A series of 6-substituted 2',3'-dideoxypurine ribofuranosides (ddP) was enzymatically synthesized with live E. coli in an effort to enhance the lipophilicity of this class of anti-human immunodeficiency virus (HIV) compounds and thereby facilitate drug delivery into the central nervous system. All 6-halo-substituted ddPs were substantially more lipophilic, as defined by their octanol-water partition coefficient (P), than their nonhalogenated congeners 2',3'-dideoxyinosine (ddI) or 2',3'-dideoxyguanosine (ddG). For this class of compounds, log P's ranged from +0.5 to -1.2 in the following order: 6-iodo, 2-amino-6-iodo greater than 6-bromo, 2-amino-6-bromo greater than 6-chloro, 2-amino-6-chloro greater than 6-fluoro, 2-amino-6-fluoro much greater than ddG greater than ddI. These compounds were evaluated in vitro for ability to suppress the infectivity, replication, and cytopathic effect of HIV. 2-Amino-6-fluoro-, 2-amino-6-chloro-, and 6-fluoro-ddP exhibited a potent activity against HIV comparable to that of ddI or ddG and completely blocked the infectivity of HIV without affecting the growth of target cells. The comparative order of in vitro anti-HIV activity was 2-amino-6-fluoro, 2-amino-6-chloro, 6-fluoro greater than 2-amino-6-bromo greater than 2-amino-6-iodo, 6-chloro greater than 6-bromo greater than 6-iodo. These compounds also exhibited potent in vitro activity against HIV-2 and 3'-azido-3'-deoxythymidine-resistant HIV-1 variants. All 2-amino-6-halo-ddPs and 6-halo-ddPs were substrates for adenosine deaminase (ADA) and were converted to ddG or ddI, respectively. In the presence of the potent ADA inhibitor 2'-deoxycoformycin, 6-halo-substituted ddPs failed to exert an in vitro antiretroviral effect. These dideoxypurine nucleoside analogues represent a new class of lipophilic prodrugs of ddG and ddI that possess the potential for more effective therapy of HIV-induced neurologic disorders.