A NEW APPROACH TO THE CHEMICAL SYNTHESIS OF THE TRISACCHARIDE, AND THE TERMINAL DISACCHARIDE AND MONOSACCHARIDE UNITS OF THE MAJOR, SEROLOGICALLY ACTIVE GLYCOLIPID FROM MYCOBACTERIUM-LEPRAE

The key intermediates for the synthesis of p-trifluoroacetamidophenyl O-(3,6-di-O-methyl-beta-D-glucopyranosyl)-(1 --> 4(-O-(2,3-di-O-methyl-alpha-L-rhamnopyranosyl)-(1 --> 2)-3-O-methyl-alpha-L-rhamnopyranoside (15), as well as p-trifluoroacetamidophenyl O-(3,6-di-O-methyl-beta-D-glucopyranosyl)-(1 --> 4)-2,3-di-O-methyl-alpha-L-rhamnopyranoside (29), were the methyl and ethyl O-(2-O-benzyl-4,6-O-benzylidene-3-O-methyl-beta-D-glucopyranosyl)-(1 -->4)-2,3-O-diphenylmethylene-1-thio-alpha-L-rhamnopyranosides (10 and 24). Dichloroalane treatment of 10 and 24 removed the diphenylmethylene group, liberating HO-2 and HO-3 of the rhamnopyranoside residue, and opened the benzylidene acetal regioselectively to give the 4-O-benzyl-glucopyranosyl disaccharides. Methylation of the free OH groups resulted in the tetra-O-methyl 1-thio disaccharides (12 and 26), useful as glycosyl donors. Introduction of these temporary blocking groups allowed a drastic reduction in the number of synthetic steps to the target compounds.