Unexpected stereocontrolled access to 1α,1′β-disaccharides from methyl 1,2-ortho esters

摘要

Mannopyranose-derived methyl 1,2-orthoacetates (R = Me) and 1,2-orthobenzoates (R = Ph) undergo stereoselective formation of 1α,1′β-disaccharides, upon treatment with BF _3?Et _2O in CH _2Cl _2, rather than the expected acid-catalyzed reaction leading to methyl glycosides by way of a rearrangement-glycosylation process of the liberated methanol.