Synthesis of purine and purine riboside derivatives

Abstract

$N^1$-Ethyl-$N^3$-methyl-isoguanosine, doridosine derivative was prepared. Reaction of 5-amino-(2``, 3``, 5``-tri-O-acetyl- $\beta$ -D-ribofuranosyl) -imidazole-4-carbonitrile with paraformaldehyde following by treatment of sodium borohydride gave 5-methylamino-1-$\beta$-D-ribofuranosyl-imidazole-4-carbonitrile. After protecting the hydroxyl groups in riboside moiety by acetic anhydride-pyridine the reaction of the acetylated compound with ethylisocyanate in dimethylformamide afforded the carbamoyl derivative. Cyclization and deacetylation in methanolic ammonium hydroxide solution at 3-4$^\circ$C yielded $N^1$-ethyl-$N^3$-methyl-isoguanosine, which is a fixed model compound in keto-enol and amino-imino tautomerism. $N^1$-Ethyl-$N^3$-methyl-isoguanosine has a fixed 2-keto and 6-imino form.