An organocatalytic synthesis of N-sulfonyl imines using chloramine-T in aqueous medium

Article abstract of DOI:10.1016/j.tetlet.2014.04.096

N-Sulfonyl imines have been synthesized in good to excellent yields from aldehydes and chloramine-T using proline as an organocatalyst in aqueous medium at ambient temperature. The protocol is applicable to a wide range of aldehydes, especially enals exhi

Full text of DOI:10.1016/j.tetlet.2014.04.096

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
An organocatalytic synthesis of N-sulfonyl imines
using chloramine-T in aqueous medium
⇑
Ruchi Chawla, Atul K. Singh, Lal Dhar S. Yadav
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
N-Sulfonyl imines have been synthesized in good to excellent yields from aldehydes and chloramine-T
using proline as an organocatalyst in aqueous medium at ambient temperature. The protocol is applicable
to a wide range of aldehydes, especially enals exhibit remarkable efficiency in the reaction. The reaction
presumably occurs via iminium activation and opens new avenues for the synthesis of N-sulfonyl imines
under environmentally benign and mild conditions.
Received 14 March 2014
Revised 26 April 2014
Accepted 28 April 2014
Available online xxxx
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Aldehydes
Chloramine-T
N-Sulfonyl imines
Aqueous medium
Organocatalysis
Colossal advances have been achieved in the field of organoca-
talysis over the past several years. In particular, aminocatalytic
processes involving the use of enamine and iminium activation
have led to the realization of important organocatalytic synthetic
protocols.1 These great advances can be accredited to the easier
experimental procedures, the economic use of time, energy, and
money and reduction of chemical wastes in organocatalysis.
Despite the huge success met in this field, organocatalytic routes
to certain synthetic frameworks are still elusive and there is a lot
of scope for new developments in this area.
Imines are a well recognized class of compounds in the fields of
chemistry and biology.2 Their electrophilic nature has been
exploited to synthesize several nitrogen containing compounds,
viz. aziridines, piperidines etc., which are widely distributed in nat-
ure and exhibit diverse biological activity.3 Especially, N-sulfonyl
imines have been attracting considerable attention during the last
two decades because of their potent reactivity, which is well com-
plemented by their stability; characteristics not easily found in
other imines. These imines are synthetically very important as they
efficiently undergo nucleophilic addition,4 hetero-Diels–Alder,5
and ene reactions.6 Consequently, numerous methods have been
developed for the synthesis of N-sulfonyl aldimines. In the last
few years, the reports that have come up for their synthesis gener-
ally include the reaction of sulfonamides with aldehydes catalyzed
by di-sulfonic acid imidazolium chloroaluminate,7 WCl6,8 CeCl3,9
InCl3,10 cyanuric chloride,11 Selectfluor™,12 [Et3NSO3H]Cl,13 and
3-methyl-1-sulfonic acid imidazolium chloride.14 Previously, we
have developed a saccharin–lithium bromide catalyzed direct syn-
thesis of N-tosylimines from alcohols.15 Very recently, Cid and co-
workers have reported an aminocatalytic synthesis of aldimines in
chlorinated solvent using molecular sieves in sealed vials16 while
we were working on the substrate scope of our reaction and pre-
paring the manuscript. The previously reported methods generally
require harsh acidic conditions, high temperatures, use of expen-
sive and difficult to remove metal catalysts, non-green solvents,
and above all, extremely dry conditions. To overcome these limita-
tions and in continuation of our work on organocatalytic17 and
environmentally benign synthesis,18 we disclose herein a simple
proline catalyzed synthesis of aldimines from aldehydes and chlo-
ramine-T on water (Scheme 1). To the best of our knowledge, this
is the first organocatalytic route to aldimines using chloramine-T,
and the reaction presumably, occurs via iminium activation.
The simplest route available for the synthesis of N-sulfonyl imi-
nes is the condensation of aldehydes with sulfonamides. The main
challenges associated with the reaction are (i) the low nucleophi-
COOH
N
H
O
10 mol%
Cl
Na
R
N
N
Ts
Ts
R
H
Water, rt, 18-24h
⇑
Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533.
E-mail address: ldsyadav@hotmail.com (L.D.S. Yadav).
Scheme 1. Organocatalytic synthesis of aldimines.
http://dx.doi.org/10.1016/j.tetlet.2014.04.096
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
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2
R. Chawla et al. / Tetrahedron Letters xxx (2014) xxx–xxx
licity of the sulfonamide nitrogen due to the strong electron-with-
drawing character of the sulfonyl group and (ii) the relatively less
stability of N-sulfonyl imines, which very easily undergo hydrolysis
if strong acidic activation of carbonyl group is carried out to over-
come the first limitation. A smart strategy to defeat both of these
challenges can be the replacement of the sulfonamido group
(ASO2NH2) with a different N-sulfonyl group furnishing reagent,
which contains a more nucleophilic nitrogen atom than sulfona-
mides and avoids the use of strong acidic activation of the carbonyl
group. We hypothesized to employ chloramine-T as a sulfonamide
surrogate for the efficient synthesis of N-sulfonyl imines from alde-
hydes under milder conditions. Chloramine-T has been used for the
synthesis of aldimines only on one occasion by B.M. Trost, way
back in 1992 in the presence of stoichiometric amount of tellurium
metal in toluene.19 Thereafter, no report on the use of chloramine-
T for N-sulfonyl imine synthesis is available in the literature.
To test the feasibility of our hypothesis, a model reaction of
benzaldehyde (1a) with chloramine-T (2) on water was performed
which furnished the desired product 3a but in extremely low yield
(Table 1, entry 1). Subsequently, we decided to activate the car-
bonyl group of aldehyde under mild conditions. Iminium ion catal-
ysis is a well-established mode of activation of carbonyl function in
organocatalysis and the pyrrolidine-based catalysts are a privi-
leged class of catalysts used for this activation. Consequently, a ser-
ies of different aminocatalysts were tested for the transformation.
Among the catalysts tested, proline (4a) furnished the best results
in aqueous medium (Table 1, entry 2). Acyclic amines such as
diisopropylamine (4f) and dibenzylamine (4g) showed poor cata-
lytic activity as compared to the cyclic secondary amines 4a–e
(Table 1, entries 2–6 vs 7 and 8). No significant amount of the prod-
uct was obtained in the presence of a tertiary amine 4h (Table 1,
entry 9). Next, screening of different solvents was carried out. Of
the solvents tested, water was the best in terms of the yield and
reaction time (Table 1, entries 2 and 10–16). This might be attrib-
uted to the greater solubility of chloramine-T and proline in water
than in organic solvents. Generally, water is not employed as a sol-
vent for the synthesis of N-sulfonyl imines because it involves a
reversible dehydration step but there are limited reports where
imines have been synthesized or used as substrates in aqueous
medium.21 The optimum catalyst loading for 4a was found to be
10 mol % (Table 1, entries 2, 17, and 18).
Under the established optimized reaction conditions the pro-
cess was extended to a variety of aldehydes and the results are
summarized in Table 2. Substituted benzaldehydes with electron-
withdrawing and electron-donating substituents (Table 2, entries
2–7) provided good yields. Aliphatic aldehydes, for example, prop-
anal and butanal, were not amendable to the synthetic protocol.
Noteworthy is the remarkable efficiency displayed by
a
,b-unsaturated aldehydes (enals) in the reaction. Proline proved
to be an excellent organocatalyst for the synthesis of N-sulfonyl
imines of ,b-unsaturated aldehydes on water. Aromatic enals
a
reacted to form the imines 3h–o in excellent yields (Table 2,
entries 8–13). The incorporation of electron-donating and elec-
tron-withdrawing groups on the aromatic ring of these enals has
no significant consequence on the efficiency of the process (Table 2,
entries 8–13). Aliphatic enals did not furnish any encouraging
results, neither under our conditions nor under the conditions of
Cid and co-workers16 (Table 2, entries 14 and 15). Complete
regioselectivity observed, in the attack of chloramine-T at C-1
and not C-3 of enals resulting in the formation of imines, is in
accordance with earlier observations.16,22
Table 1
Optimization of reaction conditionsa
The iodine catalyzed transformations of chloramine-T have
been used to carry out various organic transformations during past
few years but to the best of our knowledge, this catalytic transfor-
O
4
catalyst
Cl
Na
Ph
N
N
Ts
Ts
Ph
1a
H
solvent, rt, time
mation has never been studied with a
,b-unsaturated enals.23 Thus,
we conducted the reaction of enals with chloramine-T in the
3a
2
O
Ph
O
N
H
N
H
OTMS
N
H
N
H
N
Ph
OH
H
Table 2
Synthesis of N-sulfonyl imines using prolinea
4b
4e
4a
4c
4d
O
catalyst 4a
O
H
10 mol%
N
Cl
Na
R
N
H
N
N
Ts
Ts
R
H
N
Water, rt, time
4g
1
2
4f
3
4h
b
Yieldb (%)
Entry
R
Product
Time (h)
Yield (%)
Entry
Catalyst (mol %)
Solvent
Time (h)
1
2
3
4
5
6
7
8
9
10
11
12
11
12
13
14
15
C6H5
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
24
24
24
24
24
24
24
18
20
20
18
20
20
20
18
24
24
78
59
81
63
60
67
72
90
87
85
95
93
87
83
94
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
None
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
36
24
24
36
36
36
36
36
36
28
26
24
26
30
27
24
24
24
12
78
72
41
36
33
23
21
11
62
57
65
40
43
51
67
78
53
4-CH3OC6H4
4-CH3C6H4
4-ClC6H4
3-CH3OC6H4
4-NO2C6H4
4-NCC6H4
4a (10)
4b (10)
4c (10)
4d (10)
4e (10)
4f (10)
4g (10)
4h (10)
4a (10)
4a (10)
4a (10)
4a (10)
4a (10)
4a (10)
4a (10)
4a (15)
4a (5)
C6H5CH@CH
4-CH3OC6H4CH@CH
4-CH3C6H4CH@CH
4-NO2C6H4CH@CH
4-ClC6H4CH@CH
3-CH3OC6H4CH@CH
3-CH3C6H4CH@CH
3-NO2C6H4CH@CH
CH3CH@CH
H2O
CH3CN
EtOAc
CH2Cl2
THF
Dioxane
CH3OH
Toluene
H2O
10 (8c)
12 (9c)
C2H5CH@CH
H2O
a
b
c
For experimental procedure, see Ref. 20.
Isolated yield of purified product 3.
Under Cid’s conditions.16
a
For experimental procedure, see Ref. 20.
Isolated yield of purified product 3a.
b
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R. Chawla et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Table 3
References and notes
Synthesis of N-sulfonyl imines using iodinea
1. For reviews on enamine and iminium catalysis, see: (a) Pihko, P. M.; Majander,
I.; Erkkil, A. Asymmetric Organocatalysis; Springer: Berlin, 2010. pp 29–75; (b)
Melchiorre, P.; Marigo, M.; Carlone, A.; Bartoli, G. Angew. Chem., Int. Ed. 2008,
47, 6138; (c) Mukherjee, S.; Yang, J. W.; Hoffmann, S.; List, B. Chem. Rev. 2007,
107, 5471; (d) Erkkil, A.; Majander, I.; Pihko, P. M. Chem. Rev. 2007, 107, 5416.
2. Belowich, M. E.; Stoddart, J. F. Chem. Soc. Rev. 2012, 41, 2003. and references
cited therein.
3. (a) Bloch, R. Chem. Rev. 1998, 98, 1407; (b) Martin, S. F. Pure Appl. Chem. 2009,
81, 195; (c) Urushima, T.; Sakamoto, D.; Ishikawa, H.; Hayashi, Y. Org. Lett.
2010, 12, 4588; (d) Wang, Y.; Yu, D.-F.; Liu, Y.-Z.; Wei, H.; Luo, Y.-C.; Dixon, D.
J.; Xu, P.-F. Chem. Eur. J. 2010, 16, 3922; (e) Chiral Amine Synthesis: Methods,
Developments and Applications; Nugent, T. C., Ed.; Wiley-VCH: Weinheim,
Germany, 2010.
O
Na
Ts
K2CO3
N
Cl
2
I2
R'
N
R'
Ts
Water, rt, time
1
3
Entry
R0
Product
Time (h)
Yieldb (%)
1
2
3
4
5
C6H5
3h
3i
3j
3l
3m
6
7
7
7
7
87
85
82
91
83
4-CH3OC6H4
4-CH3C6H4
4-ClC6H4
3-CH3OC6H4
4. For selected examples: (a) Ooi, T.; Uematsu, Y.; Maruoka, K. J. Am. Chem. Soc.
2006, 128, 2548; (b) Duan, H. F.; Jia, Y. X.; Wang, L. X.; Zhou, Q. L. Org. Lett. 2006,
8, 2567; (c) Fujisawa, H.; Takahashi, E.; Mukaiyama, T. Chem.-Eur. J. 2006, 12,
5082; (d) Shi, M.; Chen, L.-H.; Li, C.-Q. J. Am. Chem. Soc. 2005, 127, 3790; (e)
Soeta, T.; Kuriyama, M.; Tomioka, K. J. Org. Chem. 2005, 70, 297; (f) Hayashi, T.;
Kawai, M.; Tokunaga, N. Angew. Chem., Int. Ed. 2004, 43, 6125; (g) Yim, H. K.;
Wong, H. N. C. J. Org. Chem. 2004, 69, 2892; (h) Wipf, P.; Kendall, C.;
Stephenson, C. R. J. J. Am. Chem. Soc. 2003, 125, 761; (i) Yamada, K.-I.; Fujihara,
H.; Yamamoto, Y.; Miwa, Y.; Taga, T.; Tomioka, K. Org. Lett. 2002, 4, 3509; (j)
Wang, D. K.; Zhou, Y. G.; Tang, Y.; Hou, X.-L.; Dai, L.-X. J. Org. Chem. 1999, 64,
4233.
5. For selected examples: (a) Garcia-Mancheno, O.; Gomez-Arrayas, R.; Carretero,
J. C. J. Am. Chem. Soc. 2004, 126, 456; (b) Morgan, P. E.; McCague, R.; Whiting, A.
J. Chem. Soc., Perkin Trans. 1 2000, 515; (c) Yao, S.; Johannsen, M.; Hazell, R. G.;
Jorgensen, K. A. Angew. Chem., Int. Ed. 1998, 37, 3121; (d) Bauer, T.; Szymanski,
S.; Jezewski, A.; Gluzinski, P.; Jurczak, J. Tetrahedron: Asymmetry 1997, 8, 2619;
(e) Boger, D. L.; Corbett, W. L.; Curran, T. T.; Kasper, A. M. J. Am. Chem. Soc. 1991,
113, 1713; (f) Boger, D. L.; Curran, T. T. J. Org. Chem. 1990, 55, 5439; (g) Sisko, J.;
Weireb, S. M. Tetrahedron Lett. 1989, 30, 3037.
a
For experimental procedure, see Ref. 24.
Isolated yield of purified product 3.
b
COOH
4a
N
O
R
Ts
N
H
H
R
H
3
1
-NaClO
AMINOCATALYTIC
CYCLE
COOH
Ts
COOH
6. (a) Yamanaka, M.; Nishida, A.; Nakagawa, M. Org. Lett. 2000, 2, 159; (b)
Melnick, M. J.; Freyer, A. J.; Weinreb, S. M. Tetrahedron Lett. 1988, 29, 3891; (c)
Taschaen, D. M.; Turos, E.; Weinreb, S. M. J. Org. Chem. 1984, 49, 5058.
7. Moosavi-Zare, A. R.; Zolfigol, M. A.; Noroozizadeh, E.; Khakyzadeh, V.; Zare, A.;
Tavasoli, M. Phosphorus, Sulfur Silicon Relat. Elem. 2014, 189, 149.
8. Zolfigol, M. A.; Tavasoli, M.; Moosavi-Zare, A. R.; Arghavani-Hadi, P.; Zare, A.;
Khakyzadeh, V. RSC Adv. 2013, 3, 7692.
N
H
N
OH
N
R
II
H
R
I
Na
Cl
N
2
Na
9. Zhu, X.; Wei, Y. J. Chem. Res. 2012, 36, 363.
HClO
10. Deng, G. S.; Zou, J. Y.; Sun, T. F. Chin. Chem. Lett. 2011, 22, 511.
11. Wu, L.; Yang, X.; Wang, X.; Yan, F. J. Sulfur Chem. 2010, 31, 509.
12. Wu, L.; Cui, T.; Ma, W.; Yan, F. Asian J. Chem. 2010, 22, 8209.
13. Zare, A.; Bahrami, F.; Merajoddin, M.; Bandari, M.; Moosavi-Zare, A. R.; Zolfigol,
M. A.; Hasaninejad, A.; Shekouhy, M.; Beyzavi, M. H.; Khakyzadeh, V.;
Mokhlesi, M.; Asgari, Z. Org. Prep. Proced. Int. 2013, 45, 211.
Ts
Scheme 2. Mechanism for the formation of N-sulfonyl imines.
14. Zolfigol, M. A.; Khazaei, A.; Moosavi-Zare, A. R.; Zare, A. J. Iran. Chem. Soc. 2010,
7, 646.
15. Patel, R.; Srivastava, V. P.; Yadav, L. D. S. Adv. Synth. Catal. 2010, 352, 1610.
16. Morales, S.; Guijarro, F. G.; Ruano, J. L. G.; Cid, M. B. J. Am. Chem. Soc. 2014, 136,
1082.
17. (a) Singh, A. K.; Chawla, R.; Rai, A.; Yadav, L. D. S. Chem. Commun. 2012, 3766;
(b) Chawla, R.; Singh, A. K.; Yadav, L. D. S. Tetrahedron Lett. 2012, 53, 3382; (c)
Chawla, R.; Rai, A.; Singh, A. K.; Yadav, L. D. S. Tetrahedron Lett. 2012, 53, 5323;
(d) Singh, A. K.; Chawla, R.; Yadav, L. D. S. Tetrahedron Lett. 2013, 54, 5099.
18. (a) Chawla, R.; Kapoor, R.; Singh, A. K.; Yadav, L. D. S. Green Chem. 2012, 14,
1308; (b) Singh, A. K.; Yadav, L. D. S. Synthesis 2012, 591; (c) Singh, A. K.;
Chawla, R.; Yadav, L. D. S. Synthesis 2012, 2353; (d) Chawla, R.; Singh, A. K.;
Yadav, L. D. S. Tetrahedron 2013, 69, 1720; (e) Chawla, R.; Singh, A. K.; Yadav, L.
D. S. Eur. J. Org. Chem. 2014, 2032.
presence of iodine in aqueous medium, which afforded N-sulfonyl
imines 3 (C-1 attack) as the only products. However, slightly lower
yields were obtained with iodine as compared to the yields in the
case of proline catalysis; but the reaction time could be reduced to
6–7 h (Table 3).
A plausible mechanism explaining the nucleophilic catalysis
exerted by proline in the synthesis of different types of aldimines
is depicted in Scheme 2, which is in accordance with the literature
precedents.16,25 The secondary amine 4a activates the aldehyde 1
by iminium ion I formation, which is subsequently attacked by
chloramine-T (2). The aminal species II so formed transforms into
the imine 3 by liberating 4a to complete the catalytic cycle.
In conclusion, we have developed a proline catalyzed simple
route to N-sulfonyl imines using aldehydes and chloramine-T in
aqueous medium. To the best of our knowledge, this is the first
organocatalytic route to aldimines using chloramine-T. Remark-
able efficiency has been exhibited by aromatic enals in the present
protocol. Operational simplicity, ambient temperature, metal-free
conditions, and aqueous medium add to the green credentials of
the developed synthetic procedure.
19. Trost, B. M.; Marrs, C. J. Org. Chem. 1991, 56, 6468.
20. General procedure for the synthesis of N-sulfonyl imines 3a–q using proline: A
mixture of aldehyde (1a–q, 1 mmol), chloramine-T (2, 1 mmol), proline (4a,
0.1 mmol), and water (2 mL) was stirred at room temperature for 18–24 h
(Table 2). After completion of the reaction (monitored by TLC), water (5 mL)
was added, and the mixture was extracted with EtOAc (3 ꢀ 5 mL). The
combined organic phase was dried over anhyd Na2SO4, filtered, and
evaporated under reduced pressure. The resulting crude product was purified
by silica gel column chromatography using a mixture of EtOAc/n-hexane (1:9)
as eluent to afford an analytically pure sample of 3a–q (Table 2).
Characterization data of representative compounds.
Compound 3a: White solid, yield 78%. 1H NMR (400 MHz, CDCl3) d: 2.43 (s, 3H),
7.32 (d, J = 8.0 Hz, 2H), 7.48 (t, J = 7.6 Hz, 2H), 7.64 (t, J = 7.6 Hz, 1H), 7.86 (d,
J = 8.0 Hz, 2H), 7.92 (d, J = 7.6 Hz, 2H), 9.01 (s, 1H). 13C NMR (100 MHz, CDCl3)
d: 21.7, 128.0, 128.3, 129.1, 129.3, 132.3, 134.5, 136.4, 144.7, 170.2. EI-MS (m/z)
259 (M+). Anal. Calcd for C14H13NO2S: C, 64.84; H, 5.05; N, 5.40; Found: C,
64.97; H, 4.90; N, 5.17. Compound 3h: White solid, yield 90%. 1H NMR
(400 MHz, CDCl3) d: 2.45 (s, 3H), 6.98 (dd, J = 11.8, 9.4 Hz, 1H), 7.34 (d,
J = 8.1 Hz, 2H), 7.41–7.45 (m, 3H), 7.49 (d, J = 11.8 Hz, 1H), 7.54–7.56 (m, 2H),
7.85 (d, J = 8.1 Hz, 2H), 8.77 (d, J = 9.4 Hz, 1H). 13C NMR (100 MHz, CDCl3) d:
21.7, 124.7, 128.0, 128.8, 129.2, 129.9, 131.7, 134.2, 135.3, 144.6, 153.9, 170.9.
EI-MS (m/z) 285 (M+). Anal. Calcd for C16H15NO2S: C, 67.34; H, 5.30; N, 4.91;
Found: C, 67.12; H, 5.47; N, 4.82. Compound 3k: Yellow solid, yield 95%. 1H
Acknowledgments
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra. A.K.S. is thankful to CSIR
for the award of a Junior Research Fellowship (File No. 09/
001(0379)/2013-EMR-I) and R.C. is thankful to DST (File No. SR/
S1/OC-22/2010) for the award of a Senior Research Fellowship.
Please cite this article in press as: Chawla, R.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.04.096

4
R. Chawla et al. / Tetrahedron Letters xxx (2014) xxx–xxx
NMR (400 MHz, CDCl3) d: 2.43 (s, 3H), 7.06 (dd, J = 15.9, 9.3 Hz, 1H), 7.34 (d,
24. General procedure for the synthesis of N-sulfonyl imines 3h–j, 3l, 3m using
iodine: mixture of ,b-unsaturated aldehyde (1h–j, 1l, 1m, 1 mmol),
J = 7.9 Hz, 2H), 7.52 (d, J = 15.9 Hz, 1H), 7.72 (d, J = 8.4 Hz, 2H), 7.85 (d,
J = 7.9 Hz, 2H), 8.28 (d, J = 8.4 Hz, 2H), 8.83 (d, J = 9.3 Hz, 1H). 13C NMR
(100 MHz, CDCl3) d: 21.6, 124.3, 128.1, 128.5, 129.2, 129.9, 129.9, 134.7, 139.9,
145.1, 149.4, 169.7. EI-MS (m/z) 330 (M+). Anal. Calcd for C16H14N2O4S: C,
58.17; H, 4.27; N, 8.48; Found: C, 58.48; H, 4.02; N, 8.76.
A
a
chloramine-T (2, 1 mmol), iodine (0.5 mmol), and water (3 mL) was
stirred at room temperature for 2 h with subsequent addition of K2CO3
(1 mmol). The reaction mixture was then stirred for another 4–5 h
(Table 3). After completion of the reaction (monitored by TLC), aqueous
Na2S2O3 (0.2 M, 5 mL) was added to the reaction mixture and the
resulting solution was extracted with EtOAc (3 ꢀ 5 mL). The combined
organic phase was dried over anhyd Na2SO4, filtered, and concentrated
under reduced pressure. The resulting crude product was purified by
21. (a) Saggiomo, V.; Lüning, U. Tetrahedron Lett. 2009, 50, 4663. and references
cited therein; (b) Li, Z.; Ren, X.; Wei, P.; Wan, H.; Shi, Y.; Ouyang, P. Green Chem.
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Please cite this article in press as: Chawla, R.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.04.096