C-nucleoside studies. Part 9. Synthesis of 3(5)-α-D-ribofuranosyl pyrazole and related compounds

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Abstract
3-(1,2:4,5-Di-O-isopropylidene-D-allo-1,2,3,4,5-pentahydroxypentyl)-1-(2,4-dinitrophenyl)pyrazole (8) was prepared in four steps (40% overall yield) from 2,3:5,6-di-O-isopropylidene-D-allose (4). Treatment of (8) with acetone and concentrated sulphuric acid formed a mixture of (8) and its 1,3:4,5-(12) and 2,3:4,5-di-O-isopropylidene (13) isomers. On treatment with methanesulphonyl chloride in pyridine, the isomer (13) formed an unstable methanesulphonate ester which spontaneously underwent cyclisation with loss of acetone to give 1-(2,4-dinitrophenyl)-3-(2,3-O-isopropylidene-α-D-ribofuranosyl)pyrazole (21). Reaction of 2,3,5-tri-O-benzyl-D-ribofuranosyl chloride with 3,3-diethoxyprop-1-ynylmagnesium bromide, and treatment of the major product with aqueous acid and hydrazine, yielded 3(5)-(2,3,5-tri-O-benzyl-α-D-ribofuranosyl)pyrazole (25), which could be converted in 3 steps (62% overall yield) into (21). Treatment of (25) with boron trichloride and subsequent methanolysis produced 3(5)-α-D-ribofuranosyl pyrazole (3) in 61% yield.