An efficient and mild deprotection of 1,3-oxathiolanes to carbonyl compounds using the superoxide ion

Article abstract of DOI:10.1080/10426500802465850

An efficient deprotection of 1,3-oxathiolanes to carbonyl compounds has been achieved under the mild reaction conditions of tetraethylammonium superoxide in an aprotic medium at room temperature.

Full text of DOI:10.1080/10426500802465850

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Phosphorus, Sulfur, and Silicon
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An Efficient and Mild
Deprotection of 1,3-
Oxathiolanes to Carbonyl
Compounds Using the
Superoxide Ion
a
a
Satish Kumar Singh & Krishna Nand Singh
a
Department of Chemistry, Faculty of Science ,
Banaras Hindu University , Varanasi, India
Published online: 04 Sep 2009.
To cite this article: Satish Kumar Singh & Krishna Nand Singh (2009) An Efficient and
Mild Deprotection of 1,3-Oxathiolanes to Carbonyl Compounds Using the Superoxide
Ion, Phosphorus, Sulfur, and Silicon and the Related Elements, 184:9, 2339-2343
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Phosphorus, Sulfur, and Silicon, 184:2339–2343, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802465850
An Efficient and Mild Deprotection of 1,3-Oxathiolanes
to Carbonyl Compounds Using the Superoxide Ion
Satish Kumar Singh and Krishna Nand Singh
Department of Chemistry, Faculty of Science, Banaras Hindu
University, Varanasi, India
An efficient deprotection of 1,3-oxathiolanes to carbonyl compounds has been
achieved under the mild reaction conditions of tetraethylammonium superoxide
in an aprotic medium at room temperature.
Keywords Deprotection; 1,3-oxathiolane; phase-transfer catalyst; superoxide ion
INTRODUCTION
Protection and deprotection are common features of multistep organic
synthesis. Among the various functional groups, the protection of the
carbonyl group using 1,3-oxathiolanes has attracted a great deal of
interest owing to their remarkable applications. They can be used as
1
acyl carbanion equivalents for carbon–carbon bond forming reactions.
The utility of chiral 1,3-oxathiolanes for the synthesis of optically ac-
tive tertiary alcohols having a carbonyl functionality at the α-position
2
,3
has been nicely demonstrated in organic synthesis. Moreover, the
use of oxathiolanes is much more convenient than O, O-acetals or
S, S-acetals. Although a large number of methods has been reported
4
for the deprotection of 1,3-dithiolanes, only a few are known for the
5
deprotection of 1,3-oxathiolanes. Usual and current methods for the
6
deprotection of 1,3-oxathiolanes employ ammonium tribromide, IBX-
7
8
9
10
11
β-cyclodextrin, BNBTS, isoamylnitrite, chloramine-T, TMSOTf,
1
2
13
14
NCS-AgNO NBS, and Amberlys-15/glyoxylic acid.
3
,
Received 30 July 2008; accepted 10 September 2008.
The authors are thankful to UGC, New Delhi, for financial support.
Address Correspondence to K. N. Singh, Department of Chemistry, Faculty of Science,
Banaras Hindu University, Varanasi 221005, India. E-mail: knsinghbhu@yahoo.co. in
2339

2340
S. K. Singh and K. N. Singh
TABLE I Deprotection of 1,3-Oxathiolanes with In Situ Generated
Tetraethylammonium Superoxide
Entry
1
Substrate
Time (h)
1.5
Yield (%)a
83
2
3
2.0
2.0
81
79
4
5
6
2.5
2.0
2.5
77
80
76
7
2.5
73
8
9
2.75
3.0
84
91
1
0
1
3.0
3.0
90
88
1
12
3.0
65
aYields are pure isolated product.

Deprotection of 1,3-Oxathiolanes
2341
SCHEME 1 Superoxide-induced deprotection of 1,3-oxathiolanes.
RESULTS AND DISCUSSION
In light of the above, and as a part of our ongoing research on the use of
the superoxide ion15 in organic synthesis, we report herein a mild and
efficient method to restore carbonyl compounds from 1,3-oxathiolanes
using in situ generated tetraethylammonium superoxide in dry DMF
at room temperature (Scheme 1).
A wide variety of structurally different 1,3-oxathiolanes (1a–1l)
were prepared by the reaction of the appropriate carbonyl com-
pound with 2-mercaptoethanol in the presence of Montmorillonite K-
1
6
17
1
0 clay (1a–1d, 1f, 1j–1l) and PAS (1e, 1g, 1h, 1i) as a catalyst in
dichloromethane. The reaction conditions were then optimized for the
transformation of 1,3-oxathiolanes to the parent carbonyl compounds.
As an outcome, a mixture of substrate, potassium superoxide, and
tetraethylammonium bromide in the molar ratio 1:2:1 in dry DMF was
found to give the best result. Subsequently various 1,3-oxathiolanes
(
(
1a–1l) were smoothly converted to their parent carbonyl compounds
2a–2l) under the optimized conditions at room temperature (Table I).
The products 2a–2h were accompanied by the formation of the corre-
sponding acids, albeit in low yield (10–15%).
CONCLUSION
The present findings demonstrate the role of the superoxide ion as
a convenient and safe deprotecting agent for 1,3-oxathiolanes to the
parent carbonyl compounds.
EXPERIMENTAL
Potassium superoxide and tetraethylammonium bromide were pro-
duced from E. Merck, Germany and were used as received. Dry N, N-
dimethylformamide (DMF), from Aldrich, USA, was stored over molec-
ular sieves (4Å) prior to use. The other reagents and solvents were of
A.R. grade. All the protected and deprotected compounds were fully
1
13
characterized by their IR, H NMR, and C NMR spectra. IR spectra

2342
S. K. Singh and K. N. Singh
were recorded on a JASCO FT/IR-5300 spectrophotometer. NMR spec-
tra were run on a JEOL AL300 FTNMR spectrometer; chemical shifts
are given in δ ppm, relative to TMS as the internal reference. All the
products were purified by column chromatography.
REACTION OF IN SITU GENERATED
TETRAETHYLAMMONIUM SUPEROXIDE WITH
1,3-OXATHIOLANES (1a–1l): GENERAL PROCEDURE
A mixture of potassium superoxide (1.13 g, 16 mmol) and tetraethylam-
monium bromide (1.68 g, 8 mmol) (weight under a nitrogen atmosphere
using an atmosbag) in dry DMF (25 mL) was stirred for 15 min, and
then the substrate 1,3-oxathiolane (1a-1l) (8 mmol) was added. After
the reaction was over (1.5 to 3 h) as indicated by TLC, a cold brine so-
lution (10 mL) was introduced to decompose the unreacted potassium
superoxide, followed by the addition of saturated sodium hydrogen car-
bonate solution (10 mL). The reaction mixture was extracted with di-
ethyl ether (3 × 15 mL). The combined organic extract was dried over
anhydrous Na2SO4, filtered, and evaporated to give the crude product
(
2a-2l), which was purified by column chromatography on silica gel us-
ing n-hexane:ethyl acetate (9:1) as an eluent. The aqueous phase was
acidified with hydrochloric acid and extracted with diethyl ether (3 ×
10 mL) to isolate the acidic product.
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