SYNTHESIS OF FURANOSE GLYCOSIDES OF ABEQUOSE (3,6-DIDEOXY-D-XYLO-HEXOSE)

Hydrogenolysis of 2,3,5-tri-O-benzoyl-6-O-trityl-D-galactono-1,4-lactone (2) gave the corresponding 3-deoxy-D-xylo-hexono-1,4-lactone derivative (3), which on treatment with HBr in acetic acid afforded 2,5-di-O-benzoyl-6-bromo-3,6-dideoxy-D-xylo-hexono-1,4-lactone (4) . Hydrogenation of 4 led to 3, 6-dideoxy-D-xylo-hexono-1,4-lactone dibenzoate (6). The overall yield of 6 from D-galactono-1,4-lactone (1) was about 59%. Alternatively, compound 6 was prepared (67% overall yield from 1) by hydrogenolysis of 6-bromo-6-deoxy-D-galactono-1,4-lactone tribenzoate (5), obtained by treatment of 2 with HBr in dry dichloromethane. Diisoamylborane reduction of 6 gave an anomeric mixture of 2,5-di-O-benzoyl-3,6-dideoxy-alpha,beta-D-xylo-hexofuranose (7), which on O-debenzoylation afforded 3,6-dideoxy-D-xylo-hexose (abequose, 8) whose tautomeric equilibrium was studied by C-13 NMR spectroscopy. Acetylation of 7 gave the 1-O-acetyl derivative (9) mainly in the beta-anomeric configuration. Tin (IV) chloride promoted glycosylation of 9 with methanol and ethanol afforded stereoselectively methyl (10) and ethyl 2,5-di-O-benzoyl-3,6-dideoxy-beta-D-xylo-hexofuranoside (11), respectively.