Highly diastereoselective alpha-mannopyranosylation in the absence of participating protecting groups

S-Phenyl 2,6-di-O-benzyl-3,4-O-(2',3'-dimethoxybutane-2', 3'-diyl)-1-thia-alpha-D-mannopyranoside and its sulfoxide, following activation at -78 degrees C with benzenesulfenyl triflate or triflic anhydride, respectively, provide the corresponding alpha-mannosyl triflate as demonstrated by NMR spectroscopy. On addition of an acceptor alcohol alpha-mannosides are then formed. Similarly, S-phenyl 2,3-O-carbonyl-4, 6-O-benzylidene-1-thia-alpha-D-mannopyranoside and ethyl 3-O-benzoyl-4, 6-O-benzylidene-2-O-(tert-butyldimethylsilyl)-1-thia-alpha-D-mannopyr anoside both provide alpha-mannosides on activation with benzenesulfenyl triflate followed by addition of an alcohol. These results stand in direct contrast to the highly beta-selective couplings of comparable glycosylations with 2,3-di-O-benzyl-4, 6-O-benzylidene protected mannosyl donors and draw attention to the subtle interplay of reactivity and structure in carbohydrate chemistry.