The synthesis of pyrimidine unsaturated keto and exomethylene arabinopyranonucleoside analogs as potential antitumor and antiviral agents is described. Commercially available 1,2,3,4-tetra-O-acetyl-D-arabinopyranose (1) was condensed with silylated thymine, uracil, 5-fluorouracil, N(4)-benzoyl cytosine and 5-(trifluoromethyl)uracil, respectively, deacetylated and acetylated to afford 1-(3,4-O-isopropylidene-α-D-arabinopyranosyl)pyrimidine analogs 4. Two different synthetic routes were investigated for the conversion of compounds 4 into the new 1-(2,3,4-trideoxy-2-methylene-α-pent-3-enopyranosyl)nucleoside derivatives of thymine (10a), uracil (10b), 5-fluorouracil (10c) and N(4)-benzoyl cytosine (10d). Only the first approach could afford derivative 10d. Debenzoylation of 10d afforded 1-(2,3,4-trideoxy-2-methylene-α-pent-3-enopyranosyl)cytosine (10f). The first approach resulted also to the 2-keto-3,4-unsaturated analogs 9. The new analogs did not show inhibition of DNA and RNA virus replication in cell culture. The 2'-ketonucleoside derivatives 9 were found to be more cytostatic than the corresponding 2'-exomethylene nucleosides 10. The 5-fluorouracil unsaturated keto derivative 9c and the exomethylene derivatives 10c and 13c showed antiproliferative activity in the lower micromolar range. Experimental evidence revealed that 9c, 10c and 13c may act as novel types of 5-fluorouracil releasing prodrugs, and points to thymidylate synthase as target for their cytostatic action.