A new synthetic method for functionalized cyclopentanones wasdeveloped on the basis of a [3+2]cycloaddition reaction of a 1-(methylthio)-2-siloxyallyl cationicspecies and olefins. Allyl acetates
1a and
1b,which are the precursors of the allyl cationic species, are easilyprepared in three or four steps from commerciallyavailable compounds. Under the influence of EtAlCl
2 orAlCl
3,
1a or
1b reacted with variouskinds of olefinssuch as enol ethers, vinyl sulfides, styrenes, and trialkylolefins toafford the corresponding cyclopentanones ingood yields. It is noteworthy that the sterically more hinderedregioisomer was predominantly formed inevery case. Furthermore, the reactions of
1b with vinylsulfides exhibited surprisingly high stereoselectivity,which can be rationalized by the six-membered transition state modelsinvolving an orbital interaction betweenthe sulfur atom of the vinyl sulfide and the
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-carbon of the allylcation.