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

Abstract

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-(β-D-arabinofuranosyl)pyrazole (16) in up to 58% yield. Treatment of dinitrophenyl derivative (16) with methanolic ammonia gave an 86% yield of 3(5)-β-D-arabinofuranosylpyrazole (2).