Domino aziridine ring opening and Buchwald-Hartwig type coupling-cyclization by palladium catalyst

Article abstract of DOI:10.1016/j.tet.2012.08.054

Highly important trans-3,4-dihydro-2H-1,4-benzoxazine moieties were easily synthesized by domino aziridine ring opening with o-bromophenols and o-chlorophenols followed by the palladium catalyzed coupling-cyclization (intramolecular C_(aryl)-N

Full text of DOI:10.1016/j.tet.2012.08.054

Tetrahedron 68 (2012) 9090e9094
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Domino aziridine ring opening and BuchwaldeHartwig type coupling-cyclization
by palladium catalyst
*
R. Koteshwar Rao, I. Karthikeyan, Govindasamy Sekar
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 March 2012
Received in revised form 15 August 2012
Accepted 17 August 2012
Available online 24 August 2012
Highly important trans-3,4-dihydro-2H-1,4-benzoxazine moieties were easily synthesized by domino
aziridine ring opening with o-bromophenols and o-chlorophenols followed by the palladium catalyzed
coupling-cyclization (intramolecular C(aryl)eN(amide) bond formation) with good to excellent yields.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Cross-coupling
C(aryl)eN(amide) bond formation
BuchwaldeHartwig coupling
Domino reaction
Coupling-cyclization
1. Introduction
catalyst. Using Hantzsch 1,4-dihydropyridine (HEH) and Pd/C as
catalyst, 2H-1,4-benzoxazine derivatives were synthesized from
For the past few years tremendous advances have been made in
the development of cross-coupling methodology.1 The leading ap-
plication of cross-coupling chemistry, particularly CeN bond-
forming reactions are important in medicinal chemistry, pharma-
ceutical companies and in academic laboratories. The cross-
coupling reactions by utilization of less reactive, inexpensive aryl
bromides and aryl chlorides are a challenging task. For the im-
mense majority of these reactions, the scope, experimental ease
and reliability of a method are much more important.
In pharmaceutical industry, compounds containing phenox-
azine and 1,4-benzoxazine moieties have known to be potent drugs
as ion channel activators, vasodilator agents and anti-oxidants
(Fig. 1).2 Usually, 1,4-benzoxazine compounds were synthesized
by multistep processes like cyclocondensation of o-aminophenols
with suitable dihalo derivatives,3 reaction of o-aminophenols with
1,2-epoxy-3-(2-nitroaryloxy)propanes. Also, 2-[N-benzyl(or alkyl)-
N-prop-2-ynyl]-aminophenyl tosylate was used to synthesize 1,4-
benzoxazine skeleton using palladium catalyzed CeO bond for-
mation reaction.7
Recently, we have developed copper-catalyzed oxidation
chemistry,8 and reported 1,10-binaphthyl-2,20-diamine (BINAM)e
CuI as an efficient catalyst for the synthesis of diaryl ethers and aryl
alkyl ethers through Ullmann coupling.9 To strengthen this field,
we have developed a simple, efficient and alternative method to the
conventional multistep process to prepare 1,4-benzoxazine skele-
ton in a single step process from readily available o-iodophenols
a
-halo acyl bromides then subsequent reduction of carbonyl group
by BH3,4 and alkylation of o-nitrophenol by haloester followed by
reductive cyclization to yield 1,4-benzoxazine moiety.5
Later on 1,4-benzoxazine moieties were synthesized by reacting
epoxides with o-halosulfonamides followed by subsequent cycli-
zation,6 epoxide ring opening with o-aminophenols followed by
cyclocondensation and reacting aryl/vinyl iodides with 1-azido-2-
(prop-2-ynyloxy)benzene in the presence of palladiumecopper
* Corresponding author. E-mail address: gsekar@iitm.ac.in (G. Sekar).
Fig. 1. Biologically important compounds having 1,4-benzoxazine as a core skeleton.2
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.08.054

R. Koteshwar Rao et al. / Tetrahedron 68 (2012) 9090e9094
9091
Table 1
and aziridines using copper-catalyzed domino ring opening fol-
lowed by Goldberg cyclization.10 However, less reactive and in-
expensive bromo and chlorophenols refused to react under the
optimized reaction condition to give 1,4-benzoxazine derivatives.
To devise a more efficient and general catalyst system for the
domino synthesis of trans-1,4-benzoxazines, we explored the use of
simple and commercially available chelating ligand using palla-
dium as metal source and readily available aziridines, inexpensive
o-bromophenols and o-chlorophenols as starting materials
(Scheme 1).
Ligand and palladium source screening for the domino aziridine ring opening and
coupling-cyclization for the synthesis of 3
Entry
Pd salts
Ligands
Time
Yielda (%)
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd2(dba)3
PdCl2
Pd(OCOCF3)2
K2PdCl4
Pd(acac)2
PdCl2(dppf)
BINAP
PPh3
dppe
dppp
DIOP
BINAP
BINAP
BINAP
BINAP
BINAP
BINAP
48
72
72
72
72
48
72
48
72
72
72
71
0b
0b
0b
0b
99
55
84
60
43
29
9
10
11
Scheme 1. Proposed scheme for palladium-catalyzed domino reaction.
a
Isolated yields.
Only aziridine opened product was observed.
b
2. Results and discussion
In the initial studies, the synthesis of a 1,4-benzoxazine moiety 3
was carried out using 5 mol % of Pd(OAc)2 and 10 mol % of
(ꢀ)-BINAP in toluene at 110 ꢁC from 7-tosyl-7-azabicyclo[4.1.0]
heptane 1 with o-bromophenol 2 and Cs2CO3 by domino aziridine
ring opening followed by palladium catalyzed BuchwaldeHartwig
coupling-cyclization and the reaction mixture provided 3 in 71%
yield in 48 h. The trans stereochemistry of the product was deduced
from the coupling constants of the methine protons in the 1H NMR
spectrum.
Table 2
Effect of solvents, bases and catalyst loading on the domino reaction
Entry
Solvents
Bases
Time
Yielda (%)
We screened the domino reaction with several phosphorous-
based ligands, which are shown in Fig. 2 to increase the efficiency
of the reaction, and the results are summarized inTable 1. Among the
phosphine ligands examined for the domino reaction, (ꢀ)-BINAP
was foundtobe best ligand (entry 1). Thenthe reactionwas screened
with several palladium sources, although all of them catalyzed the
domino reaction, the Pd2(dba)3 turned out to be the best palladium
source of choice as it gave quantitative yield (entry 6).
1
2
3
4
5
6
7
8
Toluene
DMF
Acetonitrile
THF
Dioxane
DMSO
Toluene
Toluene
Toluene
Toluene
Toluene
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
NaOMe
K3PO4
Na2CO3
Cs2CO3
48
72
72
72
60
72
72
48
72
72
72
99
71
45
47
99
26
31
83
45
28
29b
9
10
11
a
Isolated yields.
Pd2(dba)3 (5 mol%) was used without ligand.
b
Several aziridines in combination with o-bromophenols and o-
chlorophenols gave the corresponding 1,4-benzoxazine moieties in
good to excellent yields. In case of aziridine, when the aziridine ring
is fused with six-membered ring, it gave quantitative yield with
simple o-bromophenol. When electron-withdrawing or electron-
donating groups are present on the o-bromophenol, the yields
were slightly reduced. When the aziridine ring is fused with five- or
seven-membered rings, the yields were slightly reduced. In the
case of o-chlorophenols the catalyst loading and the reaction
temperature were increased to increase the yields of the corre-
sponding 1,4-benzoxazine products (entries 11 and 12). The acyclic,
simple aziridine also reacted well under the optimized reaction
conditions and gave moderate to good yields and based on 1H NMR
of crude reaction mixture it was confirmed that only one
regioisomer formed (entries 13 and 14).
In previous report we have demonstrated coupling kinetic res-
olution for the synthesis of optically active 1,4-benzoxazines
through single-step process.11 In order to explore the asymmetric
version of the reaction, further, we screened the reaction with
optically active (S)-BINAP instead of (ꢀ)-BINAP. Only partial reso-
lution is observed and the moderate selectivity factor (s¼3.3)12 was
obtained (Scheme 2).
Fig. 2. Ligands screened for the palladium catalyzed-domino reaction.
Further, the reaction was optimized by changing the solvents
and bases to increase the efficiency of the domino reaction and the
results are summarized in Table 2. Toluene and Cs2CO3 continued to
be choice of solvent and base for the domino reaction. When the
reaction was carried out using Pd2(dba)3 without ligand (ꢀ)-BINAP,
the reaction did not proceed to completion and gave only 29% yield
after 3 days (Table 2, entry 11).
Using the above mentioned optimized reaction conditions, we
initiated
our
investigation
into
the
scope
of
the
(ꢀ)-BINAPePd2(dba)3 complex catalyzed domino aziridine ring
opening and BuchwaldeHartwig type coupling-cyclization. By us-
ing this methodology various types of 1,4-benzoxazine derived
from several aziridines and substituted o-bromophenols/o-chlor-
ophenols and the results are summarized in Table 3. The trans
stereochemistry of the products was determined by 1H NMR.

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R. Koteshwar Rao et al. / Tetrahedron 68 (2012) 9090e9094
Table 3 (continued)
Table 3
The scope of the palladium catalyzed domino ring opening and coupling-cyclization
Entry
Aziridines
Products
Time (h)
72
Yielda (%)
62
13
Entry
1
Aziridines
Products
Time (h)
48
Yielda (%)
99
14
72
45
a
Isolated yield.
b
c
Reaction was carried out at 130 ꢁC.
o-Chlorophenol was used instead of o-bromophenol.
d
Pd2(dba)3 (10 mol%), and 20 mol % (ꢀ)-BINAP were used at 130 ꢁC.
2
60
60
48
52
72
72
48
48
48
96
62
76
61
66
76
84
90
76
67
78
3
Scheme 2. Domino ring opening/coupling kinetic resolution.
The possible reaction pathway is described in Scheme 3 based
4
on the previous literature1a and the observed stereochemistry of
the products. Initially, Cs2CO3 abstracts the proton from o-bromo-
phenol 2 to provide corresponding phenoxide ion 15. The phen-
oxide 15, on reaction with aziridine
1 provided a trans-
intermediate 16. Then the palladium(0) catalyst oxidatively added
to the trans-intermediate 16 and produced another intermediate
17. Then nucleophilic displacement took place at palladium(II)
centre to produce a cyclic intermediate 18. The cyclic intermediate
18 upon reductive elimination gave the corresponding trans-
product 3 and simultaneously regenerated the palladium(0) cata-
lyst, which will be utilized for the next catalytic cycle.
5
6
7b
8
9
10
Scheme 3. The plausible mechanistic explanation.
11c,d
3. Conclusions
In conclusion, for the first time we have developed a novel and
practical protocol for the synthesis of the trans-1,4-benzoxazine moi-
ety by domino ring opening followed by BuchwaldeHartwig type
coupling-cyclization using the easily available Pd2(dba)3e(ꢀ)-BINAP
complex as catalyst and Cs2CO3 as base. A variety of trans-1,4-
12c,d
96
40

R. Koteshwar Rao et al. / Tetrahedron 68 (2012) 9090e9094
9093
benzoxazinemoietieswere synthesized from correspondingaziridines
133.6, 135.3, 140.4, 144.1, 152.2; HRMS: MS (ESI, m/z): [MH]þ, found
with less reactive, inexpensive o-bromophenols and o-chlorophenols.
420.1652. C25H26NO3S requires 420.1633.
4. Experimental section
4.1. General
4.2.3. trans-8-Methoxy-10-tosyl-2,3,4,4a,10,10a-hexahydro-1H-phe-
noxazine (10, entry 8, Table 3). White solid, mp 88e90 ꢁC, Rf (10%
ethyl acetate/hexanes) 0.61; IR (CDCl3): 2934, 2863, 1602, 1499,
1455, 1354, 1264, 1216, 1168, 1057, 815, 730, 666 cmꢂ1 1H NMR
;
All reactions were carried out in reaction tubes under nitrogen
atmosphere. Ligands and palladium salts, o-bromophenol and
Cs2CO3 were purchased from Aldrich Chemical Company and used
without further purification. The aziridines and substituted o-bro-
mophenols were made using literature procedures.13 Toluene was
purchased from SRL chemicals, India and dried over sodium wire.
Reactions were performed by using Aldrich Stirrer. Thin-layer chro-
matography (TLC) was performed using Merck silica gel 60 F254 pre-
coated plates (0.25 mm) and visualized by UV fluorescence
quenching. Silica gel (particle size 100e200 mesh) purchased from
SRL Indiawasused forchromatography.1Hand13C NMR spectrawere
recorded on a Bruker 400 MHz instrument. 1H NMR spectra were
(400 MHz, CDCl3): d 1.10e1.49 (4H, m, CH2CH2), 1.66e1.77 (1H, m,
CH),1.98e2.08 (1H, m, CH), 2.27 (3H, s, CH3), 2.55e2.64 (1H, m, CH),
3.27 (1H, t, J¼10.0 Hz, NCH), 3.411 (1H, t, J¼10.8 Hz, OCH), 3.74 (3H,
s, OCH3), 6.49e6.58 (2H, m, CHCH), 7.05 (2H, d, J¼7.2 Hz, CHCH),
7.21 (2H, d, J¼8.4 Hz, CHCH), 7.33 (1H, s, CH); 13C NMR (100 MHz):
d
21.7, 24.2, 24.7, 32.0, 33.7, 56.0, 65.5, 81.3, 111.1, 112.4, 117.5, 127.6,
128.4, 129.4, 133.5, 144.0, 146.8, 154.5; HRMS: MS (ESI, m/z):[MH]þ,
found 374.1432. C20H24NO4S requires 374.1426.
4.2.4. trans-3a,9a-7-Methoxy-9-tosyl-1,2,3,3a,9,9a-hexahydrobenzo
[b]cyclopenta[e][1,4]oxazine (11, entry 9, Table 3). White solid, mp
103e105 ꢁC, Rf (10% ethyl acetate/hexanes) 0.64; IR (CDCl3): 2924,
1608,1496,1356,1267,1215,1168,1097, 952, 733, 668 cmꢂ1; 1H NMR
reported relative to Me4Si (
d
0.0 ppm) or residual CHCl3 (
d 7.26 ppm).
13C NMR spectra were reported relative to CDCl3 (
d
77.16 ppm). FTIR
(400 MHz, CDCl3): d 1.49e1.61 (1H, m, CH), 1.73e1.90 (3H, m,
spectra were recorded on a Nicolet 6700 spectrometer and are re-
ported in frequency of absorption (cmꢂ1). High-resolution mass
spectra (HRMS) were recorded on Q-Tof Micro mass spectrometer.
CHCH2), 1.91e2.02 (1H, m, CH), 2.29 (3H,s, CH3), 2.44e2.54 (1H, m,
CH), 3.21e3.31 (1H, m, NCH), 3.73 (3H, s, OCH3), 3.76e3.86 (1H, m,
OCH), 6.49 (1H, dd, J¼8.8 and 2.8 Hz, CH), 6.62 (1H, d, J¼8.8 Hz, CH),
7.11 (2H, d, J¼8.0 Hz, CHCH), 7.41 (2H, d, J¼8.0 Hz, CHCH), 7.61 (1H, d,
4.2. Typical experimental procedure for the domino ring
opening/coupling-cyclization reaction (entry 1, Table 3)
J¼2.8 Hz, CH); 13C NMR (100 MHz):
d 18.5, 21.7, 25.5, 28.6, 56.0, 63.1,
80.8,108.1, 111.7,118.4,127.8,129.7,133.9,143.4,144.3,154.3; HRMS:
MS (ESI, m/z): found 360.1261. C19H22NO4S requires 360.1270.
Pd2(dba)3 (22.9 mg, 0.025 mmol), (ꢀ)-BINAP (31 mg, 0.05 mmol),
aziridine (125.5 mg, 0.5 mmol), o-bromophenol (58
mL, 0.5 mmol)
4.2.5. trans-2-Methoxy-11-tosyl-5a,6,7,8,9,10,10a,11-octahydrobenzo
[b]cyclohepta[e][1,4]oxazine (12, entry 10, Table 3). Gummy solid, Rf
(10% ethyl acetate/hexanes) 0.63; IR (CDCl3): 2930, 2863, 1603,
and Cs2CO3 (325.8 mg, 1.0 mmol) were taken under nitrogen atmo-
sphere in a 10 mL reaction tube equipped with a septum. To this re-
action mixture, 2.2 mL toluene was added and heated at 110 ꢁC for
48 h. After completion of the reaction (the reaction progress was
monitored by TLC), the reaction mixture was allowed to cool, toluene
was evaporated under rotavapor and the crude reaction mixture was
purified directly using column chromatography on silica gel using
ethyl acetate/hexane as eluent to afford trans-10-tosyl-
2,3,4,4a,10,10a-hexahydro-1H-phenoxazine 171 mg (99%).
1499, 1454, 1351, 1265, 1214, 1164, 1030, 813, 727, 666 cmꢂ1 1H
;
NMR (400 MHz, CDCl3):
d 1.32e1.54 (5H, m, CHCH2CH2), 1.54e1.66
(3H, m, CHCH2), 1.94e2.05 (1H, m, CH), 2.27 (3H, s, CH3), 2.23e2.38
(1H, m, CH), 3.42 (1H, ddd, J¼9.8, 9.7 and 4.0 Hz, NCH), 3.73 (3H, s,
OCH3), 3.96 (1H, ddd, J¼9.8, 9.7 and 4.0 Hz, OCH), 6.56 (2H, d,
J¼1.6 Hz, CHCH), 7.05 (2H, d, J¼8.0 Hz, CHCH), 7.18e7.21 (1H, m,
CH), 7.23 (2H, d, J¼8.4 Hz, CHCH); 13C NMR (100 MHz):
d 21.7, 23.8,
24.8, 25.1, 33.6, 34.5, 55.9, 64.8, 84.0, 112.4, 113.0, 117.4, 127.3, 128.5,
129.4, 134.7, 143.7, 147.4, 154.9; HRMS: MS (ESI, m/z): found
388.1599. C21H26NO4S requires 388.1583.
4.2.1. trans-8-Methyl-10-tosyl-2,3,4,4a,10,10a-hexahydro-1H-phe-
noxazine (6, entry 4, Table 3). White solid, mp 118e120 ꢁC, Rf (10%
ethyl acetate/hexanes) 0.71; IR (CDCl3): 2936, 2865, 2358, 1600,
1454, 1353, 1266, 1169, 1056, 903, 819, 729, 663 cmꢂ1
;
1H NMR
4.2.6. trans-3-Phenyl-4-tosyl-3,4-dihydro-2H-benzo[b][1,4]oxazine
(13, entry 13, Table 3). White solid, mp 138e141 ꢁC, Rf (10% ethyl
acetate/hexanes) 0.62; IR (CDCl3): 2930, 1599, 1497, 1361, 1169, 906,
(400 MHz, CDCl3): d1.13e1.49 (m, 4H, CH2CH2), 1.66e1.76 (2H, m,
CHCH2), 1.99e2.07 (1H, m, CH), 2.26 (3H, s, CH3), 2.27 (3H, s, CH3),
2.53e2.61 (CH, m, 1H), 3.28 (1H, ddd, J¼10.6, 10.6 and 4.0 Hz, NCH),
3.39 (1H, ddd, J¼10.6, 10.5 and 3.2 Hz, OCH), 6.52 (1H, d, J¼8.4 Hz,
CH), 6.77 (1H, d, J¼8.0 Hz, CH), 7.04 (2H, d, J¼8.0 Hz, CHCH), 7.17
736, 662, 581 cmꢂ1 1H NMR (400 MHz, CDCl3):
; d2.41 (3H, s, CH3),
3.25 (1H, dd, J¼14.4, 10.4 Hz, NCH), 4.38 (1H, dd, J¼14.4 Hz, 2.4,
OCHaHb), 4.21 (1H, dd, J¼10.2, 2.4 Hz, OCHaHb), 6.94 (1H, dd, J¼8.2,
1.6 Hz, CH), 7.0 (1H, td, J¼8.2, 1.6 Hz, CH), 7.10e7.15 (1H, m, CH), 7.21
(2H, dd, J¼8.0, 2.0 Hz, CHCH), 7.30 (2H, d, J¼8.0 Hz, CHCH), 7.34e7.41
(3H, m, CHCHCH), 7.58 (2H, d, J¼8.4 Hz, CHCH), 7.93 (1H, dd, J¼8.2,
(2H, d, J¼8.0 Hz, CHCH), 7.54 (1H, s, CH); 13C NMR (100 MHz):
d 21.1,
21.7, 24.1, 32.0, 33.6, 65.5, 81.1, 116.8, 126.7, 126.8, 127.6, 127.7, 129.4,
131.7, 133.7, 143.9, 150.7; HRMS: MS (ESI, m/z): [MNa]þ, found
380.1284. C20H23NO3NaS requires 380.1296.
1.6 Hz, CH); 13C NMR (100 MHz):
d 21.6, 50.4, 73.3, 117.8, 121.3, 123.6,
125.0,126.1,126.5,127.5,128.9,130.2,135.7,137.2,144.6,147.5; HRMS:
4.2.2. trans-4a,10a-8-Phenyl-10-tosyl-2,3,4,4a,10,10a-hexahydro-
1H-phenoxazine (9, entry 7, Table 3). White solid, mp 109e111 ꢁC, Rf
(10% ethyl acetate/hexanes) 0.68; IR (CDCl3): 2931, 2861, 1600, 1482,
1353,1268,1166,1059, 818, 703, 664 cmꢂ1;1HNMR(400 MHz, CDCl3):
MS (ESI, m/z): found 366.1148. C21H20NO3S requires 366.1164.
4.2.7. trans-6-Chloro-3-phenyl-4-tosyl-3,4-dihydro-2H-benzo[b]
[1,4]oxazine (14, entry 14, Table 3). White solid, mp 154e156 ꢁC, Rf
(10% ethyl acetate/hexanes) 0.60; IR (CDCl3): 2930,1588,1488,1357,
d
1.16e1.30 (2H, m, CH2), 1.30e1.45 (2H, m, CH2), 1.45e1.58 (2H, m,
CH2), 1.69e1.79 (2H, m, CH2), 2.03e2.11 (1H, m, CH), 2.26 (3H, s, CH3),
2.57e2.66 (1H, m, CH), 3.36 (1H, ddd, J¼10.8, 10.6 and 4.0 Hz, NCH),
3.46 (1H, ddd, J¼11.0,10.6 and 3.6 Hz, OCH), 6.69 (1H, d, J¼8.4 Hz, CH),
7.04 (1H, d, J¼8.0 Hz, CH), 7.21 (2H, d, J¼8.0 Hz, CHCH), 7.26 (1H, t,
J¼7.6 Hz, CH), 7.37(2H, t, J¼7.6 Hz, CHCH), 7.55 (2H, d, J¼7.6 Hz, CHCH),
1165, 1060, 755, 701, 664 cmꢂ1 1H NMR (400 MHz, CDCl3):
; d 2.43
(3H, s, CH3), 3.21 (1H, dd, J¼14.6, 10.6 Hz, NCH), 4.17 (1H, dd, J¼9.2,
2.4 Hz, OCHaCHb), 4.36 (1H, dd, J¼7.2, 2.4 Hz, OCHaCHb), 6.87 (1H,
d, J¼8.8 Hz, CH), 7.08 (1H, dd, J¼4.4, 2.4 Hz, CH), 7.19 (2H, dd, J¼3.8,
2.0 Hz, CHCH), 7.33 (2H, d, J¼8.0 Hz, CHCH), 7.35e7.41 (3H, m,
CHCHCH), 7.61 (2H, d, J¼8.4 Hz, CHCH), 7.96 (1H, d, J¼2.4 Hz, CH);
8.0 (1H, d, J¼2.0 Hz, CH); 13C NMR (100 MHz):
d 21.7, 24.1, 24.7, 32.0,
33.6, 65.4, 81.1,117.4,124.7,124.9,127.0,127.2,127.7,128.1,128.9,129.5,
13C NMR (100 MHz):
d 21.8, 50.1, 73.5, 118.8, 124.4, 126.1, 126.4,

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42, 680.
7. (a) Brown, D. W.; Ninan, A.; Sainsbury, M. Synthesis 1997, 895; (b) Chowdhury,
C.; Sasmal, A. K.; Dutta, P. K. Tetrahedron Lett. 2009, 50, 2678; (c) Meng, Q.-Y.;
Liu, Q.; Li, J.; Xing, R.-G.; Shen, X.-X.; Zhou, B. Synlett 2009, 3283; (d) Chaudhuri,
G.; Chowdhury, C.; Nitya, G. K. Synlett 1998, 1273.
127.6, 129.0, 129.1, 130.3, 135.3, 136.8, 145.0, 146.1; HRMS: MS (ESI,
m/z): found 400.0789. C21H19NO4S requires 400.0774.
Acknowledgements
We thank CSIR (Project No.: 01(2378)/10/EMR-II), DST, New
Delhi, for the financial support, and R.K.R. and I.K. thank CSIR, New
Delhi, for senior research fellowship. We thank DST, New Delhi, for
the funding towards the 400 MHz NMR instrument to the De-
partment of Chemistry, IIT Madras under the IRPHA scheme and
ESI-MS facility under the FIST programme.
8. (a) Alamsetti, S. K.; Mannam, S.; Muthupamdi, P.; Sekar, G. Chem.dEur. J. 2009,
29, 1086; (b) Mannam, S.; Sekar, G. Tetrahedron Lett. 2008, 49, 1083.
9. (a) Naidu, A. B.; Raghunath, O. R.; Prasad, D. J. C.; Sekar, G. Tetrahedron Lett.
2008, 49, 1057; (b) Naidu, A. B.; Sekar, G. Tetrahedron Lett. 2008, 49, 3147; (c)
Naidu, A. B.; Jaseer, E. A.; Sekar, G. J. Org. Chem. 2009, 74, 3675; (d) Prasad, D. J.
C.; Sekar, G. Org. Biomol. Chem. 2009, 7, 5091.
Supplementary data
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.08.054.
10. Koteshwar Rao, R.; Naidu, A. B.; Sekar, G. Org. Lett. 2009, 11, 1923.
11. Koteshwar Rao, R.; Sekar, G. Tetrahedron: Asymmetry 2011, 22, 948.
12. The selectivity factor (s) was determined using the equation.
ðsÞ ¼ ðkfast=kslowÞ ¼ ððln½ð1 ꢂ CÞð1 ꢂ eeRSMÞꢃÞ=ðln½ð1 ꢂ CÞð1 þ eeRSMÞꢃÞÞ; C ¼
Conversion ¼ ðeeRSM=ðeeRSM þ eeproductÞÞ; eeRSM¼enantiomeric excess of re-
covered starting material.
References and notes
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