Chlorination of an S-methyl-5-cyano-6-arylthiouracil derivative afforded a chloropyrimidine which underwent aminolysis followed by cyclization to give pyrimidothiazolpyrimidine and imidazopyrimidines. The same thiouracil derivative was also used to prepare oxadiazolopyrimidines, triazolopyrimidines, and pyrimidotriazepines via a diaminopyrimidine and its cyclization with a variety of reagents. Some of the newly synthesized compounds showed a high antibacterial activity against Gram (+ve) and Gram (–ve) bacteria.