Enantioselective Michael addition of α,α-disubstituted aldehydes to nitroolefins catalyzed by a pyrrolidine-pyrazole

Article abstract of DOI:10.1016/j.tetasy.2014.06.009

An efficient protocol for the asymmetric catalytic Michael additions of α,α-disubstituted aldehydes to nitroolefins with a pyrrolidine-pyrazole is described. The desired products γ-nitrocarbonyl compounds possessing an all-carbon quaternary center, were o

Full text of DOI:10.1016/j.tetasy.2014.06.009

Tetrahedron: Asymmetry 25 (2014) 1129–1132
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Enantioselective Michael addition of a,a-disubstituted aldehydes
to nitroolefins catalyzed by a pyrrolidine-pyrazole
⇑
Togapur Pavan Kumar , Kothapalli Haribabu
Division of Natural Products Chemistry, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 May 2014
Accepted 16 June 2014
An efficient protocol for the asymmetric catalytic Michael additions of
nitroolefins with a pyrrolidine-pyrazole is described. The desired products
possessing an all-carbon quaternary center, were obtained in good yields and with high levels of enanti-
oselectivities under solvent-free reaction conditions, employing benzoic acid as an additive.
Ó 2014 Elsevier Ltd. All rights reserved.
a
,
a
-disubstituted aldehydes to
c-nitrocarbonyl compounds
1. Introduction
asymmetric Michael addition reaction of ketones to nitroolefins
via an enamine mechanism under solvent-free conditions using
Over the past few years, remarkable advancements have been
made in the development of organocatalytic asymmetric transfor-
mations. As a result, asymmetric organocatalysis1 has gained sig-
nificant attention from the scientific community and been
realized as a powerful alternate and/or complementary protocol
to the enzymatic and organometallic catalysis. In particular, asym-
metric aminocatalysis has emerged as a useful tool for the con-
struction of chiral building blocks by means of an enamine or
iminium mechanism.2 Among the wide range of organic transfor-
mations, the study of asymmetric Michael reactions has been the
focus of many research groups, as it results in the formation of
functionalized products with multiple stereogenic centers in a sin-
gle step. A large number of designed organocatalysts bearing a pyr-
rolidine moiety have been developed and investigated in a wide
variety of donor and acceptor substrates and found to be effi-
TFA as an additive.9e
Having been encouraged by this study and considering the
consistency of catalytic mechanisms, wherein the privileged chiral
pyrrolidine backbone acts as the catalytically active site and the
pyrazole appendage serves as the stereo-control unit,10 we there-
fore started our investigations by testing whether catalyst 1 would
also catalyze asymmetric Michael reactions of
a,a-disubstituted
aldehydes to nitroolefins. Herein we report on the use of pyrroli-
dine-pyrazole 1 as an effective organocatalyst for the asymmetric
Michael reaction of
a,a-disubstituted aldehydes to nitroolefins
providing Michael adducts with an all-carbon quaternary center
under solvent-free conditions.
cient.3–5 Among the Michael products,
c-nitrocarbonyls are more
N
prominent as they serve as versatile templates for the construction
of various bioactive compounds.5,6 The generation of all-carbon
quaternary stereogenic centers is considered to be a challenging
task in asymmetric synthesis.7 Although a large number of organ-
ocatalysts are known to be highly efficient for the Michael addition
of unmodified carbonyls to nitroolefins, only several of them have
been found to be effective for Michael reactions between
N
N
H
1
Figure 1. Pyrrolidine-pyrazole.
a,a
-disubstituted aldehydes and nitroolefins.8 Therefore, an
investigation into new catalytic methods for this relatively
underexplored transformation is highly desirable. In continuation
of our research on organocatalysis,9 we have recently developed
the pyrrolidine-pyrazole 1 (Fig. 1) as an efficient catalyst for
O
O
NO2
1
catalyst
NO2
H
H
solvent (0.5 mL)
rt
3a
2a
4a
⇑
Corresponding author. Tel.: +91 40 27191727; fax: +91 40 27160512.
E-mail address: pavantogapur@gmail.com (T.P. Kumar).
Scheme 1. Michael addition of isobutyraldehyde to nitrostyrene.
http://dx.doi.org/10.1016/j.tetasy.2014.06.009
0957-4166/Ó 2014 Elsevier Ltd. All rights reserved.

1130
T. P. Kumar, K. Haribabu / Tetrahedron: Asymmetry 25 (2014) 1129–1132
Table 1
using isobutyraldehyde 2a as a donor and nitrostyrene 3a as an
Screening of solvents and catalyst loadinga
acceptor (Scheme 1); the reaction optimization involved variation
of the solvent, additive, loading, and template. Initially, the reac-
tions were simultaneously carried out in various solvents using
10–20 mol % of the catalyst at room temperature and the results
are summarized in Table 1. As evident from the survey, the Michael
reaction worked well in all of the solvents irrespective of their
polar/non-polar nature to afford the product
c-nitrocarbonyl
compound 4a in good yield and enantioselectivity (Table 1, entries
1–20). However, the reaction performed under solvent-free
conditions was found to be more effective in terms of yield
(64–78%) and enantioselectivity (67–76% ee) among the conditions
Table 4
Enantioselective Michael addition of isobutyraldehyde to nitroolefinsa
a
Reaction conditions: isobutyaldehyde (4 mmol), nitrostyrene (1 mmol).
Isolated yields.
Determined by chiral HPLC.
b
c
Table 2
Screening of additivesa
a
Reaction conditions: isobutyaldehyde (4 mmol), nitrostyrene (1 mmol) catalyst
1 (20 mol %), neat, rt.
b
Isolated yields.
Determined by chiral HPLC.
c
Table 3
Effect of temperaturea
a
Reaction conditions: isobutyaldehyde (4 mmol), nitrostyrene (1 mmol) catalyst
1 (20 mol %), PhCOOH (10 mol %), neat.
b
Isolated yields.
Determined by chiral HPLC.
c
2. Results and discussion
In order to evaluate the efficacy of pyrrolidine-pyrazole catalyst
a
Reaction conditions: isobutyaldehyde (4 mmol), nitrostyrene (1 mmol).
Isolated yields.
Determined by chiral HPLC.
b
1 for asymmetric Michael additions of
a,a-disubstituted aldehydes
c
to nitroolefins, we started our investigations with a model reaction

T. P. Kumar, K. Haribabu / Tetrahedron: Asymmetry 25 (2014) 1129–1132
1131
tested and laid a platform for further screening studies (Table 1,
entries 7–9).
N
N
N
Encouraged by these results, we next conducted experiments to
test the effect of an acid additive in the above transformation. The
presence of an acid additive could enhance the catalytic efficiency
by accelerating enamine formation, thereby improving the overall
productivity. In anticipation, we examined the effect of various
acid additives under solvent-free conditions using 20 mol % of cat-
alyst at room temperature and the results are summarized in
Table 2. Benzoic acid turned out to be the most efficient additive
in combination with catalyst 1 (Table 2, entries 7–10) and subse-
O
N
O
H
O
O
Ar
Figure 2. Proposed transition state.
quent experiments were carried out by employing a 20:10 mol %
catalyst-additive combination (Table 2, entry 8).
Table 5
Enantioselective Michael addition of cyclopentanecarboxyaldehyde to nitroolefinsa
With the aforementioned parameters, we then conducted
experiments at different temperatures to determine the optimal
temperature conditions required to maximize the catalytic ability
of 1. As shown in Table 3, the highest catalytic performance was
observed at 0 °C (Table 3, entry 2), while the reactions conducted
at other temperatures suffered either with long reaction times or
a loss of yield with no substantial improvement in the enantiose-
lectivity (Table 3, entries 1, 3, and 4).
Having determined the optimal reaction conditions for the
Michael addition of isobutyraldehyde 2a with nitrostyrene 3a,
we next explored the generality of this transformation for other
substrate combinations under solvent-free reaction conditions
using catalyst 1 (20 mol %) in combination with benzoic acid
(10 mol %) at 0 °C. As shown in Tables 4 and 5, all substrate combi-
nations involving variations in nitroolefins 3b–k reacted smoothly
with isobutyraldehyde 2a (Table 4, entries 1–9) and cyclopentane-
carboxaldehyde 2b (Table 5, entries 1–8) under the optimized
reaction conditions and the corresponding Michael products 4b–j
and 5a–h were obtained in good yields and with high enantioselec-
tivities, regardless of the nature of the substitution pattern in the
nitroolefins. However, reactions of nitroolefins with electron
donating substitutions were found to be slightly inferior in overall
productivity compared to that of electron withdrawing substitu-
ents (Tables 4 and 5, entries 4 and 5, respectively). Overall, a good
substrate scope was observed for the conjugate addition of
a,a-disubstituted aldehydes to nitroolefins using a pyrrolidine-
pyrazole catalyst, providing access to a variety of -nitrocarbonyl
c
compounds with an all-carbon quaternary center with high
enantioselectivities.
The absolute stereochemical outcome of this transformation
could be realized by considering the possible transition state10,11
model as shown in Figure 2. The pyrrolidine ring of the catalyst
activates the aldehyde toward enamine formation and the neigh-
boring pyrazole template serves as an efficient stereo-control ele-
ment, which provides steric coverage and also participates in H-
bonding interactions with the nitroolefin by the intermediacy of
benzoic acid. The complete arrangement results in a tighter transi-
tion state, wherein the nucleophilic enamine attacks the nitroolefin
from the Si face and leads to the formation of the desired products
with high enantioselectivities.
3. Conclusion
In conclusion, we have demonstrated the application of pyrrol-
idine-pyrazole 1 as an effective organocatalyst for enantioselective
Michael additions of
a,a-disubstituted aldehydes to nitroolefins.
The catalytic protocol was found to be effective in terms of yield
and enantioselectivities when performed under solvent-free reac-
tion conditions using 20 mol % of catalyst and 10 mol % of benzoic
acid as an additive. Further investigations to extend the scope of
this transformation are currently underway in our laboratory.
a
Reaction conditions: cyclopentanecarboxaldehyde (4 mmol), nitroolefin
(1 mmol).
b
Isolated yields.
Determined by chiral HPLC.
c

1132
T. P. Kumar, K. Haribabu / Tetrahedron: Asymmetry 25 (2014) 1129–1132
2006, 71, 9244–9247; (g) Alza, E.; Cambeiro, X. C.; Jimeno, C.; Pericas, M. A. Org.
4. Experimental
4.1. General
Lett. 2007, 9, 3717–3720; (h) Cobb, A. J. A.; Longbottom, D. A.; Shaw, D. M.; Ley,
S. V. Chem. Commun. 2004, 1808–1809; (i) Cobb, A. J. A.; Longbottom, D. A.;
Shaw, D. M.; Gold, J. B.; Ley, S. V. Org. Biomol. Chem. 2005, 3, 84–96; (j) Cao, C. L.;
Ye, M. C.; Sun, X. L.; Tang, Y. Org. Lett. 2006, 8, 2901–2904; (k) Tsogoeva, S. B.;
Wei, S. Chem. Commun. 2006, 1451–1453; (l) Wei, S.; Yalaov, D. A.; Tsogoeva, S.
B.; Schmatz, S. Catal. Today 2007, 121, 151–157; (m) Wang, J.; Li, H.; Lou, B.; Zu,
L. S.; Guo, H.; Wang, W. Chem. Eur. J. 2006, 12, 4321–4332; (n) Zu, L.; Wang, J.;
Li, H.; Wang, W. Org. Lett. 2006, 8, 3077–3079; (o) Wang, J.; Li, H.; Zu, L. S.;
Wang, W. Adv. Synth. Catal. 2006, 348, 425–428; (p) Wang, W.; Wang, J.; Li, H.
Angew. Chem., Int. Ed. 2005, 44, 1369–1371; (q) Ishii, T.; Fujioka, S.; Sekiguchi,
Y.; Kotsuki, H. J. Am. Chem. Soc. 2004, 126, 9558–9559; (r) Ni, B.; Zhang, Q.;
Headley, A. D. Tetrahedron Lett. 2008, 49, 1249–1252; (s) Xu, D. Z.; Shi, S.;
Wang, Y. Eur. J. Org. Chem. 2009, 4848–4853; (t) Luo, S.; Mi, X.; Zhang, L.; Liu, S.;
Xu, H.; Cheng, J. P. Angew. Chem., Int. Ed. 2006, 45, 3093–3097; (u) Luo, S.; Mi,
X.; Zhang, L.; Liu, S.; Xu, H.; Cheng, J. P. Chem. Commun. 2006, 3687–3689; (v)
Xu, D. Q.; Luo, S. P.; Wang, Y. F.; Xia, A. B.; Yue, H. D.; Wang, L. P.; Xu, Z. Y. Chem.
Commun. 2007, 4393–4395; (w) Li, H. M.; Wang, Y.; Tang, L.; Wu, F. H.; Liu, X.
F.; Guo, C. Y.; Foxman, B. M.; Deng, L. Angew. Chem., Int. Ed. 2005, 44, 105–108;
(x) Okino, T.; Hoashi, Y.; Furukawa, T.; Xu, N. X.; Takemoto, Y. J. Am. Chem. Soc.
2005, 127, 119–125; (y) Tsogoeva, S. B.; Yalalov, D. A.; Hateley, M. J.;
Weckbecker, C.; Huthmacher, K. Eur. J. Org. Chem. 2005, 4995–5000.
All solvents and reagents were purified by standard techniques.
Crude products were purified by column chromatography on silica
gel of 60–120 mesh. IR spectra were recorded on a Perkin–Elmer
683 spectrometer. Optical rotations were obtained on a Jasco Dip
360 digital polarimeter. 1H and 13C NMR spectra were recorded
in CDCl3 solution on a Varian Gemini 200 and Bruker Avance
300. Chemical shifts were reported in parts per million with
respect to internal TMS. Coupling constants (J) are quoted in Hz.
Mass spectra were obtained on an Agilent Technologies LC/MSD
Trap SL. Chiral HPLC analysis was carried out on chiral pak OD-H,
IC, or IA columns using a mixture of isopropanol and hexanes as
the eluent.
5. (a) McCooey, S. H.; Connon, S. J. Angew. Chem., Int. Ed. 2005, 44, 6367–6370; (b)
Vakulya, B.; Varga, S.; Csampai, A.; Soos, T. Org. Lett. 2005, 7, 1967–1969; (c)
Mosse, S.; Alexakis, A. Org. Lett. 2005, 7, 4361–4364; (d) Chi, Y. G.; Gellman, S.
H. Org. Lett. 2005, 7, 4253–4256; (e) Hayashi, Y.; Gotho, H.; Hayashi, T.; Shoji,
M. Angew. Chem., Int. Ed. 2005, 44, 4212–4215; (f) Luo, S. Z.; Mi, L. X.; Zhang, L.;
Liu, S.; Xu, H.; Cheng, J. P. Angew. Chem., Int. Ed. 2006, 45, 3093–3097; (g)
Pansare, S. V.; Pandya, K. J. Am. Chem. Soc. 2006, 128, 9624–9625; (h) Mase, N.;
Watanabe, K.; Yoda, H.; Takabe, K.; Tanaka, F.; Barbas, C. F., III J. Am. Chem. Soc.
2006, 128, 4966–4967; (i) Reyes, E.; Vicario, J. L.; Badia, D.; Carrillo, L. Org. Lett.
2006, 8, 6135–6138; (j) Vishnumaya; Singh, V. K. Org. Lett. 2007, 9, 1117–1119;
(k) Clarke, M. L.; Fuentes, J. A. Angew. Chem., Int. Ed. 2007, 46, 930–933; (l) Chen,
H. B.; Wang, Y.; Wei, S. Y.; Sun, J. Tetrahedron: Asymmetry 2007, 18, 1308–1312;
(m) Zhu, S. L.; Yu, S. Y.; Ma, D. W. Angew. Chem., Int. Ed. 2008, 47, 545–548; (n)
Ban, S. R.; Du, D. M.; Liu, H.; Yang, W. Eur. J. Org. Chem. 2010, 5160–5164; (o) Lu,
D. F.; Gong, Y. F.; Wang, W. Z. Adv. Synth. Catal. 2010, 352, 644–650; (p) Yang,
W.; Du, D. M. Adv. Synth. Catal. 2011, 353, 1241–1246.
4.1.1. General procedure for the Michael addition of
stituted aldehydes to nitroolefins
a
,
a
-disub-
To a mixture of catalyst 1 (20 mol %) and
a,a-disubstituted
aldehyde (4 mmol) was added PhCOOH (10 mol %) and stirred for
20 min at 0 °C. Next, nitroolefin (1 mmol) was added to the result-
ing mixture and stirred for the appropriate time (Tables 4 and 5) at
0 °C. After completion of the reaction (monitored by TLC), the mix-
ture was purified by silica-gel column chromatography to afford
the desired product. Relative and absolute configurations of the
products were determined by comparison of 1H NMR, 13C NMR,
and specific rotation values with those reported in the literature.8
6. (a) Ikeda, S.; Shibuya, M.; Kanoh, N.; Iwabuchi, Y. Org. Lett. 2009, 11, 1833–
1836; (b) Pansare, S. V.; Lingampally, R.; Kirby, R. L. Org. Lett. 2010, 12, 556–
559; (c) Ma, H.; Liu, K.; Zhang, F. G.; Zhu, C. L.; Nie, J.; Ma, J. A. J. Org. Chem. 2010,
75, 1402–1409; (d) Nakamura, A.; Lectard, S.; Hashizume, D.; Hamashima, Y.;
Sodeoka, M. J. Am. Chem. Soc. 2010, 132, 4036–4037; (e) Hong, B. C.; Kotame, P.;
Tsai, C. W.; Liao, J. H. Org. Lett. 2010, 12, 776–779; (f) Krayer, M.; Ptaszek, M.;
Kim, H. J.; Meneely, K. R.; Fan, D.; Secor, K.; Lindsey, J. S. J. Org. Chem. 2010, 75,
1016–1039.
Acknowledgments
T.P.K. thanks DST New Delhi for INSPIRE Faculty Award (IFA12-
CH-30). Authors thank Dr. J. S. Yadav and Dr. S. Chandrasekhar for
valuable discussion and support.
7. (a) Bella, M.; Jorgensen, K. A. J. Am. Chem. Soc. 2004, 126, 5672–5673; (b) Rigby,
C. L.; Dixon, D. J. Chem. Commun. 2008, 3798–3800; (c) Zhang, Z. H.; Dong, X. Q.;
Chen, D.; Wang, C. J. Chem. Eur. J. 2008, 14, 8780–8783; (d) Li, H.; Song, J.; Liu,
X.; Deng, L. J. Am. Chem. Soc. 2005, 127, 8948–8949.
References
1. (a) Berkessel, A.; Groger, H. Asymmetric Organocatalysis; Wiley-VCH:
Weinheim, Germany, 2005; (b) Akiyama, T.; Itoh, J.; Fuchibe, K. Adv. Synth.
Catal. 2006, 348, 999–1010; (c) Perlmutter, P. Conjugate Addition Reactions in
Organic Synthesis; Pergamon: Oxford, 1992; (d) Sakthivel, K.; Notz, W.; Buli, T.;
Barbas, C. F., III J. Am. Chem. Soc. 2001, 123, 5260–5267; (e) List, B.; Pojarliev, P.;
Martin, H. J. Org. Lett. 2001, 3, 2423–2425; (f) Tokoroyama, T. Eur. J. Org. Chem.
2010, 2009–2016; (g) Dondoni, A.; Massi, A. Angew. Chem. 2008, 120, 4716–
4739. Angew. Chem., Int. Ed. 2008, 47, 4638–4660; (h) Trost, B. M.; Brindle, C. S.
Chem. Soc. Rev. 2010, 39, 1600–1632; (i) Zhang, Y.; Wang, W. Catal. Sci. Technol.
2012, 2, 42–53; (j) Aleman, J.; Cabrera, S. Chem. Soc. Rev. 2013, 42, 774–793; (k)
Scheffler, U.; Mahrwald, R. Chem. Eur. J. 2013, 19, 14346–14396.
2. (a) Melchiorre, P.; Marigo, M.; Carlone, A.; Bartoli, G. Angew. Chem., Int. Ed.
2008, 47, 6138–6171; (b) Yu, X.; Wang, W. Org. Biomol. Chem. 2008, 6, 2037–
2046; (c) Bertelsen, S.; Jorgensen, K. A. Chem. Soc. Rev. 2009, 38, 2178–2189; (d)
Seayad, J.; List, B. Org. Biomol. Chem. 2005, 3, 719–724; (e) Vicario, J. L.; Badia,
D.; Carrillo, L. Synthesis 2007, 2065–2092.
3. For recent reviews on asymmetric Michael additions see: (a) Krause, N.;
Hoffmann-Roder, A. Synthesis 2001, 171–196; (b) Berner, O. M.; Tedeschi, L.;
Enders, D. Eur. J. Org. Chem. 2002, 1877–1894; (c) Christoffers, J.; Baro, A.
Angew. Chem., Int. Ed. 2003, 42, 1688–1690; (d) Notz, W.; Tanaka, F.; Barbas, C.
F. III Acc. Chem. Res. 2004, 37, 580–591; (e) Ballini, R.; Bosica, G.; Fiorini, D.;
Palmieri, A.; Petrini, M. Chem. Rev. 2005, 105, 933–972; (f) Tsogoeva, S. B. Eur. J.
Org. Chem. 2007, 1701–1716; (g) Sulzer-Mosse, S.; Alexakis, A. Chem. Commun.
2007, 3123–3135; (h) Krkkila, A.; Majander, I.; Pihko, P. M. Chem. Rev. 2007,
107, 5416–5470; (i) Mukherjee, S.; Yang, J. W.; Hoffmann, S.; List, B. Chem. Rev.
2007, 107, 5471–5569; (j) Davie, E. A. C.; Mennen, S. M.; Xu, Y.; Miller, S. J.
Chem. Rev. 2007, 107, 5759–5812; (k) Almasi, D.; Alonso, D. A.; Gomez-Bengoa,
E.; Negel, Y.; Najera, C. Eur. J. Org. Chem. 2007, 2328–2343; (l) Enders, D.; Wang,
C.; Liebich, J. X. Chem. Eur. J. 2009, 15, 11058–11076.
8. (a) Zhang, X. J.; Liu, S. P.; Li, X. M.; Yan, M.; Chan, A. S. C. Chem. Commun. 2009,
833–835; (b) Lalonde, M. P.; Chen, Y.; Jacobsen, E. N. Angew. Chem. 2006, 118,
6514–6518. Angew. Chem., Int. Ed. 2006, 45, 6366–6370; (c) Wang, J.; Li, H.; Lou,
B.; Zu, L.; Guo, H.; Wang, W. Chem. Eur. J. 2006, 12, 4321–4332; (d) Wang, W.;
Wang, J.; Li, H. Angew. Chem. 2005, 117, 1393–1395. Angew. Chem., Int. Ed. 2005,
44, 1369–1371; (e) Mase, N.; Thayumanavan, R.; Tanaka, F.; Barbas, C. F., III Org.
Lett. 2004, 6, 2527–2530; (f) Sato, A.; Yoshida, M.; Hara, S. Chem. Commun.
2008, 6242–6244; (g) McCooey, S. H.; Connon, S. J. Org. Lett. 2007, 9, 599–602;
(h) Chang, C.; Li, S. H.; Reddy, R. J.; Chen, K. Adv. Synth. Catal. 2009, 351, 1273–
1278; (i) Chen, J. R.; Cao, Y. J.; Zou, Y. Q.; Tan, F.; Fu, L.; Zhu, X. Y.; Xiao, W. J. Org.
Biomol. Chem. 2010, 8, 1275–1279; (j) He, T.; Gu, Q.; Wu, X. Y. Tetrahedron 2010,
66, 3195–3198; (k) Chen, J. R.; Zou, Y. Q.; Fu, L.; Ren, F.; Tan, F.; Xiao, W. J.
Tetrahedron 2010, 66, 5367–5372; (l) Nugent, T. C.; Bibi, A.; Sadiq, A.; Shoaib,
M.; Umar, M. N.; Tehrani, F. N. Org. Biomol. Chem. 2012, 10, 9287–9294; (m)
Avila, A.; Chinchilla, R.; Fiser, B.; Gomez-Bengoa, E.; Najera, C. Tetrahedron:
Asymmetry 2014, 25, 462–467; (n) Junpeng, H.; Qiankun, C.; Bukuo, N.
Tetrahedron Lett. 2014, 55, 3030–3032.
9. (a) Chandrasekhar, S.; Mallikarjun, K.; Reddy, G. P. K.; Rao, K. V.; Jagdeesh, B.
Chem. Commun. 2009, 4985–4987; (b) Chandrasekhar, S.; Tiwari, B.; Parida, B.
B.; Rajireddy, C. Tetrahedron: Asymmetry 2008, 19, 495–499; (c) Chandrasekhar,
S.; Johny, K.; Rajireddy, Ch. Tetrahedron: Asymmetry 2009, 20, 1742–1745; (d)
Chandrasekhar, S.; Kumar, T. P.; Haribabu, K.; Rajireddy, Ch. Tetrahedron:
Asymmetry 2010, 21, 2372–2375; (e) Chandrasekhar, S.; Kumar, T. P.; Haribabu,
K.; Rajireddy, Ch.; Kumar, C. R. Tetrahedron: Asymmetry 2011, 22, 697–702; (f)
Kumar, T. P.; Vavle, N. C.; Patro, V.; Haribabu, K. Tetrahedron: Asymmetry 2014,
25, 457–461; (g) Kumar, T. P.; Balaji, S. V. Tetrahedron: Asymmetry 2014, 25,
473–477.
10. (a) Yoon, T. P.; Jacobsen, E. N. Science 2003, 299, 1691–1693; (b) Hayashi, Y.;
Gotoh, H.; Hayashi, T.; Shoji, M. Angew. Chem., Int. Ed. 2005, 44, 4212–4215.
11. (a) Seebach, D.; Golinski, J. Helv. Chem. Acta 1981, 64, 1413–1423; (b)
Betancort, J. M.; Sakthivel, K.; Thayumanavan, R.; Tanaka, F.; Barbas, C. F., III
Synthesis 2004, 1509–1521; (c) Ting, Y. F.; Chang, C.; Reddy, R. J.; Magar, D. R.;
Chen, K. Chem. Eur. J. 2010, 16, 7030–7038.
4. (a) Betancort, J. M.; Barbas, C. F., III Org. Lett. 2001, 3, 3737–3740; (b) Alexakis,
A.; Bernardinelli, G. Org. Lett. 2002, 4, 3611–3614; (c) Enders, D.; Seki, A. Synlett
2002, 26–28; (d) Andrey, O.; Alexakis, A.; Bernardinelli, G. Org. Lett. 2003, 5,
2559–2561; (e) Li, H. M.; Wang, Y.; Tang, L.; Deng, L. J. Am. Chem. Soc. 2004, 126,
9906–9907; (f) Luo, S.; Xu, H.; Mi, X.; Li, J.; Zheng, X.; Cheng, J. P. J. Org. Chem.