Synthesis of a novel pyrazolo[1,5-c]pyrimidine C-nucleoside and conformational analysis by NMR spectroscopy

Isopropylidenation of [4-methoxycarbonyl-5-(beta-D-ribofuranosyl)-1H-pyrazo l-3 -yl]acetamide (1a) followed by the acidic cleavage of the sugar acetoni de afforded 3-methoxycarbonyl-7, 7-dimethyl-2-(beta-D-ribofuranosyl)-4H,7H- pyrazolo[1,5-c] pyrimidine-5(6H)-one (2b), the structure of which was estab lished unequivocally by Xray structure analysis of the monocrystals. Compou nds 1a and 2b have 75% and 70% preference for the N-type puckering between C3'-endo and C3'-endo-C4'-exo forms, and a great preference of 69% and 74% for gamma(+) rotamers in solution, respectively.