Direct chemical synthesis of β-mannopyranosides and other glycosides via glycosyl triflates

High yield, highly stereoselective methods for the synthesis of beta-mannopyranosides of primary, secondary, and tertiary alcohols are presented Activation of mannosyl sulfoxides or mannosyl thioglycosides with trifluoromethanesulfonic anhydride or benzenesulfenyl triflate, respectively, leads to the efficient formation of alpha-mannosyl triflates at -78 degrees C in dichloromethane, in the presence of 2,6-di-tert-butyl-4-methylpyridine. These triflates then react S(N)2-like with alcohols to give the beta-mannosides. The use of a 4,6-O-benzylidene protected mannose is required for high selectivity, as is the use of non-participating protecting groups on O-2 and O-3 of the donors. It is further demonstrated that the thioglycoside/benzenesulfenyl triflate activation is applicable in the glucoside series. when both armed and disarmed protecting groups are tolerated. (C) 1998 Elsevier Science Ltd. All rights reserved.