Synthesis of methyl 2-O- and 3-O-α-d-talopyranosyl-α-d-mannopyranoside

Methyl 3,4,6-tri-O-benzyl-2-O-[6-O-(tert-butyldiphenylsilyl)-α-d-mannopyranosyl]-α-d-mannopyranoside (2) was synthesized by treatment of methyl 3,4,6-tri-O-benzyl-2-O-α-d-mannopyranosyl-α-d-mannopyranoside with tert-butylchlorodiphenylsilane in the presence of imidazole. Isopropylidenation, followed by oxidation with pyridinium chlorochromate, and stereoselective reduction with sodium borohydride, converted 2 into methyl 3,4,6-tri-O-benzyl-2-O-[6-O-(tert)-butyldiphenylsilyl)-2,3-O-isopropylidene-α-d-talopyranosyl]-α-d-mannopyranoside (5). Treatment of 5 with a molar solution of tetrabutylammonium fluoride in dry oxolane produced a diol which, on O-de-isopropylidenation followed by catalytic hydrogenolysis, afforded the disaccharide glycoside methyl 2-O-α-d-talopyranosyl-β-d-mannopyranoside. Synthesis of methyl 3-O-α-d-talopyranosyl-α-d-mannopyranoside was accomplished by a similar reaction-sequence. The structures of the final disaccharides, and of various other intermediates, were established by ¹H- and ¹³C-n.m.r. spectroscopy.