Higher-carbon sugars. Part 13. The catalytic osmylation of some α,β-unsaturated octuronic acid derivatives and a synthesis of (meso)-threo-gluco-octitol

Abstract

In compliance with Kishi's empirical rule, catalytic osmylation of methyl [methyl (E)-2,3,4-tri-O-benzyl-6,7-dideoxy-α-D-gluco-oct-6-enopyranosid]uronate (6) produced a mixture of methyl (methyl 2,3,4-tri-O-benzyl-β-L-threo-D-gluco-octopyranosid)uronate (9) and the corresponding α-D-threo-D-gluco isomer (10) in the ratio ca. 9 : 1, respectively. Similar osmylation of the (E)-D-manno-octenopyranosiduronate (8) furnished a mixture of methyl (methyl 2,3,4-tri-O-benzyl-β-L-threo-D-manno-octopyranosid)uronate (12) and the corresponding α-D-threo-D-manno isomer (13) in the ratio ca. 6 : 1. In each instance, the stereoselectivity of the osmylation reaction for the (E)-octenopyranosiduronate is higher than that of the corresponding (E)-octenopyranoside. The catalytic osmylation of methyl (E)-3,5-O-benzylidene-6,7-dideoxy-1,2-O-isopropylidene-α-D-gluco-oct-6-enofuranuronate (24), on the other hand, breached Kishi's empirical rule to provide a mixture of methyl 3,5-O-benzylidene-1,2-O-isopropylidene-α-D-threo-D-gluco-octofuranuronate (25) and the corresponding β-L-threo-D-gluco isomer (26) in the ratio ca. 2 : 1, respectively. Acid hydrolysis of the protected octofuranose (27) obtained from the octofuranuronate (25), and reduction of the resulting octose gave the new octitol (meso)-threo-gluco-octitol (28). The stereochemistry of the osmylation product (26) was established by X-ray crystallography.