3-d]pyrimidine-4-ones. Alkylation of ambident 2-oxo(thioxo)-thieno[2,3-d]pyrimidine-4-one anion with substituted allyl (methallyl/cinnamyl) chlorides selectively leads to the formation of N- or S-substituted pyrimidines which is totally confirmed via NMR and XRD studies. Partial atomic charges are considered for prediction of a most reactive atom of the ambident anionic pyrimidine moiety in the three