Starting with 5-iodo N,O-nucleosides, prepared by 1,3-dipolar cycloaddition of nitrones with vinyliodouracil, or vinyl acetate followed by coupling with silylated iodouracil, a series of 5-alkynyl N,O-nucleosides were synthesized via a palladium-catalyzed (Sonogashira) coupling reaction. The cytotoxic activity of modified nucleosides against HEp-2 cell lines was determined in vitro. The 5-ethynyl N,O-nucleoside, the only compound in the series containing a terminal acetylene unit, was found to be the most active compound.