Synthesis of methyl O-α-L-rhamnopyranosyl-(1 → 2)-α-d-galactopyranosides specifically deoxygenated at position 3, 4, or 6 of the galactose residue☆

Add time:08/29/2019    Source:sciencedirect.com

The title disaccharides were synthesized by condensation of 2,3,4-tri-O-benzoyl-α-l-rhamnopyranosyl bromide with suitably protected, deoxygenated derivatives of methyl α-d-galactopyranoside. Deoxygenation was achieved via activation of a protected methyl α-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-α-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.

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