416
R. V. Anand et al.
LETTER
It was observed that thioacetalization reaction proceeded complete (Table). The whole mixture was loaded over a
without solvent in an efficient manner (Table). Aldehydes silica gel column (2 x 8 cm) and eluted with EtOAc in pe-
reacted faster than ketones as expected. Ketone group of a troleum ether to obtain a pure thioacetal in an excellent
b-keto ester could also be thioacetalized in a clean man- yield (Table).5
ner. Some chemoselectivity (2.6 : 1) was also obtained
with aldehyde vs ketone (entry 19). Although the reaction
was very clean in most of the cases, hindered and unreac-
Acknowledgement
V.K.S. thanks DST (Government of India) for a Swarnajayanti Fel-
lowship (1998). P.S. thanks CSIR for an award of a Senior Research
Fellowship.
tive ketones such as benzophenone and camphor did not
react in the solvent free condition at rt. But, it could be af-
fected by doing the reaction in toluene at 80 °C (entries 20
& 21). It was also observed that the thioacetalization reac-
tion could also be catalyzed using CuCl2-SiO2 in a solvent
free condition, but the reaction was slow and the yield was
a bit inferior (for example, 4-t-butyl cyclohexanone: 10 h,
88% yield). The present methodology for solvent free
thioacetalization of carbonyl compounds will find a lot of
application in organic synthesis, and to the best of our
knowledge, is unprecedented in the literature.4
References and Notes
(1) Greene, T. W.; Wuts, P.G. M. Protective Groups in Organic
Synthesis; John Wiley and Sons, Inc.: New York, 1973; pp.
323-402.
(2) (a) Patney, H. K.; Margan, S. Tetrahedron Lett. 1996, 37,
4621. (b) Chandrasekhar, S.; Takhi, M.; Reddy, Y. R.;
Mohapatra, S.; Rao, C. R.; Reddy, K. V. Tetrahedron 1997,
53, 14997.
General Procedure for Thioacetalization reactions:
The catalyst was prepared by mixing Cu(OTf)2 (2 mmol)
with 20 g of activated silica gel (activated at 120 °C at 1
mm Hg for 6 h) and mechanically rotated for 12 h. The
procedure for thioacetalization reaction was as follows: A
mixture of a carbonyl compound (1 mmol), 1,2-
ethanedithiol (2 - 5 mmol), and the above catalyst (500
mg, 0.05 mmol) was taken in a 5 mL R.B. flask. The flask
was stoppered with a septum and tied with the rod of a Bü-
chi rotary evaporator. It was rotated till the reaction was
(3) (a) Saravanan, P.; Anand, R. V.; Singh, V. K. Tetrahedron
Lett. 1998, 39, 3823. (b) Sekar, G.; Singh, V. K. J. Org. Chem.
1999, 64, 287. (c) Sekar, G.; Singh, V. K. J. Org. Chem. 1999,
64, 0000. (d) Saravanan, P.; Singh, V. K. Tetrahedron Lett.
1999, 40, 0000.
(4) For some recent papers on solvent free formation of THP
ethers, see: (a) Ballini, R.; Bigi, F.; Carloni, S.; Maggi, R.;
Sartori, G. Tetrahedron Lett. 1997, 38, 4169 (Zeolites). (b)
Ranu, B. C.; Saha, M. J. Org. Chem. 1994, 59, 8269 (Alumi-
na-ZnCl2).
(5) All the compounds gave satisfactory spectroscopic data.
Synlett 1999, No. 4, 415–416 ISSN 0936-5214 © Thieme Stuttgart · New York