A novel and efficient one-pot method has been developed for the synthesis of 2-substituted-5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one derivative by the combination of [3 + 3] cycloaddition, reduction, deamination reactions. The fused heterocyclic compounds 2-substituted-5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-5-ones was synthesized by the diversity-oriented catalysis. As an extension of the synthetic methodology, some 4H-pyrido[1,2-a]pyrimidin-4-one 4a–c was synthesized. The π–π accumulation structure of supramolecular self-assembly was discussed in the crystal.