Unexpected Stereocontrolled Access to 1α,1′β-Disaccharides from Methyl 1,2-Ortho Esters

Mannopyranose-derived methyl 1,2-orthoacetates (R = Me) and 1,2-orthobenzoates (R = Ph) undergo stereoselective formation of 1 alpha,1'beta-disaccharides, upon treatment with BF3 center dot Et2O in CH2Cl2, rather than the expected acid-catalyzed reaction leading to methyl glycosides by way of a rearrangement-glycosylation process of the liberated methanol.