In Vivo Antiviral Activity of 5-Amino-1-Methyl-3-β-D-Ribofuranosyl-Pyrazolo[4,3-d]Pyrimidin-7(6H)-One and Related Guanosine Analogues Prepared from Formycin

Abstract

A short and simple synthesis of 5-amino-3-β-D-ribofuranosylpyrazolo4,3-d]pyrimidin-7(6H)-one, (7) was achieved from 7-amino-5-chloro-3-β-D-ribofuranosylpyrazolo4,3-d]pyrimidine (5), in two steps, first deamination of 5 with NOCI, followed by amination of 6 with MeOH/NH₃. Also, an efficient synthesis of 5-amino-1(or 2)-methyl-3-β-D-ribofuranosylpyrazolo4,3-d]pyrimidin-7(6H)-one was accomplished from the corresponding 1 (or 2)-methyloxoformycin B in four steps by a sequence consisting of (i) 2′,3′,5′ acetylation with AC₂O, (ii) 5,7-chlorination with PhP(O)Cl₂, (iii) selective hydrolysis of the 7-chloro group with aqueous Na₂CO₃, (iv) followed by amination of the 5-chloro group with MeOH/NH₃. Single crystal X-ray analysis off 11 confirmed position 7 as the site of selective hydrolysis with Na₂CO₃. The three guanosine C-nucleosides prepared were evaluated for their ability to inhibit certain RNA and DNA viral replication in vitro and Semliki Forest virus infection in vivo. Only 5-amino-1-methyl-3-β-D-ribofuranosylpyrazolo4,3-d]pyrimidin-7(6H)-one (13) provided protection (67% survivors, compared to 0% for placebo controls) against a lethal dose of Semliki Forest virus infection in mice. The antiviral effect of 13 is believed to be due to the enhancement of the host immune function.