Various 2',3'-dideoxy-L-cytidine,2',3'-dideoxy-L-uridine, and 3'-deoxy-L-thymidine analogues have been synthesized and evaluated in vitro as potential anti-HIV and anti-HBV agents. Coupling of 1-O-acetyl-5-O-(tert-butyldimethylsilyl)-2,3-dideoxy-L-ribofuranose (1) with silylated derivatives of 5-fluorocytosine, cytosine, 5-fluorouracil, uracil, and thymine in the presence of ethylaluminum dichloride gave the corresponding nucleosides 2, 3, 4, 5, 10, 11, 12, 16, 17, and 18 as a mixture of alpha- and beta-anomers, which were then deblocked to yield the corresponding 2',3'-dideoxy-L-5-fluorocytidine derivatives, 6 and 7, 2',3'-dideoxy-L-cytidine derivatives, 8 and 9, 2',3'-dideoxy-beta-L-fluorouridine (13), 2',3'-dideoxy-beta-L-uridine (14), and 3'-deoxy-L-thymidine derivatives, 15 and 19. Among these 2',3'-dideoxy-L-nucleoside analogues, 2',3'-dideoxy-beta-L-5-fluorocytidine (6, beta-L-FddC) was found to be the most active against HIV-1, which is approximately 3 and 4 times more active against HIV-1 in vitro than 2',3'-dideoxy-beta-D-cytidine (ddC) and 2',3'-dideoxy-beta-D-5-fluorocytidine (beta-D-FddC) with ED50 values of 0.5, 1.5, and 2 microM, respectively. The dose-limiting toxicity of ddC is severe neuropathy which may be caused by the inhibition of the synthesis of mitochondrial DNA. ddC has an IC50 value of 0.022 microM against host mitochondrial DNA synthesis. Conversely, the IC50 values for beta-L-FddC and beta-L-ddC are > 100 microM; therefore, neuropathy may not present itself to be a problem with beta-L-FddC and beta-L-ddC as chemotherapeutic agents. In addition, beta-L-FddC and 2',3'-dideoxy-beta-L-cytidine (8, beta-L-ddC) demonstrated equally potent activity against HBV in vitro by having the same ED50 value of 0.01 microM. Both beta-L-FddC and beta-L-ddC, which have an "unnatural" L-configuration in the sugar moiety, are approximately 1000 and 280 times more potent, respectively, against HBV than the D-configuration beta-D-FddC and ddC which have an ED50 values of 10 and 2.8 microM. In view of the potent antiviral activity of beta-L-FddC against both HIV-1 and HBV and potent antiviral activity of beta-L-ddC against HBV in vitro, their low cytotoxicity, and especially the negligible inhibitory effect on host mitochondrial DNA synthesis, beta-L-FddC and beta-L-ddC merit further development as potential anti-HIV and anti-HBV agents.