Diastereoselectivity in Reactions of Lithiated 2-Phenylthiomethylthiotetrahydropyrans

Diastereoselectivities of up to 95:5 in alkylations of lithiated O,S,S-acetals are rationalized by the locked conformation of the five-membered ring caused by lithium-to-oxygen coordination. The relative configurations of the two diastereomers obtained in benzylation of lithiated 2-(phenylthiomethylthio)tetrahydropyran were confirmed by comparison with reference compounds of known stereochemistry obtained by benzylic reduction of crystalline hydroxyalkylated derivatives.