Water-assisted organocatalysis: An enantioselective green protocol for the henry reaction

Article abstract of DOI:10.1071/CH12428

We report an enantioselective Henry (nitroaldol) reaction catalysed by an organocatalyst using water as solvent. The enantioselective synthesis of β-nitroalcohols was achieved by using a neutral chiral organocatalyst, through strong hydrogen bonding, whic

Full text of DOI:10.1071/CH12428

CSIRO PUBLISHING
Aust. J. Chem. 2013, 66, 661–666
Full Paper
http://dx.doi.org/10.1071/CH12428
Water-Assisted Organocatalysis: An Enantioselective
Green Protocol for the Henry Reaction
A
A
A
Prashant B. Thorat, Santosh V. Goswami, Wamanrao N. Jadhav,
A B
,
and Sudhakar R. Bhusare
A
Department of Chemistry, Dnyanopasak College, Parbhani-431 401,
Maharashtra State, India.
B
Corresponding author. Email: bhusare71@yahoo.com
We report an enantioselective Henry (nitroaldol) reaction catalysed by an organocatalyst using water as solvent. The
enantioselective synthesis of b-nitroalcohols was achieved by using a neutral chiral organocatalyst, through strong
hydrogen bonding, which results in the formation of corresponding products in excellent yield and enantioselectivity at
room temperature. Other attractive features of the method are the eco-friendly, non-hazardous, and mild reaction
conditions, inexpensive catalyst, and simple work up conditions.
Manuscript received: 19 September 2012.
Manuscript accepted: 14 March 2013.
Published online: 5 April 2013.
polymer-supported or dendritic systems.[23] Hence the develop-
ment of chiral organic molecules that are able to catalyse
aqueous stereoselective reactions is an important goal.
Over the past few years, chiral organocatalysts have experi-
enced a renaissance in asymmetric organic transformations.
Chiral secondary amines and amides are favoured catalysts for
the chemist so far. However most of these reactions are a based
on a nucleophilic enamine/imine and/or hydrogen bonding
catalysis to form a transition state intermediate which leads to
chiral product.[24]
Hydrogen bonding is a ubiquitous force which is utilised to
catalyse many organic transformations.[24] In most of these
reactions, an acidic N-H proton activates the substrate through
strong hydrogen bonding interactions, which increases the
electrophilicity of the carbon atom and facilitates formation
of a well organised transition state, leading to a chiral
product.[25–27] By using this concept, the asymmetric Strecker
reaction was shown to be catalysed via hydrogen bonding
interactions without formation of an enamine intermediate.[28,29]
On the basis of the success of our previous results[30] on
hydrogen bonding organocatalysis by neutral chiral pyrrolidine
compounds (Fig. 1) and escalating our research in organocata-
lysis, we sought to study the asymmetric Henry reaction using
these pyrrolidine based organocatalysts (1–7), exclusively via
hydrogen-bonding interaction.
Introduction
The Henry reaction is a fundamental reaction in organic
chemistry.[1] The products obtained in the Henry reaction are
useful in the synthesis of several products. This reaction forms a
new carbon–carbon bond, and provides a valuable synthetic
building block for further transformations.[2] Due to the
electron-withdrawing properties of the nitro group of the
nitroalkane, the a proton can be removed easily to generate a
nucleophile in a variety of transformations.
The Henry reaction is mainly carried out in the presence of a
base to give b-nitroalcohols.[1] Although plenty of new methods
have been documented, the asymmetric Henry reaction has been
studied very rarely until the last decade. The first asymmetric
Henry reaction was reported in 1997 by Shibasaki et al.[3] Since
then various metal-based catalysts, using chiral organic mole-
cules as ligands, involving Zn,[4] Cu,[5,6] Co,[7] Mg,[8] and Cr,[9]
zeolites,[10] and organo-catalysts[11] have been reported. The
reported catalytic enantioselective Henry reactions have certain
associated drawbacks, such as high catalyst loading,[4,12,13] need
for low temperatures,[14,15] use of inflammable, carcinogenic,
and toxic solvents,[16] and the requirement for strong base for
activation of the nitroalkane.[17]
Asymmetric syntheses using water as solvent has recently
been the subject of interest in recent years.[18] Water has many
advantages, such as low cost; it can replace many hazardous
organic solvents, it is safe to use, and is environmentally
benign.[19,20] However the use of water has not been explored
widely for organic transformations, especially for chiral synthe-
sis, as it leads to racemic products.[21] Water has a high dielectric
constant and strong protic interactions, which may disrupt polar
interactions and hydrogen bonding, to affect catalytic activity
and stereocontrol of the reaction.[22] To overcome these
difficulties reactions are performed in mixed aqueous-organic
solvents or in the presence of surfactants, and some are
Result and Discussion
Initially, the catalytic activities of chiral organocatalysts 1–7
for the asymmetric Henry reaction were investigated by per-
forming a model reaction using stoichiometric amounts of
p-nitrobenzaldehyde 8a with nitromethane 9 in the presence
of 10 mol-% organocatalyst and 5 mol-% triethylamine at room
temperature in tetrahydrofuran (Scheme 1). As the data
Journal compilation Ó CSIRO 2013
www.publish.csiro.au/journals/ajc

662
P. B. Thorat et al.
O
HN
1
O
HN
2
O
HN
3
O
N
N
N
N
HN
Cl
F
Ts
Ts
Ts
Ts
4
O
O
O
O
O
S
O
S
N
N
N
HN
Me
NO2
HN
S
Me
HN
O
O
O
O
O
O
5
6
7
Fig. 1. L-Proline based organocatalysts.
OH
OH
CHO
O
CHO
O
Organocatalyst
Solvent, rt
Organocatalyst 4
Water, base
NO2
NO2
Nϩ
Nϩ OϪ
OϪ
ϩ
ϩ
O2N
O2N
O2N
O2N
8a
9
10a
8a
9
10a
Scheme 2. The effect of base and its concentration on the reaction between
nitrobenzaldehyde 8a and nitromethane 9.
Scheme 1. The effect of catalysts, solvent, and catalytic loading on the
reaction between nitrobenzaldehyde 8a and nitromethane 9.
chloroform, the reaction yield and ee was decreased. In particu-
lar, the ee was reduced to 24 % when THF was replaced by
dichloromethane (Table 1, entries 5 and 6). When polar protic
solvents ethanol or methanol were used as solvent, the corre-
sponding reaction rates and yields were enhanced markedly to
69 % and 71 % respectively (Table 1, entries 7 and 8). As for ee,
ethanol showed more favourable enantioselectivity (71 % ee)
than methanol (66 % ee). Water was found to be superior to all
solvents in terms of yields and enantioselectivities of 10a. The
reaction was completed in 17 h at room temperature in water,
achieving a good yield of 78 % and high ee of 80 % (Table 1,
entry 9). Using dioxane as solvent, ee decreased to 61 % and
yield was only a moderate 57 % after 24 h at room temperature
(Table 1, entry 10). When DMF or dimethyl sulfoxide were used
as solvents, moderate yields (66 and 60 %) and enantioselec-
tivity 68 and 63 % respectively for product 10a was observed
(Table 1, entries 11 and 12).
Next we opted to determine optimum catalyst loading for the
Henry reaction. The performance of the reaction declined
slightly with an increased quantity of organocatalyst 4 (catalyst
loading of 12 mol-%); after 15 h product 10a was obtained in
75 % yield and with 79 % ee. Subsequently, when catalyst
loading was decreased to 7.5 mol-% the reaction was complete
in 24 h and the yield obtained for 10a was very poor (61 %) with
a significant decrease in enantioselectivity (49 %). All the
experimental data demonstrated water to be a superior solvent
for the reaction in terms of yield and ee for 10a, in presence of
organocatalyst 4 and triethylamine base.
Moreover, the organic base has an important influence on
this reaction. On surveying the literature we found several
reports describing base as an additive used to enhance perfor-
mance of the Henry reaction.[3,31] Delighted by the above
success, the reaction was further examined in the presence of
stoichiometric quantities of various commercially available
organic bases along with organocatalyst 4 in water to obtain
10a (Scheme 2). The results are summarised in Table 2.
The strong base piperidine was able to reduce the reaction
time at 5 mol-% concentration, but enantiomeric excess
obtained was an unacceptable 38 % (Table 2, entry 1). In the
case of pyridine at 5 mol-% the yield of 10a obtained was 68 %,
also with low enantioselectivity of 43 % in water (Table 2,
Table 1. Effect of catalyst quantity on reaction conditionA
Entry Catalyst Solvent
Mol-% Time [h] YieldB [%] eeC
1
1
2
3
4
5
6
7
4
4
4
4
4
4
4
4
4
4
THF
10
10
10
10
10
10
10
10
10
10
10
10
10
10
48
36
12
18
27
24
22
26
28
18
19
17
24
24
28
15
24
56
45
49
69
55
61
57
41
44
68
71
78
61
66
60
75
61
28
47
49
70
41
50
44
24
31
71
66
80
57
68
63
79
49
2
THF
3
THF
4
THF
5
THF
6
THF
7
THF
8
Dichloromethane
Chloroform
Ethanol
Methanol
Water
Dioxane
DMF
9
10
11
12
13
14
15
16
17
Dimethyl sulfoxide 10
Water
Water
12
7.5
AProgress of reaction was determined by thin layer chromatography.
BIsolated yield.
CDetermined by chiral HPLC.
collected in Table 1 show, when compounds 1, 2, or 3 were
employed as the organocatalyst, a moderate yield for product
10a was obtained with low enantioselectivity, with the reaction
proceeding slowly (Table 1, entries 1–3). However, organo-
catalyst 4 was prominent in providing nitroaldol products 10a
in good yield (69 %) and most importantly with 70 % ee
(Table 1, entry 4). The reaction with (S)-1-acetylpyrrolidine-2-
carboxamide based catalysts 5, 6, or 7 bearing different sulfonyl
groups afforded moderate yields for 10a, along with low enan-
tiomeric excesses.
The reaction was further explored in order to search for the
optimal solvent conditions. A series of reaction solvents, such as
dichloromethane, ethanol, methanol, water, DMF, dimethyl
sulfoxide and dioxane were tested in the catalytic enantioselec-
tive Henry reaction. When the reaction solvent was changed
from THF to aprotic solvents such as dichloromethane or

Organocatalysis: An Enantioselective Henry Reaction
663
Table 2. Effect of base on the reactionA
and also increase the reaction rate.[11b] The aldehyde is activated
through double hydrogen-bonding and the nitromethane is
activated by the triethylamine (Fig. 2). Further, for electron-
rich aromatic aldehydes, non-covalent p–p interactions
between the aromatic rings of the catalyst and the aldehydes
are established which provide additional strength to the transi-
tion state to achieve higher stereoselectivity (Fig. 2b).[34] The
results obtained from non-aromatic and aliphatic aldehydes also
supports the latter transition state, where the p–p bonding
interactions do not operate and hence reduced yield and enan-
tioselectivites for products were observed. Water brings the
hydrophobic catalyst and reactants closer together; by vigorous
stirring, an emulsion-like medium forms and facilitates the
reaction.[19a,20a,20b]
Entry
Base
Mol-%
Time [h]
YieldB [%]
eeC
1
Piperidine
Pyridine
5
5
15
22
20
26
14
12
30
28
30
24
75 (S)
68 (S)
70 (S)
51 (S)
78 (S)
88 (S)
64 (S)
47 (S)
53 (S)
79 (S)
38
43
73
49
35
92
72
66
32
83
2
3
DMAP
5
4
Morpholine
Piperidine
Triethylamine
DMAP
5
5
10
10
10
10
10
10
6
7
8
Pyridine
9
10D
Morpholine
Triethylamine
AProgress of reaction was determined by thin-layer chromatography.
BIsolated yield. Configuration of major enantiomer shown in parentheses.
CDetermined by chiral HPLC.
DReaction was performed at 08C temperature.
Recyclability of the Catalyst
The recyclability of the organocatalyst 4 was checked in the
asymmetric Henry reaction to obtain 10a. The catalyst was
isolated by simple filtration at completion of the reaction. The
catalyst was washed well with diethyl ether, then dissolved in
dichloromethane and dried over anhydrous Na2SO4, and then
concentrated. The solid catalyst obtained after solvent evapo-
ration was further dried under vacuum at 808C and reused for the
next cycle of reaction.
entry 2). When 4-dimethylaminopyridine (DMAP) was used in
5 mol-% quite a good yield (70 %) and enantioselectivity (73 %)
was obtained, with a drawback of extended reaction time
(Table 2, entry 3). Morpholin as a base increased the reaction
time to 26 h and decreased yield of 10a to 51 % with 49 % ee
(Table 2, entry 4).
As evident from Table 4, in the next cycles of reactions the
yield and enantioselectivities were drastically reduced. On
reusing the catalyst 4 in the next cycle, the yield and ee obtained
for 10a was 67 % and 74 % respectively. We assumed that the
decrease in yield was caused by the residual CH3NO2, which
remained with the catalyst after the diethyl ether wash. In this
experiment, the catalyst was only able to be reused four times,
providing a lower yield of 28 % and a poor ee value of 34 % in
the fourth cycle.
Increase in the base concentration showed some enhanced
effects on the reaction. At 10 mol-% of piperidine, although
reaction time was drastically reduced, no significant change was
observed in yield and enantioselectivity (Table 2, entry 5). In the
case of triethylamine or DMAP at 10 mol-%, the enantioselec-
tivity of 10a could reach to 92 or 72 % respectively in water
(Table 2, entries 6 and 7). Other bases pyridine and morpholine
were less effective than triethylamine or DMAP at 10 mol-%
under identical conditions (Table 2, entries 8 and 9). Neverthe-
less, the best conditions for the reaction in water at room
temperature were obtained with organocatalyst 4 in the presence
of 10 mol-% triethylamine (Table 2, entry 6). This may be due to
more favourable basicity and less steric hindrance compared
with other bases. In order to see the effect of temperature the
reaction was carried out at 08C, resulting in a comparable yield
of 79 % and enantioselectivity of 83 % for 10a; the reaction was
also completed in 24 h (Table 2, entry 10). Thus, a decrease in
temperature did not show a substantial effect on the reaction.
With these optimised conditions in hand, we next examined
the asymmetric Henry reaction using a variety of arylaldehydes
8a–j with nitromethane 9 and the results are summarised in
Table 3. The results show that yields of the product are
uniformly high, irrespective of the substituent on the aromatic
ring. In the case of ortho substituted chloro and bromo com-
pounds, yields and enantioselectivities were less than para
substituted compounds, due to steric hindrance. However, ortho
substituted hydroxy and methoxy groups showed greater enan-
tioselectivity, which is due to hydrogen bonding interactions
between the nitronate oxygen and hydrogen. In the cases of non-
aromatic and aliphatic aldehydes, lower enantioselectivities and
yields relative to aromatic aldehydes were obtained with extend-
ed reaction time.
Conclusion
Wehavedevelopedanewfacilemethodforasymmetricsynthesis
via the Henry reaction (nitroaldol). The reaction was studied
using various organocatalysts 1–7. The organocatalyst (S)-N-(4-
fluorophenyl)-1-tosylpyrrolidine-2-carboxamide proved to be
the best organocatalyst in water to yield product 10a in up to
88 % yield and with excellent ee (92 %). Water plays a crucial
role in determination of stereoselectivity, as it assists the orga-
nocatalyst in providing strong hydrogen bonding. The method
was studied for a broad range of aromatic, non-aromatic, and
aliphatic aldehydes. The yield of products for aromatic alde-
hydes were consistently high regardless of the type of substit-
uent on the aromatic ring; however, for non-aromatic and
aliphatic aldehydes yields obtained were only good to moderate.
Mild reaction conditions, an eco-friendly solvent, high yields
with excellent stereoselectivity with a wide range of substrates,
and recyclability are some striking features of the reaction. The
synthesis and applications of the organocatalyst in other asym-
metric reactions are currently being carried out in our laboratory
and will be reported in due course.
Experimental
Based on our previous[30] work and literature docu-
ments[32,33] the mechanism of reaction was deduced. The
proposed transition states for the Henry reaction is given in
Fig. 2. The positioning of the reactants seems the most favour-
able in a staggered orientation taking into account both steric
and electronic interactions which control the outcome of product
General
All solvents were commercial anhydrous grade, and used
without further purification. Column chromatography was car-
ried out over silica gel (100–120 mesh). Optical rotations were
measured on a Polax-2L digital polarimeter. Melting points

664
P. B. Thorat et al.
Table 3. Nitroaldol reaction with aldehydesA
O
OH
O
Organocatalyst 4
Nϩ OϪ
ϩ
NO2
R
H
Water,
10 mol-% Et3N
R
8(a–r)
9
10(a–r)
Entry
R
Time [h]
Product 10
YieldB [%]
eeC [%]
[a]DD
1
4-NO2-C6H4
4-F-C6H4
12
a
b
c
d
e
f
88
89
87
86
83
81
89
87
90
81
85
86
92
94
91
89
87
87
89
94
88
90
89
92
þ26.0 (S)[36a]
þ28.6 (S)[14]
þ28.0 (S)[36a]
þ42.0 (S)[36a]
þ23.8 (S)[36a]
ꢀ27.3 (S)[36b]
þ22.4 (S)[35a]
þ36.6 (S)[14]
þ19.0 (S)[14]
þ23.7 (S)[36a]
þ68.4 (S)[4c]
þ25.7 (S)
2
11.30
11
3
3-NO2-C6H4
4-ClꢀC6H4
2-ClꢀC6H4
2-Br-C6H4
4-OH-C6H4
2-OCH3-C6H4
4-OCH3-C6H4
C6H5
4
12.30
13
5
6
13.30
12
7
g
h
i
8
13
9
12
10
11
12
13
j
4-Br-C6H4
2-OH-C6H4
13
k
l
13
13
14
14
m
n
72
78
76
81
ꢀ21.9 (S)[5c]
CHO
CHO
13.30
þ22.4 (S)[35c]
O
þ16.5 (S)[35b]
H
15
18.30
o
67
70
þ19.6 (S)[35b]
ꢀ2.2 (S)[13]
þ6.2 (S)[14]
O
16
17
18
18.30
19
p
q
r
70
64
59
65
62
64
O
O
20
AThe progress of reaction was determined by thin layer chromatography.
BIsolated yield.
CDetermined by chiral HPLC.
DThe absolute configuration was assigned by comparing specific rotation with literature data (refs. [14] and [36]) or analogy.
(a)
(b)
O
O
N
O
N
O
O
O
S
S
N
N
H
O
H
O
H
O
H
O
H
H
F
F
ϩ
ϩ
N
N
Ϫ
O
Ϫ
Ϫ
O
EDG
EWG
O
O
Electron withdrawing group
Electron donating group
Fig. 2. Proposed transition state model for the Henry reaction.

Organocatalysis: An Enantioselective Henry Reaction
665
Table 4. Reusability of organocatalyst 4 for the synthesis of 10a
(c) S. Liu, C. Wolf, Org. Lett. 2008, 10, 1831. doi:10.1021/OL800442S
(d) A. Bulut, A. Aslan, O. Dogan, J. Org. Chem. 2008, 73, 7373.
doi:10.1021/JO8010073
Run no.
Reaction time
YieldA [%]
eeB
[5] (a) T. Arai, N. Yokoyama, A. Yanagisawa, Chem. – Eur. J. 2008, 14,
1
2
3
4
12
88
67
46
28
92
74
52
34
2052. doi:10.1002/CHEM.200701439
(b) G. Q. Zhang, E. Yashima, W. D. Woggon, Adv. Synth. Catal. 2009,
351, 1255. doi:10.1002/ADSC.200900069
14.30
15
15
(c) C. Gan, G. Lai, Z. Zhang, Z. Wang, M. M. Zhou, Tetrahedron:
Asymmetry 2006, 17, 725. doi:10.1016/J.TETASY.2006.01.032
[6] (a) S. Selvakumar, D. Sivasankaran, V. K. Singh, Org. Biomol. Chem.
2009, 7, 3156. doi:10.1039/B904254G
AIsolated yield.
BDetermined by chiral HPLC.
(b) M. Breuning, D. Hein, M. Steiner, V. H. Gessner, C. Strohmann,
Chem. – Eur. J. 2009, 15, 12764. doi:10.1002/CHEM.200901789
(c) A. Noole, K. Lippur, A. Metsala, M. Lopp, T. Kanger, J. Org.
Chem. 2010, 75, 1313. doi:10.1021/JO902664V
were determined in open capillary tubes and are uncorrected.
1H and 13C NMR spectra were recorded on an Avance 300 MHz
spectrometer in CDCl3. Mass spectra were recorded on a
Polaris-Q Thermoscientific gas chromatography–mass spec-
trometer (GC-MS). Enantiomeric purity was determined on a
Perkin Elmer Series 200 HPLC system.
[7] Y. Kogami, T. Nakajima, T. Ashizawa, S. Kezuka, T. Ikeno,
T. Yamada, Chem. Lett. 2004, 33, 614. doi:10.1246/CL.2004.614
[8] B. M. Choudary, K. V. S. Ranganath, U. Pal, M. L. Kantam,
B. Sreedhar, J. Am. Chem. Soc. 2005, 127, 13167. doi:10.1021/
JA0440248
[9] (a) R. Kowalczyk, P. Kwiatkowski, J. Skarzewski, J. Jurczak, J. Org.
General Procedure for the Synthesis of Organocatalysts 1–7
Chem. 2009, 74, 753. doi:10.1021/JO802107B
(b) A. Zulauf, M. Mellah, E. Schulz, J. Org. Chem. 2009, 74, 2242.
doi:10.1021/JO802769Y
The synthesis and characterisation of the organocatalysts 1–7 is
reported in our earlier communications.[30]
[10] N. H. Khan, M. B. Ansari, E. A. Prasetyanto, H. Jin, S. E. Park,
Tetrahedron: Asymmetry 2011, 22, 117. doi:10.1016/J.TETASY.
2010.12.016
[11] (a) H. M. Li, B. M. Wang, L. Deng, J. Am. Chem. Soc. 2006, 128, 732.
doi:10.1021/JA057237L
General Procedure for the Henry Reaction
The aromatic aldehyde (2 mmol) was added in a solution
of nitromethane (4 mmol) in distilled water (20 mL) and
organocatalyst
(S)-N-(4-fluorophenyl)-1-tosylpyrrolidine-2-
(b) T. Marcelli, R. N. S. van der Haas, J. H. van Maarseveen,
H. Hiemstra, Angew. Chem. 2006, 118, 943. doi:10.1002/ANGE.
200503724
(c) T. Marcelli, R. N. S. van der Haas, J. H. van Maarseveen,
H. Hiemstra, Angew. Chem. Int. Ed. 2006, 45, 929. doi:10.1002/
ANIE.200503724
carboxamide (0.1 mmol). The reaction mixture was stirred for
the appropriate time at room temperature (see Table 3). The
progress of reaction was monitored with thin layer chromato-
graphy. After completion of reaction as indicated by TLC,
25 mL cool distilled water was added. This reaction mixture was
then extracted with dichloromethane (3 ꢁ 15 mL). Solvent was
removed under vacuum to obtain crude product. The crude
mixture was purified by column chromatography. Spectral data
of all compounds is compared with those reported in the liter-
ature[34,35] and it is well in agreement with the structure.
[12] (a) Y. Sohtome, Y. Hashimoto, K. Nagasawa, Eur. J. Org. Chem. 2006,
2894. doi:10.1002/EJOC.200600307
(b) T. Mandal, S. Samanta, C. G. Zhao, Org. Lett. 2007, 9, 943.
doi:10.1021/OL070209I
(c) D. Uraguchi, S. Sakaki, T. Ooi, J. Am. Chem. Soc. 2007, 129,
12392. doi:10.1021/JA075152þ
[13] B. M. Trost, V. S. C. Yeh, Angew. Chem. Int. Ed. 2002, 41, 861.
doi:10.1002/1521-3773(20020301)41:5,861::AID-ANIE861.3.0.
CO;2-V
Supplementary Material
Full experimental procedures for synthesis of catalysts and
characterisation of compounds are available on the Journal’s
website.
[14] B. Zheng, M. Wang, Z. Li, Q. Bian, J. Mao, S. Li, S. Liu, M. Wang,
J. Zhong, H. Guo, Tetrahedron: Asymmetry 2011, 22, 1156.
doi:10.1016/J.TETASY.2011.06.032
[15] T. Nitabaru, N. Kumagai, M. Shibasaki, Tetrahedron Lett. 2008, 49,
Acknowledgement
272. doi:10.1016/J.TETLET.2007.11.055
[16] (a) P. A. Grieco, Organic Synthesis in Water 1998 (Blackie Academic
and Professional: London).
We acknowledge Dr P. L. More, Dnyanopasak College, Parbhani for pro-
viding necessary facilities, IICT Hyderabad for providing spectral data is
highly appreciated.
(b) C. J. Li, T. H. Chan, Organic Reactions in Aqueous Media 1997
(John Wiley & Sons: New York, NY).
(c) U. M. Lindstrom, Chem. Rev. 2002, 102, 2751. doi:10.1021/
CR010122P
[17] T. Risgaard, K. V. Gothelf, K. A. Jorgensen, Org. Biomol. Chem. 2003,
References
[1] (a) L. Henry, Compt. Rend. Hebd. Seances Acad. Sci. 1895, 120, 1265.
(b) G. Rosini, in Comprehensive Organic Synthesis (Eds B. M. Trost,
C. H. Heathcock) 1991, Vol. 2, 321–340 (Pergamon: Oxford).
(c) N. Ono, The Nitro Group in Organic Synthesis 2001 Ch. 3, p. 30
(Wiley VCH: New York, NY).
1, 153. doi:10.1039/B208859M
[18] Y. J. An, Y. X. Zhang, Y. Wu, Z. A. Liu, C. Pi, J. C. Tao, Tetrahedron:
Asymmetry 2010, 21, 688. doi:10.1016/J.TETASY.2010.04.019
[19] (a) Y. Wu, Y. Zhang, M. Yu, G. Zhao, S. Wang, Org. Lett. 2006, 8,
4417. doi:10.1021/OL061418Q
(b) V. Maya, M. Raj, V. K. Singh, Org. Lett. 2007, 9, 2593.
doi:10.1021/OL071013L
[20] (a) S. Guizzetti, M. Benaglia, L. Raimondi, G. Celentono, Org. Lett.
[2] B. R. Linton, M. H. Reutershan, M. Aderman, E. A. Richardson, K. R.
Brownell, C. W. Ashley, C. A. Evans, S. J. Miller, Tetrahedron Lett.
2007, 48, 1993. doi:10.1016/J.TETLET.2007.01.073
[3] M. Shibasaki, H. Sasai, T. Arai, Angew. Chem. Int. Ed. 1997, 36, 1236.
doi:10.1002/ANIE.199712361
2007, 9, 1247. doi:10.1021/OL070002P
[4] (a) C. Palomo, M. Oiarbide, A. Laso, Angew. Chem. 2005, 117, 3949.
doi:10.1002/ANGE.200463075
(b) D. E. Siyutkin, A. S. Kucherenko, M. I. Struchkova, S. G. Zlotin,
Tetrahedron Lett. 2008, 49, 1212. doi:10.1016/J.TETLET.2007.12.044
[21] (a) A. Cordova, W. Notz, C. F. Barbas III, Chem. Commun. 2002,
3024.
(b) C. Palomo, M. Oiarbide, A. Laso, Angew. Chem. Int. Ed. 2005, 44,
3881. doi:10.1002/ANIE.200463075

666
P. B. Thorat et al.
(b) K. A. Sakthivel, W. Notz, T. Buiand, C. F. Barbas III, J. Am. Chem.
Soc. 2001, 123, 5260. doi:10.1021/JA010037Z
[22] L. S. Zu, H. X. Xie, H. X. Li, J. Wang, W. Wang, Org. Lett. 2008, 10,
[32] (a) H. Sasai, T. Suzuki, S. Arai, T. Arai, M. Shibasaki, J. Am. Chem.
Soc. 1992, 114, 4418. doi:10.1021/JA00037A068
(b) R. Chinchilla, C. Najera, P. S. Agullo, Tetrahedron: Asymmetry
1994, 5, 1393. doi:10.1016/0957-4166(94)80183-5
1211. doi:10.1021/OL800074Z
[23] (a) S. Luo, X. Mi, S. Liu, H. Xu, J. P. Cheng, Chem. Commun. 2006,
3687. doi:10.1039/B607846J
(c) R. Ballini, G. Bosica, P. Forconi, Tetrahedron 1996, 52, 1677.
doi:10.1016/0040-4020(95)00996-5
(b) D. Font, C. Jimeno, M. A. Pericas, Org. Lett. 2006, 8, 4653.
doi:10.1021/OL061964J
[24] M. T. Robak, M. A. Herbage, J. A. Ellman, Chem. Rev. 2010, 110,
(d) D. Simoni, F. P. Invidiata, S. Manfredini, R. Ferroni, I. Lampronti,
M. Roberti, G. P. Pollini, Tetrahedron Lett. 1997, 38, 2749.
doi:10.1016/S0040-4039(97)00461-9
3600. doi:10.1021/CR900382T
[25] F. Ferreira, C. Botuha, F. Chemla, A. Perez-Luna, Chem. Soc. Rev.
2009, 38, 1162. doi:10.1039/B809772K
[26] D. Morton, R. A. Stockman, Tetrahedron 2006, 62, 8869. doi:10.1016/
J.TET.2006.06.107
[27] P. Zhou, B. C. Chen, F. A. Davis, Tetrahedron 2004, 60, 8003.
doi:10.1016/J.TET.2004.06.071
[28] M. Sigman, E. N. Jacobsen, J. Am. Chem. Soc. 1998, 120, 4901.
doi:10.1021/JA980139Y
[29] E. J. Corey, M. J. Grogan, Org. Lett. 1999, 1, 157. doi:10.1021/
OL990623L
[30] (a) P. B. Thorat, S. V. Goswami, B. C. Khade, S. R. Bhusare,
Tetrahedron Lett. 2012, 53, 6083. doi:10.1016/J.TETLET.2012.08.123
(b) P. B. Thorat, S. V. Goswami, B. C. Khade, S. R. Bhusare,
Tetrahedron: Asymmetry 2012, 23, 1320. doi:10.1016/J.TETASY.
2012.08.013
[33] (a) J. G. Hernandez, E. Juaristi, J. Org. Chem. 2011, 76, 1464.
doi:10.1021/JO1022469
(b) J. G. Hernandez, E. Juaristi, Tetrahedron 2011, 67, 6953.
doi:10.1016/J.TET.2011.06.042
(c) Z. Jiang, Z. Liang, X. Wu, Y. Lu, Chem. Commun. 2006, 2801.
doi:10.1039/B606154K
(d) Z. Jiang, H. Yang, X. Han, J. Luo, M. W. Wong, Y. Lu, Org.
Biomol. Chem. 2010, 8, 1368. doi:10.1039/B921460G
[34] R. Kowalczyk, L. Sidorowicz, J. Skarzewski, Tetrahedron: Asymme-
try 2007, 18, 2581. doi:10.1016/J.TETASY.2007.10.023
[35] (a) K. Dhahagani, J. Rajesh, R. Kannan, G. Rajagopal, Tetrahedron:
Asymmetry 2011, 22, 857. doi:10.1016/J.TETASY.2011.04.020
(b) N. Sanjeevakumar, M. Periasamy, Tetrahedron: Asymmetry 2009,
20, 1842. doi:10.1016/J.TETASY.2009.07.040
(c) G. Lai, S. Wang, Z. Wang, Tetrahedron: Asymmetry 2008, 19,
1813. doi:10.1016/J.TETASY.2008.06.036
[36] (a) X. G. Liu, J. J. Jianga, M. Shi, Tetrahedron: Asymmetry 2007, 18,
[31] (a) S. Syu, T. Kao, W. Lin, Tetrahedron 2010, 66, 891. doi:10.1016/
J.TET.2009.11.093
2773. doi:10.1016/J.TETASY.2007.11.008
(b) M. F. A. Adamo, S. Suresh, Tetrahedron 2009, 65, 990.
doi:10.1016/J.TET.2008.11.089
(b) B. V. S. Reddy, S. M. Reddy, M. Swain, M. Chinnala, Tetrahedron:
Asymmetry 2011, 22, 530. doi:10.1016/J.TETASY.2011.02.019
(c) D. Xin, Y. Ma, F. He, Tetrahedron: Asymmetry 2010, 21, 333.
doi:10.1016/J.TETASY.2010.02.007