A new synthetic method for functionalized cyclopentanones was developed on the basis of a [3+2] cycloaddition reaction of a 1-(methylthio)-2-siloxyallyl cationic species and olefins. Allyl acetates 1a and 1b. which are the precursors of the allyl cationic species, are easily prepared in three or four steps from commercially available compounds. Under the influence of EtAlCl2 or AlCl3, 1a or 1b reacted with various kinds of olefins such as enol ethers, vinyl sulfides, styrenes, and trialkylolefins to afford the corresponding cyclopentanones in good yields. It is noteworthy that the sterically more hindered regioisomer was predominantly formed in every case. Furthermore, the reactions of 1b with vinyl sulfides exhibited surprisingly high stereoselectivity, which can be rationalized by the six-membered transition state models involving an orbital interaction between the sulfur atom of the vinyl sulfide and the alpha-carbon of the allyl cation.