Sugar amide-pyrrolidine catalyst for the asymmetric Michael addition of ketones to nitroolefins

Article abstract of DOI:10.1016/j.tetasy.2014.02.003

New sugar amide-pyrrolidine derivatives possessing the furano form of the carbohydrate template were designed and developed as efficient and stereoselective organocatalysts for asymmetric Michael additions of ketones to nitroolefins at room temperature. G

Full text of DOI:10.1016/j.tetasy.2014.02.003

Tetrahedron: Asymmetry 25 (2014) 473–477
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Sugar amide-pyrrolidine catalyst for the asymmetric Michael
addition of ketones to nitroolefins
⇑
Togapur Pavan Kumar , Sirinyam Venugopal Balaji
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 21 January 2014
Accepted 5 February 2014
New sugar amide-pyrrolidine derivatives possessing the furano form of the carbohydrate template were
designed and developed as efficient and stereoselective organocatalysts for asymmetric Michael addi-
tions of ketones to nitroolefins at room temperature. Good yields and high selectivities were achieved
with catalyst 2 under solvent-free and additive-free reaction conditions.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
‘privileged’ chiral pyrrolidine backbone (derived from
which acts as the catalytically active site and the carbohydrate
template (derived from furano -glucose), which provides a bulky
environment and has additional hydrogen bonding sites for the
activation of nitroolefins to furnish the Michael products with high
stereoselectivity. Herein we report the synthesis and development
of carbohydrate–pyrrolidine based amide catalysts for asymmetric
Michael additions of ketones with various nitroolefins.
L-proline),
Over the past few years, organocatalysis has witnessed remark-
able advances and great attention has been given to the design and
application of small privileged organic molecules to construct
asymmetric carbonAcarbon and carbonAhetero atom bonds for
the preparation of enantiomerically pure compounds.1 The Michael
addition is widely recognized as one of the most efficient and pow-
erful synthetic tools for the stereoselective construction of car-
bonAcarbon bonds for the formation of stereoenriched adducts
with multiple stereogenic centers in a single step.2 In particular,
the use of nitroolefins as Michael acceptors has received attention
D
O
O
O
O
O
O
N
H
N
H
for the efficient formation of chiral c-nitro carbonyl compounds,
NH
O
NH
which serve as versatile building blocks for the synthesis of com-
O
H2N
HO
plex organic molecules.3 A wide variety of proline based organo-
1
2
catalysts have been developed with
a distinct range of
selectivities. Among these pyrrolidine–triazoles,4 pyrrolidine–
tetrazoles,5 pyrrolidine–thioureas,6 pyrrolidine–sulfonamides,7
pyrrolidine–pyridines,8 pyrrolodine–pyrazoles,9 pyrrolidine–imi-
dazoliums,10 2,2-bipyrrolidines11, and phosphoprolines12 represent
the major organocatalyst categories for asymmetric Michael addi-
tions. However, the use of carbohydrates as chiral templates with a
pyrrolidine ring is very limited. To the best of our knowledge there
have been only a few sugar-based pyrrolidine organocatalysts with
the pyranose form of the carbohydrate that have been used
successfully, while the furanose form of the carbohydrate in
organocatalysis is unexplored.13 In a continuation of our research
interests,4h,9,14 we have developed new sugar based pyrrolidine–
Figure 1. Structure of new sugar based organocatalysts.
2. Results and discussion
The sugar amide-pyrrolidine catalysts 1 and 2 were synthesized
from the known pyrrolidine amine 3 (readily obtained from -pro-
line) and sugar acids 4 and 4a, respectively (derived from furano
-glucose),15 as illustrated in Scheme 1. Accordingly, the Cbz-pro-
tected proline amine 3 was subjected to peptide coupling with
-glucose derived acids 4a and 4b followed by hydrogenation using
L
D
D
Pd/C in methanol to give the desired catalysts 1 and 2 in 84% and
82% yield, respectively (Scheme 1).
amide catalysts (Fig. 1) derived from
L-proline and the furanose
With both catalysts in hand, we tested their efficiency in a
model reaction of cyclohexanone 6a with b-nitrostyrene 7a
(Scheme 2). At first, the reaction was performed with 10 mol % of
the catalyst under solvent-free conditions at room temperature.
Both catalysts 1 and 2 promoted the addition with good yield
form of
D-glucose. Structurally, the designed catalysts possess a
⇑
Corresponding author. Tel.: +91 40 27191727; fax: +91 40 27160512.
E-mail address: pavantogapur@gmail.com (T.P. Kumar).
http://dx.doi.org/10.1016/j.tetasy.2014.02.003
0957-4166/Ó 2014 Elsevier Ltd. All rights reserved.

474
T. P. Kumar, S. V. Balaji / Tetrahedron: Asymmetry 25 (2014) 473–477
O
O
O
O
NH2
EDCI, HOBt
O
O
N
H
HO
+
N
CH2Cl2, rt,
8 h, 81%
N
O
O
R
R
Cbz
Cbz
R=N3
R=OBn
4a
4b
5a R=N3
3
5b
R=OBn
O
O
O
O
O
O
O
Pd/C, H2, CH3OH
rt, 12 h
N
H
N
H
NH
NH
O
HO
H2N
2
1
(82%)
(84%)
Scheme 1. Synthesis of sugar amide-pyrrolidine catalysts.
Table 2
Screening of solvents using 2a
O
O
NO2
Entry
Solvent
Time (h)
Yieldb (%)
syn/antic
eed (%)
catalyst
NO2
1
2
3
4
5
6
7
8
CHCl3
Toluene
Hexane
CH3CN
THF
Dioxan
MeOH
H2O
60
72
72
55
60
52
56
50
72
69
73
76
85
82
78
64
95:5
8:2
8:2
93:7
92:8
85:15
93:7
92:8
92
63
68
79
86
90
88
60
solvent free
rt
6a
7a
8a
Scheme 2. Michael addition of cyclohexanone to nitrostyrene.
Table 1
Screening of catalystsa
a
Reaction conditions: cyclohexanone (5 mmol), nitrostyrene (1 mmol), solvent
Yieldb (%)
syn/antic
eed (%)
(0.5 mL), catalyst 2 (20 mol %).
Entry
Catalyst
mol %
Time (h)
b
Isolated yields.
c
Determined by the 1H NMR of the crude product.
1
2
3
4
5
6
1
2
1
2
2
2
10
10
15
15
20
30
36
36
36
36
24
24
85
88
90
91
94
95
9:1
92:8
9:1
91:9
98:2
98:2
18
66
21
70
91
91
d
Determined by chiral HPLC.
as TFA, HCOOH, PhCOOH, CSA, CH3COOH, and pTSA under
solvent-free reaction conditions. As shown in Table 3, these exper-
iments resulted in the formation of the desired Michael product in
moderate to good yields and diastereoselectivities, whereas the
enantioselectivities obtained in all respects were very low. This
may be due to the fact that catalysts bearing intramolecular hydro-
gen bonding sites do not require any additives for high reactivity
and stereoselectivity, while an acidic co-catalyst is critical for
catalysts lacking an intramolecular proton donor.
In order to explore the scope and the limitations of the Michael
reaction, various nitroolefins and ketones were studied using cata-
lyst 2 with the optimized reaction conditions and the results are
summarized in Table 4. All of the b-nitrostyrenes, irrespective of
the nature of the substituents on the aryl group, reacted efficiently
with cyclohexanone (Table 4, entries 1–8) to give the correspond-
ing Michael adducts in good yields and with high diastereoselectiv-
a
b
c
Reaction conditions: cyclohexanone (5 mmol), nitrostyrene (1 mmol).
Isolated yields.
Determined by the 1H NMR of the crude product.
Determined by chiral HPLC.
d
and diastereoselectivity, while the enantioselectivity obtained was
moderate for catalyst 2 (Table 1, entry 2) and very low for catalyst
1 (Table 1, entry 2). In order to improve the efficacy, screening
experiments were conducted by varying the catalyst loading and
the results are summarized in Table 1. The best result was
observed with 20 mol % of catalyst 2 (Table 1, entry 5); catalyst 1
gave good yields and diastereoselectivities but poor enantioselec-
tivities (Table 1, entry 3). The observed low efficacy of catalyst 1
may be due to the formation of enamines in the presence of a
primary amine group in the carbohydrate template, which also
activates the ketone along with the secondary amine of the pyrrol-
idine ring.
With these observations, we then conducted solvent screening
experiments using catalyst 2 (20 mol %) and investigated the scope
of different solvents such as CH2Cl2, toluene, hexane, CH3CN, THF,
dioxane, and H2O. The reaction times, yields, and selectivities of
2 differed significantly and the results are summarized in Table 2.
The reaction proceeded well in solvents such as CHCl3, THF, diox-
ane, and MeOH resulting in the Michael adduct in good yield,
diastereoselectivity, and enantioselectivity (Table 2, entries 1, 5,
6, and 7). However in other solvents, the reaction was found to
be less productive (Table 2, entries 2–4 and 8). The results of the
solvent screening experiments were found to be inferior in all
respects when compared to the solvent free conditions.
Table 3
Screening of additivesa
Entry
Additive
Time (h)
Yieldb (%)
syn/antic
eed (%)
1
2
3
4
5
6
TFA
24
24
30
24
30
30
81
78
84
70
75
72
82:18
75:25
85:15
8:2
7:3
85:15
42
39
61
53
57
48
HCOOH
PhCOOH
CSA
CH3COOH
pTSA
a
Reaction conditions: cyclohexanone (5 mmol), nitrostyrene (1 mmol), additive
(5 mol %), solvent-free.
b
Isolated yields.
c
Determined by the 1H NMR of the crude product.
In order to evaluate the effect of additives, we next conducted
additive screening experiments using various acid additives such
d
Determined by chiral HPLC.

T. P. Kumar, S. V. Balaji / Tetrahedron: Asymmetry 25 (2014) 473–477
475
Table 4
Asymmetric Michaels addition of ketones to nitroolefin using organocatalyst 2a
Entry
Nitroolefin
Time (h)
Product
Yieldb (%)
(syn/anti)c
eed
NO2
7a
O
1
24
94
98:2
91
89
87
85
NO2
NO2
8a
O2N
NO2
O
O
2
3
4
24
24
26
95
92
88
96:4
99:1
93:7
7b
NO2
8b
8c
NO2
Cl
Cl
NO2
7c
O2N
NO2
O2N
O
Cl
Cl
NO2
8d
7d
7e
NO2
OCH3
H3CO
5
28
90
93:7
90
O
NO2
8e
8f
NO2
OCH3
H3CO
OCH3
6
26
96
94:6
88
O
O
O
OCH3
NO2
7f
NO2
7g
O
S
O
S
7
8
28
24
85
92
97:3
97:3
93
85
NO2
NO2
8g
8h
NO2
7h
NO2
O
O
7a
9
48
56
86
62
92:8
—
82
57
NO2
8i
8j
NO2
10
7a
NO2
a
b
c
Reaction conditions: cyclohexanone (5 mmol), nitrostyrene (1 mmol), solvent-free.
Isolated yields.
Determined by the 1H NMR of the crude product.
Determined by chiral HPLC.
d

476
T. P. Kumar, S. V. Balaji / Tetrahedron: Asymmetry 25 (2014) 473–477
O
was added and stirred at room temperature for 8 h. The reaction
was diluted with water and the aqueous layer was separated.
The organic layer was washed with water (2 ꢁ 25 mL), dried over
Na2SO4, and concentrated under vacuum. The crude product was
purified by silica gel column chromatography (hexane/ethyl ace-
tate—1:1) to give amide 5a (0.77 g, 81% yield) as a thick liquid;
O
N
H
N
O
O
H
O
O
N
O
½
a 2D5
ꢂ
¼ ꢃ42:8 (c 0.25, CHCl3); 1H NMR (CDCl3, 300 MHz): d 7.51
Ar
Figure 2. Proposed transition state.
(br s, 1H), 7.41–7.28 (m, 5H), 6.05–5.92 (m, 1H), 5.28–5.08 (m,
2H), 4.79–4.68 (m, 1H), 4.65–4.53 (m, 1H), 4.38 (d, J = 3.2 Hz,
1H), 4.07–3.93 (m, 1H), 3.70–3.25 (m, 4H), 2.08–1.60 (m, 4H),
1.49 (s, 3H), 1.33 (s, 3H); 13C NMR (CDCl3, 75 MHz): d 167.4,
144.1, 136.6, 129.6, 128.4, 127.9, 115.8, 114.2, 105.2, 83.1, 79.6,
66.4, 67.0, 66.8, 57.4, 55.3, 46.8, 43.0, 29.1, 26.7, 26.3, 23.8; ESIMS:
m/z 446 [M+H]+; HRMS Calcd for C21H28N5O6: 446.2034, found:
446.2030.
ity and enantioselectivity. The reaction of b-nitrostyrene with
cyclopentanone (Table 4, entry 9) was comparatively less produc-
tive, while the reaction with acetone was very slow and afforded
the desired product in low yield and with low selectivity even after
a prolonged reaction time (Table 4, entry 10).
We propose the following transition state16 (Fig. 2) to account
for the stereochemical outcome of the Michael reaction performed
by catalyst 2. The secondary amine of the pyrrolidine ring activates
the ketone via the formation of an enamine intermediate, whereas
the carbohydrate template acts as the steric controller, contribut-
ing towards the facial selectivity. Furthermore it also provides an
additional hydrogen bonding site (free hydroxyl group) along with
the amide group, thereby stabilizing the nitroolefin through hydro-
gen bonding interactions leading to the Michael products with
high selectivities. The observed low efficacy of catalyst 1 may be
due to problems in the formation of an enamine due to the pres-
ence of a primary amine group in the carbohydrate template,
which also activates the ketone along with the secondary amine
of the pyrrolidine ring.
4.1.2. (S)-Benzyl 2-(((3aR,5S,6R,6aR)-6-(benzyloxy)-2,2-dimeth-
yltetrahydrofuro[2,3-d][1,3]dioxole-5-carbox amido)methyl)-
pyrrolidine-1-carboxylate 5b
To a stirred solution of acid 4b (0.49 g, 2.13 mmol) in CH2Cl2
(10 mL) were added EDCꢀHCl (0.5 g, 2.6 mmol) and HOBt (0.35 g,
2.6 mmol) at 0 °C and stirred for 15 min. A solution of amine 3
(0.63 g, 2.13 mmol) in CH2Cl2 was added and stirred at room tem-
perature for 10 h. Water (25 mL) was then added to the reaction
mixture and stirred. The layers were separated and the organic
layer was washed with water (2 ꢁ 25 mL), dried over Na2SO4,
and concentrated under vacuum. The crude was purified by silica
gel column chromatography (hexane/ethyl acetate—1:1) to give
amide 5b (0.84 g, 77% yield) as a thick liquid; ½a D25
¼ ꢃ31:6 (c
ꢂ
0.2, CHCl3); 1H NMR (CDCl3, 300 MHz): d 7.38–7.18 (m, 10H),
6.01 (d, J = 2.6 Hz, 1H), 5.23–5.0 (m, 2H), 4.76–4.67 (m, 1H),
4.61–4.51 (m, 3H), 4.35–4.28 (m, 1H), 3.93–3.81 (m, 1H), 3.55–
3.24 (m, 4H), 1.93–1.61 (m, 4H), 1.44 (s, 3H), 1.28 (s, 3H); 13C
NMR (CDCl3, 75 MHz): d 167.4, 154.9, 136.7, 136.1, 127.9, 127.7,
127.3, 127.2, 127.0, 118.8, 104.9, 81.6, 80.5, 72.2, 66.2, 56.9, 46.2,
41.5, 28.1, 26.4, 25.7, 23.1; ESIMS: m/z 511 [M+H]+; HRMS Calcd
for C28H35N2O7: 511.2439, found: 511.2441.
3. Conclusion
In conclusion, we have developed new sugar amide-pyrrolidine
organocatalysts using
a simple protocol for the asymmetric
Michael addition of ketones to nitroolefins. The reactions were per-
formed under solvent-free and additive-free conditions leading to
the corresponding Michael adducts in good yield and with high
selectivity. Further investigations to extend the scope of these
sugar derived catalysts are currently underway in our laboratory.
4.1.3. (3aR,5S,6R,6aR)-6-Amino-2,2-dimethyl-N-((S)-pyrrolidin-
2-lmethyl)tetrahydrofuro[2,3-d][1,3]dioxole-5-carboxamide 1
At first, Pd/C (0.20 g) was added to a solution of amide 5a
(0.70 g, 1.5 mmol) in methanol and the resulting suspension was
stirred under H2 atmosphere for 12 h. The reaction mass was fil-
tered through a pad of Celite and washed with ethyl acetate. The
filtrate was concentrated under reduced pressure to afford 1
4. Experimental
4.1. General
(0.54 g, 84% yield) as a white solid; ½a D25
¼ ꢃ39:6 (c 1.4, CHCl3);
ꢂ
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 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 Brucker 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.
1H NMR (CDCl3, 300 MHz): d 7.54 (s, 1H) 5.96 (d, J = 3.4 Hz, 1H),
4.64 (d, J = 3.4 Hz, 1H), 4.36 (d, J = 3.4 Hz, 1H), 3.74 (d, J = 3.4 Hz,
1H), 3.60–3.35 (m, 5H), 3.23–2.91 (m, 3H), 1.98–1.66 (m, 3H),
1.51–1.38 (m, 4H), 1.30 (s, 3H); 13C NMR (CDCl3, 75 MHz): d
169.9, 112.0, 105.0, 85.6, 81.2, 58.8, 51.2, 44.8, 40.5, 27.9, 26.7,
26.1, 23.8; ESIMS: m/z 286 [M+H]+; HRMS Calcd for C13H24N3O4:
286.1761, found: 286.1758.
4.1.4. (3aR,5S,6R,6aR)-6-Hydroxy-2,2-dimethyl-N-((S)-pyrroli-
din-2-ylmethyl)tetrahydrofuro[2,3-d][1,3]dioxole-5-carbox-
amide 2
To a stirred solution of amide 5b (0.80 g, 1.57 mmol) in metha-
nol, Pd/C (0.20 g) was added and the resulting suspension was stir-
red under H2 atmosphere for 15 h. The reaction mass was filtered
through a pad of Celite and washed with ethyl acetate. The filtrate
was concentrated under reduced pressure to afford 2 (0.37 g, 82%
4.1.1. (S)-Benzyl 2-(((3aR,5S,6R,6aR)-6-azido-2,2-dimethyltetra-
hydrofuro[2,3-d][1,3]dioxole-5-carbox amido) methyl) pyrroli-
dine-1-carboxylate 5a
To a stirred solution of acid 4a (0.49 g, 2.13 mmol) in CH2Cl2
(10 mL) was were added EDCꢀHCl (0.5 g, 2.6 mmol) and HOBt
(0.35 g, 2.6 mmol) at 0 °C and the resultant mixture was stirred
for 15 min. A solution of amine 3 (0.5 g, 2.13 mmol) in CH2Cl2
yield) as a white solid; ½a D25
ꢂ
¼ ꢃ33:2 (c 1.1, CHCl3); 1H NMR (CDCl3,
300 MHz): d 6.89 (br s, 1H), 6.03 (d, J = 3.2 Hz, 1H), 4.68 (d,
J = 2.6 Hz, 1H), 4.53 (d, J = 3.2 Hz, 1H), 4.48 (d, J = 2.4 Hz, 1H),
3.90–3.78 (m, 1H), 3.31–3.20 (m, 1H), 3.05–2.79 (m, 3H), 2.60 (br

T. P. Kumar, S. V. Balaji / Tetrahedron: Asymmetry 25 (2014) 473–477
477
s, 1H), 1.85–1.65 (m, 4H), 1.50 (s, 3H), 1.33 (s, 3H); 13C NMR (CDCl3,
75 MHz): d 169.5, 112.4, 105.9, 84.6, 82.0, 75.4, 57.6, 45.5, 41.6,
28.2, 26.9, 26.3, 24.5; ESIMS: m/z 287 [M+H]+; HRMS Calcd for
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C13H23N2O5: 287.1601 found: 287.1607.
4.1.5. General procedure for the Michael addition of cyclohexa-
none to b-nitrostyrene
To a mixture of catalyst 2 (20 mol %) and cyclohexanone
(5 mmol) was added the corresponding nitroolefin (1 mmol) and
stirred for an appropriate time (Table 3) at room temperature.
After completion of the reaction (monitored by TLC), the reaction
mixture was purified by silica-gel column chromatography to
afford the desired product. The relative and absolute configuration
of the products were was determined by comparison of 1H NMR,
13C NMR, and specific rotation values with those reported in the lit-
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Acknowledgments
T.P.K. thanks DST, New Delhi for the INSPIRE Faculty Award
(IFA12-CH-30). S.V.B. thanks CSIR, New Delhi for the award of a
research fellowship. Authors are thankful to Dr. J. S. Yadav and
Dr. S. Chandrasekhar for their valuable suggestions and support.
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