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-l-alpha-yl]guanine (CBV) 5, (-)-beta-D-(2R,4R)-1,3-dioxo-lane-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. (C) 2009 Elsevier Masson SAS. All rights reserved.