The Synthesis of Novel Regioisomeric Ring-Expanded Xanthine Nucleosides Containing The 5:7-Fused Imidazo[4,5-e][1,2,4]Triazepine Ring System

The syntheses of novel regioisomeric ring-expanded purine nucleosides containing the imidazo[4,5-e][1,2,4]triazepine nucleus are reported. The glycosylation of the heterocycle 3,4,6,7-tetrahydroimidazo[4,5-e][1,2,4]triazepine-5,8-dione (2a) by the stannic chloride procedure gave nucleosides 3 and 4, with the sugar moiety attached at the 7-and 3-positions of the heterocycle, respectively. On the other hand, the mercuric cyanide procedure for glycosylation of 2a yielded nucleosides 4 and 5, with the sugar attached at the 1-position in the latter. In either procedure, 4 was the minor isomer and was obtained only in trace amounts. While debenzoylation of 3 and 5 provided the respective parent nucleosides 8 and 10, that of 4 resulted in ring-opening to produce 9. Attempted enzymic glycosylation of 2a with purine nucleoside phosphorylase failed to yield any nucleoside product.