Tetrakis(triphenylphosphine)palladium(0) effects the regio- and stereo-selective alkylation of 2-acetoxy-5,6-dihydro-2 H-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-2 H-pyran, 2-[acetamidobis(methoxycarbonyl)methyl]-6-ethoxy-5,6-dihydro-2 H-pyran, and 2-[acetamidobis(methoxycarbonyl)methyl]-6-methoxymethyl-5,6-dihydro-2 H-pyran. Use of nonstabilized carbanions, such as arylzinc chlorides, resulted in the formation of alkylated dihydropyrans and C-glycosyl compounds with net inversion at the oxygen-bearing carbon atom. Examples include the preparation of 2-[(6-ethoxy-5,6-dihydro-2 H-pyranyl)methyl]-4,4-dimethyl-2-oxazoline, trans-methoxy-methyl-2-phenyl-5,6-dihydro-2 H-pyran, trans-methoxymethyl-2-vinyl-5,6-dihydro-2 H-pyran, rans-2-[2,2-bis(ethoxy)ethyl]-6-methoxymethyl-5,6-dihydro-2 H-pyran, (4,6-di- O-methyl-2,3-dideoxy-α- d- erythro-hex-2-enopyranosyl)benzene, (2,3-dideoxy-4,6-di- O-methyl-β- d- erythro-hex-2-enopyranosyl)benzene, 1-(2,3-dideoxy-4,6-di- O-methyl-α- and -β- d- erythro-hex-2-enopyranosyl)naphthalene, 4-(2,3-dideoxy-4,6-di- O-methyl-α- and -β- d- erythro-hex-2-enopyranosyl)toluene, and 1-(2,3,6-trideoxy-4- O-methyl-α- l- erythro-hex-2-enopyranosyl)naphthalene.