Synthesis of a terminal A-B-C disaccharide fragment of flambamycin, curamycin, and avilamycin.

Methyl 2,6-dideoxy-4-O-[2,6-dideoxy-4-O-(3,5-dichloro-4-hydroxy-2-methoxy-6- methylbenzoyl)-beta-D-arabino-hexopyranosyl]-alpha-D-arabino-he xopyranoside (25) corresponds to an A-B-C disaccharide subunit of the title antibiotics. It was synthesized from suitably protected monosaccharide and aromatic precursors. The readily available 4,6-O-benzylidene-1,2-O-(R)-propylidene-alpha-D-glucopyranose was converted in six steps into 2-O-acetyl-4-O-benzoyl-3-O-benzyl-6-deoxy-alpha-D-glucopyranosyl bromide, which was condensed with methyl 3-O-benzyl-2,6-dideoxy-alpha-D-arabino-hexopyranoside in the presence of mercury(II) cyanide. Methyl 4-O-(2-O-acetyl-4-O-benzoyl-3-O-benzyl-6-deoxy-beta- D-glucopyranosyl)-3-O-benzyl-2,6-dideoxy-alpha-D-arabino-hexopyranosi de was converted either into methyl 2,6-dideoxy-4-O-(6-deoxy-beta-D- glucopyranosyl)-alpha-D-arabino-hexopyranoside by removal of the protective groups or into methyl 3-O-benzyl-4-O-(3-O-benzyl-2,6-dideoxy-beta-D-arabino-hexopyranosyl)-2,6 - dideoxy-alpha-D-arabino-hexopyranoside (22) by selective deacetylation, Barton deoxygenation, and Zemplén debenzoylation. Disaccharide 22 was deprotonated with butyllithium and treated with 4-benzyloxy-3,5-dichloro-2-methoxy-6-methylbenzoyl chloride to give the title compound 25 after hydrogenolysis.