C-Nucleoside studies. Part 8. Synthesis of 3-β-D- arabinofuranosylpyrazole from D-mannose

Treatment of 3(5)-(1,2:4,5-di-O-isopropylidene-D-manno-pentahydroxypentyl) pyrazole (6) with acetone and concentrated sulphuric acid caused isomerisation to 3(5)-(2,3:4,5-di-O-isopropylidene-D-manno-pentahydroxypentyl)pyrazole (10). Reaction with 1-fluoro-2,4-dinitrobenzene and triethylamine, and subsequent treatment with methanesulphonyl chloride afforded 1-(2,4-dinitrophenyl)-3-(1-O- methylsulphonyl-2,3:4,5-di-O-isopropylidene-D-manno-pentahydroxypentyl)pyrazole (14) in 71% overall yield. On treatment with dilute hydrochloric acid in dioxan, or, preferably, with boron trichloride in dichloromethane followed by methanolysis, (14) afforded 1-(2,4-dinitrophenyl)-3-(ß-D-arabinofuranosyl) pyrazole (16) in up to 58% yield. Treatment of dinitrophenyl derivative (16) with methanolic ammonia gave an 86% yield of 3(5)-ß-D- arabinofuranosylpyrazole (2).