Efficient protocol for stereoselective epoxidation of cinnamic esters using TsNBr2

Article abstract of DOI:10.1080/00397910903318617

An efficient method has been developed for the synthesis of epoxide from cinnamic esters without any catalyst. The reaction was performed in CH ₃CN-water (4:1) using N,N-dibromo-p-toluenesulfonamide (TsNBr ₂) in alkaline conditions. This procedure can be utilized for stereoselective synthesis of epoxides from cinnamic esters in excellent yield in a shorter reaction time with exclusive formation of the trans-isomer. The method was further extended successfully for styrenes. Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910903318617

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Efficient Protocol for Stereoselective
Epoxidation of Cinnamic Esters Using
TsNBr2
Indranirekha Saikia a , Bishwapran Kashyap a & Prodeep Phukan a
a Department of Chemistry, Gauhati University, Guwahati, Assam,
India
Version of record first published: 05 Aug 2010.
To cite this article: Indranirekha Saikia , Bishwapran Kashyap & Prodeep Phukan (2010): Efficient
Protocol for Stereoselective Epoxidation of Cinnamic Esters Using TsNBr2 , Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 40:17, 2647-2652
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Synthetic Communications1, 40: 2647–2652, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903318617
EFFICIENT PROTOCOL FOR STEREOSELECTIVE
EPOXIDATION OF CINNAMIC ESTERS USING TsNBr2
Indranirekha Saikia, Bishwapran Kashyap,
and Prodeep Phukan
Department of Chemistry, Gauhati University, Guwahati, Assam, India
An efficient method has been developed for the synthesis of epoxide from cinnamic
esters without any catalyst. The reaction was performed in CH3CN–water (4:1) using
N,N-dibromo-p-toluenesulfonamide (TsNBr2) in alkaline conditions. This procedure can
be utilized for stereoselective synthesis of epoxides from cinnamic esters in excellent yield
in a shorter reaction time with exclusive formation of the trans-isomer. The method was
further extended successfully for styrenes.
Keywords: Catalyst-free; N,N-dibromo-p-toluenesulfonamide (TsNBr2); epoxidation; epoxide; olefin;
styrene; a,b-unsaturated carbonyl compounds
INTRODUCTION
Epoxides are versatile building blocks for the manufacture of a range of
pharmaceuticals, natural products, and functional materials.[1] There are several
methods for epoxidation available in the literature.[2] Recently, new methods have
been developed for epoxidation of olefins using various metallic catalysts.[3–8]
Derivatives of ketones[9] or amines[10] also found application in epoxidation.
Although several catalytic methods have been developed in the recent past to activate
the olefin for epoxide synthesis, new reagent-based noncatalytic method has rarely
been reported. Geng et al. developed a procedure for epoxidation of olefins using
sodium chlorite as an oxidant.[11] However, this method suffers from the drawbacks
such as long times, high temperatures, and poor yield. Synthesis of epoxide by using
bromohydrin in the presence of a base has been extensively used.[12] Recently, we have
demonstrated the use of N,N-dibromo-p-toluenesulfonamide (TsNBr2) for the
synthesis of bromohydrin, alkoxybromide, and bromoazide.[13] We report herein a
procedure for the synthesis of epoxides using TsNBr2 and a base (Scheme 1).
RESULTS AND DISCUSSION
The reagent TsNBr2 employed for this purpose was prepared from
chloroamine-T by following a literature procedure.[14] We found that bromohydrins
Received May 2, 2009.
Address correspondence to Prodeep Phukan, Department of Chemistry, Gauhati University,
Guwahati 781014, Assam, India. E-mail: pphukan@yahoo.com
2647

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I. SAIKIA, B. KASHYAP, AND P. PHUKAN
Scheme 1. Epoxidation using TsNBr2.
form instantaneously upon addition of TsNBr2 to olefins in aqueous acetonitrile.[13a]
We desired to extend the method for epoxidation of olefins by treating the bromohy-
drin in situ with a base. Thus, TsNBr2 (1.2 mmol) and the substrate (1.1 mmol) in
acetonitrile–water (4:1) were stirred for 10 min at room temperature, and then
K2CO3 was added. After 40 min, the epoxide was isolated in good yield (Table 1).
When the reaction was evaluated with varying amounts of K2CO3, we found that
addition of 1.4 equivalents of the base (based on the substrate) is enough to get
the best result. Epoxidation of cinnamic esters is stereoselective, and only the trans
1
isomer was observed as evidenced by the analysis of H NMR data. After initial
success with various cinnamic esters, the procedure could be extended to other
olefins such as styrenes (Table 1, entries 8–10).
a
Table 1. Epoxidation of various olefins using TsNBr2
Entry
1
Olefins (a)
Products (b)
Time (min) Yieldb (%)
50
45
85
70
2
3
40
40
45
73
75
4
5
74
(Continued )

STEREOSELECTIVE EPOXIDATION OF CINNAMIC ESTERS
2649
Table 1. Continued
Entry
Olefins (a)
Products (b)
Time (min) Yieldb (%)
6
45
40
70
75
7
8
9
30
30
50
80
10
60
50
aReaction conditions: ethyl cinnamate (1.1 mmol), MeCN (4 mL), H2O (1 mL), TsNBr2 (1.2 mmol),
K2CO3 (1.5 mmol), room temperature.
bIsolated yield after chromatographic purifications.
CONCLUSION
In conclusion, an efficient protocol for epoxidation of olefins has been
established by using N,N-dibromo-p-toluenesulfonamide in the presence of K2CO3
as base.
EXPERIMENTAL
General Procedure for Synthesis of Epoxides
TsNBr2 (1.2 mmol) was added at room temperature to a solution of olefin
(1.1 mmol) in a mixture of acetonitrile (4 mL) and water (1 ml). After stirring the
reaction mixture for 10 min, K2CO3 (1.5 mmol) was added, and stirring was contin-
ued for another 45 min at room temperature. The reaction was quenched by adding
sodium thiosulfate (200 mg approximately), and stirring continued for another
20 min. The reaction mixture was extracted with ethyl acetate (3 Â 20 mL), dried
(Na2SO4), and concentrated. The crude product was purified by flash chromato-
graphy on silica gel (230–400 mesh) with petroleum ether–ethyl acetate (5%) as

2650
I. SAIKIA, B. KASHYAP, AND P. PHUKAN
eluent. The product was confirmed by analyzing NMR, infrared, and mass
spectroscopic data.
ACKNOWLEDGMENT
Financial support from the Department of Science and Technology (Grant
No. SR=S1=RFPC-07=2006) is gratefully acknowledged.
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