A method for the synthesis of novel 5-methylsulfanylazolo[1,5-a]pyrimidin-7(4H)-ones by heterocyclization of 3-aminoazoles and 5-[bis(methylsulfanyl)methylidene]-2,2-dimethyl-1,3-dioxane-4,6-diones was developed. 5-Methylsulfanyl-7-oxo-4,7-dihydroazolo- [1,5-a]pyrimidine-6-carboxylic acid was isolated during optimization of the process, which allowed us to establish the sequence of transformations of this heterocyclization reaction. The reactivity of the resulting 5-methylsulfanylazolo[1,5-a]pyrimidin-7(4H)-ones in classical electrophilic substitution reactions was studied. The cytotoxic effect of these compounds toward A549, HepG2, and RD tumor cell lines as well as normal HEK-293 cells was assessed.