Stereoselective Synthesis of Protected Galactopyranosyl(α1→2)Glucopyranoside

OBJECTIVE To achieve a protected disaccharide galactopyranosyl(α1→2) glucopyranoside fragment with a 1,2-cis galactosyl linkage and protected groups easily selectively deprotected.METHODS Through multiple protecting and coupling protocol,methyl 3-O-allyl-2-O-benzoyl-4,6-O-benzylidene-α-D-galactopyranosyl-(1→2)-3,4,6-tri-O-benzoyl-α-D-glucopyranoside(11) was synthesized with D-galactose and D-glucose as raw materials.RESULTS Methyl 3,4,6-tri-O-benzoyl-α-D-glucopyranoside was achieved via selective acetylation,benzoylation,debenzylidenation,benzoylation and selective deacetylation of methyl 4,6-O-benzylidene-α-D-glucopyranoside from D-glucose.A donor isopropyl 3-O-allyl-2-O-benzoyl-4,6-O-benzylidene-1-thio-β-D-galactopyranoside from D-galactose and an acceptor methyl 3,4,6-tri-O-benzoyl-α-D-glucopyranoside were coupled to obtain methyl 3-O-allyl-2-O-benzoyl-4,6-O-benzylidene-α-D-galactopyranosyl-(1→2)-3,4,6-tri-O-benzoyl-α-D-glucopyranoside and methyl 3-O-allyl-2-O-benzoyl-4,6-O-benzylidene-β-D-galactopyranosyl-(1→2)-3,4,6-tri-O-benzoyl-α-D-glucopyranoside,and the stereoselective ratio of α-galactosyl linkage to β-galactosyl linkage was 3∶1.All products were characterized with NMR,MS etc.CONCLUSION The protected disaccharide Gal(α1→2) Glc fragment 3-O-allyl-2-O-benzoyl-4,6-O-benzylidene-α-D-galactopyranosyl-(1→2)-3,4,6-tri-O-benzoyl-α-D-glucopyranoside was stereoselectively synthesized for the first time.