Heterocyclization of 2,4-disubstituted thiosemicarbazides with haloketones

2,4-Disubstituted thiosemicarbazides were allowed to react with α-bromoacetophenone to give the anticipated 2,3-dihydro-6H-1,3,4-thiadiazines, whereas with β-propiophenone and γ-butyrophenone to afford the unexpected 2,3-dihydro-1,3,4-thiadiazole. The reaction mechanism was also discussed.