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University of Groningen
Asymmetric catalysis in the synthesis of cis-cyclopropyl containing fatty acids and the additionof Grignard reagents to carbonyl compoundsHanstein, Miriam
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Publication date:2014
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Citation for published version (APA):Hanstein, M. (2014). Asymmetric catalysis in the synthesis of cis-cyclopropyl containing fatty acids and theaddition of Grignard reagents to carbonyl compounds. [S.n.].
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Asymmetric synthesis of a cis-configured cyclopropyl building block: studies towards the synthesis of mycolic acid
In this chapter our efforts to synthesize an -mycolic acid, a main component found in the cell wall of Mycobacterium tuberculosis, are described. For the cyclopropyl moieties in the mycolic acid, the synthesis of a common building block via intramolecular cyclopropanation of allyl diazoactetate was studied. For the -hydroxy ester motif we aimed to apply an asymmetric hydrogenation to introduce enantioselectivity.
Chapter 2
20
2.1 Introduction
In 2009, the WHO (World Health Organization) published an up-date of the global control of tuberculosis (TB): each year over nine million new cases are reported, with nearly two million deaths annually caused by infection by Mycobacterium tuberculosis (M. tuberculosis).1 In the last decade, M. tuberculosis strains that are multidrug-resistant and even extensively-drug resistant have been found, therefore the need to understand this resistance becomes urgent.2
The persistence of M. tuberculosis can partly be explained by the low permeability of the cell wall for antibiotics and other drugs. The impermeability is found in most mycobacteria and caused by a thick and robust outer membrane.3,4 The highly complex cell envelope consists of three structural components: the plasma membrane, the cell wall core and the capsule (Figure 2.1).5,6
Figure 2.1. Schematic structure of the cell envelope of M. tuberculosis.
The main compounds forming the cell wall (40-60% of the cells dry weight) are mycolic acids 1 (Figure 2.2), which are either found as free fatty acids, as monoesters or as diesters of trehalose, glucose and glycerol or covalently bound to the arabinogalactan layer.7,8 The initial evaluation and characterization of mycolic acids from M. tuberculosis was reported by Lederer et al. and later Minnikin and co-workers described the detailed structure of the major classes of mycolic acids, including their stereochemistry.6,9 Mycolic acids are -branched -hydroxyl fatty acids with up to ninety carbons. A mycolic acid can be divided into two parts; an un-functionalized mycolic chain and a meromycolate chain with up to two functional groups [X] and
Studies towards the synthesis of mycolic acid
21
[Y] (Figure 2.2). Depending on the functional group in the distal position [X] one can distinguish between the three major classes of mycolic acids: -mycolic acids, methoxy-mycolic acids and keto-mycolic acids.
Me
OMe
Me
O
Me
[X] [Y] OH
OH O
a c db
mycolic chainmeromycolate chain
[X]
- methoxy- keto-
[Y] or orn
orMe
or
a: 15, 17, 18, 19 b: 10,14, 16c: 11, 15, 17, 19, 21 d: 21,23
mycolic acid 1
Figure 2.2. Major types of mycolic acids from M. tuberculosis.
Watanabe et al. reported the detailed composition of mycolic acid components of different M. tuberculosis strains and other mycobacteria species. Analysis of the cell wall extract by TLC gives rise to a specific pattern for each Mycobacterium species.10 This characteristic makes mycolic acids attractive as straightforward diagnostic markers for mycobacterial infections. Different groups published the possibility to detect mycolic acids by either HPLC or mass spectra analysis in sputum samples of possible TB infected patients.11,12 Thus, we envisioned that synthesized, (enantiopure) mycolic acids could be employed as an internal standard in those measurements. With such an internal standard the amount of M. tuberculosis in the sputum can be quantified, as well the identification of the right fractions isolated from the sputum would be easier.
2.2 Reported syntheses of mycolic acids
In the field of mycolic acid synthesis, two research groups have been mainly involved: Baird et al. and Minnikin et al.. These groups were able to synthesize examples of all the different mycolic acid classes,13–16 but since -mycolic acids are
Chapter 2
22
the major component of the cell wall of M. tuberculosis, we focused only on the synthesis of this mycolic acid.17
The first step of their reported synthesis is the desymmetrization reaction of meso diester 2 to monoester 3 with pig liver esterase at pH 6.5 (Scheme 2.1). The free hydroxyl group is oxidized with PCC to aldehyde 4, which was connected to an alkyl chain in a Wittig reaction giving olefin 5 with 81% yield and a Z/E ratio of 6:1.
Me(CH2)18PPh3Br BuLi, THF
pig liver esterase ethylene glycol, water, pH 6.5
2
O Pr
O
OPr
O3
O Pr
O
HO80%
4
O Pr
O
O
PCC, CH2Cl288%
H81%
6:1 (Z/E)5
O Pr
O
H3C(H2C)17
1) LiAlH4, THF2) N2H4, NaIO4 AcOH, CuSO4, i-PrOH3) PCC, CH2Cl2
68%
H3C(H2C)19
6
O
H(CH2)12CHOH3C(H2C)19
7
8
O Pr
O
SO
ON
S
LiHMDS
(CH2)12H3C(H2C)19
9
CH2OCOPr
43%
77%
1) LiAlH4, THF2) N2H4, NaIO4 AcOH, CuSO4, i-PrOH
(CH2)14H3C(H2C)19
10
OH
Scheme 2.1. Synthesis of the meromycolate chain in an -mycolic acid.
In the subsequent steps, the ester is reduced to the corresponding alcohol, the double bond is reduced under mild conditions because of the presence of the cyclopropyl unit, and the free alcohol is oxidized to give aldehyde 6. Then again the alkyl chain is
Studies towards the synthesis of mycolic acid
23
extended via Wittig reaction with an ester group at the end, applying the same repetitive steps as described before to get aldehyde 7. In a Julia-Kocienski olefination of aldehyde 7 with sulfone 8, the second cyclopropyl moiety of the meromycolate chain was installed. Sulfone building block 8 was made from the same starting material 2. Alcohol 10 was obtained using the same steps as before: reduction of the ester group to the corresponding alcohol and reduction of the double bond with diimide.
The -hydroxy ester part was prepared from epoxide 11 by ring opening with a Grignard reagent to give secondary alcohol 12 (Scheme 2.2). In the next steps the free alcohol was protected and the benzyl group was removed. After oxidation, the resulting acid was transformed into methyl ester 13, and under these acidic conditions the THP-group was removed.
O
BnOH
BrMg(CH2)9OTHPCuI, 2 h, 30 °C
BnO (CH2)10OTHP
OH
1) imidazole, DMF, tBuSiMe2Cl
2) H2, Pd/C, MeOH
3) NaIO4, RuCl3.H2O
CH3CN, H2O, CCl44) MeOH, H2SO4
MeO (CH2)10OH
OHO
11 12
13
1) tBuPh2SiCl, DMAP, Et3N2) LDA, CH3(CH2)23I, HMPA
86%
54%
26%MeO (CH2)10OSiPh2
tBu
OHO
14
(CH2)23CH3
1) Ac2O, pyridine2) F3) PCC
61%
MeO (CH2)9CHO
OAcO
15
(CH2)23CH3
Scheme 2.2. Synthesis of the -hydroxy ester moiety in mycolic acid, using epoxide 11.
Next, the free primary alcohol was protected with a TBDPS-group and in a Fráter alkylation the long mycolic chain was introduced. Then the secondary alcohol was
Chapter 2
24
protected, followed by deprotection of the primary alcohol and oxidation of this to give aldehyde 15.
In the final part of the synthesis, the meromycolate chain is attached to the -hydroxy ester part (Scheme 2.3), and in order to do this, alcohol 10 was converted to sulfone 16. In a Julia-Kocienski reaction the two parts were coupled to give olefin 17 as an E/Z mixture. Reduction of the double bond with diimide in situ generated from dipotassium azodicarboxylate led to the protected -mycolic acid 18.
MeO (CH2)9CHO
OAcO
15(CH2)23CH3
(CH2)14H3C(H2C)19
10
OH
S
NSH PPh3, DEAD1)
2) MCPBA, CH2Cl2
(CH2)14H3C(H2C)19
16
SO
OS
N
LiHMDS
(CH2)14H3C(H2C)19
17
(CH2)9OMe
OAc
(CH2)23CH3
O
Potassium azodicarboxylate,CH3COOH, THF
(CH2)14H3C(H2C)19
18
(CH2)11 OMe
OAc
(CH2)23CH3
O
Scheme 2.3. Coupling of the meromycolate chain to the -hydroxy ester part.
Studies towards the synthesis of mycolic acid
25
2.3 Retrosynthetic analysis
A closer look at the mycolic acid methyl ester 18 shows, that there are not many functional groups in the molecule. Therefore, it is challenging to connect the different building blocks in a limited number of steps (Scheme 2.4). For the coupling of the different fragments we preferably chose olefination reactions followed by diimide reduction, as this is tolerate by cyclopropyl moieties and also have been applied in the syntheses reported by Baird et al..17
In the first retro synthetic step, the mycolic chain is cleaved giving -hydroxy ester 19 (Scheme 2.4). We envisioned to obtain -hydroxy ester 19 via asymmetric hydrogenation of -keto ester 20. Subsequently the meromycolate chain is disconnected leading to alkyl bromide 21 and and -keto ester 22. The alkyl bromide can be divided into the aldehyde 23 and the sulfone building block 24.
Chapter 2
26
Scheme 2.4. Retrosynthetic analysis of the -hydroxy ester moiety with the meromycolate chain.
-my coli c acid est e r 18
1920
Studies towards the synthesis of mycolic acid
27
The meromycolate chain derivative 23 can be built from four building blocks: two alkyl chains and two cyclopropyl units (Scheme 2.5). Both cyclopropyl moieties can be prepared from the common cyclopropyl lactone 29, which is obtained in an intramolecular cyclopropanation reaction of allyl diazoacetate 30. For the distal cyclopropyl moiety, lactone 29 can be reduced and reaction of the resulting lactol with Wittig reagent 25 would lead to the first part of the chain. The free hydroxyl group can be transformed into a good leaving group and in a copper-catalyzed Grignard cross-coupling alkyl chain 26 can be linked. The alcohol terminus is subsequently deprotected and oxidized to the corresponding aldehyde. After protection of the hydroxyl group of building block 28 the ester can be reduced to an alcohol and converted into a sulfone group. Then the cyclopropyl building block can be connected in a Julia-Kocienski olefination to give meromycolate derivative 23 after selective oxidation of the free hydroxyl group.
23
O
O
O
OPPPh3Br
O OH
Br
OHO
O
25 26
27 28
29
O
O
30
H
N2
Scheme 2.5. Retrosynthetic analysis of the meromycolate chain.
We also considered the cobalt-catalyzed cyclopropanation described by Katsuki and co-workers that gives a building block similar to cyclopropyl unit 29. However, this approach was rejected due to difficulties in the synthesis of the required salen ligand, which is used as ligand in the cobalt-catalyzed cyclopropanation.18
Chapter 2
28
2.4 Results and discussion
2.4.1 Synthesis of the meromycolate chain
With this approach we tried to shorten the number of steps and searched for efficient reactions in comparison to the synthesis reported by Baird.17 In the process of designing a synthesis of enantiopure -mycolic acids, we required an asymmetric approach to prepare a cis-cyclopropyl building block. One option is an intramolecular cyclopropanation of allylic diazoacetates. We chose for the conditions described by Doyle et al., since the yield is high and an excellent enantioselectivity was reached (Scheme 2.6).19,20 In addition, the synthesis of Rh-catalyst 31 was shorter than that of the Co-salen catalyst Katsuki uses for the cyclopropanation.18 Lactone 29 obtained in this reaction, can be used as common building block for both cyclopropyl moieties of the meromycolate chain.
O
CHN2
O O O95%
0.1 mol% 31, CH2Cl2 reflux, 12-18 h
Rh2(4S-MEOX)4 31
30 29
Rh Rh
N O
O N COOMe
HMeOOC
H
OH
MeOOC
O N COOMe
H
Scheme 2.6. Intramolecular cyclopropanation as reported by Doyle et al..
Rh-catalyst 31 is synthesized in two steps starting from D or L-serine depending on the configuration of the cyclopropyl unit required. For our synthesis we started from L-serine (Scheme 2.7), and got oxazolidinone 33 in 69% yield and observed the same optical rotation as earlier reported. Next, ligand 33 was reacted with dirhodium(II) tetraacetate in refluxing chlorobenzene, which led to Rh2(4S-MEOX)4 by replacing the acetate ligands.21,22
HO NH3Cl
COOMe
69%O NH
O
COOMe
+ Rh2(OAc)4
NaCO3, sandchlorobenzene, reflux, 18 h
55%
(Cl3CO)2CO, Et3N, CH2Cl20 °C, 2 h, -78 °C, 30 min
32 3398% ee
34 31
Rh Rh
O N COOMeH
Scheme 2.7. Synthesis of Rh2(4S-MEOX)4 following Doyle’s procedure.
Studies towards the synthesis of mycolic acid
29
The starting material for the intramolecular cyclopropanation, diazoacetate 30, was formed in three steps from diketene acetone adduct 35 and allyl alcohol 36 (Scheme 2.8). After considerable optimization, -keto ester 37 was obtained in 72% yield. The two most significant changes to the literature procedure were to leave out the solvent and carry out the reaction neat, and to change the ratio of allyl alcohol to the diketene acetone from 1.5 eq to 1 eq allyl alcohol.23 In the next step, -keto ester 37 was reacted with acetaminobenzenesulfonyl azide to give the corresponding -diazo- -dicarbonyl compound as intermediate, which after addition of aq. LiOH and stirring at room temperature for 10 h gave diazoacetate 30.24 As an alternative to the expensive acetaminobenzenesulfonyl azide, freshly prepared p-toluenesulfonyl azide was used, but the yield obtained was only 50% on average and the reproducibility was poorly.
O
O
O+ OH
120 °C, 15 h
72%
46%
0.5 mol% Rh2(4R-MEOX)4CH2Cl2, reflux, 20 h
O
O
O
OH
DIBALH, CH2Cl275 °C, 1 h
OH t-BuOK THF, rt, 3 h
19PPh3Br
O
O O
O
ON2
Et3N, LiOH, CH2Cl2, rt, 18 h73%
35 36 37
302987% ee
38 39
SO
ON3N
H
O
Scheme 2.8. Synthesis of common building block lactone 30.
The cyclopropanation reaction turned out to be very difficult to perform, because it required a complex reaction set-up, in which diazoacetate 30 could be added slowly to a refluxing solution of the catalyst in dichloromethane. A solution to this problem was the use of a long cannula, which ended in the reaction flask passing the condenser. In this way lacton 29 was obtained with 46% yield. Unfortunately, even
Chapter 2
30
the smallest leakage in the reaction set-up allowed the volatile lactone to evaporate together with the solvent.
Our efforts to synthesize the end part of the meromycolate chain, alcohol 39, failed so far. The reduction to lactol 38 seems to occur, but there was an unidentified side product, which was difficult to separate by column chromatography. Even with the impurity, the lactol opening with the Wittig reagent was not observed. The significant NMR signals for the double bond were not detected.25
2.4.2 A model substrate of the -branched -hydroxy ester motif
In this study we synthesized a model substrate, testing the different steps to form the -hydroxy ester moiety as we envisioned for building block 19.26 The synthesis
described here was planned for a linear synthesis, in which first the meromycolate chain is connected to a -keto ester before the asymmetric hydrogenation is performed (Scheme 2.9).
[(RuCl(T-BINAP))2(μ-Cl)3NH2Me2]EtOH, 20 bar, 50 °C, 25 h
95%
OH
20
I2, PPh3, ImidazolTHF, 55 °C, overnight
72%
I
20
O
OH O
17 C22H45
DIPA, MeLi, HMPATHF, 40 °C, 7 h
40%
+O
O O NaH, n-BuLiTHF, 0 °C to rt, 3 h
O
O O
1759%
O
OH O
17
+
40 41 42
43>95% ee
44
45
Br
17
Scheme 2.9. Synthesis of the -branched -hydroxy ester motif.
Studies towards the synthesis of mycolic acid
31
The alkylation of -keto ester 41 with 1-bromooctadecane gave -keto ester 42. In the subsequent step an asymmetric hydrogenation was performed. First we followed the procedures described by Brückner et al. and Genet et al., using (R)-BINAP together with RuCl2(p-cymene), as the metal precursor, but no product formation was observed.27,28 Due to better results in other projects of our group, we changed the catalyst to the commercially available (R)-[(RuCl(Tol-BINAP))2( -Cl)3NH2Me2] complex. Complete conversion was subsequently observed and the isolated yield was high with 95%, but the ee determination was tricky since there is no UV-active group in the molecule. After protection of the secondary alcohol with a TBDPS group, still no separation on either chiral GC or chiral HPLC was found. As an alternative, we prepared the Mosher ester of the enantiopure compound and the racemate. In the end we observed for the enantioenriched compound only one signal in the 19F-NMR (compared to the racemate), which means that the ee is at least 95%. The long alkyl iodide 44 was prepared in one step from the corresponding alcohol. In a Fráter alkylation the mycolic chain 44 is introduced and a high diastereoselectivity is expected, due to the chiral induction of the hydroxyl group. Till now the reaction was only carried out with the racemate. The low yield of -hydroxy ester motif 45 is in accordance with earlier reported yields.17,26
2.5 Conclusion
The synthesis of an enantiopure -mycolic acid has not been completed, due to problems in the scaling-up of the cyclopropanation and the difficulties to perform the reaction with reproducible results. Sadder and wiser, for a future approach, meso diester 2 should be considered as common cyclopropyl building block, since this methodology of desymmetrization is well established and gives many options for connecting the alkyl chain linkers between the cyclopropyl units in the meromycolate chain. For the -branched -hydroxy ester motif, we applied a new commercially available ruthenium catalyst in the asymmetric hydrogenation of -keto esters with an C18 alkyl chain reaching high yields and an excellent enantioselectivity. Still it needs to be studied, whether similar results can be obtained with an additional functional group in the alkyl chain, since the synthetic route was changed. The Fráter alkylation needs to be investigated with an enantiopure compound, and the diastereoselectivity needs to be reviewed. The low yield can be avoided by first introducing an allyl group and elongation of the alkyl chain to the full length of the mycolic chain, as reported by Baird et al. in their latest synthesis.29
Chapter 2
32
2.6 Experimental
2.6.1 General information
All reactions were carried out in flame dried glassware under N2 atmosphere and using standard Schlenk technique. All dry solvents were taken from an MBraun solvent purification system (SPS-800). All chemicals were purchased from Acros, Sigma-Aldrich or TCI Europe, and used without further purification. Flash chromatography was performed using Screening Devices silica gel type SiliaFlash P60 (230 – 400 mesh). TLC analysis was performed on Merck silicagel 60/Kieselguhr F254, 0.25 mm and visualized by UV and staining with Seebach’s reagent. 1H and 13C NMR were recorded on a Varian 400-MR (400, 100.59 MHz, respectively) using CDCl3 as solvent. Chemical shift values are reported in ppm with the solvent resonance as the internal standard (CDCl3: 7.26 for 1H, 77.0 for 13C). Data are reported as follows: chemical shifts, multiplicity (s=singlet, d=doublet, t=triplet, q=quartet, br=broad, m=multiplet), coupling constants J (Hz), and integration. Carbon assignments are based on APT 13C NMR experiments. Progress of the reaction and conversion were determined by GC-MS (GC, HP6890: MS HP5973) with HP1 or HP5 columns (Agilent Technologies, Palo Alto, CA). Enantiomeric excess (ee value) was determined by chiral HPLC analysis using a Shimadzu LC-10ADVP HPLC equipped with a Shimadzu SPD-M10AVP diode array detector and chiral columns as indicated. Retention times (tR) and integrated ratios were obtained using Agilent Chemstation Software. High resolution mass spectra (HRMS) were recorded on a ThermoScientific LTQ Oribitrap XL spectrometer. Optical rotations were measured on a Schmidt + Haensch polarimeter (Polartronic MH8) with a 10 cm cell (concentration c given in g/100 mL).
Studies towards the synthesis of mycolic acid
33
2.6.2 Synthesis of the meromycolic chain
(S)-methyl 2-oxazolidine-4-carboxylate (33)30
Methyl L-serine (1.26 g, 8.09 mmol, 2 eq) was dissolved in 40 mL of CH2Cl2 and cooled to 0 °C with an ice bath. To the reaction mixture was added triethyl amine (3.4 mL, 24.2 mmol, 6 eq) within 3 min. The solution was stirred for 10 min before a solution of triphosgene (1.2 g,
4.04 mmol, 1 eq) in 10 mL of CH2Cl2 was slowly added over 30 min. The reaction was stirred at 0 °C for another 2 h. Then 60 mL of diethyl ether were added and the reaction mixture was cooled to -78 °C to precipitate all Et3NHCl. After 30 min, the reaction mixture was allowed to reach rt and then filtered. The filter cake was washed with diethyl ether. The filtrate was concentrated in vacuo and the crude product was purified by column chromatography (SiO2, ethyl acetate) to yield oxazolidine 33 as an oil (582 mg, 3.74 mmol, 69%). 1H NMR (400.0 MHz, CDCl3): ppm 6.85 (brs, 1H), 4.56 (dd, J=12.9, 7.4 Hz, 1H), 4.51–4.39 (m, 2H), 3.76 (s, 3H).13C NMR (100.6 MHz, CDCl3): ppm 170.7 (C), 159.2 (C), 66.6(CH2), 53.6 (CH), 52.8 (CH3). Optical rotation [ ]D = -1.27 (c = 1.22, EtOH).
Rh2(4S-MEOX)4 (31)22
A one-neck round-bottom flask charged with ligand 33 (300 mg, 2.07 mmol, 8.3 eq), Rh2(OAc)4 (110.1 mg, 0.25 mmol, 1 eq) and 150 mL chlorobenzene was fitted to a Soxhlet extractor. The extractor was equipped with a thimble containing 6 g of a mixture of Na2CO3 and sand (2:1). The reaction mixture was heated to reflux and stirred for 16 h. Next the solvent was
removed under reduced pressure and the blue solid was dissolved in a minimal volume of methanol. The excess of ligand was separated from catalyst 31 by column chromatography (CN Bound silica purchased from SiliCycle, methanol to methanol:acetonitrile 98:2). The red colored fractions were combined and concentrated in vacuo. The solid was then recrystallized from dry acetonitrile (1.0 mL per 100 mg) to give bright red crystals of Rh2(4S-MEOX)4 31 (68.4 mg, 0.138 mmol, 55%). Anal. Calcd. for C20H24N4O2Rh2: C, 30.71; H, 3.09; N, 7.16. Found: C, 32.68; H, 3.45; N, 9.27. The elementary analysis is not matching considering the tolerance value of ± 0.4.
O NH
O
COOMe
Rh Rh
N O
O N COOMe
HMeOOC
H
OH
MeOOC
O N COOMe
H
Chapter 2
34
Allyl 3-oxobutanoate (37)23
A suspension of diketene acetone adduct 35 (33 mL, 0.25 mol, 1 eq) in allyl alcohol (17 mL, 0.25 mol, 1 eq) was heated to 120 °C and stirred at this temperature for 15 h. After cooling to rt, the
product was purified by vacuum distillation. The main fraction was collected at 80 °C (0.3 mbar) to afford -keto ester 37 (25.6 g, 0.18 mol, 72%). 1H NMR (400.0 MHz, CDCl3): ppm 5.95-5.77 (m, 1H), 5.35-5.12 (m, 2H), 4.56 (d, J=6.9 Hz, 2H), 3.42 (s, 2H), 2.20 (s, 3H). 13C NMR (100.6 MHz, CDCl3): ppm 200.2 (C), 166.5 (C), 131.4 (CH), 118.5 (CH2), 65.6 (CH2), 49.7 (CH2), 29.9 (CH3).
Allyl 2-diazoacetate (30)24
In 5 mL dry acetonitrile, -keto ester 37 (909 mg, 6.4 mmol, 1 eq) was dissolved and Et3N (1.1 mL, 8.32 mmol, 1.3 eq) was added. A solution of acetaminobenzensulfonyl azide (2.00 g, 8.32 mmol, 1.3
eq) in 5 mL acetonitrile was added dropwise in 5 min. This reaction mixture was stirred for 40 min at rt. Then aq. LiOH (805 mg, 19 mmol LiOH in 7 mL H2O) was added and the mixture was stirred for 10 h at rt. Next, the layers were separated and the water layer was extracted three times with a mixture of Et2O:EtOAc (2:1), the combined organic phases were dried over MgSO4, filtered and the filtrate was concentrated in vacuo. The crude product was purified by column chromatography (SiO2, pentane/diethyl ether 9:1) to give diazoacetate 30 as bright yellow oil (590 mg, 4.67 mmol, 73%). 1H NMR (400 MHz, CDCl3 5.91-5.86 (m, 1H), 5.24 (dd, J=31.6, 13.8 Hz, 2H), 4.76 (br s, 1H), 4.61 (d, J=4.7 Hz, 2H). 13C NMR (100 MHz, CDCl3 170.8 (C), 132.4 (CH), 118.4 (CH2), 65.5 (CH2), 46.3 (CH). IR (film, cm)-1: 3122, 2948, 2887, 2111, 1691, 1445, 1383, 1181, 992, 836, 741.
(1R,5S)-3-oxabicyclo[3.1.0]hexan-2-one (29)19
To a refluxing solution of Rh2(4S-MEOX)4 (4 mg, 0.008 mmol, 0.5 mol%) in 10 mL CH2Cl2 was added diazoacetate 30 (200 mg, 1.58 mmol, 1 eq) in 10 mL CH2Cl2 over 10 h. After the addition, the reaction mixture was stirred for 15 h under reflux (precaution was taken that the reaction flask was
closed air tight; the product is volatile). The reaction mixture was cooled to rt and most of the CH2Cl2 was removed by distillation at atmospheric pressure. The catalyst was removed by column chromatography (SiO2, CH2Cl2) and gave lactone 29 as pale yellow oil (71 mg, 0.73 mmol, 46%, 87% ee). The enantiomeric ratio was determined by chiral GC analysis, Chiraldex -cyclodextrin column, 90 °C isotherm, retention time (min): 23.5 (major) and 24.3 (minor). 1H NMR (400 MHz, CDCl3 4.30-
O
O O
O
ON2
O
O
Studies towards the synthesis of mycolic acid
35
4.23 (m, 1H), 4.15-4.11 (m, 1H), 2.21-2.14 (m, 1H), 1.99-1.92 (m, 1H), 1.22-1.16 (m, 1H), 0.80-0.75 (m,1H). 13C NMR (100 MHz, CDCl3 176.3 (C), 69.3 (CH2), 17.3 (CH), 17.1 (CH), 12.0 (CH2).
2.6.3 Model substrate of the -branched -hydroxy ester motif26
Methyl 3-oxohenicosanoate (42)31
NaH (250 mg, 6.20 mmol, 1.5 eq) was suspended in 10 mL THF and cooled to 0 °C with an ice bath. At this temperature, -keto ester 41 (479 mg, 4.13 mmol, 1 eq) was added and the reaction mixture was
stirred for 10 min before n-BuLi (3.2 mL, 1.3 M in hexane, 1.1 eq) was added dropwise. After stirring the reaction mixture for 40 min at 0 °C, a solution of alkylbromide 40 (1.51 g, 4.54 mmol, 1.1 eq) in 5 mL THF was added dropwise. The reaction mixture was stirred for 3 h and during this time rt was reached, then the reaction was quenched by addition of aq. 2 M HCl. The layers were separated and the water layer was extracted three times with diethyl ether, the combined organic phases were dried over MgSO4, filtered and the filtrate was concentrated in vacuo. The crude product was purified by column chromatography (SiO2, pentane/diethyl ether 9:1) to give -keto ester 42 as bright yellow oil (864 mg, 2.44 mmol, 59%). 1H NMR (400 MHz, CDCl3 3.74 (s, 3H), 3.44 (s, 2H), 2.54 (t, J=7.4 Hz, 2H), 1.64 (s, 2H), 1.26 (s, 30H), 0.88 (t, J=7.1 Hz, 3H). 13C NMR (100 MHz, CDCl3 203.0 (C), 167.8 (C), 52.5 (CH3), 49.2 (CH2), 43.3 (CH2), 32.2 (CH2), 29.9 (CH2), 29.8 (CH2), 29.7 (CH2), 29.6 (CH2), 29.2 (CH2), 23.7 (CH2), 22.9 (CH2), 14.3 (CH3).
(R)-methyl 3-hydroxyhenicosanoate (43)
-Keto ester 42 (177 mg, 0.50 mmol, 1 eq) was dissolved in 5 mL EtOH. Then (R)-(RuCl(T-BINAP))2( -Cl)3[NH2Me2] 5 mol% was added and the reaction mixture was stirred for 30 min at rt. The
reaction vial was placed in a stainless steel autoclave and first three times flushed with nitrogen, before a hydrogen pressure of 20 bar was applied. The reaction mixture was heated to 50 °C and stirred at this temperature for 72 h. After cooling to rt the autoclave was flushed with nitrogen. The crude product was filtered over silica and the column was washed with CH2Cl2. -Hydroxy ester 43 (169 mg, 0.47 mmol, 94%, 95% ee) was obtained after purification by column chromatography (SiO2, pentane/diethyl ether 3:2). The enantiomeric ratio was determined by Mosher ester analysis using 19F-NMR (racemate: -71.4, -71.5; enantiopure: -71.4).1H NMR (400 MHz, CDCl3 J=7.4 Hz, 2H), 1.64 (s, 2H), 1.26 (s, 30H), 0.88 (t, J=7.1 Hz, 3H). 13C NMR (100 MHz, CDCl3
O
O O
17
O
OH O
17
Chapter 2
36
(CH), 51.7 (CH3), 41.1 (CH2), 36.5 (CH2), 31.9 (CH2), 29.7 (CH2), 29.6 (2XCH2), 29.5 (2XCH2), 29.3 (CH2), 25.5 (CH2), 22.7 (CH2), 14.1 (CH3). HRMS was not measured.
1-iododocosane (44)32
In dry THF, iodine (2.79 g, 11.0 mmol, 1.1 eq), PPh3 (2.62, 10 mmol, 1 eq), imidazole (661 mg, 10 mmol, 1 eq) and docosan-1-ol (3.5 g, 11 mmol, 1.1 eq) were dissolved. This reaction mixture was stirred at 55 °C for 4 d.
After cooling, the reaction was quenched by addition of aq. NaHCO3. The layers were separated and the water layer was extracted three times with diethyl ether, the combined organic phases were dried over MgSO4, filtered and the filtrate was concentrated in vacuo. The crude product was purified by column chromatography (SiO2, pentane/diethyl ether 9:1) to give 1-iododocosane 44 as light yellow solid (3.15 g, 7.23 mmol, 72%). 1H NMR (400 MHz, CDCl3 3.18 (t, J=6.0 Hz, 2H), 1.89-1.76 (m, 2H), 1.33 (s, 38H), 0.88 (t, J=7.6 Hz, 3H). 3C NMR (100 MHz, CDCl3
33.6 (CH2), 32.0 (CH2), 30.6 (CH2), 29.7 (CH2), 29.6 (2XCH2), 29.5 (CH2), 29.4 (CH2), 28.6 (CH2), 22.7 (CH2), 14.1 (CH3), 6.9 (CH2).
Methyl 2-(1-hydroxynonadecyl)tetracosanoate (45)
To a stirred solution of diisopropylamine (0.25 mL, 1.76 mmol, 3.3 eq) in 6 mL THF was added MeLi (1.4 M solution in hexane, 1.2 mL, 1.7 mmol, 3.2 eq) at -78 °C. After the addition, the solution was stirred for 30 min at 0 °C. Then the reaction mixture was cooled to
-40 °C and a solution of -hydroxy ester 43 (120 mg, 0.53 mmol, 1 eq) in 2 mL THF was added. After stirring for 30 min at the same temperature HMPA (0.58 mL, 3.34 mmol, 6.3 eq) and a solution of 1-iododocosane 44 (220 mg, 0.50 mmol, 1 eq) in 2 mL THF were added, and the reaction was stirred for 6 h at -40 °C. The reaction mixture was quenched by adding sat. aq. NH4Cl. The layers were separated and the water layer was extracted three times with diethyl ether, the combined organic phases were dried over MgSO4, filtered and the filtrate was concentrated in vacuo. The crude product was purified by column chromatography (SiO2, pentane/diethyl ether 9:1) to give alkylation product 45 as light yellow solid (142 mg, 0.21 mmol, 40%). 1H NMR (400 MHz, CDCl3 -3.44 (m, 1H), 2.56-2.36 (m, 1H), 1.55-1.49 (m, 2H), 1.43 (m, 2H), 1.26 (s, 72H), 0.88 (t, 6H). 13C NMR (100 MHz, CDCl3 (CH3), 41.1 (2xCH2), 36.5 (2xCH2), 31.9 (2xCH2), 29.7 (2xCH2), 29.6 (2XCH2), 29.4 (2xCH2), 25.5 (CH2), 22.7 (CH2), 14.1 (CH3). HRMS was not measured.
I
20
O
OH O
17 C22H45
Studies towards the synthesis of mycolic acid
37
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