A convenient new pathway for stereospecific epimerization of monosaccharide moieties in disaccharides

The disaccharides benzyl 4,6-O-benzylidene-2-O-α-d-mannopyranosyl-β-d-glucopyranoside (2), 6-O-β-d-galactopyranosyl-1,2:3,4-di-O-isopropylidene-α-d-galactopyranose (4), and phenyl 4-O-β-d-galactopyranosyl-1-thio-β-d-glucopyranoside (7) were selectively acetalated with chloral-dicyclohexylcarbodiimide in a nonclassical pathway. During acetalation, the d-mannopyranosyl moiety of the disaccharide 2 and the unprotected β-d-galactopyranosyl moieties of 4 and 7 were epimerized at their 3-positions, generating d-altro- and d-gulo-pyranosyl moieties, respectively.