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N. C. Ganguly, M. Datta
LETTER
Technology; Clark, J. H.; Macquarrie, D., Eds.; Blackwell
Science: Oxford, 2002.
(15) Varma, R. S.; Meshram, H. M. Tetrahedron Lett. 1997, 38,
7973.
(16) Chen, F.-E.; Ling, X.-H.; He, Y.-P.; Peng, X.-H. Synthesis
2001, 1772.
assume that dioxygen is involved in the cleavage process.
The cation radical 1a may trap molecular oxygen, possi-
bly in its rearranged carbon radical form 2a, to give finally
2 (route 2; 1a→2a→2b→2). However, peroxy com-
pounds are known to oxidize sulfides to sulfoxides under
various conditions.26,27 By analogy, we favour a mecha-
nism of cleavage involving oxidation of substrate to sul-
foxides 3a (or disulfoxides) with oxygen. These labile
sulfoxides undergo facile hydrolytic cleavage to 2 (route
3; 1→3a→3b→2). However, these routes of cleavage are
not mutually exclusive and may co-exist and overlap in
the cleavage sequence. It is noteworthy that photosensi-
tized oxygenation of these derivatives resulting in cleav-
age to carbonyl compounds is documented.28,29 However,
microwave-assisted solid-state oxygen-mediated dethio-
acetalization has not yet been reported to the best of our
knowledge.
(17) Varma, R. S. Microwave-Assisted Reactions under Solvent-
Free ‘Dry’ Conditions, In Microwaves: Theory and
Applications in Material Processing IV, 80; Clark, D. E.;
Sutton, W. H.; Lewis, D. A., Eds.; American Ceramic
Society, Ceramic Transactions: Washington DC, 1997, 365–
375.
(18) Varma, R. S. Expeditious Solvent-Free Organic Syntheses
Using Microwave Irradiation, In Green Chemical Syntheses
and Processes; Anastas, P. A.; Heine, L. G.; Williamson, T.
C., Eds.; ACS Symposium Series 767, American Chemical
Society: Washington DC, 2000, 292–312.
(19) Clark, J. H. Catalysis of Organic Reactions by Supported
Inorganic Reagents; VCH Publisher Inc.: New York, 1994.
(20) Balogh, M.; Laszlo, P. Organic Chemistry Using Clays;
Springer-Verlag: Berlin, 1993.
(21) Cave, G. W. V.; Raston, C. L.; Scott, J. L. Chem. Commun.
2001, 2159.
In conclusion, a clean, microwave-assisted deprotection
of cyclic S,S-acetals under solvent-free conditions em-
ploying ammonium persulfate on wet montmorillonite K-
10 support has been delineated. High efficiency, manipu-
lative simplicity, absence of overoxidation, compatibility
with a number of functional groups and generality are the
advantageous features of this protocol. We hope this to be
a method of choice for cleavage of 1,3-dithianes and 1,3-
dithiolanes, particularly for high throughput chemistry.
(22) Wu, Y.; Chen, X.; Huang, J.-H.; Tang, C.-J.; Liu, H.-H.; Hu,
Q. Tetrahedron Lett. 2002, 43, 6443.
(23) Pienta, N. J. Photoinduced Electron Transfer, Part C; Fox,
M. A.; Chanon, M., Eds.; Elsevier: Amsterdam, 1988.
(24) Kamata, M.; Sato, M.; Hasegawa, E. Tetrahedron Lett.
1992, 33, 5085.
(25) Kamata, M.; Murakami, Y.; Tamagawa, Y.; Kato, Y.;
Hasegawa, E. Tetrahedron 1994, 50, 12821.
(26) Richardson, D. E.; Yao, H.; Frank, K. M.; Bennet, D. A. J.
Am. Chem. Soc. 2000, 122, 1729.
(27) Vayssie, S.; Elias, H. Angew. Chem. Int. Ed. 1998, 37, 2088.
(28) Nakamura, C. Y.; Satoh, J. Y. J. Chem. Soc., Chem.
Commun. 1977, 680.
Acknowledgment
We sincerely thank the University of Kalyani for a fellowship to
Mrityunjoy Datta and DST, New Delhi for grants, which enabled
the purchase of an IR spectrometer (Perkin Elmer FT-IR L120-
000A) used in this study.
(29) Fasani, E.; Freccero, M.; Mella, M.; Albini, A. Tetrahedron
1997, 53, 2219.
(30) 1,3-Dithianes and 1,3-dithiolanes were prepared by
literature procedures: (a) Hatch, R. P.; Shringarpure, J.;
Weinreb, S. M. J. Org. Chem. 1978, 43, 4172. (b)Marshall,
J. A.; Belletire, J. L. Tetrahedron Lett. 1971, 871.
(31) Representative Procedure for the Deprotection of 1,3-
Dithianes and -1,3-Dithiolanes under Solvent-Free
Conditions.
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2,2-Diphenyl-1,3-dithiane (330 mg, 1.2 mmol) was
dissolved in the minimum amount of CH2Cl2 (1.5 mL) and
to it was added montmorillonite K-10 clay (0.4 g). Removal
of volatiles in vacuo gave a free-flowing powder. A
concentrated solution of ammonium persulfate (1.64 g, 7.2
mmol) in H2O (0.3 mL) was separately adsorbed on
montmorillonite K-10 clay (0.6 g). It was thoroughly mixed
with the free-flowing powder and the mixture was irradiated
at 300 W power level for 2 min (TLC-monitored) in a
conical flask. The cooled reaction mixture was extracted
with CH2Cl2 (3 × 15 mL) and the clay was removed via
filtration through a sintered glass funnel. Careful
concentration of the combined organic extracts under H2O
suction gave the crude product showing a single TLC spot.
Chromatographic filtration over silica gel (60–120 mesh)
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be identical with an authentic sample of benzophenone in all
respects.
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