SYNTHESIS OF LIGANDS RELATED TO THE O-SPECIFIC ANTIGEN OF SHIGELLA-DYSENTERIAE TYPE-1 .6. SYNTHESIS OF METHYL O-ALPHA-L-RHAMNOPYRANOSYL-(1 -] 2)-ALPHA-D-GALACTOPYRANOSIDES SPECIFICALLY DEOXYGENATED AT POSITION-3, POSITION-4, OR POSITION-6 OF THE GALACTOSE RESIDUE

The title disaccharides were synthesized by condensation of 2,3,4-tri-O-benzoyl-a -L-rhamnopyranosyl bromide with suitably protected, deoxygenated derivatives of methyl alpha-D-galactopyranoside. Deoxygenation was achieved via activation of a protected methyl alpha-D-gluco- or galacto-pyranoside with N,N'-thiocarbonyldiimidazole followed by treatment with tributyltin hydride and azobisisobutyronitrile. At position 3, the deoxygenation was more successful when performed with the tri-O-benzoylated precursor, rather than the tri-O-benzylated one. The corresponding nucleophile was obtained by benzylidenation of the methyl 3-deoxy-alpha-D-xylo-hexopyranoside. The preparation of the glycosyl acceptor deoxygenated at position 4 could be pursued starting from derivatives having either the D-galacto or the D-gluco configuration. The pathway involving the former was found superior.