dithiol (940 mg, 10 mmol) and clay (100 mg) in either benzene
or CCl (25 ml) was refluxed for 4 h. After the reaction was
4
S
S
complete (TLC), the clay catalyst was filtered off and the filtrate
worked up to give the product which was purified by flash
chromatography to afford the 1,3-dithiolane derivative (1.9 g,
N
i
9
0%) as a colourless liquid.
7
8 75%
General procedure for transdithioacetalization of oximes,
enamines and tosylhydrazones
The oxime, enamine or tosylhydrazone (10 mmol), ethane-1,2-
dithiol (10 mmol) and clay (10% by wt. of starting material) in
O
S
i
N
S
CCl was refluxed for 4–6 h. After completion of the reaction
4
(
TLC), the clay catalyst was filtered off and the filtrate worked
9
10 80%
up to give a crude product, purification of which by flash
chromatography gave the dithiolanes.
N
NHTs
S
H
Acknowledgements
i
H
S
R
R
The authors J. G. K and N. B. B. thank CSIR (New Delhi) for
the award of research fellowship. Author A. S. G. thanks the
Director, NCL, for providing facilities to carry out the present
work. We all thank Dr T. Ravindranathan for constant support
and encouragement.
1
2
1
1
R = Cl,NO2
Scheme 3 Reagents and conditions: i = HS
SH, Clay, CCl , reflux,
4
4
h
References
transdithiocaetalization, although Al O3 was found to be
inactive. A variety of aryl acetals having substituents such as
2
1 T. W. Green and P. G. M. Wuts, Protective Groups in Organic
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therein.
NO , CO H, OH, Cl etc. including unsaturated acetals have
2
2
been successfully transformed into 1,3-dithianes in high yields.
Table 3 summarizes the transdithioacetalization of a variety of
aliphatic acetals and ketals catalysed by kaolinitic clay. A
remarkable feature observed here is that keto acetals under-
went chemoselective transdithioacetalization in preference
to ketone in excellent yields (entries 5 and 6). Even sterically
hindered ketones (entries 7–10), halogeno and unsaturated
acetals (entries 2 and 4) have been successfully transformed
into dithianes and dithiolanes in high yields (see Table 4 for
spectroscopic data on selected products).
2
3 B. Ku and D. Y. Oh, Synth. Commun., 1989, 19, 433; L. Garlaschelli
and G. Vidari, Tetrahedron Lett., 1990, 31, 5815.
4
5
(a) P. Kumar, R. S. Reddy, A. P. Singh and B. Pandey, Tetrahedron
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6 T. Satoh, S. Uwaya and K. Yamakawa, Chem. Lett., 1983, 667.
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1
993, 49, 199.
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3ϩ
3ϩ
4ϩ
of coordinated hydroxy groups of Al , Fe and Ti ions
9
18,19
relocated in the interlamellar space of the clay.
In conclu-
1
0 G. Rosini and R. Ballini, Synthesis, 1988, 833.
sion, natural clay is an effective and convenient catalyst for
transdithioacetalization of acetals, ketals, oximes, enamines
and tosylhydrazones, a novel method likely to have wide appli-
cation in organic synthesis.
1
1 D. H. R. Barton, J. M. Beaton, L. E. Geller and M. M. Pechet,
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1
1
2 G. Simchen, Chem. Ber., 1970, 103, 389.
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1
4 D. E. Ponde, H. B. Borate, A. Sudalai, T. Ravindranathan and V. H.
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Experimental
Preparation of 6-bromo-4-methoxycarbonylmethyl-2-pyridyl
15 T. T. Upadhya, T. Daniel, K. R. Sabu, T. Ravindranathan and
A. Sudalai, Synth. Commun., 1996, 26, 4539.
3
,3-dimethoxypropyl ether 2
1
6 A. S. Gajare, V. R. Kulkarni, N. B. Barhate, M. S. Shingare and
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A mixture of bromopyridone (245 mg, 1 mmol), 3-bromo-
propionaldehyde dimethyl acetal (183 mg, 1 mmol) and potas-
sium carbonate (207 mg, 1.5 mmol) in dry acetonitrile (25 ml)
was refluxed on a water-bath for 6 h. After the reaction was
complete (TLC), acetonitrile was removed from the mixture
under reduced pressure and the residue was diluted with water
1
1
1
8 P. Laszlo, Pure Appl. Chem., 1990, 62, 2027.
9 P. Laszlo, Science, 1987, 235, 1473.
(
10 ml) and extracted with chloroform. Evaporation of the
extract gave the crude product, which was purified by column
chromatography to give the pure title compound 2 (247 mg,
8
7%).
Typical procedure for the preparation of 2-(4-methoxyphenyl)-
Paper 7/06475F
Received 4th September 1997
Accepted 2nd December 1997
1
,3-dithiolane
4
-Methoxyphenyl-1,3-dioxolane (1.8 g, 0.01 mol), ethane-1,2-
9
68
J. Chem. Soc., Perkin Trans. 1, 1998