Pyrazolo[4,3-d]pyrimidine nucleosides. Synthesis and antiviral activity of 1-beta-D-ribofuranosyl-3-methyl-6-substituted-7H-pyrazolo[4,3-d]pyr imi din-7-ones.

N1 analogues of formycin B, with substituents at the 3 and 6 positions of the pyrazolo[4,3-d]pyrimidine moiety were synthesized by the direct SnCl4-catalyzed ribosylation method. The site of the glycosidic linkage and the anomeric configurations were established on the basis of X-ray crystallography, as well as 1H and 13C nuclear magnetic resonance spectroscopy. Preliminary results of the antiviral testing of these derivatives in vitro are described.