4‘-C-Ethynyl-β-d-arabino- and 4‘-C-ethynyl-2‘-deoxy-β-d-ribo-pentofuranosylpyrimidine and -purine nucleosides were synthesized and evaluated for their in vitro anti-HIV activity. The key intermediate, 4-C-ethynyl- or 4-C-triethylsilylethynyl-d-ribo-pentofuranose, was prepared from d-glucose and glycosidated with various pyrimidine or purine bases. The arabino pyrimidine derivatives were prepared from the corresponding ribo derivatives via O2,2‘-anhydro nucleosides. The 2‘-deoxy-ribo derivatives were synthesized by radical reduction of 2‘-bromo or 2‘- phenoxythiocarbonyloxy nucleosides. Among these 4‘-C-ethynyl nucleosides, seven analogues proved to be potent against HIV-1 in vitro with EC50 values ranging from 0.0003 to 0.03 μM. These compounds also exerted activity against clinical and multi-dideoxy-nucleoside-resistant HIV-1 strains with comparable EC50 values. Three such 4‘-C-ethynyl-2‘-deoxypurine analogues including 4‘-C-ethynyl-2‘-deoxyadenosine and 4‘-C-ethynyl-2,6-diamino-2‘-deoxypurine were less ...