Novel synthesis of 1,2,4-triazolo[4,3-a]pyrimidin-5-one derivatives

1,2-Dihydro-2-thioxopyrimidin-4(3H)ones 6 react with C-ethoxycarbonyl-N-arylhydrazonoyl chlorides 1 to give 1,2,4-triazolo[4,3-a]pyrimidines 11 in good yield. The latter products 11 react with benzenediazonium chloride, nitrous acid, acetyl chloride and chloroacetyl chloride to give the corresponding substitution products 16–18. The structures of the newly synthesized compounds are established on the basis of chemical and spectroscopic evidence.