A new asymmetric alkylation of cyclic β-keto esters with optically active sulfonium salts, which are easily prepared by optical resolution methods, has been investigated. Relative reactivities for alkyl substituents are found to be quite different from those for SN2 alkylation. Stereochemical reaction course via S–O sulfurane intermediate for this new reaction is proposed.
Methods for preparation and physical properties of oxosulfonium salts have been discussed. Optically active ethylmethylphenyloxosulfonium salts have been prepared and their absolute configurations have been determined.