Synthesis of pyrrolidin-2-ones via tandem reactions of vinyl sulfonium salts under mild conditions

Article abstract of DOI:10.1016/j.tetlet.2010.07.108

A novel and efficient synthesis of pyrrolidin-2-ones through the tandem reactions of vinyl sulfonium salts with malonyl amides under very mild conditions is reported. The reaction demonstrates a wide tolerance toward various aryls, acyl, sulfonyl, and benzyl while aliphatic groups delivering the tetrahydro-2-oxofuran product.

Full text of DOI:10.1016/j.tetlet.2010.07.108

Tetrahedron Letters 51 (2010) 5238–5241
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Tetrahedron Letters
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Synthesis of pyrrolidin-2-ones via tandem reactions of vinyl sulfonium salts
under mild conditions
Chunsong Xie , Deyu Han , Yue Hu , Jinhua Liu a,*, Tian Xie
a
b
b
a,*
a
Research Center of Biomedicine and Health, Hangzhou Normal University, Hangzhou 310012, PR China
College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310012, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel and efficient synthesis of pyrrolidin-2-ones through the tandem reactions of vinyl sulfonium salts
with malonyl amides under very mild conditions is reported. The reaction demonstrates a wide tolerance
toward various aryls, acyl, sulfonyl, and benzyl while aliphatic groups delivering the tetrahydro-2-oxofu-
ran product.
Received 28 May 2010
Revised 15 July 2010
Accepted 20 July 2010
Available online 24 July 2010
Ó 2010 Published by Elsevier Ltd.
Introduction
pyrrolidin-2-ones from readily accessible materials is still highly
desirable and of much significance.
Pyrrolidin-2-ones (also
c
-lactams) are structurally ubiquitous
In the past few years, vinyl sulfonium salts have exhibited great
utilizations in organic synthesis, and many heterocyclic motifs
have been efficiently accessed by the tandem assemblies of these
motifs in many natural products and pharmaceuticals.2 During
the past few years, many patented pharmaceutical ingredients
based on these units have been approved for their potential treat-
ment toward many common diseases such as hepatitis, diabetes,
1
1
5
reactive electrophiles. During our continuous explorations on
the heterocycle synthesis by the domino reactions of vinyl sulfo-
3
16
and cancers. The SAR studies have shown that they are structural
nium salts, we excitedly found that the pyrrolidin-2-one frame-
necessities in many biologically active molecules since they play
important roles in the binding with receptor proteins.4 Besides
these, pyrrolidin-2-ones have also been widely used as monomers
works could be synthesized straightforwardly via the reactions of
vinyl sulfonium salts with malonyl amides in high efficiency and
under very mild conditions. In view of the easy availability of these
5
15b,17
for many functional polymer materials. The utilization of them in
two reactants
and the mild conditions involved, as well as the
6
organic synthesis has also attracted wide attention and many pyr-
cleavability of the ester group in the products, this novel synthesis
represents a good complement to the existing preparation meth-
ods for pyrrolidin-2-ones.
7
8
rolidines as well as
c
-amino acids have been efficiently accessed
from them.
As a result, the synthesis of pyrrolidin-2-ones has been widely
We started our researches by choosing the reaction between
ethyl 2-(phenylcarbamoyl)butanoate 1a and diphenyl vinyl sulfo-
nium triflate 2a as the model (Scheme 1). To our delight, in the
presence of TEA, after being conducted in DCM at room tempera-
ture for 6 h, the reaction indeed took place and delivered pyrroli-
din-2-one 3aa in 49% yield. However, in addition to the
formation of 3aa, an isomeric tetrahydro-2-iminofuran product
9
studied and many promising methods have been developed. For
2
b
example, the direct amination of either dihydrofuran-2(3H)-ones
or 1,4-dicarbonyl compounds (also their acetals)
10
has been
frequentlyused. However, bothmethodshaveproventobedefective
becausetheyarealwaysperformedunderacidicconditionsand/orat
high temperatures. In the latest years, many novel cyclization
0
protocols such as the intramolecular cyclization of b,
c
-unsaturated
3aa was also formed in 50% yield.
1
1
amides,
the thermal ring-closure of oxirane-containing
1
2
13
enamides, the cascades of imines with 3-nitropropanoate, as
well as the transition-metal promoted multi-component assembly
involving imines, b-halo allylic alcohols, and CO,14 have been
proposed for the synthesis of various pyrrolidin-2-ones. Although
these procedures have been well documented, they still suffer
from the drawbacks that elevated temperatures and/or the use
of contaminative metals are needed. Accordingly, the development
of efficient, mild, and general reactions for the synthesis of
O
Et
EtO C
2
Ph
N
3
aa, 49%
O
Ph
Et
3
N
EtO C
Ph
2
N
H
+
S
Ph
OTf
+
Ph
RT, DCM, 6 h
Et
N
Et
1
a
2a
EtO
2
C
3aa', 50%
O
Column Separatable
Scheme 1. Initial experiment.
*
Corresponding authors. Tel.: +86 571 28862281; fax: +86 571 28865630 (J.L.).
E-mail addresses: ljh@hznu.edu.cn (J. Liu), tianxie@hznu.edu.cn (T. Xie).
0
040-4039/$ - see front matter Ó 2010 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2010.07.108

C. Xie et al. / Tetrahedron Letters 51 (2010) 5238–5241
5239
Table 1
NaH had proven to be positive to the selectivity and the efficiency
of the tandem reaction (Table 1, entries 1–2). When the reaction
was conducted in the presence of DBU as the base, a high yield
(93%, Table 1, entry 1) of the pyrrolidin-2-one 3aa was exclusively
formed. Similarly, in the case of NaH, a completely selective forma-
a
Optimization of the conditions
O
O
Ph
Base
Et
EtO
2
C
Ph
+
S
Ph
OTf
2
EtO C
Ph
N
N
H
T ºC, 6 h,
Solvent
Et
0
tion of 3aa and 3aa could also be observed albeit with a moderate
1
a
2a
3aa
yield (47%, Table 1, entry 2). A control experiment revealed that the
base was very crucial and the reaction could not work in the ab-
sence of a base (Table 1, entry 3). On the other hand, the screening
of the reaction temperature showed that the room temperature
was optimal and a slightly lower yield (88% and 86%, respectively)
was afforded when the tandem reaction was performed either at
T b (°C)
Yieldsc (%)
Entry
Base
Solvent
1
2
3
4
5
6
DBU
NaH
No
DBU
DBU
DBU
DCM
DCM
DCM
DCM
DCM
THF
rt
rt
rt
45
0
93
47
0
88
86
90
4
5 °C or in an ice-water bath (Table 1, entries 4–5). However, sol-
rt
vents seemed to have little influence on the reaction. When con-
ducted in THF at the same temperature, the reaction could give a
comparable yield (90%) with that in DCM (Table 1, entry 6).
With the optimal conditions in hand, we next studied the scope
of the reaction and the results are summarized in Table 2. Various
N-aryl malonyl amides like ethyl 2-(4-methoxylphenylcarba-
moyl)butanoate 1b and ethyl 2-(4-chlorophenylcarbamoyl)but-
anoate 1c as well as ethyl 2-(4-nitrophenylcarbamoyl)butanoate
a
Reaction conditions: 1a (235 mg, 1.0 mmol), 2a (543 mg, 1.5 mmol), base
(
2.0 mmol), solvents (5.0 mL).
b
Bath temperature.
Isolated yields.
c
To improve the selectivity as well as the efficiency of the reac-
tion, we next performed an optimization on the reaction conditions
Table 1). Changing the base to be a stronger one such as DBU and
(
Table 2
Scope of the reactiona
O
O
R3
Ph
R2
R1
DBU
R2
R1
N
S
Ph
OTf
N
H
+
R3
RT, DCM, 6 h
R4
R4
1
2
3
Entry
1
1
R4
Products
Yieldb (%)
93
O
O
O
O
O
O
Et
EtOOC
Ph
EtOOC
Ph
N
H
H 2a
H 2a
N
N
Et
Et
Et
Et
3aa
1
a
OMe
OMe
Et
EtOOC
EtOOC
EtOOC
EtOOC
2
3
98
93
N
H
3ba
1
c
1
b
Cl
O
O
O
Cl
Et
EtOOC
H 2a
N
N
N
H
3ca
1
NO2
NO
2
Et
EtOOC
4
5
H 2a
H 2a
83
77
N
H
3da
d
O
O
O
N
Et
N
EtOOC
EtOOC
EtOOC
EtOOC
N
H
N
N
N
Et
Et
Et
3ea
1
e
O
O
O
O
Et
6
7
8
9
N
H
Ph
H 2a
H 2a
H 2a
H 2a
EtOOC
74
88
85
Ph
3fa
1f
O
O
O
O
Et
S
EtOOC
S
N
H
Ph
Ph
3ga
1g
O
O
Et
NC
Ph
NC
Ph
N
N
H
Et
1h
3ha
O
O
Ph
EtOOC
Ph
EtOOC
Ph
N
79
N
H
Ph
3ia
1
i
(
continued on next page)

5
240
C. Xie et al. / Tetrahedron Letters 51 (2010) 5238–5241
Table 2 (continued)
Entry
1
R4
Products
Yieldb (%)
41
O
O
O
O
Et
EtOOC
EtOOC
Ph
Ph
Ph
Ph
N
H
N
N
1
0
1
Ph 2b
Et
Et
1
a
Ph
Et
3ab
3ac
EtOOC
EtOOC
N
H
1
a
1
4-Cl–C
H
6 4
2c
63
Cl
O
O
EtOOC
O
Ph
EtOOC
Ph
77c
32
1
2
3
H 2a
H 2a
N
H
N
H
1j
3
ja
O
H
N
1
Ph
O
N
H
3ka
1k
a
b
c
Reaction conditions: amides (1.0 mmol), vinyl sulfoniums (1.5 mmol), DBU (2.0 mmol), DCM (5.0 mL), room temperature, 6 h.
Isolated yield based on amides.
The reaction was performed on a 0.5 mmol scale.
1
d had proven to be the reliable reaction partners with vinyl sulfo-
O
R1 = Bn 57%
Et
nium, leading to N-aryl pyrrolidin-2-ones in high yields (Table 2,
entries 2–4). In addition, N-heteroaryl malonyl amide like ethyl
EtO C
Bn
2
N
O
O
EtO C
2
R1
Ph
DBU
RT
2
-(pyridin-4-ylcarbamoyl)butanoate 1e also reacted smoothly
N
H
+
S
Ph
OTf
with vinyl sulfonium resulting in pyrrolidin-2-one 3ea bearing 4-
pyridyl in 77% yield (Table 2, entry 5). When using the malonyl
amides bearing an acyl and sulfonyl as the substrates, the reaction
could also proceed efficiently leading to the overwhelming forma-
tion of N-acyl and N-sulfonyl pyrrolidin-2-one in 74% and 88%
yield, respectively (Table 2, entries 6 and 7). This is of particular
importance that these removable groups will allow the preparation
of diversely N-substituted pyrrolidin-2-ones feasible.
Et
O
DCM, 6 h R1 = n-C
H13 43%
6
Et
EtO C
2
R1 = cyclohexyl 36%
Scheme 2. The reactions of N-alkyl amides.
giving the corresponding pyrrolidin-2-one in 57% yield. However,
other N-alkyl amides such as ethyl 2-(hexylcarbamoyl)butanoate
and ethyl 2-(cyclohexylcarbamoyl)butanoate exclusively led to the
Besides these substrates bearing substituents at the nitrogens,
2
-cyano-N-phenylbutanamide 1h was also found to be able to un-
dergo the tandem reaction efficiently, affording the desired pyrroli-
same c-lactone product ethyl 3-ethyl-tetrahydro-2-oxofuran-3-
din-2-one 3ha bearing a cyano at the 3-position in 85% yield (Table
carboxylate in variable yields (43% and 36%). It was evident that
the tetrahydro-2-oxofuran product was produced by the hydrolysis
of the corresponding tetrahydro-2-iminofuran intermediates due to
the instability of this kind of N-alkyl tetrahydro-2-iminofuran prod-
ucts. These indicated that the N-substitution also had a dramatic im-
pact on the product distribution possibly due to the fact that it would
affect the acidity of the N–H bonds of the amide moieties.
2, entry 8). Analogously, ethyl 2-(phenylcarbamoyl)-2-phenylace-
tate 1i could also be used in the tandem reaction and produce
the pyrrolidin-2-one 3ia in 79% yield (Table 2, entry 9).
On the other hand, substituted vinyl sulfonium salts such as di-
phenyl styryl sulfonium triflate 2b and diphenyl 4-chlorostyryl sul-
fonium triflate 2c had also been used for the tandem reaction and
yielded 4-substituted pyrrolidin-2-ones 3ab and 3ac in moderate
yield (Table 2, entries 10 and 11). These reactions were of much
importance since they implied the assemblies of the pyrrolidin-
Based upon these results, we proposed a plausible mechanism
for the pyrrolidin-2-one synthesis which was illustrated in Scheme
3. As we mentioned above, under the promotion of the bases, the
assemblies were possibly initiated by the nucleophilic addition of
the enolates I to the electrophilic vinyl sulfonium salts and gener-
ated the sulfur ylids II. Then depending on the N-substitution and
the bases, the reaction would proceed toward two directions,
namely the intramolecular proton transfer to form the imino an-
ion-containing sulfoniums III and the intermolecular protonation
to produce the sulfoniums IV. Under the circumstances of III, the
nitrogens of the imino anion moieties would attack the sulfoniums,
leading to the formation of pyrrolidin-2-ones 3. On the contrary,
the carbonyl oxygens of the amide parts would attack the sulfoni-
ums, yielding the tetrahydro-2-iminofurans V which would be fur-
ther hydrolyzed to give the tetrahydro-2-oxofuran product when
2
-ones bearing substituents at 4-positions were viable. Meanwhile,
the regioselectivity revealed that the reactivity of the methines
was superior to the amides in the malonyl amides, and the reaction
was initiated by the nucleophilic attack of the active methines to
the vinyl sulfoniums. The same conclusion could also be drawn
by the exclusive formation of the cyclopropane derivative 3ja from
the reaction of ethyl 2-(phenethylcarbamoyl)acetate 1j with vinyl
sulfonium (Table 2, entry 12).
In order to explore the reactions of the vinyl sulfoniums with
simple acetamides, we next performed the reaction of N-phenyl-
acetamide 1k with vinyl sulfonium under the standard conditions.
To our astonishment, this reaction did not produce the anticipated
pyrrolidin-2-one product and, instead, an ester of aminoethanol
1
R were aliphatic groups.
18
3
ka was isolated in 32% yield (Table 2, entry 13).
Lastly, the reactions between vinyl sulfonium salts and N-alkyl
In conclusion, we have developed an expeditious efficient syn-
thesis of pyrrolidin-2-ones by the assemblies of vinyl sulfonium
salts and malonyl amides under very mild conditions. Various sub-
stituents such as aryl, acyl, sulfonyl, and benzyl can be tolerated
malonyl amides had also been studied (Scheme 2). N-Benzyl amide
was found to be able to work with diphenyl vinyl sulfonium triflate

C. Xie et al. / Tetrahedron Letters 51 (2010) 5238–5241
5241
References and notes
O
O
H
N
R2
R1
R2
R1
Ph
N
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2
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Ph S
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R4
Scheme 3. Proposed mechanism.
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V
7
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2
a (543 mg, 1.5 mmol) in DCM (3 mL) was added dropwise into
2461.
the above solution at room temperature. The reaction mixture
was then allowed to react at the same temperature for 6 h. After
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CDCl
t, J = 7.4 Hz, 1H), 4.21 (m, 2H), 3.96 (q, J = 9.2 Hz, 1H), 3.76 (t,
J = 9.2 Hz, 1H), 2.65 (m, 1H), 2.18 (m, 1H), 2.10 (m, 1H), 1.88 (m,
3
, TMS) d 7.64 (d, J = 8.4 Hz, 2H), 7.37 (t, J = 7.8 Hz, 2H), 7.16
2003, 32, 392; (g) Kim, K. H.; Jimenez, L. S. Tetrahedron: Asymmetry 2001, 12,
(
999; (h) Wang, Y. F.; Zhang, W. H.; Colandrea, V. J.; Jimenez, L. S. Tetrahedron
999, 55, 10659.
1
1
1
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1
3
1
(
5
H), 1.27 (t, J = 7.6 Hz, 3H), 0.98 (t, J = 7.4 Hz, 3H); C NMR
100 MHz, CDCl ) d 171.4, 171.3, 139.3, 128.8, 124.8, 120.0, 61.5,
7.7, 46.2, 27.3, 27.1, 14.1, 8.9. HRMS (EI) Calcd for C15
3
Ac
N
1
8. Another possible structure of 3ka was
(CAS 28358-86-3).
H
3
19NO :
+
Ph
OH
[
M] 261.1365. Found, 261.1381.
However, after the comparison of the spectroscopic data we obtained and the
reference (H. Firouzabadi, N. Iranpoor, F. Nowrouzi, K. Amani, Chem. Commun.
2
003, 764–765) reported, we finally assigned the structure of 3ka to be the
Acknowledgments
ester of the aminoethanol as described in Table 2. A possible pathway for the
formation of 3ka was as follows:
The authors gratefully thank the grant support from the Na-
tional Natural Science Foundation of China (20675022) and the
Zhejiang Provincial Major Project of China (2007C01004-2). The
financial support from the Zhejiang Provincial Natural Science
Foundation of China (Y4090408) and the Hangzhou City Research
Project (20090331N04) as well as the Hangzhou Normal University
Research Program (YS05203121) are simultaneously acknowl-
edged by Dr. C.S.X.
H
N
N
Ph
N
O
H
Ph
Ph
Ph
S
OH
OTf
Ph
Ph OTf
O
S
1k
Ph
O
DBU
2a
Supplementary data
H
N
2
H O
Ph
N
O
Ph
O
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.07.108.
3
ka