C-nucleoside studies. Part 23. New and more direct synthesis of 3-(β-D-xylofuranosyl)pyrazole

Reaction of 2,3,4-tri-O-benzyl-D-xylopyranose 5 with ethynylmagnesium bromide gave the D-idodiol 7 with high stereoselectivity. Diol 7 was converted into its 7-O-pivaloyl derivative 9, which, on treatment with toluene-p-sulfonyl chloride in pyridine, underwent cyclization with benzyloxy participation to yield 2,3-di-O-benzyl-5-O-pivaloyl-ß-D-xylofuranosylethyne 10 in 75% yield. A similar reaction sequence involving substrate 5 and the Grignard derivative of 1,1-diethoxyprop-2-yne led to 1-(2,3-di-O-benzyl-5-O-pivaloyl-ß-D- xylofuranosyl)-3,3-diethoxyprop-1-yne 14, which, on treatment with acetic acid-dil. hydrochloric acid, followed by hydrazine, gave 3(5)-(2,3-di-O-benzyl- 5-O-pivaloyl-ß-D-xylofuranosyl)pyrazole 15. Base treatment of compound 15, followed by transfer hydrogenation, produced 3(5)-(ß-D-xylofuranosyl) pyrazole 4.