Tetrahedron Letters 54 (2013) 5677–5681

Contents lists available at ScienceDirect

Tetrahedron Letters

journal homepage: www.elsevier .com/ locate/ tet le t

Synthesis of a bipyridine-derived achiral thioureatrifluoromethanesulfonic acid salt and its applicationas an additive in organocatalytic asymmetric reactions

0040-4039/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.tetlet.2013.08.004

⇑ Corresponding author. Tel.: +90 (0)364 227 7000; fax: +90 (0)364 227 7005.E-mail address: sinanbasceken@hitit.edu.tr (S. Basceken).

� Deceased on June 24th 2012.

N

N

NH2

H2N

N

N

HN

HN

S

CF

NH

NH

S

CF3

F3C

F3C CF3

NCS

acetone, 48 h, rt

II

CF3SO3HCHCl3, rt

N

N

HN

HN

S

CF

NH

NH

S

CF3

F3CIII

H

H

CF3SO3

CF3SO3

I

Scheme 1. Synthesis of bipyridine trifluoromethanesulfonic acid salt

Ayhan Sıtkı Demir a,�, Sinan Basceken a,b,⇑a Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkeyb Department of Chemistry, Hitit University, 19030 Corum, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 February 2013Revised 18 July 2013Accepted 1 August 2013Available online 11 August 2013

Keywords:Host–guest complexProline–thiourea interactionAsymmetric aldol reactionEnantioselective Michael reactionAsymmetric Mannich reaction

The host–guest complex of a proline–thiourea bipyridine trifluoromethanesulfonic acid salt can catalyzeorganocatalytic asymmetric reactions such as aldol, Michael, and Mannich in polar protic medium withhigh stereoselectivities. The privileged bipyridine backbone and the thiourea motif are essential to theactivity and enantioselectivity through hydrogen bonding interactions.

� 2013 Elsevier Ltd. All rights reserved.

3

CF3

3

CF3

III.

Organic reactions in water and aqueous medium have attracteda great deal of attention and have become highly desirable.Although organocatalytic asymmetric reactions are normally car-ried out in organic solvents, it has been demonstrated that theaddition of water has a positive effect on the process,1,2 but theuse of an excess amount decreases the reactivity and stereocontrolof the reaction.3 The search for simple chiral molecules capable ofpromoting enantioselective reactions in water and in aqueousmedium has seen significant activity.4

Barbas and co-workers developed a catalytic direct asymmetricaldol reaction that was performed in water without the addition oforganic solvents.5 The use of proline furnished low selectivities andthe bifunctional catalyst system demonstrated excellent reactivity,diastereoselectivity, and enantioselectivity in water. Kobayashiet al. reported the catalytic diastereo- and enantioselective Man-nich-type reaction of a hydrazono ester with silyl enol ethers inaqueous medium with zinc fluoride and a chiral diamine ligand.6

Prolinamide and its derivatives appear to be the most efficientorganocatalysts for Barbas–List aldol reactions.7 Prolinamides de-rived from amino alcohols or amino phenols,8 amines, diamines,9

and prolyl sulfonamides10 have been explored as bifunctional cat-alysts to promote stereoselective aldol reactions in water.1 Accord-ingly, we decided to explore bipyridine-derived achiral thiourea

II11 in water soluble form, that is, water soluble salt III, and to testits utility in Barbas–List aldol reactions in water.

The bipyridine derived achiral thiourea II was converted into itstrifluoromethanesulfonic acid salt III as shown in Scheme 1.

Aldol reaction. Initially, the catalytic activity of the salt III inwater was tested. Various reaction conditions were examined asdescribed for Barbas–List aldol reactions (Table 1). The aldol prod-uct was obtained in ee: 99% and dr: 98/2 and 85% yield.

Table 1The enantioselective direct aldol reaction of various aldehydes 1 with cyclohexanone2 in watera

Ar

OO

Ar

OOH

+

anti -3a-f1a-e 2H2O, rt

III(S)-proline

Entry Aldehyde (Ar) Yield (%) dr (anti/syn)b eec (%) (Conf.)d

1 4-NO2C6H4 85 98/2 99 (2R,10S)2 3-NO2C6H4 80 98/2 99 (2R,10S)3 4-BrC6H4 60 91/9 98 (2R,10S)4 4-CF3C6H4 65 91/9 98 (2R,10S)5 2-MeOC6H4 70 99/1 96 (2R,10S)

6e 4-NO2C6H4 78 44/56 98 (2R,10S)

a The reactions were conducted with (S)-proline (20 mol %), bipyridinium tri-fluoromethanesulfonate salt III (10 mol %), cyclohexanone or cyclopentanone(16 equiv), aldehyde (0.125 mmol), and H2O (20 lL) for 12 h.

b Determined by chiral HPLC analysis of the mixture of anti/syn product.c Determined by chiral HPLC.d Determined by comparison of their specific rotations with those reported for

anti-3a–f.e Cyclopentanone was used.

N

N

N

SCF3

CF3

H

N OO

HH

H

Ar H

O

F3C SO

O

O

Guest

Host

Figure 1. Proposed TS for the enantioselective aldol reaction.

OHN O

O+

-H2O N O

O

N

OOH

H

OR

re -facial attacksi -enamine + re -aldehyde

TS

NH O

OOH

ROH

OHN

O

O+R

OH

L*H

L*

+H2O

L* =

N

HN

HN

SCF3

CF3

L*H

L*

L*H

anti

HCF3SO3

gues

t

host

O

R

Scheme 2. Proposed mechanism of the aldol reaction.

5678 A. S. Demir, S. Basceken / Tetrahedron Letters 54 (2013) 5677–5681

The isolated products showed the same stereochemical out-come as those obtained via Barbas–List reactions. To the best ofour knowledge, this is the first organocatalytic asymmetric aldolreaction in water with unmodified proline (host) and a bipyri-dine-derived achiral thiourea trifluoromethanesulfonic acid salt(guest), which gives excellent diastereo- and enantioselectivities.Various examples were studied and the aldol products were ob-tained in comparably good yields (60–85%) and with very highselectivity (up to 99% ee) (Table 1). The reactions with aldehydesbearing electron-withdrawing groups proceeded smoothly to af-ford the aldol adducts in 12 h (Table 1, entries 1–4), with a goodlevel of diastereoselectivity. For the electron-rich aromatic alde-hyde (Table 1, entry 5), the reaction required a longer time(24 h). In entry 6, after the addition of cyclopentanone, p-nitro-benzaldehyde was added, and stirring was continued for 12 h.The isolated aldol product showed excellent enantioselectivity(98%), but much lower diastereoselectivity.

To test the ability and flexibility of the catalytic system, (S)-tert-leucine (host) was used instead of proline under similar conditions.The aldol reactions furnished anti products with excellent ee(>98%) and good yield (80%). The reactions were conducted with(S)-tert-leucine (20 mol %), bipyridine trifluoromethanesulfonicacid salt III (10 mol %), cyclohexanone (16 equiv), 4-nitrobenzalde-hyde (0.125 mmol), and water (20 lL) for 12 h.

‘To make a good asymmetric catalyst perfect’, the role ofsuitable additives can be crucial in enhancing the reactivity andstereoselectivity of the catalytic system.11 Therefore, we decidedto use, instead of bipyridinium trifluoromethanesulfonate salt III,pyridine, 2,6-dimethylpyridine, and bipyridine as additives in theproline-catalyzed organocatalytic asymmetric reactions. The re-sults should provide information on the discriminating hydrogenbonding effect of thiourea or the basic effects of the pyridine moi-ety. However, when pyridine and pyridine derived additives wereused in place of the thiourea, no reaction took place and we didnot obtain any product. This fact demonstrates clearly that thehydrogen bonding effect of proline–thiourea plays the most impor-tant role, and not the basicity of the pyridine moiety. In addition,under similar conditions, the aldol reaction without bipyridiniumtrifluoromethanesulfonate salt III did not lead to any product.Therefore, the use of water as a reaction solvent is not alwayspractical for asymmetric catalytic reactions because water ofteninhibits the catalyst activity or alters the enantioselectivity byinterrupting ionic interactions and hydrogen bonds critical forstabilizing the transition states of the reaction intermediates.5

Thus, special design is required for performing asymmetricreactions in water. Here we report the efficient enamine-basedorganocatalytic direct asymmetric aldol reactions between cyclicketones and aldehydes using bipyridinium trifluoromethanesulfo-nate salt III as the additive in water without any organic solvent.

Proline-catalyzed direct aldol reactions have been shown,experimentally and computationally, to proceed through enamineintermediates. Based on this initial proposal, we propose that theinteraction of the thiourea–bipyridine moiety with proline in thetransition state (TS) is as shown in Figure 1. The aldehyde is acti-vated by hydrogen bonding with the carboxyl group of the prolinein a manner such that carbon–carbon (C–C) bond formation takesplace from its re face. The presence of the bipyridine moiety re-stricts the conformation and makes the carboxyl group a betterhydrogen-bond donor. The main advantage of this model is thatthe transition state is stabilized through hydrogen bonding. There-fore, a small change in the pKa value of an organic compoundwould affect its catalytic activity and selectivity in the aldolreaction.11,12

As proposed for the Hajos–Eder–Sauer–Wiechert reaction,11 weassume that the key intermediate of the direct intermolecularasymmetric aldol reactions described is an enamine formedbetween proline and the corresponding ketone donor substrate.In analogy to the mechanism of aldolase antibody 38C2,11 thisenamine with a bipyridine thiourea moiety in the backbone attacksthe carbonyl group of the aldehyde acceptor with high enantiofa-cial selectivity, which is imposed by a highly organized tricyclichydrogen bonded framework resembling a metal-free Zimmer-man–Traxler type transition state (Scheme 2).11 This readily

N OO

H

ON O

Ar NN

SN

CF3

CF3

HH

Host

GuestHS

O O

OF3C

Figure 2. Proposed TS for the enantioselective Michael reaction.

OHN O

O+

-H2O N O

O

N

OOH

NH O

OOH

Ar

H

OHN

O

O+

Ar

L*H

L*

+H2O

L*H

L*

L*H

syn

gues

t

host

ArNO2

N O

O

ArO2N

O2N

TS

4a

5a-d

6a-f

Scheme 3. Proposed mechanism of the Michael reaction.

A. S. Demir, S. Basceken / Tetrahedron Letters 54 (2013) 5677–5681 5679

explains the observed re facial enantioselectivity. Typically, anti-diastereoselectivity is observed in the proline–thiourea catalyzedaldol reaction.11

Michael reaction. To expand further the scope of amino acidbased catalysis, we examined other important C–C bond-formingreactions (Table 2). In this study, we tested the activity of this cat-alyst for Michael addition reactions in water. In contrast to the al-dol reaction, Michael addition with this water soluble catalyst didnot form any product. Next, we carried out this reaction in a mix-ture of water/organic solvent (toluene) and the reaction was mon-itored by thin layer chromatography. After 24 h, the expectedMichael product was formed in 98% ee and 18/82 dr as shown inTable 2, entry 4. The Michael reaction with (S)-tert-leucine fur-nished the corresponding product with 36/64 dr, but with reversedenantioselectivity (80% ee) (Table 2, entry 5). We also used (S)-tryptophan as the catalyst, which interestingly furnished the samediastereoselectivity (36/64) but with opposite enantioselectivity(78% ee) (Table 2, entry 6).

We continued to evaluate the scope of the reaction by testingthe Michael addition of butyraldehyde 4b to nitroalkene 5b, whichled to a good isolated yield (65%) and high diastereoselectivity (16/84) with high enantioselectivity (85%) (Table 2, entry 7). Addition-ally, the reaction between isovaleraldehyde 4c and trans-4-bromo-b-nitrostyrene (5b) gave the Michael adduct in good yield (60%)and high stereoselectivity (Table 2, entry 8). Accordingly, we car-ried out the Michael reaction in the absence of bipyridinium tri-fluoromethanesulfonate salt III in toluene/water under the sameconditions, but unfortunately, no product was formed. This resultindicates that when the catalyst includes an acid group,5 the reac-tion does not proceed in a toluene/water system. This may be be-cause the proline is soluble, whereas the reactants are less misciblein water. In a biphasic system, the interactions required for thereaction do not occur. In the case of the Michael reaction usingthe proline–thiourea host–guest complex, the catalyst additiveassembles with the reactants through hydrophobic interactions,excluding water molecules from the organic phase. In this concen-trated organic phase the reaction occurs efficiently to afford Mi-chael adducts with high enantioselectivities, probably facilitatedby hydrogen bonds between the thiourea salt and proline in the TS.

We assume that the Michael reaction would proceed accordingto a modified Seebach’s model,13 in which the carboxylate moiety

Table 2Michael reaction of aldehydes and nitroalkenes in toluene/watera

NO2+

O

R1

(S

tolue

5a-dR2

4a-c

Entry AldehydeR1

NitrostyreneR2

Yield (

1 CH3 H 672 CH3 4-Br 723 CH3 4-OMe 704 CH3 2-Br 71

5e CH3 4-Br 666f CH3 4-Br 627 CH2CH3 4-Br 65

8 CH(CH3)2 4-Br 60

a The reactions were conducted with (S)-proline (20 mol %), bipyridinium trifluo(0.125 mmol), toluene (1.0 mL), and H2O (20 lL) for 24 h.

b Determined by chiral HPLC analysis of the mixture of anti/syn product.c Determined by chiral HPLC.d Determined by comparison of their specific rotations with those reported.e (S)-tert-Leucine was used.f (S)-Tryptophan was used.

of the proline forms an assembly with the thiourea, in turn enhanc-ing the reactivity and selectivity of the catalyst (Fig. 2).

Based on the observed stereoselectivities, we have proposed aplausible catalytic mode for this asymmetric conjugate addition(Scheme 3). Presumably, the in situ formed enamine intermediatebetween aldehydes 4 and the proline–thiourea catalyst adopts theE-conformation. Subsequently, a new C–C bond is formed by the

(S)(R) NO2

O

R1

III)-proline

ne/H2O, rt

6a-f

R2

%) dr (anti/syn)b eec (%) (Conf.)d

18/82 93 (2R,3S)34/66 97 (2R,3S)20/80 82 (2R,3S)18/82 98 (2R,3S)36/64 80 (2S,3R)36/64 78 (2S,3R)16/84 85 (2R,3S)5/95 90 (2R,3S)

romethanesulfonate salt III (10 mol %), aldehyde (5 equiv, 20 lL), nitrostyrene

+

O

III(S)-proline

toluene/H2O, rt7

1011

O HN

OMe

NO2

H

N

NO2

87% ee and 60% yield

50% ee and 36% yield

(S)-tert -leucineIII

MeO

Scheme 4. Two-component Mannich reaction.

NO

OH

H R

NPMP

R'N

N

S

CF3

CF3

HH

Host

5680 A. S. Demir, S. Basceken / Tetrahedron Letters 54 (2013) 5677–5681

addition to the unsaturated electrophile 5. Here, the proline–thio-urea moiety supports the substrate orientation and provides thebasis for the enantioselective coupling. Next, the re face of the en-amine intermediate attacks the re face of trans-b-nitrostyrene togive the corresponding preferred syn Michael adducts6a–f(Scheme 3). Nevertheless, the precise catalytic mechanism needsfurther investigation.13

Mannich reaction. Another representative application was theMannich reaction in water. The catalytic, diastereo-, and enantio-selective Mannich-type reaction of a hydrazono ester with silylenol ethers in aqueous media has been achieved with zinc fluorideand a chiral diamine ligand. The use of water and a small amountof TfOH was essential for the reactions to proceed in high yields.6

In our initial experiment, we found that the host–guest proline–thiourea system could catalyze the three-component Mannichreaction of aliphatic aldehydes 4c–e, p-anisidine 8, and acetone 7in toluene/water to give the expected b-amino-carbonyl com-pounds 9a–c with high enantioselectivities (up to 85% ee) in goodyields, as shown in Table 3. Accordingly, under similar conditions,the Mannich reaction in the absence of bipyridinium trifluoro-methanesulfonate salt III did not lead to any product. This resultis the same as those previously described in this study on enam-ine-based organocatalytic asymmetric reactions, such as aldoland Michael. The effectiveness of the proline–thiourea catalystscompared to proline can be attributed to the solubility of the cat-alysts. Although proline dissolves in water, the thiourea salt is onlypartially soluble in water and forms an organic phase with thealdimine and ketone in which the Mannich reaction proceedsefficiently.

More effective was the two-component Mannich reaction ofaldimine 10, derived from the aromatic aldehyde, p-nitrobenzalde-hyde (1a), and p-anisidine (8), with acetone 7 in toluene/water inthe presence of 10 mol % of the catalyst. The Mannich productwas isolated in 87% ee and 60% yield as shown in Scheme 4. Thereactions were conducted with (S)-proline (20 mol %), bipyridini-um trifluoromethanesulfonate salt III (10 mol %), acetone (50 lL),aldimine (0.0625 mmol), toluene (2 mL), and water (10 lL) for48 h. Accordingly, the same conditions were applied to the asym-metric Mannich reaction of acetone 7 with aldimine 10 in water.Unfortunately, no product was formed. However, when the

Table 3Three-component Mannich reactions with various aliphatic aldehydes (PMP =p-methoxyphenyl)a

+

OIII

(S)-proline

toluene/H2O, rtR

O HN+ R

O

OMe

9a-c

4c-e

H2N

OMe78

Entry Aldehyde Product Yield (%) eeb (%) (Conf.)c

1O O HN

PMP

60 85 (S)

2

OO HN

PMP55 80 (R)

3

OO HN

PMP58 80 (R)

a The reactions were conducted with (S)-proline (20 mol %), bipyridinium tri-fluoromethanesulfonate salt III (10 mol %), acetone (50 lL), p-anisidine(0.0625 mmol), aldehyde (0.0625 mmol), toluene (2 mL), and H2O (10 lL) at roomtemperature for 48 h.

b Determined by chiral HPLC with an OD-H column.c Determined by comparison of their specific rotations with those reported.

reaction conditions were modified using toluene/water as the reac-tion solvent, the desired product 11 was formed. The reaction with(S)-tert-leucine resulted in the formation of the Mannich product,however, with low enantioselectivity (50%) and in low yield (36%).

In order to explain the observed stereoselectivity, it was pro-posed that the reaction follows an enamine mechanism and in-volves a transition state as shown in Figure 3. This is similar tothe earlier proposed transition state of the corresponding aldolreaction (see Fig. 1).

The proposed mechanism of the proline–thiourea catalyzedMannich reaction is depicted in Scheme 5. On the basis of theprevious mechanism for proline-catalyzed Mannich reactions, wereasoned that this asymmetric Mannich reaction could alsoproceed via an enamine pathway because the nucleophilic additionof the in situ generated enamine would be faster to an imine thanto an aldehyde.14 The (E)-configurations are assumed for both theenamine and the imine. Typically, Mannich products are formed

OHN

O

O+

-H2O N O

O

N

OOHR

NH

RCHO

si-facial attacksi-enamine + si-imine

TS

NH O

OOH

RNH

OHN

O

O

+R

NH

L*

H

L*

+H2O

L*H

L*

L*H

R H

NAr+

ArNH2

-H2O

ArAr

Ar

Scheme 5. Proposed mechanism for the Mannich reaction.

NH

SF3C O

O O Guest

Figure 3. Proposed TS for the enantioselective Mannich reaction.

A. S. Demir, S. Basceken / Tetrahedron Letters 54 (2013) 5677–5681 5681

via si face attack on an imine. The si face of the imine is attackedselectively by the enamine to allow for protonation of its lone pairand compensation of negative charge formation. This mechanismis similar to that proposed for the proline-catalyzed aldol reaction(Scheme 2).14

In summary, we have found that the host–guest proline–thio-urea bipyridinium trifluoromethanesulfonate salt III complex cancatalyze directly enantioselective aldol reactions in water withunmodified proline and with excellent stereoselectivities (up to98:2 dr, >99% ee). In contrast to aldolreactions, the Michael andMannich reactions were not successful in water. We found thatthese reactions in water/toluene gave the desired products withhigh stereoselectivities. More detailed studies on the mechanismsand synthetic applications of this supramolecular catalyst in enan-tioselective organocatalytic reactions are currently underway.

Acknowledgments

We gratefully acknowledge the Scientific and Technological Re-search Council of Turkey (TÜBITAK), the Turkish Academy of Sci-ences (TÜBA), and Middle East Technical University (METU).Thanks are also due to Professor Metin Balci for editorialassistance.

Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.tetlet.2013.08.004.

References and notes

1. Pedrosa, R.; Andres, J. M.; Manzano, R.; Roman, D.; Tellez, S. Org. Biomol. Chem.2011, 9, 935.

2. (a) Tang, Z.; Yang, Z.-H.; Cun, L.-F.; Gong, L.-Z.; Mi, A.-Q.; Jiang, Y.-Z. Org. Lett.2004, 6, 2285; (b) Torii, H.; Nakadai, M.; Ishihara, K.; Saito, S.; Yamamoto, H.Angew. Chem., Int. Ed. 1983, 2004, 43; (c) Nyberg, A. I.; Usano, A.; Pihko, P. M.Synlett 2004, 1891; (d) Dziedzic, P.; Zou, W.; Ibrahem, I.; Sunden, H.; Cordova,A. Tetrahedron Lett. 2006, 47, 6657; (e) Gruttadauria, M.; Giacalone, F.;Marculescu, A. M.; Noto, R. Adv. Synth. Catal. 2008, 350, 1397.

3. (a) Darbre, T.; Machuqueiro, M. Chem. Commun. 2003, 1090; (b) Peng, Y.-Y.;Ding, Q.-P.; Li, Z.; Wang, P. G.; Cheng, J.-P. Tetrahedron Lett. 2003, 44, 3871; (c)Chimni, S. S.; Mahajan, D.; Babu, V. V. S. Tetrahedron Lett. 2005, 46, 5617; (d)Hayashi, Y.; Aratake, S.; Itoh, T.; Okano, T.; Sumiya, T.; Shoji, M. Chem. Commun.2007, 957.

4. (a) Chanda, A.; Fokin, V. V. Chem. Rev. 2009, 109, 725; (b) Raj, M.; Singh, V. K.Chem. Commun. 2009, 6687; (c) Gruttadauria, M.; Giacalone, F.; Noto, R. Adv.Synth. Catal. 2009, 351, 33; (d) Paradowska, J.; Stodulski, M.; Mlynarski, J.Angew. Chem., Int. Ed. 2009, 48, 4288.

5. Mase, N.; Nakai, Y.; Ohara, N.; Yoda, H.; Takabe, K.; Tanaka, F.; Barbas, C. F. J.Am. Chem. Soc. 2006, 128, 734.

6. Kobayashi, S.; Hamada, T.; Manabe, K. J. Am. Chem. Soc. 2002, 124, 5640.7. Aratake, S.; Itoh, T.; Okano, T.; Usui, T.; Shoji, M.; Hayashi, Y. Chem. Commun.

2007, 2524.8. (a) Tang, Z.; Jiang, F.; Yu, L.-T.; Cui, X.; Gong, L.-Z.; Qiao, A.; Jiang, Y.-Z.; Wu, Y.-

D. J. Am. Chem. Soc. 2003, 125, 5262; (b) Wang, C.; Jiang, Y.; Zhang, X.-X.; Huang,Y.; Li, B.-G.; Zhang, G.-L. Tetrahedron Lett. 2007, 48, 4281; (c) Gandhi, S.; Singh,V. K. J. Org. Chem. 2008, 73, 9411; (d) Zhao, J.-F.; He, L.; Jiang, J.; Tang, Z.; Cun, L.-F.; Gong, L.-Z. Tetrahedron Lett. 2008, 49, 3372; (e) Vishnumaya, M. R.; Singh, V.K. J. Org. Chem. 2009, 74, 4289; (f) Fu, Y.-Q.; Li, Z.-C.; Ding, L.-N.; Tao, J.-C.;Zhang, S.-H.; Tang, M. S Tetrahedron: Asymmetry 2006, 17, 3351; (g) Zhang, S. P.;Fu, X. K.; Fu, S. D. Tetrahedron Lett. 2009, 50, 1173.

9. (a) Guillena, G.; Hita, M. C.; Najera, C. Tetrahedron: Asymmetry 2006, 17, 1493;(b) Huang, W. P.; Chen, J. R.; Li, X. Y.; Cao, Y. J.; Xiao, W. J. Can. J. Chem. 2007, 85,208; (c) Jia, Y. N.; Wu, F. C.; Ma, X.; Zhu, G.-J.; Da, C. S. Tetrahedron Lett. 2009, 50,3059.

10. (a) Zu, L.; Xie, H.; Li, H.; Wang, J.; Wang, W. Org. Lett. 2008, 10, 1211; (b) Zhang,S. P.; Fu, X. K.; Fu, S. D.; Pan, J. F. Catal. Commun. 2009, 10, 401; (c) Fu, S. D.; Fu,X.-K.; Zhang, S.-P.; Zou, X. C.; Wu, X. J. Tetrahedron: Asymmetry 2009, 20, 2390.

11. (a) Eder, U.; Sauer, G.; Wiechert, R. Angew. Chem., Int. Ed. Engl. 1971, 10, 496; (b)Hajos, Z. G.; Parrish, D. R. J. Org. Chem. 1974, 39, 1615; (c) Zimmerman, H. E.;Traxler, M. D. J. Am. Chem. Soc. 1920, 1957, 79; (d) Agami, C.; Platzer, N.;Sevestre, H. Bull. Soc. Chim. Fr. 1987, 2, 358–360; (e) Agami, C.; Puchot, C.;Sevestre, H. Tetrahedron Lett. 1986, 27, 1501; (f) Agami, C. Bull. Soc. Chim. Fr.1988, 3, 499; (g) Sakthivel, K.; Notz, W.; Bui, T.; Barbas, C. F., III J. Am. Chem. Soc.2001, 123, 5260–5267; (h) Cobb, A. J. A.; Shaw, D. M.; Longbottom, D. A.; Gold,J. B.; Ley, S. V. Org. Biomol. Chem. 2005, 3, 84; (i) Hayashi, Y.; Sumiya, T.;Takahashi, J.; Gotoh, H.; Urushima, T.; Shoji, M. Angew. Chem. Int. Ed. 2006, 45,958; (j) Wu, Y.; Zhang, Y.; Yu, M.; Zhao, G.; Wang, S. Org. Lett. 2006, 8, 4417; (k)Reis, Ö.; Eymur, S.; Reis, B.; Demir, A. S. Chem. Commun. 2009, 1088; (l)Basceken, S.; Demir, A. S Tetrahedron: Asymmetry 2013, 24, 515.

12. (a) Hoang, L.; Bahmanyar, S.; Houk, K. N.; List, B. J. Am. Chem. Soc. 2003, 125, 16;(b) Bahmanyar, S.; Houk, K. N.; Martin, H. J.; List, B. J. Am. Chem. Soc. 2003, 125,2475; (c) Clemente, F. R.; Houk, K. N. Angew. Chem., Int. Ed. 2004, 37, 5766; (d)Allemann, C.; Gordillo, R.; Clemente, F. R.; Cheong, P. H.; Houk, K. N. Acc. Chem.Res. 2004, 37, 558.

13. (a) Blarer, S. J.; Seebach, D. Chem. Ber. 1983, 116, 2250; (b) Seebach, D.; Golinski,J. Helv. Chim. Acta 1981, 64, 1413; (c) Husmann, R.; Jörres, M.; Raabe, G.; Bolm,C. Chem. Eur. J. 2010, 16, 12549; (d) Chen, J.-R.; Cao, Y.-J.; Zou, Y.-Q.; Tan, F.; Fu,L.; Zhu, X.-Y.; Xiao, W.-J. Org. Biomol. Chem. 2010, 8, 1275; (e) Betancort, J. M.;Barbas, C. F., III Org. Lett. 2001, 3, 3737; (f) Gotoh, H.; Hayashi, T.; Hayashi, Y.;Shoji, M. Angew. Chem., Int. Ed. 2005, 44, 4212; (g) Barros, M. T.; Phillips, A. M. F.Eur. J. Org. Chem. 2007, 178; (h) Reddy, R. J.; Kuan, H. H.; Chou, T. Y.; Chen, K.Chem. Eur. J. 2009, 15, 9294; (j) Wiesner, M.; Revell, J. D.; Wennemers, H.Angew. Chem., Int. Ed. 1871, 2008, 47; (k) Xiao, J.; Xu, F. X.; Lu, Y. P.; Loh, T. P.Org. Lett. 2010, 12(6), 1220.

14. (a) List, B. J. Am. Chem. Soc. 2000, 122, 9336; (b) List, B.; Pojarliev, P.; Biller, W.T.; Martin, H. J. J. Am. Chem. Soc. 2002, 124, 827; (c) Zheng, X.; Qian, Y.-B.;Wang, Y. Eur. J. Org. Chem. 2010, 515; (d) Sasaoka, A.; Uddin, M. I.; Shimomoto,A.; Ichikawa, Y.; Shiro, M.; Kotsuki, H. Tetrahedron: Asymmetry 2006, 17, 2963.