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-al pha-D-mannopyranoside and its sulfoxide, following activation at -78 degree s C with benzenesulfenyl triflate or triflic anhydride, respectively, provi de the corresponding alpha-mannosyl triflate as demonstrated by NMR spectro scopy. 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-mannop yranoside and ethyl 3-O-benzoyl-4,6-O-benzylidene-2-O-(tert-butyldimethylsi lyl)-1-thia-alpha- both provide alpha-mannosides on activation with benzene sulfenyl triflate followed by addition of an alcohol. These results stand i n direct contrast to the highly beta-selective couplings of comparable glyc osylations 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.