Derivatives of 2-methylidene-1,3-dihydropyrimidin-4-ones 2a, 2b, 2c, 2d, 2e, 2f, 2g were synthesized by interaction of 6-methyl-2-thiouracil and 6-phenyl-2-thiouracil 1a, 1b with some activated halogenides: diethyl bromomalonate, ethyl 2-chloro-3-oxobutanoate, ethyl 2-bromocyanoacetate, 2-bromo-5,5-dimethylcyclohexan-1,3-dione, and bromomalononitrile. The boiling of 1a with ethyl 2-bromocyanoacetate in mixture of ethanol and EtONa results in intramolecular cyclization and formation of thiazolo[3,2-a]pyrimidin-5-one 3. Interaction of 1a with 3-chloropentane-2,4-dione and 2-bromo-1,3-diphenylpropane-1,3-dione yielded corresponding S-substituted thiopyrimidines 4a,4b. In general, the products of 1b S-alkylation are less prone to sulfur extrusion. Reaction of 1b with diethyl bromomalonate in the absence of EtONa stops at the S-alkylation step, while in the presence of EtONa in ethanol or PPh₃in dioxane 2-(ethoxycarbonylmethyl)thio-6-phenyl-1,3-dihydropyrimidin-4(1H)-one 6 is formed exclusively. Molecular structure and crystal structure of 2-(1,1-diethoxycarbonylmethyliden)-6-methyl-1,3-dihydropyrimidin-4(1H)-one 2a are discussed.