Intramolecular Cyclization of Mannich Reaction for Synthesis of Pyrimido[2,1-b]-1,3,5-tiadiazines

Reaction of 6-aryl-5-cyano-2-mercapto-3,4-dihydropyrimidin-4-ones with formaldehyde and primary amines in suitable solvent via a double Mannich reaction gave the corresponding 8-aryl-7-cyano-3-N-substituted-pyrimido[2,1-b]-1,3,5-thiadiazin-6-ones rather than the isomeric products 6-aryl-7-cyano-3-N-substituted-pyrimido[2,1-b]-1,3,5-thiadiazin-8-ones. The cyclization method was found to be the most favored for the formation of the linear products rather than the angular isomers. This was confirmed not only by using spectral analysis and molecular mechanical calculations but also by X-ray single crystal structure determination.