A series of 3,9-dihydro-9-oxo-5H-imidazo[1,2-A]purine nucleosides (tricylic nucleosides) were synthesized from 9-[4-alpha-(hydroxymethyl)cyclopent-2-ene-1-alpha-yl]guanine (CBV) 5, (-)-beta-D-(2R,4R)-1,3-dioxolane-guanosine (DXG) 6, 3'-azido-3'-deoxy-guanosine (AZG) 7, and 2'-C-methylguanosine 8. Their in vitro activity against HIV and HCV was evaluated and correlated to their ability to degrade to their purine counterpart.