C-nucleosides. Part 9. Synthesis of carbazole and pyrrole C-nucleosides from 2-(2,3,5-tri-O-benzoyl-β-D-ribofuranosyl)furan
The synthesis of carbazole and pyrrole C-nucleosides from 2-(2,3,5-tri-O-benzoyl-β-D-ribofuranosyl)furan (1) is described. The key synthetic intermediate γ-keto aldehyde (7) can be obtained from the tetrahydrofuran (5), which was prepared by hydrogenation of the dihydrofuran (3) using 10% palladium–carbon. Treatment of compound (7) with indole in dichloromethane under acidic conditions afforded the protected carbazole (9). Deprotection of compound (9) with methanolic sodium hydroxide afforded compound (11) in 55% yield. Treatment of the furan (5) with ethyl carbamate in the presence of 50% trifluoroacetic acid in acetone afforded, in 26% yield, ethyl-2-(2,3,5-tri-O-benzoyl-β-D-ribofuranosyl)pyrrole-1-carboxylate (15) and its α isomer (16) in a 7 : 1 ratio. Deprotection of compounds (15) and (16) with methanolic sodium hydroxide afforded compounds (19) and (20).