Aerobic oxysulfonylation of alkenes using thiophenols: An efficient one-pot route to β-ketosulfones

Article abstract of DOI:10.1039/c4ob00776j

We have developed a highly efficient synthetic route to β-ketosulfones via AgNO₃ catalyzed oxysulfonylation of alkenes using thiophenols in the presence of air (O₂) and K₂S₂O₈ as eco-friendly oxidants. Thiophenols have been used as sulfonylation precursors for the first time in a dioxygen activation based radical process. Moreover, the protocol also offers a new and convenient method for the synthesis of β-hydroxysulfides at room temperature without the use of any initiator. This journal is

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Aerobic oxysulfonylation of alkenes using thiophenols: An efficient oneꢀ
pot route to βꢀketosulfones
†
Atul K. Singh, Ruchi Chawla, Twinkle Keshari, Vinod K. Yadav, and Lal Dhar S. Yadav*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
synthesis of sulfones include the intermolecular addition of
We have developed a highly efficient synthetic route to βꢀ
ketosulfones via AgNO3 catalyzed oxysulfonylation of
alkenes using thiophenols in the presence of air (O2) and
K2S2O8 as ecoꢀfriendly oxidants. Thiophenols have been
10 used as sulfonylation precursors for the first time in a
dioxygen activation based radical process. Moreover, the
protocol also offers a new and convenient method for the
synthesis of βꢀhydroxysulfides at room temperature without
use of any initiator.
sulfonyl radicals to alkenes using sulfonyl chlorides,6a,c
45 sulfonyl selenides,6d,e sulfonyl cyanides,6f sulfonyl azides,6g
sulfonyl
hyadrazides,6hꢀj
sulfinic
acids6k,l
and
arenesulfinates6mꢀo as a source of sulfonyl radical (Scheme
1a).
Considering the above points and in continuation of our
50 efforts for the synthesis of sulfones,6mꢀo,8 we were prompted
to develop a new and efficient protocol for the synthesis of
βꢀketosulfones from easily and commercially available
reagent and catalyst. Herein, we report an AgNO3 catalyzed
direct oxysulfonylation of alkenes with dioxygen and
55 thiophenols leading to βꢀketosulfones, which constitute a
chemically and biologically relevant class of compounds.
Chemical transformations that use thiophenols in oxygen
capture reaction for functionalization of alkenes are limited
to the synthesis of βꢀhydroxysulfides (Scheme 1b).9 To the
60 best of our knowledge, this is the first report on the
synthesis of βꢀketosulfones via AgNO3 catalyzed
oxysulfonylation of alkenes with thiophenols using K2S2O8
as an additive in a oneꢀpot operation (Scheme 1c).
15 Introduction
A major challenge at the forefront of synthetic chemistry is
to design multistep bond formation and bond cleavage in
minimum number of synthetic steps. In this regard, oneꢀpot
cascade reaction is an attractive strategy for minimizing the
20 energy cost and chemical waste.
Dioxygen is an ideal source of oxygen because it is
nontoxic, cheap, abundantly available, which is attractive
from both economic and environmental standpoints. In this
connection direct oxidative functionalization of organic
25 molecules with dioxygen is one of the most ideal strategies
for constructing oxygenꢀcontaning compounds.1 On the
other hand, alkenes are common, versatile and readily
available starting materials, which have been extensively
used in organic synthesis. Particularly, transitionꢀmetalꢀ
30 catalyzed oxidative difunctionalization of alkenes with
dioxygen such as dioxygenation,2 aminooxygenation,3
oxyalkylation4 and oxyphosphorylation,5 has become a
powerful methodology for the construction of oxygenated
Previous work
O
O
S
R1
O
R2
R3
R1
R3
initiator or
or
a)
b)
XO2S
O2
photolysis
ref. 6
R2
OH
O
S
R1
O
R2
X = halogen, SePh, N
3
, CN, NHNH
2
, H, Na
R3
base catalyst with
UV / peroxide
R1
compounds.
Dioxygenꢀtriggered
oxidative
radical
R3
OH
HS
O2
S
35 sulfonylation of alkenes has been utilized as a robust
synthetic method for construction of sulfones.6 Sulfone
compounds are important materials because they include
valuable biologically active compounds and useful building
blocks in synthetic organic chemistry.7 To date, limited
40 methods are available for the construction of βꢀketosulfone
compounds via oxidative difuntionalization of alkenes with
dioxygen. The previously reported methods for the
R1
R2
or
stoichiometric
reagent/catalyst
ref. 9
R2
R3
Present work
O
O
S
R1
AgNO3 (20 mol%)
c)
HS R3
R1
O2
R3
O
R2
K2S2O8 (3 equiv)
DMF, rt
R2
65
Scheme 1 Aerobic oxysulfonylation of alkenes.
Results and discussion
Green Synthesis Lab, Department of Chemistry, University of
Allahabad, Allahabad 211 002, India; E-mail:ldsyadav@hotmail.com;
Fax: (+91)532-246-0533; Tel.: (+91)532-250-0652
Initially, a model reaction was performed by stirring a
mixture of styrene (1a, 0.25 mmol), thiophenol (2a, 0.25
mmol), FeCl3 (20 mol%) and K2S2O8 (0.75 mmol) in DMF
†
Electronic
supplementary
information
(ESI)
available:
1
experimental details,characterization data and copies of H and 13C
NMR spectra for the products. See DOI:……….
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at rt overnight. The corresponding βꢀketosulfone 3a was
obtained in 70% yield (Table 1, entry 1). After the
formation of βꢀketosulfone, we focused our efforts to know
the minimum time required for completion of the reaction
and it was found to be 18 h. Encouraged by the result, we
next focused our efforts on increasing the yield of βꢀ
ketosulfone by changing the additive. However, it was
observed that the use of other additives like TBHP, DTBP,
and (NH4)2S2O8 in place of K2S2O8 did not improve the
produced the ketosulfone 3t efficiently (yield 75%) under
60 the reaction conditions. The scope of the reaction was also
examined with other alkenes such as alkylethenes, 1,2ꢀ
dialkylethenes, 1,2ꢀdiarylethenes, trisubstituted alkenes, and
electronꢀdeficient alkenes, but either the reaction did not
proceed or the desired products were formed in traces. The
65 reaction was also successfully tried with a thiol, e.g.
propanethiol, to afford the corresponding βꢀketosulfone 3u
in 84% yield (Table 2).
5
10 yield (Table 1, entries 1ꢀ4). After the optimization of
additive, we performed the screening of different metal
salts. It was found that AgNO3 gave better result in
comparison to FeCl3 and other tested metal salts (Table 1,
entries 5ꢀ10). For comparison purpose, the reaction was also
15 performed in various solvents but DMF remained the best in
terms of yield and reaction time (Table 1, entries 8 and 11ꢀ
17). Subsequently, the quantitative optimization of catalyst,
additive and thiophenol was done. Firstly, we optimized the
quantitative ratio of thiophenol and it was found that the
20 best yield of βꢀketosulfone (92%) was obtained with 1 equiv
of thiophenol (Table 1, entry 8). In the presence of 1.5
equiv of thiophenol, the yield of 3a did not change but the
use of 0.8 equiv of thiophenol decreased the yield of βꢀ
ketosulfone to 70% even when the reaction time was
25 extended up to 24 h (Table 1, entries 18,19). With the best
solvent, metal salt and additive in hand, we further tried to
optimize the molar ratio of AgNO3 and K2S2O8 and the
results are summarized in Table 1, entries 20ꢀ24. As can be
seen from Table 1, equimolar amounts of styrene (1a) and
30 thiophenol (2a) with 20 mol% of AgNO3 and 3 equiv of
K2S2O8 delivered the maximum yield (92%) of βꢀ
ketosulfone 3a (Table 1, entry 8).
Table 1 Optimization of reaction conditions for the synthesis of βꢀketo
70 sulfonesa
O
O
S
Catalyst/ additive
Solvent, rt, 18 h
O2
HS
O
2a
3a
1a
Entry Solvent Metal salt (mol%) Additive (equiv) Yieldb (%)
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
MeOH
EtOH
THF
FeCl3 (20)
FeCl3 (20)
K2S2O8 (3)
TBHP (3)
70
35
28
62
40
65
10
92
15
40
83
20
28
ꢀ
1
2
3
4
5
6
7
8
FeCl3 (20)
DTBP (3)
FeCl3 (20)
(NH4)2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (2.5)
K2S2O8 (3.5)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
Fe2O3 (20)
[Fe(Pc)] (20)
FeCl2 (20)
AgNO3 (20)
CuCl (20)
9
Cu(OAc)2 (20)
AgNO3 (20)
AgNO3 (20)
AgNO3 (20)
AgNO3 (20)
AgNO3 (20)
AgNO3 (20)
AgNO3 (20)
AgNO3 (20)
AgNO3 (20)
AgNO3 (20)
AgNO3 (20)
AgNO3 (15)
AgNO3 (25)
ꢀ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a
CH3CN
DCM
Dioxane
DMF
DMF
DMF
DMF
DMF
DMF
DMF
ꢀ
To evaluate the scope and limitations of this optimized
reaction protocol, we investigated a variety of different
35 alkenes 1 and thiophenols 2 (Table 2). As shown in Table 2,
the alkenes 1 as well as thiophenols 2 having different
electronꢀwithdrawing and electronꢀdonating substituents
efficiently reacted to afford the corresponding βꢀ
ketosulfones 3 in good to excellent yields, demonstrating
40 that this new AgNO3 catalyzed aerobic oxysulfonylation is a
general and practically useful method for the preparation of
10
26
92c
70d
75
92
80
92
23
such
a valuable class of compounds. A variety of
Reaction conditions: styrene (1a, 0.25 mmol), thiophenol (2a, 0.25
mmol), solvent (3 mL). b Isolated yield after column chromatography. c
75 2a (0.38 mmol). d 2a (0.20 mmol) and reaction time 24 h.
functionalities such as methyl, methoxy, fluoro, chloro,
bromo, and cyano on alkenes were all well tolerated under
45 the reaction conditions (3aꢀ3j). Notably, the variation of
position (oꢀ, mꢀ and pꢀ) of substituents on the phenyl ring of
styrene also gave excellent yield of the corresponding βꢀ
ketosulfones (3bꢀ3f). The bicyclic substrate, 2ꢀ
naphthylethylene could also be employed in the reaction
50 leading to βꢀketosulfone 3k in 83% yield. Propꢀ1ꢀ
enylbenzene, an internal alkene, also served as a suitable
reaction partner in this protocol and the desired product 3l
was obtained in 84% yield. Subsequently, the scope of the
reaction was explored for the synthesis of βꢀketosulfones
55 3mꢀ3t using different thiophenols 2. A series of functional
groups on thiphenols including methyl, methoxy, bromo,
chloro and fluoro were tolerable under the present mild
reaction conditions. Even the bulky 2ꢀnaphthylthiophenol
In order to gain reasonable insight into the reaction
mechanism, we conducted
a number of experiments
(Scheme 3). Initially, we studied the progress of reaction
80 using styrene (1a, 0.25 mmol), thiophenol (2a, 0.25 mmol),
AgNO3 (20 mol%) and K2S2O8 (0.75 mmol) as model
substrate in DMF at rt and the progress was monitored by
TLC. After 4 hours, a new spot appeared on TLC and the
corresponding intermediate product was isolated in 94%
85 yield. It was confirmed to be βꢀhydroxysulfide 4a, as its
spectral data were in full agreement with those reported in
the literature.10 Next, we checked the role of
catalyst/additive in the formation of βꢀhydroxysulfide. The
control experiments indicated that the intermediate βꢀ
2
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hydroxysulfide was also formed in the absence of any
additive in 94% yield (Scheme 3a). However, in the absence
of the catalyst AgNO3, βꢀhydroxysulfide was not formed.
Further, we performed the reaction without any additive
using styrene (0.25 mmol), thiophenol (0.25 mmol), and
AgNO3 (20 mol%) in DMF at rt for 18 h, but only βꢀ
hydroxysulfide 4a was obtained instead of βꢀketosulfone 3a.
On the basis of above experiments, it was clear that for the
formation of βꢀketosulfone the additive K2S2O8 was
10 necessary, whereas it does not seem to play any role in the
formation of the intermediate βꢀhydroxysulfides. Inspired
by the previous reports that dioxygen activation based
processes mostly involve radical species,6 a radical trapping
experiment was performed using styrene (0.25 mmol),
15 thiophenol (0.25 mmol), and AgNO3 (20 mol%) in DMF for
the formation of βꢀhydroxysulfide. βꢀhydroxysulfide 4a was
not at all formed in the presence of TEMPO under the
oxygen atom of βꢀhydroxysulfide originates from dioxygen
(Scheme 3d). To further support that βꢀketosulfones are
35 formed by oxidation of βꢀhydroxysulfide 4a, it was
subjected to the standard reaction conditions and product
was isolated in 95% yield (Scheme 3e). This experiment
also confirms that the reaction proceeds via βꢀ
hydroxysulfide as an intermediate. It is in conformity with
40 our earlier observation6m that βꢀhydroxysulfones are not
oxidized into βꢀketosulfones under the given reaction
conditions, so it clearly indicates that βꢀhydroxysulfides are
first oxidized into βꢀketosulfides which in turn are oxidized
into βꢀketosulfones.
5
45
a) synthesis of intermediate:
SH
OH
AgNO3 (20 mol%)
DMF, rt, 4 h
S
Table 2 Synthesis of βꢀketosulfonesa
4a, 94%
b) evidence in support of radical pathway:
O
O
S
R1
AgNO3 (20 mol%)
HS R3
O2
R1
R3
AgNO3 (20 mol%)
SH
OH
S
R2
K
2S2
O8
(3 equiv)
O
R2
TEMPO (0.25 mmol)
DMF, rt, 4 h
DMF, rt
3
1
2
20
4a, 0%
Time Yieldb
Entry
R1
R2
R3
Product
c) role of air as the oxidant:
(h)
18
(%)
92
90
88
90
85
82
86
91
92
76
83
84
94
93
86
81
79
82
80
75
84
OH
S
SH
Ph
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
1
2
3
4
5
6
7
8
AgNO3 (20 mol%)
3ꢀMeC6H4
2ꢀMeC6H4
4ꢀMeC6H4
4ꢀMeOC6H4
2ꢀMeOC6H4
4ꢀBrC6H4
4ꢀClC6H4
4ꢀFC6H4
4ꢀCNC6H4
2ꢀnaphthyl
Ph
H
H
H
H
H
H
H
H
H
H
Me
H
H
H
H
H
H
H
H
H
20
20
21
22
22
18
19
18
18
20
22
20
22
19
20
18
19
18
20
18
DMF, rt, 4 h
N2 atmosphere
4a, traces
d) 18Oꢀlabelling experiment:
18OH
SH
S
AgNO3 (20 mol%)
DMF, rt, 4 h
18O2/N2
9
4a', 88%
Ph
Ph
Ph
10
11
12
13
14
15
16
17
18
19
20
21
e) intermediate probe:
OH
O
O
S
Ph
4ꢀMeC6H4
4ꢀMeC6H4
4ꢀMeC6H4
4ꢀMeOC6H4
4ꢀBrC6H4
4ꢀClC6H4
4ꢀFC6H4
2ꢀnaphthyl
C3H7
K2S2O8
S
(3 equiv mol%)
4ꢀMeC6H4
4ꢀBrC6H4
Ph
O
DMF, rt, 16 h
3a, 95%
Scheme 3. Preliminary mechanistic investigations.
Ph
Ph
Ph
Ph
On the basis of our experimental results and the literature
50 precedents,6h,i,k,l,m,n a plausible mechanism for the formation
of βꢀketosulfones is depicted in Scheme 4. The AgNO3
catalyst triggers the thiyl radical formation from thiophenol.
This thiyl radical attacks the double bond of alkene to
furnish carbon centered radical 5, which is ultimately
55 trapped by dioxygen to form βꢀhydroxysulfide 4. The βꢀ
hydroxysulfide 4 is oxidized into βꢀketosulfide 8 followed
by further oxidation with K2S2O8 to afford the product βꢀ
ketosulfone 3.
3t
3u
Ph
a Reaction conditions: styrene (1a, 0.25 mmol), thiophenol/thiol (2a,
0.25 mmol), AgNO3 (20 mol%), K2S2O8 (0.75 mmol), DMF (3 mL). b
Isolated yield after column chromatography.
25
standard reaction conditions, elucidating a radical pathway
(Scheme 3b). Studying the role of air in the reaction was
also vital to unfold the intricacies of the mechanism. The
reaction was considerably inhibited under nitrogen (Scheme
30 3c). We also performed oxygen labeling experiments. In the
presence of 18O2/N2, 18Oꢀlabeled product βꢀhydroxysulfide
4a’ was obtained in 88% yield, confirming that the hydroxyl
βꢀHydroxysulfides are important building blocks for the
60 synthesis of other functionalized organic molecules.11 They
exhibit great synthetic utility in the field of pharmaceuticals
and natural products.12 Generally, they are synthesized by
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of βꢀhydroxysulfides at rt without use of any initiator.
Table 3 Synthesis of βꢀhydroxysulfidesa
R2
OH
R1
OO
AgNO3 (20 mol%)
O2
HS R3
S
O2
R1
R3
S
1
R1
S
H
R1
R3
R3S
R1
R3
DMF, rt
R2
4
R2
5
1
2
6
Ag0
O2
45
Time Yieldb
AgI
Entry
R1
R3
Ph
Product
OOH
(h)
(%)
R3SH
S
Ph
4ꢀMeC6H4
4ꢀMeOC6H4
4ꢀBrC6H4
4ꢀClC6H4
Ph
4a
4b
4c
4d
4e
4f
4
4
5
4
4
4
4
5
5
5
5
6
2
92
91
88
88
91
95
80
83
82
90
81
78
87
R1
R3
1
2
3
4
5
6
7
8
2
Ph
Ph
Ph
Ph
R2
7
4ꢀMeC6H4
4ꢀBrC6H4
4ꢀClC6H4
4ꢀFC6H4
4ꢀMeC6H4
4ꢀClC6H4
4ꢀFC6H4
C3H7
OH
O
O
S
O
Ph
4g
4h
4i
K2S2O8
K2S2O8
S
S
R1
R3
R1
R3
R1
R3
Ph
O
3
4
R2
R2
8
R2
Ph
9
4ꢀClC6H4
4ꢀClC6H4
4ꢀClC6H4
Ph
4j
10
11
12
13
a
4k
4l
Scheme 4. Plausible mechanism for the formation of βꢀketosulfones.
4m
5
ring opening of epoxides with thiols in the presence of
promoters or catalysts but these method suffer from various
disadvantages such as drastic reaction conditions, poor
regioselectivity, lower yield and undesirable sideꢀproducts
by rearrangement of epoxides and oxidation of thiols.13
Reaction conditions: styrene (1a, 0.25 mmol), thiophenol/thiol (2a,
0.25 mmol), AgNO3 (20 mol%) and DMF (3 mL). b Isolated yield after
column chromatography.
Experimental
10 However, another method commonly used for the
straightforward synthesis of βꢀhydroxysulfides involves the
use of thiols and olefins.9 These reactions usually require a
base catalyst with a large excess of thiol and are initiated by
UV irradiation or peroxides, or require stoichiometric
15 amounts of reagents/catalysts. Advantageously, the present
study also offers a catalytic synthesis of βꢀhydroxysulfides
from styrene and thiols without use of any initiator in the
presence of only 20 mol% of AgNO3 as a mild catalyst
(Table 3). A variety of alkenes 1 and thiophenols 2 react to
20 give the corresponding βꢀhydroxysulfides in good to
excellent yield demonstrating the synthetic utility of present
method. The scope of the reaction was also examined with
other alkenes such as alkylethenes, 1,2ꢀdialkylethenes, 1,2ꢀ
diarylethenes, trisubstituted alkenes, and electronꢀdeficient
25 alkenes, but either the reaction did not proceed or the
desired products were formed in traces. However, when the
reaction was tried with a thiol, e.g. propanethiol, βꢀ
hydroxysulfide 4m was formed in 87% yield (Table 3, entry
13).
50 General
1H NMR spectra were recorded on a Bruker Avance II (400
MHz) FT spectrometer in CDCl3 using TMS as internal
reference. 13C NMR spectra were recorded on the same
55 instrument at 100 MHz in CDCl3 and TMS was used as
internal reference. Mass (EI) spectra were recorded on
JEOL Dꢀ300 mass spectrometer. Elemental analyses were
carried out in a Coleman automatic carbon, hydrogen and
nitrogen analyser. All chemicals used were reagent grade
60 and were used as received without further purification. All
reactions were performed using ovenꢀdried glassware.
Organic solutions were concentrated using a Buchi rotary
evaporator. Column chromatography was carried out over
silica gel (Merck 100–200 mesh) and TLC was performed
65 using silica gel GF254 (Merck) plates.
General procedure for the synthesis of βꢀketosulfones
3. A mixture of alkene 1 (0.25 mmol), thiophenol/thiol 2
(0.25 mmol), AgNO3 (20 mol%), K2S2O8 (0.75 mmol) and
70 DMF (3 mL) was stirred at rt in an open flask for 18ꢀ22 h
(Table 2). After completion of the reaction (monitored by
TLC), the mixture was extracted with EtOAc (3 × 5 mL).
The combined organic phases were dried over anhyd.
Na2SO4, filtered, and concentrated under reduced pressure.
75 The resulting crude product was purified by silica gel
column chromatography using a mixture of EtOAcꢀnꢀ
hexane (1:4) as eluent to afford an analytically pure sample
of βꢀketo sulfones 3 (Table 2).
30 Conclusions
In conclusion, we have demonstrated a novel, efficient and
AgNO3 catalyzed synthesis of βꢀketosulfones directly from
olefins by employing thiophenols in the presence of air (O2)
and K2S2O8 as ecoꢀfriendly oxidants at an ambient
35 temperature. The facile formation of new C=O, CꢀS and
S=O bond takes place through a radical pathway followed
by oxidation. In this process breaking of 2 old bonds (1 σ
and 1 π) and formation of 7 new bonds (4 σ and 3 π) take
place in a oneꢀpot operation. Apart from this, the protocol
40 also offers a new and convenient method for the synthesis
80
General procedure for the synthesis of βꢀ
4
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Organic & Biomolecular Chemistry
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DOI: 10.1039/C4OB00776J
Rajendar, J. Mol. Catal. A: Chem., 2007, 272, 26; (e) C.
Muangkaew, P. Katrun, P. Kanchanarugee, M. Pohmakotr,
V. Reutrakul, D. Soorukram, T. Jaipetch and C. Kuhakarn,
Tetrahedron, 2013, 69, 8847.
hydroxysulfides 4. A mixture of alkene 1 (0.25 mmol),
thiophenol/thiol 2 (0.25 mmol), AgNO3 (20 mol%), and
DMF (3 mL) was stirred at rt in an open flask for 2ꢀ6 h
(Table 3). After completion of the reaction (monitored by
TLC), the mixture was extracted with EtOAc (3 × 5 mL).
The combined organic phases were dried over anhyd.
Na2SO4, filtered, and concentrated under reduced pressure.
The resulting crude product was purified by silica gel
column chromatography using a mixture of EtOAcꢀnꢀ
70
75
80
85
10 J. X. Chen, H. Y. Wu, C. Jin, , X. X. Zhang Y. Y. Xie and
W. K. Su, Green Chem., 2006, 8, 330.
11 V. Kesavan, D. BonnetꢀDelpon and J. P. Bégué,
Tetrahedron Lett., 2000, 41, 2895.
5
12 (a) C. G. Aifheli and P. T. Kaye, Synth. Commun., 1996, 26,
4459; (b) H. Sugihara, H. Mabuchi, M. Hirata, T. Iamamoto
and Y. Kawamatsu, Chem. Pharm. Bull., 1987, 35, 1930; (c)
J. R. Lucy, N. Yi, J. Soderquist, H. Stein, J. Cohen, T. J.
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(c) F. Fringuelli, F. Pizzo, S. Tortoioli and L. Vaccaro,
Tetrahedron Lett., 2003, 44, 6785; (d) F. Fringuelli, F.
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10 hexane (1:4) as eluent to afford an analytically pure sample
of βꢀhydroxysulfides 4 (Table 3).
Acknowledgement
We sincerely thank SAIF, Punjab University, Chandigarh,
15 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)/2013ꢀEMRꢀI).
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