Abstract Tetrakis (triphenylphosphine) palladium (0) effects the regio-and stereo-selective alkylation of 2-acetoxy-5, 6-dihydro-2H-pyrans and 1-S-acetyl-1-thiohex-2-enopyranosides. Use of stabilized carbanions resulted in the formation of alkylated dihydropyrans with net retention at the oxygen-bearing carbon atom. Examples include the preparation of2- [acetamidobis (ethoxycarbonyl) methyl]-5, 6-dihydro-2H-pyran, 2-[acetamidobis ( ...