Synthesis of 3-azido-3-deoxy-beta-D-galactopyranosides.

Three efficient routes to 3-azido-3-deoxy-beta-D-galactopyranosides were developed relying on a double inversion protocol at C3. Two of the routes were demonstrated to work with both O- and S-glycosides. In all three routes, the 2-O-acetyl-3-azido-4,6-O-benzylidene-3-deoxy-beta-D-galactopyranosides were obtained by an azide inversion of the key intermediates 2-O-acetyl-4,6-O-benzylidene-3-O-trifluoromethanesulfonyl-beta-D-gulopyranosides. The intermediate gulopyranosides were in turn obtained from 2-O-acetyl-4,6-O-benzylidene-3-O-trifluoromethanesulfonyl-beta-D-galactopyranosides, installed in one pot from the 4,6-O-benzylidene-beta-D-galactopyranosides, by inversion with nitrite or acetate. For O-glycosides, the gulopyranoside configuration could alternatively be obtained from the 4,6-O-benzylidene-beta-D-galactopyranoside by elimination to give the 2,3-dianhydro derivative followed by a highly stereoselective cis-dihydroxylation.