C-Nucleoside studies. Part 19. The synthesis of the β-D-xylofuranosyl analogue of formycin

1-(2,4-Dinitrophenyl)-3-(2,3,5-tri-O-acetyl-ß-D-xylofuranosyl)pyrazole (15), prepared by improved procedures from D-gulonolactone, was treated with ammonium nitrate and trifluoroacetic anhydride in trifluoroacetic acid to give, after deprotection of the pyrazole ring, 4-nitro-3(5)-(2,3,5-tri-O-acetyl- ß-D-xylofuranosyl)pyrazole (17). N-Nitration and cine-substitution with cyanide ion gave 3(5)-cyano-4-nitro-5(3)-(2,3,5-tri-O-acetyl-ß-D- xylofuranosyl)pyrazole (19) in 66% overall yield; this was elaborated into 7-amino-3-(ß-D-xylofuranosyl)pyrazolo[4,3-d]pyrimidine (3), the xylofuranosyl analogue of the C-nucleoside antibiotic formycin.