Thiourea-catalyzed asymmetric formal [3+2] cycloaddition of azomethine ylides with nitroolefins

Article abstract of DOI:10.1016/j.tetlet.2008.09.113

A chiral thiourea catalyst possessing an amine function catalyzes an asymmetric [3+2] cycloaddition of azomethine ylides to nitroolefins to provide highly functionalized pyrrolidines with high diastereo- and enantioselectivities (up to 98:1:1 dr, 92% ee). The reaction proceeds in a stepwise manner consisting of Michael addition and subsequent intramolecular aza-Henry reaction. Both reactions are promoted by the thiourea catalyst, and the reaction rate of the latter step is efficiently enhanced by the addition of 2,2,2-trifluoroethanol.

Full text of DOI:10.1016/j.tetlet.2008.09.113

Tetrahedron Letters 49 (2008) 6910–6913
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Tetrahedron Letters
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Thiourea-catalyzed asymmetric formal [3+2] cycloaddition
of azomethine ylides with nitroolefins
*
Jianwu Xie, Kohzo Yoshida, Kiyosei Takasu, Yoshiji Takemoto
Graduate School of Pharmaceutical Sciences, Kyoto University, Shimoadachi-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A chiral thiourea catalyst possessing an amine function catalyzes an asymmetric [3+2] cycloaddition of
azomethine ylides to nitroolefins to provide highly functionalized pyrrolidines with high diastereo-
and enantioselectivities (up to 98:1:1 dr, 92% ee). The reaction proceeds in a stepwise manner consisting
of Michael addition and subsequent intramolecular aza-Henry reaction. Both reactions are promoted by
the thiourea catalyst, and the reaction rate of the latter step is efficiently enhanced by the addition of
2,2,2-trifluoroethanol.
Received 27 August 2008
Revised 11 September 2008
Accepted 18 September 2008
Available online 23 September 2008
Keywords:
Ó 2008 Elsevier Ltd. All rights reserved.
Organocatalyst
[3+2] Cycloaddition
Asymmetric catalyst
Pyrrolidines
The [3+2] cycloaddition reactions have been employed as one of
the most powerful synthetic tools to provide a variety of five-
membered carbocycles as well as heterocycles.1 For example, the
[3+2] cycloaddition of azomethine ylides with olefins affords
highly functionalized pyrrolidines, which are structural motifs of
biologically active naturally occurring substances as well as phar-
maceuticals.2 Development of the catalytic enantioselective vari-
ants with high stereoselectivity is currently of interest.3 There
are several studies on the asymmetric [3+2] cycloaddition of azo-
methine ylides using metal salts, such as silver,4 copper,5 zinc,6
and nickel,7 with a variety of chiral ligands. Quite recently, the
highly enantioselective catalytic [3+2] cycloaddition of azomethine
azomethine ylides with nitroolefins could be achieved through a
stepwise mechanism. Namely, we have recently developed chiral
thiourea catalyst 1,11 which demonstrated highly asymmetric
induction in a variety of reactions, such as Michael addition of mal-
onates to nitroolefins11d,e and aza-Henry reaction of imines with
nitroalkanes.11f,g Taking into account our recent results, we envis-
aged that the thiourea 1 can catalyze a formal [3+2] cycloaddition
of azomethine ylides with nitroolefins (Scheme 1). Thus, both the
azomethine ylide, which is generated from
a-amino malonate
imine 2 and nitroolefin 3 would be anchored and activated through
hydrogen-bonding system with the amine and thiourea moieties of
the catalyst 1, respectively. As a result, enantioselective Michael
addition of 2–3 would proceed to furnish a zwitterionic intermedi-
ate 4, and then a subsequent intramolecular aza-Henry reaction of
4 affords the desired pyrrolidine 5.
ylides with a,b-unsaturated aldehydes and maleates by chiral pro-
line derivatives8 and a chiral phosphonic acid,9 respectively, were
reported. To the best of our knowledge, there is only one report
on the enantioselective organocatalytic [3+2] cycloaddition with
nitroolefins.10 However, in this case, the level of asymmetric induc-
tion is still moderate (up to 63% ee). Herein, we describe a thio-
urea-catalyzed asymmetric [3+2] cycloaddition of azomethine
ylides with nitroolefins, reaching ee values up to 92% with high
diastereoselectivity.
In an initial study, we investigated the thiourea-catalyzed reac-
tion between 2a and 3a in a series of organic solvents (Table 1).
When the reaction was carried out in the presence of
1
(10 mol %) in CH2Cl2 at ambient temperature, no [3+2] cycloadduct
was observed but only Michael adduct 4aa was obtained in 55%
yield with good enantioselectivity (86% ee, entry 1). The reaction
in a polar aprotic solvent such as THF and CH3CN afforded only
4aa with somewhat lower enantioselectivities (entries 2 and 3).
On the contrary, the desired pyrrolidine 5aa was formed in 59%
yield along with 4aa (19% yield), when the reaction was carried
out in a protic solvent, EtOH. However, both the enantiomeric ex-
cesses of the products 4aa and 5aa were quite low (21% ee, 11% ee;
entry 4). Among the solvents examined, use of toluene gave the
best asymmetric induction, affording the Michael adduct 4aa
(81%, 86% ee; entry 5). Furthermore, both the chemical yield and
It is widely accepted that [3+2] cycloaddition proceeds through
either a concerted [ 2s+ 4s] 1,3-dipolar cycloaddition pathway or a
p
p
non-concerted stepwise addition pathway. Almost all of the enan-
tioselective catalytic [3+2] cycloaddition of azomethine ylides
were designed on the basis of the concerted mechanism.10 We
considered that good stereoselectivity in [3+2] cycloaddition of
* Corresponding author. Tel.: +81 75 753 4528; fax: +81 75 753 4569.
E-mail address: takemoto@pharm.kyoto-u.ac.jp (Y. Takemoto).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.113

J. Xie et al. / Tetrahedron Letters 49 (2008) 6910–6913
6911
R1
O2N
R2
R1
N
CO2Et
CO2Et
thiourea 1 (cat.)
* * CO2Et
NO2
+
R2
*
H
N
CO2Et
5
2
3
CF3
aza-Henry
reaction
H+
S
F3C
N
H
N
H
S
N
S
Ar
Ar
N
H
N
H
1
N
H
N
H
Michael
addition
+NHMe2
R1
NMe2
H+
-
-
O
O
-
N+
O
O
R1
O
N+
N
N
H
*
R2
OEt
CO2Et
CO2Et
CO2Et
H
R2
4
Scheme 1. A proposed mechanism for the formal [3+2] cycloaddition catalyzed by 1.
Table 1
Table 2
Enantioselective Michael addition of 2a and 2b with 3a catalyzed by 1a
Thiourea-catalyzed intramolecular aza-Henry reactiona
CO2Et
O2N
R1
O2N
R1
Ph
CO2Et
CO2Et
Ph
1
(10 mol%)
9 h
1 (10 mol%)
NO2
+
R1
N
CO2Et
Ph
CO2Et
CO2Et
additive (30 equiv.)
rt
(R1 = 4-CF3-C6H4)
3a
Ph
N
H
N
2a
2b
H
1
4aa (R1 = 4-CF3-C6H4)
(R = 4-Cl-C6H4)
5aa
O2N
Ph
O2N
R1
Entry
Solvent
Additive
Time (h)
5aa
3
CO2Et
CO2Et
CO2Et
CO2Et
+
5N
R1
% conv.b
drc,d
N
H
1
2
3
4
Toluene
CH2Cl2
Dioxane
EtOH
Toluene
Toluene
Toluene
Toluene
—
—
—
—
EtOH
TFE
TFE
TFE
48
48
48
48
24
24
24
24
0
0
0
95
6
99
85
0
—
—
—
4aa
4ba
5aa
92:7: Trace
99: Trace
89:10:1
97:2: Trace
—
Entry
Solvent
Temp.
4aa (5aa)
5
% Yieldb
% eec
6
7e
8f
1
2
CH2Cl2
THF
rt
rt
55
51
86
74
a
3
4
5
CH3CN
EtOH
Toluene
Toluene
Toluene
Toluene
rt
rt
rt
0 °C
0 °C
0 °C
77
33
The reactions were carried out with 4aa (0.10 mmol) in the absence or in the
19 (59)d
21 (11)d
86
presence of an additive (30 equiv) at ambient temperature.
b
The conversion yields were determined by 1H NMR.
81
90
0
c
6
90
—
90
The stereochemistry of the minor diastereomers was not determined.
7e
8f
d
The diastereomeric ratios were determined by 1H NMR.
e
82
The reaction was carried out at 0 °C.
The reaction was carried out in the absence of 1.
f
a
The reactions were carried out with 2a (0.30 mmol), 3a (0.30 mmol), and 1
(0.030 mmol).
b
c
Isolated yield.
were tested, no reaction occurred at all at ambient temperature
(entries 1ꢀ3). On the contrary, pyrrolidine 5aa was formed in an
excellent yield with high diastereoselectivity without loss of the
enantiomeric excess in EtOH as a protic solvent (entry 4). As tolu-
ene is the optimum solvent in the above asymmetric Michael addi-
tion, reactions in toluene in the presence of protic compounds as
an additive were investigated. Although EtOH (30 equiv) was
added to the reaction, the cyclization slightly proceeded to give
5aa in 6% yield (entry 5). On the other hand, use of 2,2,2-trifluoro-
ethanol (TFE) having a more acidic proton than EtOH resulted in
the better results, affording 5aa in a diastereomeric ratio of
89:10:1 (entry 6). Lowering the temperature to 0 °C further en-
hanced the diastereoselectivity of 5aa without any loss of its enan-
tiomeric excess (entry 7). It was clear that no reaction occurred by
the addition of TFE without thiourea catalyst 1 (entry 8). Thus, it is
noteworthy that the cooperation of both thiourea and TFE is crucial
to accelerate the intramolecular aza-Henry reaction. The stereo-
chemistry of the major diastereomer of 5aa was fully assigned by
an X-ray crystallographic analysis to be (3R,4R,5R) configuration.13
Enantiomeric excess was determined by HPLC analysis using a chiral column.
The value in parentheses means the chemical yield of 5aa and its enantiomeric
d
excess.
e
The reaction was carried out in the absence of 1 for 12 h.
The reaction was carried out with 2b and 3a to give 4ba.
f
enantioselectivity of 4aa were improved to 90% yield and 90% ee by
conducting the reaction at 0 °C (entry 6). The catalyst is essential
for the Michael addition; no formation of 4aa was observed in
the absence of thiourea 1 (entry 7). Under the optimum conditions
in hand, Michael addition of 2b with 3a afforded 4ba in 82% yield
with 90% ee (entry 8). The absolute configuration of the Michael
adduct was unambiguously determined to be R by an X-ray crys-
tallography of 4ba.12 The enantioselection is consistent with the
previous asymmetric reactions reported by us.11d
We next investigated the catalytic intramolecular aza-Henry
reaction of 4aa into 5aa in the presence of 1 (Table 2). Although
several aprotic solvents, such as toluene, CH2Cl2, and 1,4-dioxane,

6912
J. Xie et al. / Tetrahedron Letters 49 (2008) 6910–6913
Table 3
Reaction of various
a
-amino malonate imines and nitroolefinsa
1 (10 mol%)
O2N
R1
R2
CO2Et
CO2Et
R1
N
CO2Et
CO2Et
toluene, 0 ºC, 9 h;
NO2
+
R2
TFE (30 equiv.), 0 ºC, 36 h
N
H
5
2
3
(1.0 equiv.)
(1.0 equiv.)
Entry
2 (R1)
3 (R2)
Product
% Yieldb
drc,d
% eee
1
2
3
4
5
6
7
8
9
2a (4-CF3–C6H4)
2a (4-CF3–C6H4)
2a (4-CF3–C6H4)
2a (4-CF3–C6H4)
2a (4-CF3–C6H4)
2a (4-CF3–C6H4)
2a (4-CF3–C6H4)
2b (4-Cl–C6H4)
2c (C6H5)
3a (C6H5)
5aa
5ab
5ac
5ad
5ae
5af
5ag
5ba
5ca
5da
84
81
72
75
52
86
75
80
78
83
98:1:1
95:4:1
98:1:1
98:1:1
94:4:2
91:5:4
96:3:1
98:1:1
96:3:1
96:2:2
92
91
90
90
92
84
88
91
80
54
3b (4-MeO–C6H4)
3c (4-Cl–C6H4)
3d (3-Cl–C6H4)
3e (2-Cl–C6H4)
3f (2-thienyl)
3g (1-naphthyl)
3a (C6H5)
3a (C6H5)
3a (C6H5)
10
2d (4-CH3–C6H4)
a
The reactions were carried out with 2 (0.30 mmol), 3 (0.30 mmol), and 1 (0.030 mmol).
Isolated yield.
b
c
The stereochemistry of the minor diastereomers was not determined.
d
e
Diastereomeric ratios were determined by 1H NMR.
Enantiomeric excess was determined by HPLC analysis using a chiral column.
With the optimum conditions for both enantioselective Michael
quently, a good diastereoselectivity is observed in the cyclization
step.
addition and stereoselective intramolecular aza-Henry reaction in
hand, we next examined the formal [3+2] cycloaddition in one-
pot sequence to give pyrrolidine 5 from imines 2 and nitroolefins
3. The results are summarized in Table 3. After the reaction of 2a
(1.0 equiv) with 3a (1.0 equiv) in the presence of thiourea 1
(10 mol %) was carried out at 0 °C for 9 h, TFE (30 equiv) was added
to the mixture at the same temperature and then stirring was con-
tinued for additional 36 h to give the [3+2] cycloadduct 5aa in 84%
yield with high diastereo- and enantioselectivities (98:1:1 dr, 92%
ee; entry 1).14 Similarly, high diastereo- and enantioselectivities
were achieved in the reaction of 2a with various nitroolefins 3b–
3g having an electron-rich, electron-deficient aromatic group or
heteroaromatic group (entries 2ꢀ7). On the other hand, an elec-
tronic nature on the aromatic moiety of 2 affects the asymmetric
induction of the cycloaddition. Imine 2b, having p-chloro substitu-
ent on the aromatic moiety, afforded 5ba with high enantioselec-
tivity in good yield (entry 8). In contrast, removal of an electron-
withdrawing substituent or introduction of electron-donating
group led to a decrease in enantioselectivity (entries 9 and 10).15
The stereochemical outcome in the sequential [3+2] cycloaddi-
tion can be rationalized by the following plausible mechanism
(Fig. 1). According to our previous literature,11d (R)-4 would be pro-
duced predominantly through a ternary complex of 1, 2, and 3. The
resulting nitroalkane 4 would be deprotonated by the amino group
of 1, furnishing the corresponding nitronate anion, which is stabi-
lized by the assistance of the thiourea moiety of 1. Moreover, an
external acidic proton of TFE might activate the imine moiety of
4 and stabilize a transition state by the hydrogen bond. Conse-
In summary, we have developed a formal enantioselective [3+2]
cycloaddition of azomethine ylides with nitroolefins by the collab-
oration of thiourea 1 and trifluoroethanol, giving the optically
active multi-functional pyrrolidines in up to 92% ee and 96% de.
Acknowledgments
This work was supported in part by a Grant-in-Aid for Scientific
Research (B) (Y.T.) and Scientific Research on Priority Areas: Ad-
vanced Molecular Transformations of Carbon Resources (Y.T. and
K.T.), and ‘Targeted Proteins Research Program’ from the Ministry
of Education, Culture, Sports, Science and Technology of Japan.
References and notes
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Me
Me
H
CO2Et
CO2Et
(R)-4
H
H
N
O
N
H
N
H
O
N
1
H
H
N
S
O
CH2CF3
9. Chen, X.-H.; Zhang, W.-Q.; Gong, L.-Zhu. J. Am. Chem. Soc. 2008, 130, 5652–
5653.
Ar
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Rios, R. Synlett 2008, 1840–1844.
Figure 1. A plausible transition state model of the thiourea-catalyzed intramole-
cular aza-Henry reaction in the presence of TFE.

J. Xie et al. / Tetrahedron Letters 49 (2008) 6910–6913
6913
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128, 9413–9419; (d) Okino, T.; Hoashi, Y.; Furukawa, T.; Xu, X.; Takemoto, Y. J.
Am. Chem. Soc. 2005, 127, 119–125; (e) Okino, T.; Hoashi, Y.; Takemoto, Y. J. Am.
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Furukawa, T.; Takemoto, Y. Org. Lett. 2004, 6, 625–627.
12. Crystal data for 4ba (enantiomer). C22H23ClN2O5, monoclinic. Space group
P21. a = 9.8653(8) Å, b = 9.2121(6) Å, c = 11.9418(8) Å, b = 91.355(2)°, V =
1085.0(1) Å3, Z = 2, Dcalc = 1.319 g/cm3, R = 0.034, Rw = 0.057, GOF = 0.66, Flack
parameter = 0.00(3). The absolute structure was determined to be (R)-
(0.30 mmol) and 1 (12.4 mg, 0.030 mmol) at 0 °C. After being stirred at the
same temperature for 9 h, trifluoroethanol (0.66 mL, 9.0 mmol) was added to
the reaction mixture at the same temperature. The mixture was stirred at 0 °C
for additional 36 h, and then was concentrated. The resulting residue was
purified by silica gel chromatography (hexane/ethyl acetate = 9:1) to give
pyrrolidine 5. Compound 5aa: Colorless prisms. Mp 128–130 °C (ethanol). IR
(KBr) 3367, 1725, 1555 cmꢀ1 1H NMR (500 MHz, CDCl3) d (ppm) 7.60 (d,
.
J = 8.1 Hz, 2H), 7.55 (d, J = 8.1 Hz, 2H), 7.34–7.29 (m, 5 H), 5.64 (dd, J = 8.0,
7.0 Hz, 1H), 5.55 (dd, J = 8.0, 5.7 Hz, 1H), 5.14 (d, J = 7.0 Hz, 1H), 4.38 (qd,
J = 10.9, 7.4 Hz, 1H), 4.30 (qd, J = 10.9, 7.4 Hz, 1H), 3.91 (qd, J = 10.9, 7.4 Hz, 1H),
3.54 (qd, J = 10.9, 7.4 Hz, 1H), 3.25 (d, J = 5.7 Hz, 1H), 1.30 (t, J = 7.4 Hz, 3H),
0.79 (t, J = 7.4 Hz, 3H); 13C NMR (126 MHz, CDCl3) d (ppm) 171.1, 168.3, 140.8,
2
configuration with a high degree of certainty from the Flack parameter
v
=
134.7, 130.9 (q, J(C,F) = 33.6 Hz), 128.7, 128.5, 128.4, 127.7, 125.4 (q,
1
0.00(3). Flack, H. D. Acta Crystallogr. 1983, A39, 876–881. CCDC 696671
contains the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
3J(C,F) = 3.6 Hz), 123.9 (q, J(C,F) = 274 Hz), 93.5, 75.9, 63.8, 62.3, 62.1, 51.9,
14.0, 13.3. MS (FAB+) m/z: 481 (M+H+, 100). Anal. Calcd for C23H23F3N2O6: C,
57.50; H, 4.83; N, 5.83. Found: C, 57.50; H, 4.91; N, 5.84. HPLC (CHIRALCEL AD-
H, hexane/2-propanol = 85/15, flow rate 1.0 mL/min, 254 nm), tr(minor) =
11.9 min, tr(major) = 24.1 min. A sample with 92% ee by HPLC analysis gave
13. Crystal data for 5aa (enantiomer). C23H23F3N2O6, monoclinic. Space group
P212121. a = 6.914(3) Å, b = 15.714(7) Å, c = 21.718(9) Å, V = 2359.5(17) Å3, Z =
4, Dcalc = 1.352 g/cm3, R = 0.042, Rw = 0.051, GOF = 0.78. CCDC 696672 contains
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
½ ꢁ 43.80 (c 1.00, CHCl3).
a 3D2
15. Crystal data for 5ca (racemate). C22H24N2O6, monoclinic. Space group P21/c;
a = 10.8269(7) Å, b = 17.907(1) Å, c = 11.1352(8) Å, b = 100.958(2)°, V =
2119.5(3) Å3, Z = 4, Dcalc = 1.292 g/cm3, R = 0.039, Rw = 0.045, GOF = 1.74.
CCDC 696673 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
14. Typical experimental procedure for [3+2] cycloaddition of 2 with 3. To a solution
of imine
2 (0.30 mmol) in toluene (3.0 mL) were added nitroolefin 3