C-Nucleoside studies. Part 7. A new synthesis of showdomycin, 2-β-D-ribofuranosylmaleimide

Showdomycin, 2-ß-D-ribofuranosylmaleimide (1), has been prepared in six stages from 2,3,5-tri-O-benzyl-ß-D-ribofuranosylethyne (2) in 23% overall yield (8% from D-ribose). Dimethoxycarbonylation of (2) afforded the substituted maleic ester (4) which was converted, in two stages, into the crystalline anhydride (10). Ammonolysis followed by ring-closure gave the crystalline maleimide (13), which was debenzylated with boron trichloride to give showdomycin.