New acyclic pyrimidine nucleosides, analogs of the antiviral drug acyclovir, were synthesized starting from 6-chloropyrimidine-2,4(1H,3H)-dione. The addition of side carbohydrate chain to the N1 or N3 atoms of the heterocyclic base with formation of N1- and N3-substituted isomers, respectively, was confirmed by 1H NMR and X-ray analysis. N1- and N3-Substituted 6-chloropyrimidine-2,4(1H,3H)-diones were subjected to amination with secondary amines, and cyclocondensation of 6-allylamino derivative afforded N1-substituted pyrido[2,3-d]pyrimidine.