A series of tricyclic analogues of acyclovir and 9-{[cis-1',2'-bis(hydroxymethyl)cycloprop-1'-yl]methyl}guanine substituted in the 6 position with thien-2-yl, 5-bromothien-2-yl or furan-2-yl group were synthesized. The new compounds 5a-f were evaluated for their activity in vitro against varicella-zoster virus (VZV) and cytomegalovirus (CMV). The marked anti-VZV activities of 5a-f remained comparable to those of their previously described 6-phenyl-substituted counterparts.