STEREOSELECTIVE GLYCOSIDIC COUPLING REACTIONS OF FULLY BENZYLATED 1,2-ANHYDRO SUGARS WITH N-TOSYL-L-SERINE METHYL-ESTER OR N-BENZYLOXYCARBONYL-L-SERINE METHYL-ESTER

The glycosidic coupling reaction of 1,2-anhydro-3,4,6-tri-O-benzyl-beta-D-mannopyranose (7), 1,2-anhydro-3,4,6-tri-O-benzyl-alpha-D-galactopyranose (21), and 1,2-anhydro-3,4-di-O-benzyl-alpha-D-xylopyranose (18) with N-tosyl- (10) or N-benzyloxycarbonyl- (11) L-serine methyl ester provides a new stereocontrolled synthesis of 1,2-trans linked glycopeptides. The 1,2-anhydro sugars are shown to react smoothly with 10 or 11 in the presence of Lewis acid (ZnCl2 or AgOTf) as well as powdered 4A molecular sieves in CH2Cl2 at room temperature to afford glycosyl serine derivatives with high stereoselectivity and high yield in less than 30 min. An improved method using 2-O-acetyl-3,4,6-tri-O-benzyl-alpha-D-mannopyranosyl chloride (6) as the key intermediate for ring closure was applied for the synthesis of 1,2-anhydro-3,4,6-tri-O-benzyl-beta-D-mannopyranose.