Synthesis of 3-β-d-ribofuranosylpyrazole-1-carboxamide

The synthesis of 3-β-d-ribofuranosylpyrazole-1-carboxamide (12) is described. Treatment of glycosyl enaminone 4 with semicarbazide hydrochloride in dioxane afforded three cyclocondensation products, 5-(2,3,5-tri-O-benzoyl-β-d-ribofuranosyl)pyrazole-1-carboxamide (5), 3-(2,3,5-tri-O-benzoyl-β-d-ribofuranosyl)pyrazole-1-carboxamide (6), and 3(5)-(2,3,5-tri-O-benzoyl-β-d-ribofuranosyl)pyrazole (7), in 51, 5, and 16% yields, respectively. Compound 5 was gradually converted to 7 at room temperature as a result of the elimination of the carbamoyl group. Treatment of the 4 with hydrochloric acid in methanol at room temperature afforded 3,3-dimethoxy-1-(2,3,5-tri-O-benzoyl-β-d-ribofuranosyl)propane-1-one (9) in 90% yield. Treatment of 9 with semicarbazide afforded the corresponding semicarbazone 10, which was cyclized in trifluoroacetic acid to afford 6. Deblocking of 6 with 10% aq ammonia gave 12.