Highly Diastereoselective α-Mannopyranosylation in the Absence of Participating Protecting Groups

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Abstract
S-Phenyl 2,6-di-O-benzyl-3,4-O-(2‘,3‘-dimethoxybutane-2‘,3‘-diyl)-1-thia-α-d-mannopyranoside and its sulfoxide, following activation at −78 °C with benzenesulfenyl triflate or triflic anhydride, respectively, provide the corresponding α-mannosyl triflate as demonstrated by NMR spectroscopy. On addition of an acceptor alcohol α-mannosides are then formed. Similarly, S-phenyl 2,3-O-carbonyl-4,6-O-benzylidene-1-thia-α-d-mannopyranoside and ethyl 3-O-benzoyl-4,6-O-benzylidene-2-O-(tert-butyldimethylsilyl)-1-thia-α-d-mannopyranoside both provide α-mannosides on activation with benzenesulfenyl triflate followed by addition of an alcohol. These results stand in direct contrast to the highly β-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.