Synthetic C-nucleosides: 3-(α- and β-D-arabinofuranosyl)pyrazolo[4,3-d]pyrimidine-5,7-diones

2,3,5-Tri- O -benzyl- D -arabinofuranosyl bromide ( 4 ) was converted into 2,5-anhydro-3,4,6-tri- O -benzyl- D -glucononitrile ( 5 ), mixed with 20% of the D - manno epimer 6 . The mixture was reduced to the amine 7 , which via the N -nitrosoacetamide 10 afforded the 1-deoxy-l-diazo sugar 11 . Dipolar addition to dimethyl acetylene-dicarboxylate afforded the C -nucleoside derivative, dimethyl 3-(2,3,5-tri- O -benzyl-α-β- D -arabinofuranosyl)pyrazole-4,5-dicarboxylate ( 20 ). Selective ammonolysis afforded the 4-ester-5-carboxamide 21 , which was separated chromatographically into the α-(minor) and β-(major) anomers. Hydrazinolysis and Curtius reaction of the pair of 4-acid hydrazides (α- 22 and β- 22 ) afforded the anomeric 3-glycosyl-1 H -pyrazolo-[4,3- d ]pyrimidine-5,7-diones (α- 24 and β- 24 ). Hydrogenolytic debenzylation yielded the β- D )- arabino epimer ( 1 ) of oxoformycin B , and the α- D - arabino form 2 . These anomeric C -nucleosides were distinguished by circular dichroism spectra that showed the same relationship as α- and β- D - arabino anomers of normal purine nucleosides.