Is Donor-Acceptor Hydrogen Bonding Necessary for 4,6-O-Benzylidene-directed β-Mannopyranosylation? Stereoselective Synthesis of β-C-Mannopyranosides and α-C-Glucopyranosides

2,3-Di-O-benzyl-4,6-O-benzylidene-thiohexopyranosides, on activation with 1-benzenesulfinyl piperidine and triflic anhydride, react with allyl silanes and stannanes, and with silyl enolethers to give C-glycosides. In the mannose series the beta-isomers are formed selectively whereas the glucose series provides the (alpha-anomers. This selectivity pattern parallels that of O-glycoside formation and eliminates the need to consider donor-acceptor hydrogen bonding in the formation of the O-glycoside.