2-Chloro-5,6-difluorobenzimidazole (8) was prepared from 4,5-difluoro-2-nitroaniline (5) via successive reduction, cyclization, and diazotization reactions. 2-Chloro-5,6-dibromobenzimidazole (10) was obtained by a direct bromination of 2-chlorobenzimidazole (9) with bromine-water. 2-Chloro-5,6-diiodobenzimidazole (15) was synthesized by a stepwise transformation of the nitro functions of 2-chloro-5,6-dinitrobenzimidazole (11) into iodo groups via diazotization reactions. Ribosylation of 8, 10, and 15 gave the respective beta nucleosides 16a-c as the major products along with a small amount of the alpha anomers 17a-c. Deprotection of 16a-c afforded the corresponding free beta nucleosides 2-chloro-5,6-difluoro-1-beta-D-ribofuranosylbenzimidazole (2), 2-chloro-5,6-dibromo-1-beta-D-ribofuranosylbenzimidazole (3), and 2-chloro-5,6-diiodo-1-beta-D-ribofuranosylbenzimidazole (4). Similar deprotection of the alpha anomers (17a-c) resulted in a removal of the acetyl protecting groups and a concomitant cyclization to give the 2,2'-O-cyclonucleosides (18a-c). Most of the benzimidazole heterocycles, but not the difluoro analog, were active against human cytomegalovirus (HCMV) (IC50's = 3-40 microM) and herpes simplex virus type 1 (HSV-1) (IC50's = 50-90 microM). This activity, however, was not well separated from cytotoxicity, IC50's = 10-100 microM. The corresponding unsubstituted, the 5,6-dimethyl, and the 5,6-difluoro ribonucleosides (19, 20, and 2, respectively), were inactive against both viruses. Similar to the previously reported 2,5,6-trichloro analog (TCRB), the 5,6-dibromo ribonucleoside 3 was active against HCMV (IC50 approximately 4 microM) but more cytotoxic than TCRB. The 5,6-diiodo analog 4 also was active (IC50 approximately 2 microM) but more cytotoxic (IC50 = 10-20 microM) than either 3 or TCRB. The cyclonucleosides were inactive against both viruses and not cytotoxic, or slightly active with corresponding cytotoxicity. The order of activity against HCMV of the dihalobenzimidazole ribonucleosides was I approximately equal to Br approximately equal to CI > > F > H = CH3. The order of cytotoxicity among the most active compounds, however, was I > Br > Cl, thereby establishing that TCRB had the best antiviral properties.