Synthetic C-nucleosides: 3-(alpha- and beta-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-1H-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.