Unsaturated sugars. I. Decarboxylative elimination of methyl 2,3-di-O-benzyl-alpha-D-glucopyranosiduronic acid to methyl 2,3-di-O-benzyl-4-deoxy-beta-L-threo-pent-4-enopyranoside

Decarboxylative elimination of methyl 2,3-di-O-benzyl-α-D-glucopyranosiduronic acid (1) with N,N-dimethylformamide dineopentyl acetal in N,N-dimethylformamide gave methyl 2,3-di-O-benzyl-4-deoxy-β-L-threo-pent-4-enopyranoside (3). Debenzylation of 3 was effected with sodium in liquid ammonia to give methyl 4-deoxy-β-L-threo-pent-4-enopyranoside (4). Hydrogenation of 3 catalyzed by palladium-on-barium sulfate afforded methyl 2,3-di-O-benzyl-4-deoxy-β-L-threo-pentopyranoside (5), whereas hydrogenation of 3 over palladium-on-carbon gave methyl 4-deoxy-β-L-threo-pentopyranoside (6). An improved preparation of methyl 4,6-O-benzylidene-α-D-glucopyranoside is also described.