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 1α,1′β-disaccharides, upon treatment with BF₃·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