A facile synthesis of methyl 3,6-dideoxy-α-L-xylo-hexopyranoside (colitose)

Methyl 3,6-dideoxy-α-L-xylo-hexopyranoside (7) bas been synthesised in four steps from methyl α-L-fucopyranoside. The intermediate methyl 3,4-O-benzylidene-α-L-fucopyranoside was separated into the S (1a) and R (1b) diastereoisomers and the absolute configuration established by proton and carbon-13 nmr spectroscopy. Under the conditions of the Hanessian–Hullar reaction methyl 4-O-benzoyl-3-bromo-3,6-dideoxy-α-L-gulopyranoside was the major product. Each diastereoisomer 1a or 1b gave identical products and product ratios; the stereochemistry and relative proportions of these products were determined by proton and carbon-13 nmr. The synthesis of ascarylose from methyl α-L-rhamnopyranoside by an analogous route is also reported. The bromo deoxy intermediates were reduced to the dideoxy sugars by catalytic hydrogenation with palladium.