We investigated the potential of 4'-C-substituted nucleosides for the treatment of HIV-1 and HBV. Of the nucleosides we prepared, several 4'-C-ethynyl-2'-deoxypurine nucleosides showed the most potent anti-HIV activity. However, two candidates, 4'-C-ethynyl-2'-deoxyguanosine and 9-(2-deoxy-4-C-ethynyl-beta-D-ribo-pentofuranosyl)-2,6-diaminopurine, were very toxic during in vivo study. On the other hand, lamivudine (3TC) is known to show remarkable activity against HIV and HBV with lower cytotoxicity. Therefore, we attempted to synthesize the L-enantiomer of 4'-C-ethynyl-2'-deoxypurine nucleosides in 20-21 steps. These methods consisted of preparing 4-C-ethynyl-L-sugar, starting from D-arabinose and then condensing the L-sugar derivative with 2,6-diaminopurine. 4'-C-Ethynyl-2'-deoxyguanosine was also prepared by enzymatic deamination from the 2,6-diaminopurine derivative. The compounds' antiviral activity against HIV and HBV was then evaluated. Unfortunately, they demonstrated no activity and no cytotoxicity.