoxide inter-mediate. They employed a hydroxyl-directed epoxidationfrom the -face using m-CPBA, followed by alkylation of thetertiary hydroxyl to provide 100.28 The epoxide was regio-selectively opened65 to give a -disposed secondary alcoholgroup that was methylated to furnish 101 and thus thiscompleted the D-ring of weisaconitine D. The methoxycar-bonyl protecting group was removed, the nitrogen wasacylated, and the acetamide was reduced using LiAlH4. Fi-nally, removal of the MOM protecting by acid treatment ofafforded the final product weisaconitine D (48) in a total of30 steps.28

This successful total synthesis of weisaconitine D (48)by the Sarpong group28 should be viewed as a direct open-ing to other members of the C18-ranaconitine class of diter-penoid alkaloids as well as the C20-denudatine types. Im-portantly, their synthetic plan shows great promise in al-lowing for the preparation of any number of unnaturalanalogues of this natural product.


In contrast to the numerous reports of total syntheses ofC20-diterpenoid alkaloids, there are only a handful of recenttotal syntheses of C19 norditerpenoid alkaloids. The totalsyntheses of liljestrandinine (52)28 and (–)-cardiopetaline(53)67 have been reported by the Sarpong group andFukuyama, Yokoshima, and co-workers, respectively. Thetotal synthesis of aconitine (27) can be considered a holygrail in organic synthesis due to its intricate, highly oxygen-ated caged framework. The synthesis of this molecule hasremained elusive, and the most recent attempt was report-ed by Qin, Zhang, and co-workers.66 Several groups havemade strategic efforts towards the construction of the C19cores and the reported methodologies will be covered here.

2.2.1 Total Synthesis of Liljestrandinine

The synthesis of liljestrandinine (52)28 starting fromsynthon 93 is shown in Scheme 4; dienone 104 was pre-pared in 7 steps in a similar fashion to the synthesis of wei-saconitine D (48). At this point, the stage is set for an intra-molecular Diels–Alder reaction. The ketone moiety of theDiels–Alder adduct was then reduced to give the hexacyclicalcohol 105, which was converted into the C19-diterpenoidalkaloid liljestrandinine (52) in 9 steps.

In a unifying approach to the C18-, C19-, and C20-diter-penoid alkaloids, the Sarpong group accomplished thepreparations of weisaconitine D (48), liljestrandinine (52),as well as the denudatine-type compounds cochlearenine(59), N-ethyl-1-17-veratroyldictyzine (61), and panicu-lamine (63).4a The advanced intermediate 93 is common toall of these natural product syntheses.

2.2.2 Total Synthesis of (–)-Cardiopetaline

Fukuyama, Yokoshima, and co-workers disclosed a nov-el Wagner–Meerwein rearrangement of a sulfonyloxirane toconstruct the aconitine skeleton (Scheme 5).67 Sulfonylox-irane 114 was synthesized from synthetic intermediate 108en route to their previous reported total synthesis of lep-enine.51 Protection of the hydroxy group of 108 followed byremoval of the methoxy groups under reductive conditionswith SmI2 furnished ketone 109. Hydrogenation of the ole-fin was achieved using Pd(OH)2 on carbon to afford ketone110, which was then converted into -phenylsulfanyl ke-tone 111 via a silyl enolate intermediate. Stereoselective re-duction gave a secondary alcohol, which was subsequentlyacylated to yield 112. The sulfide group was oxidized into asulfone using Oxone and then base-mediated elimination ofthe acetate group furnished the vinyl sulfone 113. Stereose-lective epoxidation was then carried out with tert-butyl hy-droperoxide (TBHP) to give sulfonyloxirane 114 with theappropriate stereoconfiguration for a Wagner–Meerweinrearrangement.


3924


Scheme 5 Total synthesis of (–)-cardiopetaline via the Wagner–Meerwein rearrangement of a sulfonyloxirane

Surprising stability was observed with the sulfonylox-irane 114 where the intended rearrangement was not ob-served even under harsh acidic or thermal conditions. Thedesired rearrangement was finally realized by applying mi-crowave irradiation at 150 °C in methanol. An alkyl shiftfirst occurred to cleave the oxirane to give alcohol 116. Theresulting carbocation was trapped by methanol followed by

the extrusion of benzenesulfinic acid to form hexacyclicketone 117. Reduction of the ketone was carried out imme-diately due to its instability to give alcohol 118. The totalsynthesis of (–)-cardiopetaline (53) was completed withheating in aqueous sulfuric acid to unmask the alcoholgroups.

Scheme 4 Total synthesis of liljestrandinine

H

MeO

OH

NH OMe OMOM

O OMe

N

OMe

MeO O

OMe

OOMe

N

OH

OH

OMe

93

104

MeO

MeO1) 150 °C, p-xylene2) NaBH4, MeOH


OMe

MeO

OMeOMe

MeO OH

O105

liljestrandinine (52)

H

MeO

OMs

NH OMOM

O OMe

MsO

N

OMe

MeO OOH

MeO

1) DMSO, (COCl)2, Et3N, DCM2) (CH2O)n, KOH, H2O/MeOH3) MsCl, pyridine

(71% over 3 steps)

1) KOt-Bu, THF2) NaOMe, MeOH3) ClCO2Me, K2CO3; then HCl, i-PrOH

(26% over 3 steps)

PIDA, NaHCO3,MeOH

89% yield

N

OMe

MeO

MeO OMOM

O106

O

1) TFA, H2O/CHCl3 (quant)2) MOMCl, i-Pr2NEt, THF (90%)

N

OMOM

OH

OMe

MeO

MeO

107O

1) NaBH4, MeOH2) Tf2O, pyridine, DCM3) DBU, DMSO

(62% over 3 steps)

1) KOH, (HOCH2)22) Ac2O, pyridine, DCM3) LiAlH4, Et2O4) HCl (aq), THF

(57% over 4 steps)

OMe

OMe

102 103

O

EtN

MOMOSO2Ph

MW 150 °C, MeOH, 4 minH

SO2Ph

OH

≡OH

SO2Ph

– PhSO2H– H+

O

EtN

MOMO

HOMeOMe

OH

EtN

MOMO

OMe

cardiopetaline (53)

NaBH(OAc)3,AcOH

MeCN, 40 °C

(71% over 2 steps)

OH

EtN

HO

OH

aq H2SO4

110 °C

86% yield

115 116 117

118

EtN

OH

108

O

OMeOMe 1) MOMCl, i-Pr2NEt,

CH2Cl2, reflux (89%)

2) SmI2, MeOH THF, 0 °C (84%)

EtN

MOMO

O

H2 (50 bar),NaHCO3,

Pd(OH)2/C

MeOH, 100 °C

93% yield

EtN

MOMO

O

1) LDA, THF, 0 °C, then TMSCl

2) PhSCl, Et3N, DCM, 0 °C

(72% over 2 steps)

EtN

MOMO

O

SPh

1) NaBH4, MeOH, THF (87%)

2) Ac2O, pyridine, DMAP, DCM, 40 °C (74%)

EtN

MOMO

OAc

SPh

109 110 111

112

1) Oxone, H2O, MeOH, THF, 30 °C (98%)

2) KOt-Bu, t-BuOH, THF, 0 °C (92%)

EtN

MOMOSO2Ph

113

TBHP, NaH

THF,0 to 40 °C

89% yield

EtN

MOMOSO2Ph

114

O


3925


In 2017, Fukuyama, Yokoshima, and co-workers pub-lished another variant (shown in Scheme 6) of the (–)-car-diopetaline (53) synthesis, which does not require pre-acti-vation of the pivotal hydroxy group.34 The requisite diol 120was also synthesized from intermediate 108.51 The ketonein TBS-protected 119 was stereoselectively reduced to alco-hol 120 using alane with pyridine as an additive to preventdamage to the ketal moiety; hydrolysis of the ketal followedby exhaustive hydrogenation afforded diol 120, which wasthe target for the Wagner–Meerwein rearrangement. Anumber of reaction conditions were tested to initiate therearrangement, but it was finally found that p-toluenesul-fonic acid produced the desired rearranged product mixture

123. Pivalic acid was crucial for the suppression of the un-desired acylation of the diol substrate. Hydrolysis was thencarried out to liberate the alcohol groups to give (–)-cardio-petaline (53).

2.2.3 Attempted Total Synthesis of Aconitine

The most recent attempt at the total synthesis of aconi-tine (27) in 2019 was reported by Qin, Zhang, and co-work-ers (Scheme 7).66 The synthesis commenced with the deco-ration of (–)-(R)-carvone (124) to install the desired substit-uents on the A ring. Nitrone 126 was generated in situ from

Scheme 6 Total synthesis of (–)-cardiopetaline via the Wagner–Meerwein rearrangement of a diol without pre-activation

cardiopetaline (53)

EtN

OTBS

119

O

OMeOMe

H

1) AlCl3, LiAlH4, pyridine (80%)2) aq HCl3) H2, Pd/C, NaHCO3

(80%, 2 steps)EtN

OTBS

H

OH

OH TsOH, PivOH, 80 °C EtN

OH

H

OH

O

H

H

OH

EtN

HO

OH

OH

EtN

HO

HO-Piv

OPiv

EtN

HO

OR

120 121

122 123R = mixture of H and Piv

KOH, MeOH, 80 °C

(84% over 2 steps)

Scheme 7 Progress towards the total synthesis of aconitine

125

O

(–)-(R)-carvone (124)

BnO

O

OO

H

OMe PMB–NHOH, PhMe,MeCN, 80 °C

BnO

OMe

ONO

PMB

BnO OMe

OON

PMB H O

4

DDQ, Et3N,DCM/H2O (10:1)

67% yield51% yield

126 127

2. Me3O+BF4–,

proton sponge, 3Å MS, DCM, 10 °C (80%)

1. LiOH•H2O, MeOH/H2O (3:1); then 150 °C (95%)

BnO OMe

OON

O

18

128

BnO OMe

CN4

129

MeO

3

5A

OH12 steps

BnO OMe

MeO

A

N

PMB

OEOH

OOMeO

BnO C

LHMDS,THF, –78 to 0 °C

71% yield130

131

BnO OMe

MeO

N

PMB

OHHO

BnO

MeOOO

132

BnO OMe

MeO

N

PMB

OMeO

BnO

OMe

133

OH

O

S

SMeradical initiating conditions

BnO OMe

MeO

N

PMB

MeO

OBn

OH

O

OMe7 8

11

910

BnO OMe

MeO

N

PMB135134

MeO

O

OH

OBnOMe

1110

7 8

oxidation and aza-Prins cascade

HO OMe

MeO

N

Etaconitine (27)

MeO OH

OHOMe

AcO

OBz13

16158717

F

A

11 steps

7 steps

•


3926


glyoxylate 125 when treated with N-PMB-hydroxylamine,which then induced an intramolecular [3+2] cycloadditionto give isoxazolidine 127 as a single diastereomer.

Deprotection of the N-PMB group followed by oxidationwas achieved using DDQ to give imine 128. Hydrolysis, fol-lowed by Krapcho decarboxylation, and selective methyla-tion afforded -hydroxynitrile 129. Subsequent steps, in-cluding functional group manipulations and an intramolec-ular Mannich reaction, were carried out to build the E ringgiving aldehyde 130. Addition of the alkyne fragment 131provided propargyl alcohol 132. Further manipulationsprovided xanthate 133, which was the substrate required tocarry out the key radical cascade for the formation of theBD ring systems. The planned cascade first generates a radi-cal at C11 that subsequently cyclizes on to the C10 acceptorand is finally trapped by the alkyne moiety. Various radicalinitiating conditions were tested, but only decomposition ofthe substrate was observed without the formation of thedesired cyclized product 135. The success of the cascade

may be hampered by the premature interference of thealkyne moiety, the acceptor at C10 may be too distant, aswell as the possibility of the formation of nitrogen-centeredradicals on the tertiary amine. The final key transformationin the proposed route utilized an aza-Prins cascade to fur-nish the F ring of aconitine (27). Overall, the reported routeachieved the synthesis of the fully functionalized AE ringsystems of aconitine (27) in 27 steps with an overall yield of1.64% from (–)-(R)-carvone (124).


2.3.1 Total Synthesis of Septedine

The first and asymmetric total synthesis of septedine(68) was reported in 2018 by Li and co-workers.57 Septedi-ne (68) is a hetidine type C20-diterpenoid alkaloid bearingan oxygenated heptacyclic scaffold. Highlights of the totalsynthesis are shown in Scheme 8. Starting from Weinreb

Scheme 8 Total synthesis of septedine

OHN

O

OOAc

OHCNOMe

O

1) t-BuOK, MeI,2) aq HClO4 (63%, 2 steps)3) ethanolamine, NaBH4 (68%)

O

O

MOMO

O

MOMOO

LHMDS

77% yield

MOMO

H

OMeMOMO

OMe

HO

MeON

Me

OMOM

O

O

O

P N

[Ir(cod)Cl]2, Zn(OTf)2, 142

71% yield, 2:1 rr>99% ee

136

141

142

143

146

147

149

151 septedine(68)

7

15 16

1) NH2OMe•HCl2) m-CPBA

TBSOMgBr

87% yield OTBS O

OMOM

OMe

Ph3P

I MOMOOMe

OTBS

, n-BuLi

82% yield

1) TBAF2) DMP3) H2C=CHMgBr

(70% over 3 steps)

140138

139137

1) Na/NH3 (l)2) PPTS, ethylene glycol

(83% over 2 steps)

HO

H

O

O

1) TPAP, NMO (91%)2) H2C=CHLi; MOMCl (71%)

MOMO

H

O

O

aq HCl

75% yield

144 145

1) aq HCl2) DMP3) Ph3P=CH2

(79% over 3 steps)O

148

1) SeO2, t-BuOOH (78%, 1.7:1 dr at C-15)2) Crabtree cat. (68%, 3.1:1 dr at C-16)

15

16 1) Pd(OAc)2, PIDA (42% yield, 33% SM)2) H5IO6 (81%)

OAc

NOMe

O

150

O

(75% over 2 steps)


3927


amide 136, aryldiene 141 was accessible in 5 steps. Iridium-catalyzed polyene cyclization using the conditions ofCarreira gave tricyclic intermediate 143. Intermediate 146was prepared in order to perform a Diels–Alder reaction.Surprisingly, treatment of 146 with LHMDS gave cyclizedproduct 147. In a sequence of five steps, the protected sec-ondary alcohol was transformed into a ketone, and severalmanipulations on C-15 and C-16 led to intermediate 149.C–H activation of C-7 via Sanford’s oxidation conditions andconversion of the vinyl group, via an epoxide, into an alde-hyde gave 151. The natural product septedine (68) was ob-tained through a methylation, ester hydrolysis, and reduc-tive amination which occurred under concomitant conden-sation. In order to investigate the final steps (reductiveamination and N,O-ketalization), 7-deoxyseptedine wasprepared from a model substrate (not shown).

2.3.2 Total Synthesis of Azitine

A unified approach to assemble atisine-and hetidine-type diterpenoid alkaloids was presented by Liu and Ma.55

The total syntheses of azitine (66) and the reported struc-ture of navirine C (156) are shown in Scheme 9. Both syn-theses contain a common sequence from nitrile 157 to tet-racyclic dinitrile 152 in 10 steps (Scheme 10). Reduction of152 with LiAlH4 and Li/NH3 (liq.) led to the formation ofimine 153. The final steps towards natural product azitine(66) were a deprotection, olefination, and subsequent allyl-ic oxidation. The other synthesis accomplished from tetra-cyclic dinitrile 152 started with the formation of the bondbetween C–20 and C–14 under Shenvi’s conditions to givecyclized product 154. A further six steps were required toobtain a structure that was reported to be the natural prod-uct navirine C (156), but the spectroscopic data differedfrom the reported data after isolation.

Scheme 10 Synthesis of the common intermediate for the total syn-theses of azitine and the proposed structure of navirine C

2.3.3 Formal Synthesis of (±)-Atisine

A formal synthesis of (±)-atisine (28), utilizing a cascadeof oxidative dearomatization/intramolecular Diels–Aldercycloaddition, was reported by Wang, Chen, and co-workersin 2012.41 As shown in Scheme 11, aldehyde 167 was acces-sible from commercially available ethyl 2-oxocyclohexane-carboxylate (165) in 9 steps. Wittig olefination gave styrene

Scheme 9 Total syntheses of azitine and the proposed structure of navirine C

N

152

"navirine C"(156)

154

Me

O

CNHN

O

O

CN CNO

O

N

OH

azitine(66)

NO

O

153

1) TsOH, acetone, H2O, 40 °C2) Ph3PCH3Br, n-BuLi, THF, 70 °C (91%, 2 steps)3) SeO2, TBHP, DCM (56%)

1) LiAlH4, Et2O, 50 °C2) Li/NH3, THF, i-PrOH, –78 °C

Mn(dpm)3, TBHP,PhSiH3, i-PrOH (68%)

2014

(46% over 2 steps)

N

Me

O1) Pd/C, H2, MeOH, AcOH; then HCHO, NaCNBH32) TsOH, acetone, H2O, 40 °C

(72% over 2 steps)

155

1) LHMDS, THF; then Comins' reagent (65%)2) Pd(PPh3)4, LiCl, THF, Bu3SnCH2OH, 75 °C (60%)

3) MsCl, Et3N4) NaH, hordenine (41%, 2 steps)

Me2N

157O

CN NaBH4, CeCl3,MeOH, –78 °C

97% yield

160OH

CNt-BuMgCl, THF, –78 °C;then 159, Mg, THF, 90 °C;then AcCl;then NaBH4, MeOH

Cl

OMe

OTBS

HO

OMe

OTBSSOCl2, pyridine,

PhMe, 60 °C

80% yield

158 159

61% yield

161

OAc CN

OH

OMe

OTBS

Martin's sulfurane, DCM;then TBAF

74% yield

162

OAc CN

OMe

OH

PIDA, MeOH, NaHCO3;then PhMe, 105 °C

88% yield163

OAc CN

OOMe

OMe

1) SmI2, MeOH, THF2) (CH2OH)2, TMSCl, 4 Å MS, DCM

3) DIBAL-H, DCM4) Swern oxidation

(88% over 4 steps)

164

CNO

O

O

1) TosMIC, t-BuOK, t-BuOH, DME, 40 °C2) LDA, MeI

(68% over 2 steps) CN CNO

O

152


3928


168, which was transformed into phenol 169 in five steps.Oxidative dearomatization and intramolecular Diels–Aldercycloaddition was followed by hydrogenation, olefination,and cleavage of an acetal group to give enone 170. This in-termediate was previously used by Pelletier and co-workers, and therefore this constitutes a formal synthesis of(±)-atisine (28).15,68

Scheme 11 Oxidative dearomatization/intramolecular Diels–Alder cyc-loaddition cascade for the synthesis of (±)-atisine

2.3.4 Synthesis of the Hetidine Scaffold and Total Syn-thesis of Dihydroajaconine and Gymnandine

A bioinspired synthetic approach to the skeleton of theC20-diterpenoid alkaloid type hetidine was presented byQin, Liu, and co-workers;53,69 Scheme 12 outlines the syn-thetic path. Starting from 172,39,40 alkene 174 was obtainedin 5 steps. Then, a Corey–Seebach reaction followed by thewell-established oxidative dearomatization and intramo-lecular Diels–Alder cascade were performed to obtain inter-mediate 176. The synthesis of this intermediate also con-cluded the synthesis of the pentacyclic atisine skeleton.

Intermediate 177 was treated with MeMgBr, the dith-iane moiety was removed by reaction with Dess–Martin pe-riodinane, and the resulting ketone was reduced to give diol178. Deacetylation and subsequent amide formation gaveintermediate 179, featuring the requisite C-7 hydroxygroup and an o-aminobenzamide moiety. Aminal 181 wasobtained after reaction with NaNO2, CuCl, and HCl/Et2O re-ported by Weinreb and co-workers.70 A radical mechanismwas postulated that proceeds via diazotization and a 1,5-Hshift to form intermediate 181, which has the ajaconineskeleton. Cyclization to give the desired hetidine-type prod-uct 183 was achieved by an aza-Prins reaction. Throughoutthe investigations, additional unwanted heptacyclic prod-ucts were prepared (not shown).

170NAc

O

169

NAc

OH

OMe 1) PIDA, xylene, 150 °C (78%)2) H2, Pd/C

CO2Et

O

9 steps

167

NAc O

OMOM

165

NAc

OMOM

OMeOMOM

168

OMOM

OMe

Ph3P

Br

166

166, t-BuOK 5 steps

3) Ph3PCH3Br, n-BuLi (76%, 2 steps)4) TsOH (95%)

Scheme 12 Bioinspired synthesis of the hetidine skeleton

183

N

Ac178

HO

N

Bz

HOOHN

Bz

O

OAc BF3•OEt2, H2O

52% yield

182

OH1) KOH, NH2NH2•H2O, MeOH, H2O, MW 110 °C2) isatoic anhydride, DMAP, MeCN, 0 °C to rt

N

Bz

O

OH

181(40%)

180(45%)

N

Bz

HO

OH

HO+

Ac2O, Et3N,DMAP, DCM

93% yield

NaNO2, CuCl,HCl/Et2O, acetone7 N HO

OH

O

H2N179

N

Ac177

R = -(CH3)3-

RS

OSR

1) MeMgBr, THF (76%, 3.2:1 dr)2) DMP, MeCN, DCM, H2O, 60 °C, 85%

174

N

I

Ac

OMOM

OMe

171

H

SS

N

OH

Ac

OMOM

172

1) Ac2O, Et3N, DMAP, DCM, rt (88%)2) NaI, AlCl3, MeCN, DCM, 0 °C

3) DMP, DCM (84%, 2 steps)

N

OAc

Ac

O

1) Ph3PCH3Br, t-BuOK, THF, 0 °C – rt (66%)2) Ph3P, I2, imidazole, benzene (55%)

1) 171, n-BuLi, THF, –78 °C (85%)2) conc. HCl, MeOH (55%)

N

Ac175

R = -(CH3)3-

RS

SR OMe

OMOM

173

PIDA, NaHCO3,MeOH, 0 °C to rt

79% yield N

Ac176

R = -(CH3)3-

RS

OSR OMe

OMeSmI2,

THF, MeOH85% yield

(76% over 2 steps)3) NaBH4, MeOH, 0 °C 93%, 5.8:1 dr


3929


Pentacyclic intermediate 177 was also used by Qin, Liu,and co-workers as starting point for the syntheses of dihy-droajaconine (60) and gymnandine (62) (Scheme 13).53 Pre-cursor 186 was subjected to the method developed byWeinreb and co-workers.70 Hemiaminal 187 was then suc-cessfully transformed into the atisine-type alkaloid diter-penoid dihydroajaconine (60) in 4 steps.

Simultaneous cleavage of the amide C–N bond and theaminal C–O bond of aminal 188 gave cyclic imine 191 in ex-cellent yield. Oxidation of the secondary alcohol, as de-scribed by Iwabuchi and co-workers,71 and cyclization pro-moted by SmI2 and subsequent N-acetylation gave the hex-acyclic intermediate 192 featuring the denudatine core.After construction of the carbon skeleton, natural productgymnandine (62) was obtained after six additional synthet-ic steps.

2.3.5 Total Synthesis of Cossonidine

The Sarpong group chose the hydrindanone 874a,28,52 asthe entry point to their synthetic strategy (Scheme 14).They had previously enjoyed success towards the C18-diter-penoid alkaloids scaffold, as well as the preparation of the6-7-6 fused tricyclic core72 of the hetidine- and hetisine-types using this compound as a starting point.56 In order toconvert the ester into an aldehyde, 87 was treated with

LiAlH4, which led to the reduction of both the ester and ke-tone functionalities; Ley oxidation73 furnished the ketoal-dehyde intermediate, which was immediately transformedinto the olefin 195. Notably, the standard Wittig conditionswere observed to give poor yields, so Lebel’s Rh-catalyzedmethylenation74 was employed. Acylation of intermediate195 was accomplished using allyl cyanoformate (196),75 toafford the -keto ester 197, which was followed by an aryneinsertion assisted by CsF to provide intermediate 199. So asto later functionalize the C6 position, a deallylation reactionwas performed76 using catalytic Pd(PPh3)4 and PhSiH3 to af-ford the carboxylic acid 200, which was then treated with1,3-diiodo-5,5-dimethylhydantoin (DIH, 201) under photo-chemical conditions to achieve oxidative decarboxylation,77

and thus leave the C6 carbon as an internal olefin. Themethyl group at C4 was installed by deprotection of the pri-mary alcohol, followed by oxidation, then direct conversioninto nitrile 203, which then directed the diastereoselectivemethylation of 203 leading to intermediate 204. This key in-termediate bearing handles at C6 and C20 was now set upfor formation of the piperidinyl ring. Treatment of 204 witha cobalt boride (Co2B) and borane–tert-butylamine com-plex,78 followed by treatment with LiAlH4 to reduce the ke-tone and facilitate protodebromination, was accompaniedby a serendipitous direct cyclization to form the N–C20bond, thus accessing intermediate 205. Photochemical

Scheme 13 Syntheses of dihydroajaconine and gymnandine; AZADO = 2-azaadamantane-N-oxyl

192

gymnandine (62)

N

Bz

O

188(42%)

187(45%)

N

Bz

HOHO

NaNO2, CuCl,HCl/Et2O, acetoneN HO

O

H2N191

186

+

N HO

OH

HO

dihydroajaconine (60)

[Cp2Zr(H)Cl], THF, rt (93%)

N HO

N

Ac OH

NEt

OH

1) AZADO, CuCl, bpy, DMAP, air, (93%)2) SmI2, THF, reflux then AcCl, DCM, NaHCO3 (aq), rt, (78%)

N

Ac 185HO

1) KOH, MeOH, NH2NH2•H2O, H2O, MW 110 °C, (89%)

EtSiH, BF3•OEt2,DCM, –78 °C (64%)

N

Bz

HOHO

189

SeO2, TBAP, DCM190a = 53%, 190b = 10%

N

Bz

OHHO

190a R1 = H, R2 = OH190b R1 = OH, R2 = H

1) [Cp2Zr(H)Cl], THF2) 2-bromoethanol, Cs2CO3, MeCN(69% from 190b over 2 steps)

1) NaH, CS2, MeI, THF (94%)

193

N

Ac

15-epi-gymnandine (194)

NEt

OH

R1

R2

1) Red-Al, THF (87%)2) SeO2, t-BuO2H, DCM (71%)

15

1) DMP, TFA, DCM (79%)2) NaBH4, CeCl3•7H2O, MeOH, 0 °C (74%)

N

Ac 177R = -(CH3)3-

RS

OSR 1) H2, Pd/C, EtOAc,

55 °C (82%)

N

Ac 184R = -(CH3)3-

RS

SR 1) DMP, MeCN, DCM, H2O, 60 °C (81%)

2) Ph3PCH3Br, t-BuOK, THF, 55 °C (93%)

2) NaBH4, MeOH, 0 °C (95% yield, 6.8:1 dr)

2) isatoic anhydride, DMAP, MeCN, 0 °C to rt (81%)

2) AIBN, n-Bu3SnH, PhMe, reflux (74%)


3930


hydroamination79 afforded the tertiary amine 206. The for-mation of the bicyclo[2.2.2] rings was effected through aBirch reduction/intramolecular Diels–Alder sequence80 togive the structural core 208. The selective cleavage of the C1methyl ether was carried out using HBr/AcOH, followed bytreatment with K2CO3 in methanol to yield the free alcohol209. Next, a standard Wittig methylenation was carried outto convert the ketone of 209 into an exocyclic methylene210, and thus completing the C20 carbon scaffold for the he-tisine-type skeleton. A sequence of oxidation/reduction wasthen utilized to invert the stereochemistry at the C1 posi-tion affording 212, which simply left a selenium dioxide ox-idation to furnish the allylic alcohol and the final productcossonidine (67).56

2.3.6 Total Synthesis of Lepenine

The strategy employed by Fukuyama and co-workers in2014 focused on expanding from the scaffold of tetralone218, which was accessed over 8 steps starting from L-lacticacid methyl ester and guaiacol (213) in excellent yields(Scheme 15).51 Having established a successful route to the

tetralone, the ketone was then transformed into the diene219 through a Grignard addition to the ketone followed bysilver triflate mediated dehydration. Deprotection of thepivaloyl group in 219 was then followed by coupling to amethacrylic group to afford a key intermediate, triene 220.This intermediate is now set up for the intramolecularDiels–Alder cycloaddition reaction, and upon heating in thepresence of a radical scavenger (BHT), it afforded the tetra-cyclic lactone 221. Hydroboration–oxygenation of the in-ternal alkene in 221 followed by a half reduction of the lac-tone 222 with DIBAL-H allowed access to the aldehyde 223,which was then converted into a secondary amine by re-ductive amination and protected with AllocCl to yield 224.This intermediate was then treated with DMP to afford aketoaldehyde that was treated with Pd(PPh3)4 catalyst andacetic acid to remove the Alloc group and resulted in an in-tramolecular Mannich reaction. This sequence of manipula-tions provided the polycyclic system 225 and sets up theframework for the formation of the bicyclo[2.2.2] structuralcore. Two steps of functional group transformations ac-cessed the ortho-quinone monoketal 226 that was treatedwith ethylene at 70 °C resulting in an intermolecular Diels–

Scheme 14 Total synthesis of hetisine-type diterpenoid alkaloid cossonidine

CO2Me

H

MeO O

OTBS

1) LiAlH4, Et2O 2) TPAP, NMO3) [Rh(PPh3)3]Cl, Ph3P TMSCHN2, i-PrOH

(71% over 3 steps)H

MeO

OTBS

OTfO

CsF, MeCN 38–45% yield H

MeO

OTBS

Pd(PPh3)4, PhSiH3

98% yield 74% yield

1) TBAF, THF2) TEMPO, PIDA, NH4OAc

(96% over 2 steps)

LHMDS, THF;then MeI

58% yield

1) BH3(t-BuNH2), Co2B, MeOH2) LiAlH4, THF

n-BuLi, hνi-Pr2NH, THF

(71% over 3 steps)

Na, NH3,i-PrOH/THF

(54% over 2 steps)

1) HBr/AcOH2) K2CO3, MeOH Ph3P=CH2, THF

85% yield

[Cu(MeCN)4]OTf,MeOBpy, ABNO,

NMI

81% yield

LiAlH4, THF

81% yield

1) LHMDS; then:

NC O

O

88% yieldH

MeO

OTBS

O

O

O

TMS

Br

OMe

OBr

OMe

O

O

H

MeO

OTBS

OBr

OMe

O

OH

NN

O

O

I

I

201 (DIH), hν

H

MeO

OTBS

OBr

OMe

H

MeO

CN

OBr

OMe

H

MeO

CN

OBr

OMe

NH

OMe

HOMe

N

OMe

H

O

H

N

OMe

HOMe

N

OMe

HO N

OH

H

O

H

H

(60% over 2 steps)

N

OH

H

H

HN

H

H

H

O

NH

H

H

OH

39% yield

SeO2

NH

H

H

OHOH

87

cossonidine(67)

212211210

209208207

206205204

203202200

199

198

197

196

195

pyrrolidine

H


3931


Alder reaction to give the bicyclo[2.2.2] 227; functionalgroup manipulations and final deprotection of protectinggroups gave lepenine (58).51

2.3.7 Total Syntheses of Cochlearenine, N-Ethyl-1-hydroxy-17-veratroyldictyzine, and Paniculamine

The Sarpong group embarked on the mammoth task todesign a route that was suitably amenable to modificationsat the latest possible stage and yet still access 59, 63, and61/241; three denudatine-type diterpenoid alkaloids(Scheme 16).52 This was achieved by designing a route to in-termediate cochlearenine (59); from 59, following a fewlate-stage manipulations of the functional groups on the bi-cyclo[2.2.2] structural element, each of the intended targetswas successfully accessed. Starting from 93, which the

Sarpong group had previously designed and accessed in 10steps (25% overall yield),28 the primary alcohol was convert-ed into the corresponding aldehyde 230 via a Swern oxida-tion. An aldol–Cannizzaro sequence on 230 afforded thediol 231, which was then converted into the dimesylate 102(76%). Intermediate 102 was treated with KH to effect thecyclization yielding 232 as the sole product. Next, themethylene O-mesylate group of 232 was transformed into amethyl group via reduction with the combination of NaI/Zn.This was a key step in the strategy, as stereoselective instal-lation of the methyl group, which is present in all the C20alkaloids, was achieved. Following the removal of the MOMprotecting group, oxidative dearomatization of the result-ing phenol afforded the dienone 233. This dienone wasthen heated to 150 °C to allow formation of the intramolec-ular Diels–Alder cycloaddition product 234, thus forming

Scheme 15 Total synthesis of denudatine-type diterpenoid alkaloid lepenine

L-lactic acid methyl ester, Ph3P, DEAD

87% yield 85% yield

1) MsCl, Et3N (85%)2) O3, NaBH4 (86%)3) PivCl, DMAP (80%)

(82% over 2 steps)

1) H2C=CHMgCl (85%)2) AgOTf, reflux (63%)

1) DIBAL-H (89%)2) methacrylic acid, DCC, DMAP (85%) BHT, PhCN, 160 °C

BH3•THF, THF; then NaOH, H2O2

97% yield

DIBAL-H

93% yield

1) DMP, DCM (79%)2) Pd(PPh3)4, AcOH, DCM (75%)

1) KOH, MeOH; NaBH4 (95%)2) methyl red, AcCl, MeOH, PIDA (88%)

ethylene, DCM, 70 °C, 5 d84% yield

213

DIBAL-H;then H2C=CHMgCl

94% yield

OMe

OH

OMe

O

MeO2C

OMe

O

OH

4-O2N-C6H4-OH, (Et2O)3CMe

OMe

OH

EtO2C

OMe

OMs

EtO2COPiv

OMe

OMs

OPiv

O

1) LiOH, MeOH2) TFAA, TFA

OMe

OMs

OPiv

OMe

OMs

O

O

90% yield

OMe

OMs

O

O

H

OMe

OMs

O

O

H

OH

H

OMe

OMs

H

OH

H97% yield

O H

OMe

OMs

AllocH

OH

H

NEt

EtNH2•HCl, Et3N, AcOH, MeCN;

then NaBH(OAc)3,NaOH, AllocCl

OHOH

OMe

OMs

H

O

NEt

O

H

OH

NEt

OMe

OMe

O

H

OH

NEt

OMe

OMe

H

TBSO

NEt

OH1) TBSOTf, lutidine (91%)2) SmI2, MeOH (96%)

HO

H

1) DMP, DCM (72%)2) HCO2Et, KHMDS; then HCHO (50%)

H

TBSO

NEt

O

HO

H1) NaBH4, CeCl3•7H2O (83%)2) TBAF, THF (93%)

H

OH

NEt

HO

H

lepenine (58)

OH

225224223

222221220

219218217

216215214

228227

229

226

3) Red-Al, PhMe (88%)4) BH3•THF, H2O, NaBO3(H2O) (54%)


3932


all the requisite rings for the C20 scaffold. The addition ofthe carbon atom on the bicyclo[2.2.2] structure of 234 wasachieved by Wittig methylenation and ketal hydrolysis toafford 235 and a modified Weitz–Scheffer epoxidation81 toinstall an -epoxide thus forming 237. The epoxy ketone237 was then treated with a solution of HBr/AcOH at 110 °Ceffectively cleaving the methyl carbamate and the methylgroup on the C1 hydroxyl; the reaction mixture wasquenched with NaOH, then treated with Ac2O and pyridineto form intermediate 238. Functional group manipulationsof 238 gave cochlearenine (59) which was coupled with ve-ratroyl chloride to produce N-ethyl-1-hydroxy-17-vera-troyldictyzine (61) in 25% yield, and its protonated TFA saltform 241 upon treatment with TFA. Finally, treatment of 59with H2O2 gave paniculamine (63) in 50% yield.52

2.3.8 Total Syntheses of (±)-Spiramilactone B, (±)-Spi-raminol, (±)-Dihydroajaconine, and (±)-Spiramines C and D

The strategy employed by Xu and co-workers in 2016focused on targeting 242 as the entry point into this multi-target synthesis (Scheme 17).54 This tricyclic intermediate245 was accessed in large quantities, over two steps, as asingle diastereomer, in excellent yields.82,83 This tricyc-lo[6.2.2.01,6] structure 245 was then taken through a seriesof functional group transformations to afford intermediate246, which was converted into the ,-unsaturated methylester 247. This key scaffold 247 was alkylated84 at C10 bytreatment with lithium diisopropylamide and 1,3-dimeth-yl-3,4,5,6-tetrahydropyrimidin-2(1H)-one (DMPU) fol-lowed by addition of the electrophile 248 to give 249

Scheme 16 Total synthesis of denudatine-type diterpenoid alkaloids cochlearenine, N-ethyl-1-hydroxy-17-veratroyldictyzine, and paniculamine

H

MeO

OH

NH OMe OMOM

O OMe

N

OMe

MeO O

OMe

OMOM

N

OMe

MeO O

OMe

OOMe

p-xylene, 150 °C

80–87% yield

N

OMe

MeO O

OMeOMe

O

1) HBr, AcOH; then NaOH2) Ac2O, pyridine

KOAc, AcOH, 120 °C

(COCl)2, DMSO, Et3N

95% yield

H

MeO

O

NH OMe OMOM

O OMe

KOH, CH2O

97% yield

H

MeO NH OMe OMOM

O OMe

MsCl, pyridine

76% yield

OH OH

H

MeO NH OMe OMOM

O OMe

83% yield

OMsOMs

KH

MsO

H

1) NaI, Zn, DMF2) HCl, i-PrOH/THF3) PIDA, NaHCO3, MeOH

H(61% over 3 steps)

Ph3P=CH2, THF;then 1 M HCl

96% yield N

OMe

MeO O

O O

PhPh

PhO

Li

57% yield N

OMe

MeO O

O

O

NAc

OAcO

O

(51% over 2 steps)

NAc

OAcO

OH

OAc

1) H2, Pd/C (cat.)2) LiAlH4, THF

(77% over 3 steps) NEt

OHOH

OHOH

cochlearenine (59)

Cl

O

MeO

MeODMAP

(polymer bound)

NEt

OHOH

OOH

O

OMe

OMeN

OHOH

OOH

O

OMe

OMe

H EtN

OH

Et

OH

OHOH

O

paniculamine (63)

N-ethyl-1α-hydroxy-17-veratroyldictyzine (61)

93

241

239238

237

236

235234

233232102

231230

F3COO

240

THF

25% yield

H2O250% yield

TFA

(quant.)


3933


Scheme 17 Total synthesis of atisine-type diterpenoid alkaloids spiramilactone B, spiraminol, dihydroajaconine, spiramine C, and spiramine D

The alkynyl-substituted ester 249 was desilylated andacylated, and then subjected to Ru-catalyzed cycloisomeri-zation to give tetracyclic ketone 250 as a single diastereo-meric product. To set up for the bridged core structure, in-termediate 250 was treated with LiBEt3H, the C6–C7 olefinwas epoxidized using m-CPBA, and the free hydroxyl groupwas protected; regioselective epoxide opening catalyzed by[Ti(Oi-Pr)2Cl2] afforded the formation of hydroxy -lactone251, thereby completing the pentacyclic core structure.This -lactone 251 was rearranged to the -lactone 252

over a few synthetic manipulations. Treatment of 252 withozone gave the ketone which was converted into a terminalolefin by Takai olefination;85 this lactone was then partiallyreduced to the aldolactol, followed by condensation withMeOH to afford 253. A hydroboration–oxidation reactionon the olefin of 253 allowed access to the primary alcohol,which was converted into the aldehyde, thus allowing forselective alkylation on treatment with potassium tert-bu-toxide and methyl iodide. The lactone 255 was directly ac-cessed by a Pinnick oxidation86 of 254 and completed the

PIDA, MeOH

95% yield 91% yield

1) SmI2, THF/MeOH (72%)2) (CHOH)2, CSA (90%)3) DMP, DCM, NaCO3 (92%)4) H2, Pd/C, MeOH (96%)

(89%, 2 steps)

LDA, DMPU, THF

1) K2CO3, MeOH (95%)2) n-BuLi, ClCO2Me, THF (83%)3) [CpRu(MeCN)3]PF6, TsOH (91%)

1) DMP, NaHCO3 (90%)2) SmI2, THF/MeOH3) NaBH4, CeCl3•7H2O4) DIC, HOBt (75%, 3 steps)

1) O3, DCM (95%)2) PbCl2, Zn, TiCl4, CH2Br2 (72%)3) DIBAL-H, DCM4) PPTS, MeOH (85%, 2 steps)

1) BH3•SMe2, H2O2, NaOH (75%)2) DMP, NaHCO33) t-BuOK, MeI

242OH

mesitylene,reflux

1) NaHMDS, Tf2NPh2) Pd(PPh3)4, Et3N, CO, MeOH

1) LHMDS, (Me2N+=CH2)I–, MeI, DBU (71%)2) NaBH4, MeOH/DCM (95%)

MeO OH O

OHMeO

MeO

OOMe

OMe

OMe

OMe

O

HO

OH

O OMe

OMe

HO

O O O O OMeO2C TMS

I

75% yield

O O

TMSCO2Me

CO2Me

MeO2C

O

1) LiBEt3H, THF (85%)2) m-CPBA, NaHCO3 (71%)3) MOMCl, i-Pr2NEt (89%)4) [Ti(Oi-Pr)2Cl2], DCM (83%)

MeO2C

OH

O OOMOM

MeO2C OMOM

OO

OMOM

O

O

OMOM

O

CHO

MeO

NaClO2, NaH2PO4, 2-methylbut-2-ene,

t-BuOH/H2O OMOM

OOO

1) ZnBr2, DCM, n-PrSH2) DMP, NaHCO3

OOO

O

OOO

OH

1) PHSCl, Et3N (99%)2) P(OMe)3, MeOH (64%)

OOO

(±)-spiramilactone B (259)

OH

OO

OHHO

ON

OH

(±)-spiramine C, 19-(S) (64, 60%)(±)-spiramine D, 19-(R) (65, 36%)

(±)-spiraminol (260)

ON

OH

NHO

OH

HO

1) DIBAL-H, DCM (90%)2) H2N(CH2)2OH, THF, silica gel, MeOH (60%)

NaBH4,MeOH (85%)

DIBAL-H, DCM (90%)

H2N(CH2)2OH, THF, silica gel,

MeOH

(±)-dihydroajaconine (60)

(78% over 3 steps) (81% over 2 steps)

O

19

248

257256

255254

253252

251250

249247246

245

244

243

258

O

( )3

( )3

( )3

( )3


3934


core skeleton of spiramilactone B (259). At this point only afew tailoring steps were required to access the specific di-terpenoids of the atisine type.54

2.3.9 Total Syntheses of (–)-Methyl Atisenoate, (–)-Isoatisine, and a Hetidine Derivative

The Baran group developed a unified approach to ent-atisane diterpenoids and related alkaloids.50 As illustratedin Scheme 18, (–)-methyl atisenoate (264), (–)-isoatisine(29), and also 268, a hetidine skeleton, were synthesizedstarting from (–)-steviol (261). The synthesis of ent-atisanestarted by methylation of 261 followed by treatment withMukaiyama’s conditions,87 which led directly to diketone262. Cyclization of 262 was realized by Amberlyst-15 pro-moted aldol reaction and then dehydration reaction withMartin’s sulfurane gave exomethylene 263, which under-went Wolff–Kishner reduction and re-esterification to com-plete the synthesis of (–)-methyl atisenoate (264). Interme-diate 266 was obtained from (–)-steviol (261) in 6 steps.Neopentyl iodide 267 was obtained after a Hoffmann–Löf-fler–Freytag reaction. Condensation with allylamine led toMannich-type reaction cyclization and thus formed thehetidine skeleton 268 in good yield. The synthesis of theother diterpenoid alkaloid presented, (–)-isoatisine (29),

was also synthesized starting from intermediate 266. Theketone moiety on C13 was removed in 3 steps to give 269,C–H activation led to neopentyl iodide 270, which was sub-sequently transformed into alcohol 271. Installation of anexomethylene group into 271, followed by allylic oxidationand the subsequent condensation with ethanolamine gave(–)-isoatisine (29).

3 Strategies to Synthesize Ring Systems

3.1 Radical-Based Cyclizations

In 2016, Inoue and co-workers disclosed the use of aradical-based cyclization/translocation/cyclization processfollowed by an aldol cyclization for the synthesis of thefused hexacycle of puberuline C (278) (Scheme 19).88 Treat-ment of tricycle 272 with n-Bu3SnH and 273 (V-40) underrefluxed conditions initiated a 7-endo cyclization from C11to C10, followed by a 1,5-hydrogen abstraction from C7 toC17, and finally resulting in a transannular 6-exo cycliza-tion from C17 to C8. The efficiency of this cascade is re-markable as five stereocenters (C8, C9, C10, C11, and C17)are introduced with the desired stereoconfiguration despitethe moderate yield of 274 (54%). After treatment of 274

Scheme 18 Synthesis of (–)-methyl atisenoate, (–)-isoatisine, and hetidine derivative from the common precursor (–)-steviol

OMeO

OHO

OH

O

OMeO

O

1) TBAF, MeI2) Co(acac)2, Et3SiH, O2 (68%, steps)

1) Amberlyst®152) Martin's sulfurane (90%, 2 steps, exo/endo 8:1)

OMeO

O

1) NH2NH2, KOH, 100 °C,2) TBAF, MeI (62%, 2 steps)

1) HOBt•H2O, EDC, NH4OH, THF (80%)2) LiAlH4, THF, 70 °C (94%)3) DIPEA, (EtO)2P(O)Cl, MeCN, 60 °C (93%)

NH

P(O)(OEt)2

OAc

NH

P(O)(OEt)2

OAc

1) NaBH42) TCDI, DMAP3) (TMS)3SiH, AIBN

N

P(O)(OEt)2

OAc

PIDA, I2, hν

then K2CO3, MeOH

IO

OH

IHN

O

OH

1) Martin's sulfurane2) SeO2, TBHP (57%, 2 steps, exo/endo 8:1, >19:1 dr)3) ethanolamine (89%)

N

P(O)(OEt)2

OAc

I

O

O

PIDA, I2, hν58–71%

261

isoatisine(29)

271270

268267

264

263

269(40%, 3 steps)

266

262

N

OAc

O

NH2

MeOH, 60 °C

78% yield

13

20

NH

OHP(O)(OEt)2

1) TESH, Co(acac)2, O2, DCE, 40 °C (59%)2) Amberlyst®15, acetone, 40 °C (87%)3) Ac2O, DMAP, CHCl3/Et3N, 40 °C (90%)

265

71% yield


3935


with TBSOTf to give the silyl enol ether 275, variety ofLewis acids were screen for the intramolecular Mukaiyamaaldol reaction of 275; this reaction was realized using SnCl4to activate the acetal moiety for the formation of the D ringin 277. The stereoconfiguration of C16 can be explained bythe binding of SnCl4 between the oxygen and nitrogen at-oms in 276 which dictates the nucleophilic attack of the si-lyl enol ether on to the si-face of the oxocarbenium ion.Subsequent steps installed the stereocenter at C14 of thehexacyclic core of puberuline C.

In 2016, Inoue and co-workers detailed the formation ofthe pentacyclic core of talatisamine (49) via radical and cat-ionic cyclizations (Scheme 20).89 Treating 279 with n-Bu3SnHand 280 (V-70) resulted in the formation of a stereochemi-

cally fixed C11 bridgehead radical 283 that underwent ste-reo- and regioselective 7-endo cyclization on to C10 to formthe B ring in 284. Steric repulsion between the C5-TMSether and C14-acetal moieties favored the transition state283 over 281.

Purification of 284 with silica gel caused C14-oxygen -elimination and C9 epimerization to form enone 285 andtrans-fused 286 from the initial cis-fused 284, respectively.Treatment of 285 with NaH yields 286 in a 79% combinedyield from 279. Pentacycle 286 was then converted into 287in ten steps which were required to carry out a 6-endo cat-ionic cyclization to install the D ring. The C15–C16 doublebond was activated with PhSeCl, allowing attack by the TBS

Scheme 19 Radical-based cyclization/translocation/cyclization process followed by an aldol reaction for the synthesis of the fused hexacycle of pu-beruline C

ONEt

Br

HOCO2Et

17

1198

n-Bu3SnH, 273 (V-40),PhMe, reflux NEt

O

OBnBnO

BnO

OBn

H

H

HOCO2Et

11

17

9

8

54% yieldNEt

TBSO

BnO

OBn

H

H

HOCO2Et

90% yield

TBSOTf, Et3N, DCM

13

14

SnCl4

CH2Cl2–78 to 0 °C

TBSO

HOCO2Et

NEt

OH SnCl4

Bn

NEt

O H

HOEtO2C

H

BnO H1316

16

16

1314

46% yield NEt

MeO H

OHCO2Et

H

MeO H1316

273 (V-40)

NN

NC

CN

7

272 274 275

277 278276

O

CO2Et

9 steps

6 steps

10

165

Scheme 20 Radical and cationic cyclizations for the formation of the pentacyclic core of talatisamine

O

NEt

Br

TMSOCO2Et

11

8

O

O

Cn-Bu3SnH,280 (V-70)

NEtO

O

O

TMSOCO2Et

NEt

OCO2EtO

O

O

TMS

10

10

NEt

TMSOCO2Et

10O

O

O

HH

NEt

CO2Et

O

O OO

TMS

≡

17

NEt

H

H

TMSOCO2Et

O

O

O

10

8

911

B

SiO2

17

NEt

H

H

TMSOCO2Et

O

O

O

10

89

11 +

17

NEt

H

TMSOCO2Et

O

10

89 11

O

OH

14

NaH

79% from 279

NEt

H

TMSOCO2Et

8

TBSO

HO

TBSO

PhSeCl

13

14

1516

O

TBS

SeO

HO

H

TBS

Ph

NEt

O H

HOCO2Et

OH

1316

PhSe

TBSO

15

NEt

O H

HOCO2Et

OHTBSO

279

281

283

282

284

285286287288289

290

87% yield

8 stepsO

CO2Et

10 steps

2 steps

280 (V-70)

NN

NC

CN

MeOOMe

165


3936


enol ether to afford pentacycle 289. Finally, oxidation of sel-enide 289 triggers a selenoxide elimination to give the pen-tacyclic core 290 of talatisamine (49).

In 2017, Inoue and co-workers disclosed a three-compo-nent coupling approach to append the AE ring systems tothe C ring of C19-diterpenoid alkaloids in an expedient man-ner (Scheme 21).90 The strategy utilized a radical-polarcrossover cascade initiated with Et3B/O2 and iodide 291 toform a bridgehead radical that first underwent conjugateaddition with enone 292 on the opposite side of the OTBSsubstituent. The boron enolate intermediate 293 then un-derwent an aldol addition with alkynal 294 via the six-membered transition state 295 to form the coupled product296. High chemoselectivity was demonstrated as no addi-tion to the alkyne was observed. The simultaneous genera-tion of three new stereocenters in a single step under mildreaction conditions proved to be an efficient method toprovide the advanced intermediate for the synthesis ofC19-diterpenoid alkaloids.

Scheme 21 Three-component radical-polar crossover cascade for the formation of the ACE ring systems of C19-diterpenoid alkaloids

Wang, Xu, and co-workers utilized, in 2018, a 1,7-enynereductive radical cyclization and double condensation forthe synthesis of lycoctonine-type C19-norditerpenoid alka-loid (Scheme 22).91 1,7-Enyne 297, which was preparedfrom a previously reported intermediate,92 underwent a re-ductive cyclization to form the A ring of pentacyclic inter-mediate 298 when treated with n-Bu3SnH and AIBN. Dou-ble reductive amination of 299 with ethylamine followed byan acetonide removal furnished the hexacycle 300 whichwas the precursor to the pentacyclic core of methyllycaco-nitine (301).

Scheme 22 1,7-enyne reductive radical cyclization and double con-densation for the synthesis of lycoctonine-type C19 norditerpenoids

Another radical-based cyclization was adopted by Xuand co-workers in the synthesis of the BCD ring systems of7,17-seco-type C19-norditerpenoid alkaloids (Scheme 23).93

It was shown that similar ring systems can be accessed via a[3+2] cycloaddition of a nitrile oxide. Bicyclic iodide 304participated in a radical cyclization when treated with n-Bu3SnH and AIBN to form tricyclic lactone 305 which canthen be transformed into 306, a key intermediate to theABEF ring system of C19-norditerpenoids. The alternativeapproach utilized an intramolecular [3+2] nitrile oxide cyc-loaddition to form the tricyclic system. Oxime 307 was oxi-dized into the nitrile oxide upon treatment with NCS, andthis nitrile oxide underwent an intramolecular cycloaddi-tion to form tricycle 308. Cleavage of the N–O bond wasachieved under hydrogenolysis conditions with Raney nick-el to afford -hydroxy ketone 309, which can then be trans-formed into the key tricyclic core 310.

3.2 Ruthenium-Mediated Enyne Cycloisomeriza-tion

Song, Qin, and co-workers drew inspiration from Trost’sruthenium-mediated enyne cycloisomerization94 for thesynthesis of the CD ring systems of aconitine (27) (Scheme24).95 Chiral enyne 311 was treated with 50 mol% Cp-Ru(MeCN)3PF6 to give the five-membered ruthenacycle 312which then underwent -hydride elimination followed byreductive elimination to give stereoisomers 313 and 314 in2:3 ratio. Initial attempts were made with palladium cata-lysts, but they did not give cyclized products.

O

OTBS

O

H

TMS

NH

O

OMe

I

10

BEt3, O2

DCM

NEt

O

OMe

Et2BOOTBS

H

B

OO

Et

Et

HTBSO

PhO

EtN

OMe

H

108

9

NEt

O

CH2OMe

OOTBS

H

292

291

293

294

295

HO

TMS

9

H

6

10

55% yielddr(C8) = 4:1

296

A

E

C

7 steps

O

CO2Me

8

165

OMe

OMe

OOO

OMe

OMe

OOO

O

O

OMe

OMe

OOOO

O

1) EtNH2, 50 °C; then NaBH4

2) AcOH, THF, H2O, 80 °C

OMe

OMe

OHOHONEt

297

298

299

300

26 stepsMeO

OTHP

5 steps

OMe

OMe

OHOHNEt

301

71% yield

OH

302

NaBH4,0 °C

(39% over 4 steps)

AIBN, n-Bu3SnH, 90 °C;then SiO2, DCM, 25 °C

87% yield


3937


Scheme 24 Ruthenium-mediated enyne cycloisomerization for the construction of CD ring systems of aconitine

3.3 Reductive Coupling

In 2014, Xu and co-workers reported the synthesis ofABEF ring systems of lycoctonine-type and 7,17-seco-typeC19-norditerpenoid alkaloids starting from the trans-fused6,7-bicycle 318 formed from a ruthenium-catalyzed diaste-reoselective enyne cycloisomerization protocol developedby Trost94 (Scheme 25).96 Treatment of bicycle 318 withm-CPBA in the presence of catalytic TsOH resulted in thestereoselective epoxidation of the olefin with concurrentring-opening by the ester group to give tricyclic lactone319. A series of transformations on 319 afforded tetracycle320. Finally, reductive coupling between the ketone andN,O-acetal in 320 was accomplished using SmI2 to form tet-racycle 321 bearing the ABEF ring systems of Lycoctonine-type and 7,17-seco-type C19-norditerpenoid alkaloids.

Scheme 25 Construction of the ABEF ring system with ruthenium-catalyzed enyne cycloisomerization and SmI2-induced reductive cou-pling

Sugita and co-workers reported a reductive cyclizationstrategy of ,-epoxy ketone 322 to synthesize the CD ringsystem of aconitine (27) (Scheme 26).97 SmI2 initiates theformation of samarium enolate 323 that subsequently un-derwent an intramolecular aldol cyclization with the pen-dant aldehyde. The reaction was expected to proceedthrough cyclic transition state 324. Initial reaction condi-tions either gave low yields of bicyclic ketone 325 or sideproducts from premature protonation of 324 or with ob-served TES group migration. Optimized conditions withSmI2 (6 equiv) and HMPA (24 equiv) furnished the desiredproduct 327 in 71% yield.

Scheme 23 Radical cyclization and [3+2] cycloaddition for the forma-tion of the BCD ring system of 7,17-seco-type C19 norditerpenoids

I O

O

n-Bu3SnH,AIBN

PhH, 90 °C

85% yield

O

O

H

CO2Me

CO2Me

O

O

H

CO2Me

NOH

NCS, pyridine

DCM, 60 °C82% yield

CO2Me

N O

O

O

OH

H2, Raney Ni,H3BO3

MeOH/H2O

70% yield

CO2Me

O HO

304

305 306

308 309 310

307

4 stepsMeO2C

4 stepsMeO2C

2 steps

2 steps

303

303

O

BnO

OO

CpRu(MeCN)3PF6 (50 mol%)

acetone

O

O

OMe

OBn

Ru

i-PrO

O

O

OMe

OBni-PrO

+

O

O

OMe

OBn

O

i-Pr

2 : 330% yield (69% brsm)

311

312

313 314

MeO13 steps

O

OMe

PMBO

OBn

315

CO2Me

MeO2C

CpRu(MeCN)3PF6

(5 mol%)

acetone, 23 °C99% yield

CO2Me

H

MeO2C

m-CPBA TsOH (11 mol%)

DCM, 0 °C–rt70% yield

ONEt

O

SmI2

THF, reflux, 3 h80% yield

NEt

OH

17

7

717

HO

O

7

MeO2CO OH

317

318

319 320

321

3 stepsEtO2C

11 steps

316


3938


Scheme 26 Reductive cyclization of ,-epoxy ketone for the synthesis of CD ring systems of aconitine

3.4 Diels–Alder Reactions

Brimble and co-workers reported the enantioselectivesynthesis of the ABEF ring systems of methyllycaconitineanalogues (Scheme 27).98 The elegant approached utilizedsequential Diels–Alder reaction, aldol, Mannich, andWacker-type cyclizations to furnish the tetracyclic scaffold337. The enantioselective synthesis commenced with acobalt(III)–salen complex 331 catalyzed Diels–Alder reac-tion between enal 329 and diene 330 to give the endo-adduct 332 in excellent yield and enantioselectivity; thebenzyl group on the amine was necessary to affect high en-antioselectivity. The endo-adduct 332 was then treatedwith LDA to promote intramolecular aldol cyclization be-tween the ester and the aldehyde to give -hydroxy ester333. A three-step sequence ending with an intramolecularMannich reaction was used to construct the ABE tricyclicring system 334. Reduction of the ketone in 334 gave alco-hol 335, which underwent an intramolecular Wacker-typeoxidative cyclization to install the F ring of the tetracycle336. Subsequent transformations provided alcohol 337,representing the ABEF scaffold of methyllycaconitine ana-logues.

Xu, Wang, and co-workers initiated the synthesis of theAEF ring systems of C19-norditerpenoid alkaloids with a ste-reoselective intramolecular Diels–Alder reaction (Scheme28).99 The cycloadduct furnished the bicyclic lactone 341with 5:1 endo selectivity thus establishing the A ring of thetarget. Subsequent transformations provided the N-pro-tected aminoaldehyde 342. Deprotection of the Boc groupin 342 under acidic conditions was followed by intramolec-ular Mannich reaction to furnish the tricyclic ketone 343,thus completing the synthesis of the AEF ring systems.

Scheme 28 Stereoselective Diels–Alder reaction and intramolecular Mannich reaction for the synthesis of the AEF ring systems of C19-nordi-terpenoid alkaloids

Barker and co-workers drew inspiration from the workof Yang and co-workers100 by applying a tandem Diels–Alder/Mannich reaction for the construction of the AE andABE ring systems of Delphinium alkaloids (Scheme 29).101 Atitanium-mediated [4+2] cycloaddition was carried out be-tween the -{[(cyanomethyl)amino]methyl}acrylate 344and diene 345 to form cyclic enolate intermediate 346. Thisenolate intermediate then underwent an intramolecularMannich reaction with the unmasked iminium ion togive bicyclic ketone 347, which mapped on to the AE ring

O

O

OTESOHC

O

OTESOHC

I2SmO

SmI2 O

O

SmI2H

OTESOSmI2

OSmI2

O

OTESOSmI2

OH

O

OTESHO

H3O+10

OMOM

O

OMOMO

OTBS

O

322

323 324 325

327 328

OH

HO

8 steps

SmI2 (6 equiv),HMPA (24 equiv),PivOH (3 equiv)

THF, –78 °C

71% yield

9 steps

326

Scheme 27 Sequential Diels–Alder reaction, aldol, Mannich, and Wacker-type cyclizations for the formation of the ABEF ring systems of methyllycaconitine analogues

Bn

BocN

O

OMeOO

OMeO

BocN

Bn

DCM, 0 °C, 4 h

331 (2.5 mol%)

90% yield97.5% ee

LDA, –78 °C

OH

OMeO

BocN

Bn1) DMP, pyridine (67%)

2) TFA3) formalin, K2CO3 (67%, 2 steps)

O

OMeO

BnN

MeOH,0 °C to rt

OH

OMeO

BnNNaBH4

64% yield

DMSO, 50 °C

OMeO

BnNPd(OAc)2, O2

95% yieldO

OH

BnN

O

329332

334

335 336

337

333

330

+

72% yield

3 steps N

O

N

O

t-Bu

t-Bu

t-But-BuSbF6

Co

331

AO

OPMBO2C

xylene, BHT,130 °C O

97% yield endo/exo 5:1

H

O

CO2PMB

OH

CO2Me

O

6

11

H

OMe

EtN Boc

5TFA

NEt

O

CO2Me

MeO 56 11

73% yield

340 341

342 343

OH

CO2PMB

DIC, DMAP

89% yield

339

CO2H

19 steps

+

338


3939


systems of Delphinium alkaloids. Similarly, using the diene-containing a cyclohexene moiety 349 resulted in the forma-tion of tricyclic ketone 350, which maps on to the ABE ringsystems. Installation of the succinimide-bearing benzoylgroup furnished the methyllycaconitine analogue 351. Itwas also shown that various derivatizations of the bicycliccore 347 could be used to give a wide variety of possibleprecursors of Delphinium and Aconitum alkaloids analogues(Scheme 30). Substitution patterns of methyllycaconitine(352), grandiflorine (353), talatisamine (354), delcosine(355), eldeline (356), inuline (357), ajacine (358), delvestine(359), and majusine A (360) were amongst the presentedexamples, as well as their N-benzyl analogues 364–368.

Scheme 29 Tandem titanium-mediated Diels–Alder/Mannich reaction for the construction of the ABE ring systems of Delphinium alkaloids

3.5 Oxidative Dearomatization/Diels–Alder Se-quence

One of the extensively used strategies in syntheses of di-terpenoid alkaloids and related diterpenes is the oxidativedearomatization/Diels–Alder cycloaddition cascade.102

Xu, Wang, and co-workers also demonstrated the syn-thetic versatility of the Diels–Alder reaction by applying itto the synthesis of the BCD ring systems of C19-norditer-

penoid alkaloids (Scheme 31).92 An intramolecular Diels–Alder reaction of a masked o-benzoquinone 243 developedby Liao and co-workers was applied to synthesize the tetra-cyclic ketone 245.82 PIDA-mediated oxidative dearomatiza-tion of phenol 242 in the presence of methanol gave diene243 that underwent an intramolecular [4+2] cycloadditionto give 245. Subsequent transformations of 245 formed tri-flate 369, which underwent Wagner–Meerwein rearrange-ment, an alkyl shift with extrusion of the triflate group, inthe presence of DBU and DMSO to form carbocation 370that was trapped by DMSO giving 371. Elimination of di-methyl sulfide from 371 gave enone 372 thus constructingthe BCD ring systems of C19-norditerpenoid alkaloids.

A similar PIDA-promoted oxidative cyclization strategywas applied by Xu, Wang, and co-workers to furnish theBCD ring systems of aconitine (27) (Scheme 32).103 Treat-ment of phenol 374 with PIDA in the presence of methanolresulted in the dimer 375. Fortunately, the desired cycload-duct 376 was formed under thermodynamic conditions toinduce a retro-[4+2]/intramolecular Diels–Alder reactionfrom 375; reduction of the ketone in 376 provided alcohol377. Sulfonylation of the alcohol in 377 on exposure to tri-flic anhydride initiated the Wagner–Meerwein rearrange-ment to form tricyclic alcohol 378, containing the BCD ringsystems of aconitine (27), from intermediate 379.

BnNCO2Et

NC TMSO

+46% yield EtO2C NBn

O

Cl3Ti

≡EtO2C NBn

O

O

RN

CO2R

BnNCO2Me

NCTMSO

+

TiCl4, DCM,–78 °C to rt

20% yield

O

BnN

CO2Me

7

4

2

9

4

2

9

7

MeO

BnN

O

O

N

O

O

344 346345

347

348 349 350

351

TiCl4, DCM,–78 °C to rt

5 steps

Scheme 30 Derivatization of a bicyclic ketone to furnish various syn-thons for Delphinium and Aconitum alkaloids

O

N

CO2R

R

4

2

9

7

R1O

NR2

O

O

N

O

OR1 = Me, R1 = H, R1 = Me, R1 = Me, R1 = H,

HO

NR

HO354 R = Bn 355 R = Et

R1O

NR2

R3

R1 = Me, R1 = Me, R1 = Me, R1 = Me, R1 = Me, R1 = Me, R1 = H, R1 = H,R1 = H, R1 = H,

O

O

H2N

O

O

AcHN

A

B

347

R2 = BnR2 = BnR2 = Ph(CH2)3R2 = EtR2 = Et

R3 = HR3 = H R3 = AR3 = A R3 = BR3 = B R3 = AR3 = A R3 = BR3 = B

R2 = Bn,R2 = Et, R2 = Bn,R2 = Et, R2 = Bn,R2 = Et, R2 = Bn,R2 = Et,R2 = Bn,R2 = Et,

352353361362363

364356365357366358367359368360


3940


Scheme 32 PIDA-promoted oxidative cyclization and a Wagner–Meerwein rearrangement to furnish the BCD rings systems of aconitine

The Sarpong group presented studies in 2014 on C20-di-terpenoid alkaloids, showing the synthesis of hetidineframework and its conversion into the atisine core struc-ture.104 Starting from commercially available -keto ester380, advanced intermediate 381 was obtained in 14 steps.Treatment of 381 with the hypervalent iodide reagent[bis(trifluoroacetoxy)iodo]benzene (PIFA) facilitated oxida-tive dearomatization to give cyclic carbamate 383. A se-quence of dihydroxylation, periodate cleavage, and Michaeladdition, promoted by simple stirring with silica gel in di-chloromethane, gave aldehyde 384. Hydrogenation and al-dol reaction gave synthon 385 (Scheme 33).

Scheme 33 Synthesis of a versatile intermediate for the hetidine framework

Scheme 31 Intramolecular Diels–Alder reaction and a Wagner–Meerwein rearrangement for the construction of the BCD ring systems of C19-norditerpenoid alkaloids

MeO

OHPIDA

MeOH, reflux

51% yield

OMeO

MeOOH

3

OH

3

HO

HO

OMe

OMe

H

TBSO

HOTf

OMe

OMe

H

DBU

DMSO, 120 °C

OMe

OMeOTf

HTBSO

H

H

TBSO

MeO OMe

H

H

TBSO

MeO OMe

S

O H

OS

DBU

H

H

TBSO

MeO OMe

O

85% yield

– Me2S

10

1

5

815

1614

10

12

H

H

O

OH

OMe

O

242

243 245

370 371

372 373

369

4 stepsMeO

OTHP

3 steps

5 steps

≡

302

OH

OMeTBSO

PIDA, MeOH,0 °C to rt;

then mesitylene,180 °C

90% yield

O

OMe

OMe

O

R

ROMe

OMe

R =OTBS

O OMe

OMeH

OTBS

HLiAlH4

THF, 0 °C

OMe

OMe

OTBS

H

OH

H

50% yielddr = 5:4

then SiO2

OMe

OMe

OTBS

H

OTf

H

85% yield

Tf2O, pyridine,

DCM, 0 °C;

OHTBSO

MeO OMe

374

375

376 377

379 378

5 steps

OTHP

OMe

302

N

Boc

14 stepsMeO2C

OOMe

380

OH

381

N

O

O O

I

OAc

Ph

N

O

OO

382

383

PIFA, NaHCO3, 0 °C

1) OsO4, NMO2) NaIO4 (SiO2) (59%, 2 steps)3) SiO2, DCM (80%)

N

O

O

O

O

385

N

OO

OHO1) H2, cat. Rh/Al2O3 (55%)

2) K2CO3 (78%)

384

54% yield

Scheme 34 Construction of atisine and hetidine frameworks

385

N

OO

OHO

1) NaH, CS2, MeI, 0 °C2) n-Bu3SnH, AIBN, 95 °C (65%, 2 steps)3) LHMDS, –78 °C, 1 h; then Comins' reagent (83%)

dihydronavirine (387)

NH

HO

O

Me2N

1) Ba(OH)2•8H2O, EtOH, H2O, 160 °C MW, 30 min 58% yield

NO

OHO

386

388

N

OO

OTf

2) PhIO, NaHCO3, 75%

4 steps


3941


The versatile synthon 385 was used to synthesize com-pound 386 having the atisine carbon skeleton in 2 steps(Scheme 34). The same synthon was also used to synthesizedihydronavirine (387), which unfortunately could not beconverted into the natural product navirine.

In a methodological study, Chen and Wang presented anefficient synthesis of the C/D rings (Scheme 35) of atisine-type C20-diterpenoid alkaloids.105 The synthesis also relieson the oxidative dearomatization/intramolecular Diels–Alder strategy.

Scheme 35 Synthesis of the C/D rings of atisine-type C20-diterpenoid alkaloids

Xu and co-workers accessed the core ring systems ofSpiraea atisine-type diterpenoid alkaloids through a bio-in-spired synthetic route.106 A four-step process led from com-mercially available methyl cyclohex-1-enecarboxylate (391)to trans-decalin 392 (Scheme 36). Epoxidation of 392 in thepresence of catalytic amounts of TsOH gave -lactone 393.Intermediate 393 was converted into -lactone 394 in 4steps; a further 8 transformations led to tetracyclic lactone395. This synthon was successfully converted into penta-cyclic target 396, featuring the ABEFG rings of spiramine C(64) and D (65).

Scheme 36 Biomimetic synthesis of the ABEFG pentacyclic amine of Spiraea atisine-type diterpenoid alkaloids

3.6 Transannular Aziridination

Xu and co-workers reported a PIDA-promoted intramo-lecular transannular aziridination strategy for the forma-tion of the AEF ring systems of lycoctonine-type C19-nordi-

terpenoid alkaloids (Scheme 37).107 Starting from symmet-ric 397, intermediate 398 was obtained in 5 steps. -Ketoacrylate 398 was subjected to TiCl4-mediated conjugatepropargylation108 followed by a gold(I)-catalyzed cycliza-tion109 to afford the -keto ester 400. Subsequent transfor-mations of 400 furnished the amine 401 ready for intramo-lecular transannular aziridination. Initial conditions devel-oped by Nagata and co-workers using lead(IV) acetate didnot provide the desired product.110 It was found that usingPIDA together with silica gel as an additive gave the optimalyield of the aziridine 402. Finally, regioselective ring cleav-age was achieved with ethyl iodide and DMF followed bytreatment with NaOAc to give the tricyclic amine 403 thatmodels the AEF ring systems of lycoctonine-type C19-nordi-terpenoid alkaloids.

Scheme 37 PIDA-promoted intramolecular transannular aziridination followed by regioselective ring cleavage to deliver the AEF ring systems of lycoctonine-type C19-norditerpenoid alkaloids

The racemulsonine family has been an evasive core interms of successful attempts at its total synthesis. However,work by Wang and co-workers has made it significantlyeasier to access the 10-azatricyclo[3.3.2.04,8]decane skele-ton precursor towards the final product (Scheme 38).111

The strategy employed towards this synthesis of thecage-like azatricyclic skeleton was fashioned after the intra-molecular aziridination reaction by Nagata and co-workers,110 transforming the unsaturated primary amine409 to afford the bridged aziridine 410 that will later betransformed into 415. Starting with diol 404, it was globally

MOMO

OMe

CHO 8 steps

OOMe

OMeO

389 390

ON

OO

OO

OOMeO2CMeO2C

OH

O O

MeO2C

H

COOMe

MeO2C4 steps

391

393

395

392

394

396

m-CPBA, TsOH,0 °C to rt (76%)

1) DMP, NaHCO3, 0 °C (85%)2) SmI2, 0 °C (76%)3) NaBH4, CeCl3, 0 °C4) DIC, PhMe, 80 °C (85%, 2 steps)

8 steps

1) DIBAL-H, –78 °C (77%)2) ethanolamine, THF, reflux (90%)

MeOCO2Me

NH2TBSO

O

OAcAcO

CO2Me

2) cat. AuCl(PPh3), cat. AgOTf, DCM

(87% over 2 steps)

OCO2Me

AcOAcO

1

4

17 PIDA, K2CO3,SiO2

DCE, 50 °C

N

CO2Me

17

TBSO

7

70% yield

OMeEtI, DMF;

then NaOAc

NEt

CO2Me

17

TBSO

7

77% yield

OMe

OAc

7

1) TiCl4, DCM

HO2C CO2H

CO2Me

397

5 steps

MeO2C

6 steps

403402

401400

398

O

OAcAcO

CO2Me Ph3Sn

•+

399

398


3942


protected and then converted into the -keto ester 405 infour steps. This diprotected diol 405 was then treated withDDQ, to provide the conjugated enone,112 which underwentconjugate propargylation with allenyltriphenylstannaneyielding 406. A Au(I)-catalyzed annulation via a modifiedToste conditions109 accessed the cis-fused cyclopentenescaffold 407 as a single diastereomer, thus completing theA/F ring core. Reduction of the ketone in 407 and protectionwith MeI installed the final C3–OMe group, and this wasfollowed by deprotection of the benzyl protecting groupsand monoprotection with TBSCl to afford the product as amixture of the desired and undesired isomers (1.2:1); thealcohol was transformed into the primary amine 401. Amodification of the method by Nagata and co-workers us-ing 401 with PIDA and K2CO3 in 1,2-dichloroethane at 55 °Cgave the optimized yields of 402. The selective aziridinering cleavage was then achieved by treatment of 402 withacetic anhydride, yielding the 10-azatricyclo[3.3.2.04,8]dec-ane core (OAc analogue of 409) as a single regio- and ste-reoisomer in an excellent 90% yield. Notably, the direct in-troduction of the N-ethyl group to form the tricyclic aminewas achieved by treatment of 402 with ethyl iodide fol-lowed by NaOAc to give N-ethyl tricyclic amine 403.111 Thiswork was expanded in 2016113 by using exo-cyclization un-der radical conditions to furnish the B ring, thus expandingthe scaffold. With just a few tailoring steps they were ableto access 410 from compound 409, and cyclization occurredupon treatment of 410 with n-Bu3SnH and AIBN under re-flux conditions in benzene to give 411 in 75% yield. At thisstage, the ABEF core was completed, and with minor tailor-

ing, the scaffold was further functionalized to give 412which allows for further elaborations. This was an import-ant development as this synthesis brought the work of Xuand co-workers much closer to the final racemulosine core.

3.7 Intramolecular [5+2] Cycloaddition

The synthesis of a 6/5/6/7 ring system (Scheme 39)matching parts of the skeleton of C18/C19-diterpenoid alka-loids was reported by Liu and co-workers in 2018.114 Thesynthesis features an iron-catalyzed intramolecular [5+2]cycloaddition that they had reported earlier.115 Cycloaddi-tion adduct 415 was transformed into tetracyclic synthon416 in 3 steps. It was also demonstrated that the chemose-lective reduction of a ketone moiety is feasible. The result-

Scheme 38 Synthetic studies towards racemulsonine

1) BnBr, NaH (81%)2) KMnO4, Bu4NBr

PIDA, K2CO3, SiO2,DCE, 55 °C

404

1) NaBH4, MeOH2) MeI, Ag2O, THF (80%)3) BBr3, DCM (87%)

TsBr, DCM

OH OH

O

CO2Me

BnO

BnO

1) DDQ, K2CO3 (90%)2) TiCl4, DCM (80%)

Ph3Sn •

O

CO2Me

BnO

BnO

Au(PPh3)OTf, DCE

76% yield

O

BnO

BnO

CO2Me

HO

HO

CO2Me

OMe1) TBSCl, imidazole (49%)2) PCC, Na2CO3 (96%)3) NH3(EtOH), Ti(Oi-Pr)4; then NaBH4 (80%)

TBSO

H2N

CO2Me

OMe

72% yieldN

CO2Me

OMe

TBSO

NH

CO2Me

OMe

TBSO

Br

EtI, DMF; then NaOAc

N

CO2Me

OMe

TBSO

OAc

75% yield

NTs

OMe

TBSO

Br

OH

n-Bu3SnH, AIBN, PhH, reflux (75% yield)

NTs

OMe

TBSO

OH

NTs

OMe

TBSO

OH

1) LiAlH4, THF (84%) 2) DMP, DCM (88%)3) propargylmagnesium bromide, THF (80%)

1) OsO4, NaIO4, NMO, THF, H2O (76%)2) MsCl, Et3N, DMAP, DCM (91%)3) CeCl3•7H2O, NaBH4, MeOH, –40 °C (90%)

412 411

410409

403

402401408

407406399405

3) MeI, K2CO34) t-BuOK, THF (52%, 3 steps)

80% yield

Scheme 39 Synthesis of the tetracyclic 6/5/6/7 core skeleton of C18/C19-diterpenoid alkaloids

O

413 415

3 steps

1) allylMgBr, THF2) PhMe, 170 °C3) Grubbs-II cat.

(54% over 3 steps)

1) NaBH4, MeOH2) 4-nitrobenzoyl chloride, Et3N, DMAP, DCM

416 417

H

OallylO

OMe

OMe

H

O

OOH (63% over 2 steps)

H

O

OH OAr

Ar = 4-nitrobenzoyl

414

FeCl3, PhCO3t-Bu,DCM

79% dr > 20:1

4O

MeO

allylO Oallyl


3943


ing synthon 417 features the A, B, and F rings and the pre-cursor of the C/D ring system of the diterpenoid alkaloidskeletons.

3.8 Miscellaneous

The Sarpong group aimed to study the structural activi-ty relationship of aconitine (27) and the variation of sub-stituents on the D ring while maintaining the structure ofthe AEF ring systems (Scheme 40).116 Starting with vinylni-trile 88, aryl nucleophiles with different substitution pat-tern were appended in a conjugate fashion under Hayashi-type Rh-catalysis117 or with the Cu-mediated strategy togive conjugate addition product 418. Subsequent elabora-tions on the products afforded tetracycle 419 or the dearo-matized spirocycle 420.

Scheme 40 Conjugate arylation of vinylnitrile to study the structural activity relationship for aconitine

An expedient synthesis of the AE ring systems bearing a5-hydroxyl group for C19-norditerpenoid alkaloids was re-ported by Wang and co-workers utilizing a series of intra-molecular Claisen condensation, double Mannich reaction,and stereoselective aldol addition (Scheme 41).118 Startingwith keto carbonate 422, Claisen condensation followed bymethylation furnished the bicyclic -keto lactone 423. Dou-ble Mannich reaction with benzylamine in the presence offormaldehyde installed the E ring to give 424. Finally, stere-oselective addition of a carbonyl group dictated by the 1-substituent formed the 5-hydroxyl group in 425.

4 Conclusion

The diterpenoid alkaloids have proven to be an intrigu-ing and challenging target to researchers worldwide. A cen-tury following their discovery, synthetic strategies towardsditerpenoid alkaloids have continuously expanded. Herein,we summarized synthetic efforts over the past decade, dis-

cussing successful total syntheses and pioneering work to-wards future attempts of total syntheses. The current suc-cess can be viewed as an open door towards accessing un-natural derivatives of these diterpenoid alkaloids in aneffort to tune the prevalent biological activities.

Despite the great successes reported, there are still chal-lenging targets left to synthesize, which is especially truefor the C19-diterpenoid alkaloids with aconitine (27) as themost popular representative. Further, it can be expectedthat natural product isolation will yield additional structur-ally challenging compounds with interesting biological ac-tivities.

Funding Information

We thank the University of Toronto, and the Natural Sciences and En-gineering Research Council (NSERC), and the Audrey and Ragnar Ol-sen Fellowship for financial support. Christian Dank was supportedby an Erwin Schroedinger postdoctoral fellowship awarded by theAustrian Science Fund (FWF) J 4348-N28.Austrian Science Fund (J 4348-N28)

Acknowledgment

We would like to thank Mark Stewart and the Toronto Botanical Gar-den for providing us with the image of the Aconitum in bloom.

References

(1) (a) Wang, F.-P.; Liang, X.-T. C20-Diterpenoid Alkaloids, In TheAlkaloids: Chemistry and Biology, Vol. 59; Cordell, G. A., Ed.; Aca-demic Press: San Diego, 2002, 1–280. (b) Wang, F.-P.; Chen, Q.-H.; Liang, X.-T. The C18-Diterpenoid Alkaloids, In The Alkaloids:Chemistry and Biology, Vol. 67; Cordell, G. A., Ed.; AcademicPress: San Diego, 2009, 1–78. (c) Wang, F.-P.; Chen, Q.-H. TheC19-Diterpenoid Alkaloids, In The Alkaloids: Chemistry and Biol-ogy, Vol. 69; Cordell, G. A., Ed.; Academic Press: San Diego, 2010,1–577.

(2) (a) Atta-ur-Rahman, Iqbal, Choudhary. M. Nat. Prod. Rep. 1999,16, 619. (b) Wang, F.-P.; Chen, Q.-H.; Liu, X.-Y. Nat. Prod. Rep.2010, 27, 529. (c) Wang, X. W.; Xie, H. Drugs Future 1999, 24,877. (d) Wang, C.-F.; Gerner, P.; Wang, S.-Y.; Wang, G. K. Anes-

CNMeO CO2Me

H

(MeO)3B–-Ar +Li[Rh(cod)OH]2

H2O/1,4-dioxane, 100 °C

Method A:

BrMg-ArCuI, THF, 45 °C

Method B:

CNMeO CO2Me

H

R

OTBSOTBS

N

O

Ac

Me

OH

O

88 418

419

N

O

Ac

Me

420

OMe

O

O

H

17

PhI(OAc)2,NaHCO3

MeOH, 0–23 °C

52% yield

11 steps

Scheme 41 Sequential intramolecular Claisen condensation, double Mannich reaction and stereoselective aldol addition for the formation of the AE ring systems bearing a 5-hydroxyl group for C19-norditerpenoid alkaloids

O

OCO2Et

1) NaH, THF (93%)

2) LDA, MeI, THF, –20 °C (93%)

O

O

O

37% formalin,BnNH2

O

O

O

EtOH NBn30% yield

LDA, EtOAc

THF, –78 °C82% yield

O

O

NBn

OH

CO2Et4

5

H

H

1

O

6 steps

422 423

424 425

cyclohexenone(421)


3944


thesiology 2007, 107, 82. (e) Yang, X.; Wang, G.; Ling, S.; Qiu, N.;Wang, G.; Zhu, J.; Liu, J. J. Chromatogr., B 2000, 740, 273.(f) Sadikov, A. Z.; Shakirov, T. T. Chem. Nat. Compd. 1988, 24, 79.

(3) Catterall, W. A.; Cestèle, S.; Yarov-Yarovoy, V.; Yu, F. H.; Konoki,K.; Scheuer, T. Toxicon 2007, 49, 124.

(4) (a) Kou, K. G. M.; Kulyk, S.; Marth, C. J.; Lee, J. C.; Doering, N. A.;Li, B. X.; Gallego, G. M.; Lebold, T. P.; Sarpong, R. J. Am. Chem.Soc. 2017, 139, 13882. (b) Song, M. K.; Liu, H.; Jiang, H. L.; Yue, J.M.; Hu, G. Y.; Chen, H. Z. Neuroscience 2008, 155, 469. (c) Wada,K. Chemistry and Biological Activity of Diterpenoid Alkaloids, InStudies in Natural Products Chemistry, Vol. 38; Atta-ur-Rahman,Ed.; Elsevier: Amsterdam, 2012, 191–223. (d) Zhao, X.-Y.; Wang,Y.; Li, Y.; Chen, X.-Q.; Yang, H.-H.; Yue, J.-M.; Hu, G.-Y. Neurosci.Lett. 2003, 337, 33. (e) Wada, K.; Ohkoshi, E.; Zhao, Y.; Goto, M.;Morris-Natschke, S. L.; Lee, K.-H. Bioorg. Med. Chem. Lett. 2015,25, 1525.

(5) Cherney, E. C.; Baran, P. S. Isr. J. Chem. 2011, 51, 391.(6) The Merck Index: An Encyclopedia of Chemicals, Drugs, and Bio-

logicals, 15th ed; O’Neil, M. J.; Heckelman, P. E.; Dobbelaar, P. H.;Roman, K. J.; Kenny, C. M.; Karaffa, L. S., Ed.; Royal Society ofChemistry: Cambridge, 2013.

(7) (a) Dunstan, W. R. Ber. Dtsch. Chem. Ges. 1894, 27, 664.(b) Freund, M.; Beck, P. Ber. Dtsch. Chem. Ges. 1894, 27, 433.

(8) (a) Dunstan, W. R.; Carr, F. H. Ber. Dtsch. Chem. Ges. 1895, 28,1379. (b) Freund, M. Ber. Dtsch. Chem. Ges. 1895, 28, 2537.(c) Freund, M. Ber. Dtsch. Chem. Ges. 1895, 28, 192.

(9) (a) Freudenberg, W. Ber. Dtsch. Chem. Ges. 1936, 69, 1962.(b) Majima, R.; Suginomé, H. Ber. Dtsch. Chem. Ges. 1924, 57,1466. (c) Schneider, W. Chem. Ber. 1956, 89, 768.

(10) (a) Barger, G.; Field, E. J. Chem. Soc., Trans. 1915, 107, 231.(b) Brady, O. L. J. Chem. Soc., Trans. 1913, 103, 1821. (c) Carr, F.H. J. Chem. Soc., Trans. 1912, 101, 2241. (d) Dunstan, W. R.;Andrews, A. E. J. Chem. Soc., Trans. 1905, 87, 1636. (e) Dunstan,W. R.; Carr, F. H. J. Chem. Soc., Trans. 1894, 65, 290. (f) Dunstan,W. R.; Carr, F. H. J. Chem. Soc., Trans. 1894, 65, 176. (g) Dunstan,W. R.; Carr, F. H. J. Chem. Soc., Trans. 1895, 67, 459. (h) Dunstan,W. R.; Harrison, E. F. J. Chem. Soc., Trans. 1894, 65, 174. (i) Henry,T. A.; Sharp, T. M. J. Chem. Soc. 1931, 581. (j) Jowett, H. A. D.J. Chem. Soc., Trans. 1896, 69, 1518. (k) Lawson, A. J. Chem. Soc.1936, 80. (l) Lawson, A.; Topps, J. E. C. J. Chem. Soc. 1937, 1640.

(11) Jacobs, W. A.; Elderfield, R. C. J. Am. Chem. Soc. 1936, 58, 1059.(12) (a) Cookson, R. C.; Trevett, M. E. J. Chem. Soc. 1956, 3121.

(b) Wiesner, K.; Götz, M.; Simmons, D. L.; Fowler, L. R.; Bachelor,F. W.; Brown, R. F. C.; Büchi, G. Tetrahedron Lett. 1959, 1, 15.

(13) Pelletier, S. W.; Oeltmann, T. N. Tetrahedron 1968, 24, 2019.(14) (a) Jacobs, W. A. J. Org. Chem. 1951, 16, 1593. (b) Jacobs, W. A.;

Pelletier, S. W. J. Am. Chem. Soc. 1954, 76, 4048. (c) Jacobs, W. A.;Pelletier, S. W. J. Am. Chem. Soc. 1954, 76, 161. (d) Jacobs, W. A.;Pelletier, S. W. J. Am. Chem. Soc. 1956, 78, 3542. (e) Jacobs, W. A.;Pelletier, S. W. J. Org. Chem. 1957, 22, 1428. (f) Locke, D. M.;Pelletier, S. W. J. Am. Chem. Soc. 1958, 80, 2588. (g) Locke, D. M.;Pelletier, S. W. J. Am. Chem. Soc. 1959, 81, 2246. (h) Pelletier, S.W. J. Am. Chem. Soc. 1960, 82, 2398. (i) Pelletier, S. W.; Jacobs, W.A. J. Am. Chem. Soc. 1954, 76, 4496. (j) Pelletier, S. W.; Jacobs, W.A. J. Am. Chem. Soc. 1956, 78, 4139. (k) Pelletier, S. W.; Jacobs, W.A.; Rathgeb, P. J. Org. Chem. 1956, 21, 1514. (l) Solo, A. J.;Pelletier, S. W. J. Am. Chem. Soc. 1959, 81, 4439.

(15) Pelletier, S. W.; Jacobs, W. A. J. Am. Chem. Soc. 1956, 78, 4144.(16) (a) Guthrie, R. W.; Henry, W. A.; Immer, H.; Wong, C. M.;

Valenta, Z.; Wiesner, K. Collect. Czech. Chem. Commun. 1966, 31,602. (b) Wiesner, K.; Armstrong, R.; Bartlett, M. F.; Edwards, J. A.J. Am. Chem. Soc. 1954, 76, 6068. (c) Wiesner, K.; Edwards, J. A.Experientia 1955, 11, 255. (d) Wiesner, K.; Figdor, S. K.; Bartlett,

M. F.; Henderson, D. R. Can. J. Chem. 1952, 30, 608. (e) Wiesner,K.; Taylor, W. I.; Figdor, S. K.; Bartlett, M. F.; Armstrong, J. R.;Edwards, J. A. Chem. Ber. 1953, 86, 800.

(17) (a) Wiesner, K. Chem. Soc. Rev. 1977, 6, 413. (b) Wiesner, K. Tet-rahedron 1985, 41, 485.

(18) (a) Liu, X. Y.; Qin, Y. Asian J. Org. Chem. 2015, 4, 1010. (b) Zhu,G.; Liu, R.; Liu, B. Synthesis 2015, 47, 2691.

(19) Narita, K.; Fujisaki, N.; Sakuma, Y.; Katoh, T. Org. Biomol. Chem.2019, 17, 655.

(20) Menger, M.; Lentz, D.; Christmann, M. J. Org. Chem. 2018, 83,6793.

(21) (a) Sharpe, R. J.; Johnson, J. S. J. Org. Chem. 2015, 80, 9740.(b) Kim, D. E.; Zweig, J. E.; Newhouse, T. R. J. Am. Chem. Soc.2019, 141, 1479.

(22) Enomoto, M.; Morita, A.; Kuwahara, S. Angew. Chem. Int. Ed.2012, 51, 12833.

(23) Yang, M.; Yang, X.; Sun, H.; Li, A. Angew. Chem. Int. Ed. 2016, 55,2851.

(24) Teranishi, T.; Murokawa, T.; Enomoto, M.; Kuwahara, S. Biosci.,Biotechnol., Biochem. 2015, 79, 11.

(25) Li, H.; Chen, Q.; Lu, Z.; Li, A. J. Am. Chem. Soc. 2016, 138, 15555.(26) (a) Suzuki, H.; Aoyagi, S. Org. Lett. 2012, 14, 6374. (b) Mori, N.;

Kuzuya, K.; Watanabe, H. J. Org. Chem. 2016, 81, 11866.(c) Suzuki, H.; Aoyagi, S. Chem. Commun. 2011, 47, 7878.

(27) Shi, Y.; Wilmot, J. T.; Nordstrøm, L. U.; Tan, D. S.; Gin, D. Y. J. Am.Chem. Soc. 2013, 135, 14313.

(28) Marth, C. J.; Gallego, G. M.; Lee, J. C.; Lebold, T. P.; Kulyk, S.; Kou,K. G. M.; Qin, J.; Lilien, R.; Sarpong, R. Nature 2015, 528, 493.

(29) Wiesner, K.; Tsai, T. Y. R.; Huber, K.; Bolton, S. E.; Vlahov, R.J. Am. Chem. Soc. 1974, 96, 4990.

(30) Wiesner, K. Pure Appl. Chem. 1975, 41, 93.(31) (a) Tsai, T. Y. R.; Tsai, C. S. J.; Sy, W. W.; Shanbhag, M. N.; Liu, W.

C.; Lee, S. F.; Wiesner, K. Heterocycles 1977, 7, 217. (b) Lee, S.-F.;Sathe, G. M.; Sy, W. W.; Ho, P.-T.; Wiesner, K. Can. J. Chem. 1976,54, 1039.

(32) Wiesner, K.; Tsai, T. Y. R.; Nambiar, K. P. Can. J. Chem. 1978, 56,1451.

(33) (a) Tsai, T. Y. R.; Nambiar, K. P.; Krikorian, D.; Botta, M.; Marini-Bettolo, R.; Wiesner, K. Can. J. Chem. 1979, 57, 2124.(b) Wiesner, K. Pure Appl. Chem. 1979, 51, 689.

(34) Nishiyama, Y.; Yokoshima, S.; Fukuyama, T. Org. Lett. 2017, 19,5833.

(35) Nagata, W.; Sugasawa, T.; Narisada, M.; Wakabayashi, T.;Hayase, Y. J. Am. Chem. Soc. 1963, 85, 2342.

(36) Masamune, S. J. Am. Chem. Soc. 1964, 86, 291.(37) Guthrie, R. W.; Valenta, Z.; Wiesner, K. Tetrahedron Lett. 1966, 7,

4645.(38) Nagata, W.; Sugasawa, T.; Narisada, M.; Wakabayashi, T.;

Hayase, Y. J. Am. Chem. Soc. 1967, 89, 1483.(39) Ihara, M.; Suzuki, M.; Fukumoto, K.; Kametani, T.; Kabuto, C.

J. Am. Chem. Soc. 1988, 110, 1963.(40) Ihara, M.; Suzuki, M.; Fukumoto, K.; Kabuto, C. J. Am. Chem. Soc.

1990, 112, 1164.(41) Liu, X.-Y.; Cheng, H.; Li, X.-H.; Chen, Q.-H.; Xu, L.; Wang, F.-P.

Org. Biomol. Chem. 2012, 10, 1411.(42) (a) Masamune, S. J. Am. Chem. Soc. 1964, 86, 290. (b) Nagata, W.;

Narisada, M.; Wakabayashi, T.; Sugasawa, T. J. Am. Chem. Soc.1964, 86, 929. (c) Valenta, Z.; Wiesner, K.; Wong, C. M. Tetrahe-dron Lett. 1964, 5, 2437.

(43) Nagata, W.; Narisada, M.; Wakabayashi, T.; Sugasawa, T. J. Am.Chem. Soc. 1967, 89, 1499.

(44) Wiesner, K.; Uyeo, S.; Philipp, A.; Valenta, Z. Tetrahedron Lett.1968, 9, 6279.


3945


(45) (a) Wiesner, K.; Ho, P.-T.; Tsai, C. S. J. Can. J. Chem. 1974, 52,2353. (b) Wiesner, K.; Ho, P.-T.; Tsai, C. S. J.; Lam, Y.-K. Can. J.Chem. 1974, 52, 2355.

(46) Sethi, S. P.; Atwal, K. S.; Marini-Bettolo, R. M.; Tsai, T. Y. R.;Wiesner, K. Can. J. Chem. 1980, 58, 1889.

(47) Muratake, H.; Natsume, M. Angew. Chem. Int. Ed. 2004, 43, 4646.(48) (a) Muratake, H.; Natsume, M.; Nakai, H. Tetrahedron 2006, 62,

7093. (b) Peese, K. M.; Gin, D. Y. J. Am. Chem. Soc. 2006, 128,8734.

(49) Peese, K. M.; Gin, D. Y. Chem. Eur. J. 2008, 14, 1654.(50) Cherney, E. C.; Lopchuk, J. M.; Green, J. C.; Baran, P. S. J. Am.

Chem. Soc. 2014, 136, 12592.(51) Nishiyama, Y.; Han-ya, Y.; Yokoshima, S.; Fukuyama, T. J. Am.

Chem. Soc. 2014, 136, 6598.(52) Kou, K. G. M.; Li, B. X.; Lee, J. C.; Gallego, G. M.; Lebold, T. P.;

DiPasquale, A. G.; Sarpong, R. J. Am. Chem. Soc. 2016, 138, 10830.(53) Li, X.-H.; Zhu, M.; Wang, Z.-X.; Liu, X.-Y.; Song, H.; Zhang, D.;

Wang, F.-P.; Qin, Y. Angew. Chem. Int. Ed. 2016, 55, 15667.(54) Cheng, H.; Zeng, F.-H.; Yang, X.; Meng, Y.-J.; Xu, L.; Wang, F.-P.

Angew. Chem. Int. Ed. 2016, 55, 392.(55) Liu, J.; Ma, D. Angew. Chem. Int. Ed. 2018, 57, 6676.(56) Kou, K. G. M.; Pflueger, J. J.; Kiho, T.; Morrill, L. C.; Fisher, E. L.;

Clagg, K.; Lebold, T. P.; Kisunzu, J. K.; Sarpong, R. J. Am. Chem.Soc. 2018, 140, 8105.

(57) Zhou, S.; Guo, R.; Yang, P.; Li, A. J. Am. Chem. Soc. 2018, 140,9025.

(58) Curran, T. T.; Hay, D. A.; Koegel, C. P.; Evans, J. C. Tetrahedron1997, 53, 1983.

(59) Al Dulayymi, A. R.; Al Dulayymi, J. R.; Baird, M. S.; Gerrard, M. E.;Koza, G.; Harkins, S. D.; Roberts, E. Tetrahedron 1996, 52, 3409.

(60) Forsyth, C. J.; Clardy, J. J. Am. Chem. Soc. 1990, 112, 3497.(61) Prabhakaran, J.; Lhermitte, H.; Das, J.; Sasi-Kumar, T. K.;

Grierson, D. S. Synlett 2000, 658.(62) Marx, J. N.; Cox, J. H.; Norman, L. R. J. Org. Chem. 1972, 37, 4489.(63) Bandyopadhyaya, A. K.; Manion, B. D.; Benz, A.; Taylor, A.; Rath,

N. P.; Evers, A. S.; Zorumski, C. F.; Mennerick, S.; Covey, D. F.Bioorg. Med. Chem. Lett. 2010, 20, 6680.

(64) Lee, J.; Kim, M.; Chang, S.; Lee, H.-Y. Org. Lett. 2009, 11, 5598.(65) Cuerva, J. M.; Campaña, A. G.; Justicia, J.; Rosales, A.; Oller-

López, J. L.; Robles, R.; Cárdenas, D. J.; Buñuel, E.; Oltra, J. E.Angew. Chem. Int. Ed. 2006, 45, 5522.

(66) Zhou, R.-J.; Dai, G.-Y.; Zhou, X.-H.; Zhang, M.-J.; Wu, P.-Z.; Zhang,D.; Song, H.; Liu, X.-Y.; Qin, Y. Org. Chem. Front. 2019, 6, 377.

(67) Nishiyama, Y.; Yokoshima, S.; Fukuyama, T. Org. Lett. 2016, 18,2359.

(68) Pelletier, S. W.; Parthasarathy, P. C. Tetrahedron Lett. 1963, 4,205.

(69) Zhu, M.; Li, X.; Song, X.; Wang, Z.; Liu, X.; Song, H.; Zhang, D.;Wang, F.; Qin, Y. Chin. J. Chem. 2017, 35, 991.

(70) Han, G.; LaPorte, M. G.; McIntosh, M. C.; Weinreb, S. M.; Parvez,M. J. Org. Chem. 1996, 61, 9483.

(71) Sasano, Y.; Nagasawa, S.; Yamazaki, M.; Shibuya, M.; Park, J.;Iwabuchi, Y. Angew. Chem. Int. Ed. 2014, 53, 3236.

(72) Pflueger, J. J.; Morrill, L. C.; deGruyter, J. N.; Perea, M. A.;Sarpong, R. Org. Lett. 2017, 19, 4632.

(73) Griffith, W. P.; Ley, S. V.; Whitcombe, G. P.; White, A. D. J. Chem.Soc., Chem. Commun. 1987, 1625.

(74) Lebel, H.; Paquet, V. J. Am. Chem. Soc. 2004, 126, 320.(75) Donnelly, D. M. X.; Finet, J.-P.; Rattigan, B. A. J. Chem. Soc., Perkin

Trans. 1 1993, 1729.(76) Dessolin, M.; Guillerez, M.-G.; Thieriet, N.; Guibé, F.; Loffet, A.

Tetrahedron Lett. 1995, 36, 5741.

(77) Kulbitski, K.; Nisnevich, G.; Gandelman, M. Adv. Synth. Catal.2011, 353, 1438.

(78) Heinzman, S. W.; Ganem, B. J. Am. Chem. Soc. 1982, 104, 6801.(79) Trost, B. M.; Tang, W. J. Am. Chem. Soc. 2002, 124, 14542.(80) Zhang, Q.; Zhang, Z.; Huang, Z.; Zhang, C.; Xi, S.; Zhang, M.

Angew. Chem. Int. Ed. 2018, 57, 937.(81) Weitz, E.; Scheffer, A. Ber. Dtsch. Chem. Ges. 1921, 54, 2327.(82) Chen, Y.-K.; Peddinti, R. K.; Liao, C.-C. Chem. Commun. 2001,

1340.(83) Chittimalla, S. K.; Shiao, H.-Y.; Liao, C.-C. Org. Biomol. Chem.

2006, 4, 2267.(84) Herrmann, J. L.; Kieczykowski, G. R.; Schlessinger, R. H. Tetrahe-

dron Lett. 1973, 14, 2433.(85) Hibino, J. H.; Okazoe, T.; Takai, K.; Nozaki, H. Tetrahedron Lett.

1985, 26, 5579.(86) Bal, B. S.; Childers, W. E.; Pinnick, H. W. Tetrahedron 1981, 37,

2091.(87) Shigeru, I.; Teruaki, M. Chem. Lett. 1989, 18, 1071.(88) Hagiwara, K.; Tabuchi, T.; Urabe, D.; Inoue, M. Chem. Sci. 2016, 7,

4372.(89) Tabuchi, T.; Urabe, D.; Inoue, M. J. Org. Chem. 2016, 81, 10204.(90) Minagawa, K.; Urabe, D.; Inoue, M. J. Antibiot. 2017, 71, 326.(91) Liu, M.; Cheng, C.; Xiong, W.; Cheng, H.; Wang, J.-L.; Xu, L. Org.

Chem. Front. 2018, 5, 1502.(92) Cheng, H.; Xu, L.; Chen, D.-L.; Chen, Q.-H.; Wang, F.-P. Tetrahe-

dron 2012, 68, 1171.(93) Li, Y.-L.; Liu, M.-C.; Meng, Y.-J.; Xu, L. Tetrahedron 2016, 72,

3171.(94) (a) Trost, B. M.; Ferreira, E. M.; Gutierrez, A. C. J. Am. Chem. Soc.

2008, 130, 16176. (b) Trost, B. M.; Gutierrez, A. C.; Ferreira, E. M.J. Am. Chem. Soc. 2010, 132, 9206.

(95) Zhou, X.-H.; Liu, Y.; Zhou, R.-J.; Song, H.; Liu, X.-Y.; Qin, Y. Chem.Commun. 2018, 54, 12258.

(96) Cheng, H.; Zeng, F.-H.; Ma, D.; Jiang, M.-L.; Xu, L.; Wang, F.-P.Org. Lett. 2014, 16, 2299.

(97) Matsuzawa, A.; Kasamatsu, A.; Sugita, K. Tetrahedron Lett. 2016,57, 4585.

(98) Sparrow, K.; Barker, D.; Brimble, M. A. Tetrahedron 2011, 67,7989.

(99) Liu, Z.-G.; Xu, L.; Chen, Q.-H.; Wang, F.-P. Tetrahedron 2012, 68,159.

(100) Yang, T.-K.; Hung, S.-M.; Lee, D.-S.; Hong, A.-W.; Cheng, C.-C.Tetrahedron Lett. 1989, 30, 4973.

(101) Goodall, K. J.; Brimble, M. A.; Barker, D. Tetrahedron 2012, 68,5759.

(102) Liu, X.-Y.; Qin, Y. Nat. Prod. Rep. 2017, 34, 1044.(103) Yang, X.; Cheng, B.; Cheng, H.; Xu, L.; Wang, J.-L. Chin. Chem.

Lett. 2017, 28, 1788.(104) Hamlin, A. M.; Lapointe, D.; Owens, K.; Sarpong, R. J. Org. Chem.

2014, 79, 6783.(105) Chen, D. L.; Wang, F. P. Chin. Chem. Lett. 2012, 23, 1378.(106) Tang, D.-H.; Ma, D.; Cheng, H.; Li, Y.-L.; Xu, L. Org. Biomol. Chem.

2016, 14, 2716.(107) Liu, Z.-G.; Cheng, H.; Ge, M.-J.; Xu, L.; Wang, F.-P. Tetrahedron

2013, 69, 5431.(108) Haruta, J.; Nishi, K.; Matsuda, S.; Akai, S.; Tamura, Y.; Kita, Y.

J. Org. Chem. 1990, 55, 4853.(109) Staben, S. T.; Kennedy-Smith, J. J.; Toste, F. D. Angew. Chem. Int.

Ed. 2004, 43, 5350.(110) Nagata, W.; Hirai, S.; Kawata, K.; Aoki, T. J. Am. Chem. Soc. 1967,

89, 5045.(111) Mei, R.-H.; Liu, Z.-G.; Cheng, H.; Xu, L.; Wang, F.-P. Org. Lett.

2013, 15, 2206.


3946


(112) Chou, H.-H.; Wu, H.-M.; Wu, J.-D.; Ly, T. W.; Jan, N.-W.; Shia, K.-S.; Liu, H.-J. Org. Lett. 2008, 10, 121.

(113) Jiang, M.-L.; Meng, Y.-J.; Xiong, W.-Y.; Xu, L. Tetrahedron Lett.2016, 57, 1610.

(114) Liu, Y.; Zhang, Y.; Wang, X.; Fu, S.; Liu, B. Synlett 2018, 29, 1978.(115) Liu, Y.; Wang, X.; Chen, S.; Fu, S.; Liu, B. Org. Lett. 2018, 20, 2934.

(116) Doering, N. A.; Kou, K. G. M.; Norseeda, K.; Lee, J. C.; Marth, C. J.;Gallego, G. M.; Sarpong, R. J. Org. Chem. 2018, 83, 12911.

(117) Takaya, Y.; Ogasawara, M.; Hayashi, T. Tetrahedron Lett. 1999, 40,6957.

(118) Yang, Z.-K.; Chen, Q.-H.; Wang, F.-P. Tetrahedron 2011, 67, 4192.


Documents

Recent Advances Towards Syntheses of Diterpenoid Alkaloids · Wagner–Meerwein rearrangement, affording the ent-kau-rane 23 and ent-atisane 24 diterpene skeletons. These scaf-folds