C-nucleoside studies. Part 6. Synthesis of 3-[2,3,5-tri-O-benzyl-β-(and α)-D-ribofuranosyl]prop-2-yn-1-ol and related compounds; a new synthesis of 3(5)-(2,3,5-tri-O-benzyl-β-D-ribofuranosyl)pyrazole

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Abstract
Treatment of 2,3,5-ri-O-benzyl-β-D-ribofuranosylethyne (2) with a large excess of paraformaldehyde and potassium hydroxide in ethanol gave 3-(2,3,5-tri-O-benzyl-β-D-ribofuranosyl)prop-2-yn-1-ol (5) in 70% yield. Oxidation of (5) with chromic oxide (Jones reagent) afforded the carboxylic acid (4)(75%), esterification of which with diazomethane gave the known ester (7). Similar reactions and correlations have been carried out in the α-series. Reaction of the Grignard reagent (14) of 3-(tetrahydropyran-2-yloxy)propyne with 2,3,5-tri-O-benzyl-D-ribofuranose (19) followed by ring closure and removal of the tetrahydropyranyl ether group gave the alcohol (5) in 52% overall yield. Careful oxidation of (5) gave the corresponding aldehyde (3) which yielded the known pyrazole (1)(72%) on treatment with hydrazine. When 1,2-dideoxy-4,5:7,8-di-O-isopropylidene-D-manno-oct-1-yn-3-ulofuranose (22) was treated with hydrazine, 3(5)-(1,2:4,5-di-O-isopropylidene-D-manno-pentahydroxypentyl)pyrazole (23) was isolated in 93% yield.

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