Synthesis and fungicidal activity evaluation of 5-benzylidene-5H-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazines

A series of 5-benzylidene-5H-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazines were synthesized by the reaction of benzaldehyde 1,2,3-thiadiazol-5-ylhydrazones with a-bromo-4-chloroacetophenone and chloroacetone in the presence of triethylamine. The key steps of the reaction are the Dimroth rearrangement of the 1,2,3-thiadiazole ring, the formation of 6,7-dihydro-5H-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazines, and the transformation of the thiadiazine ring giving 5-arylidene-5H-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazines. It was demonstrated that 6,7-dihydro-5H-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazines, which do not undergo the transformation in the presence of triethylamine, can be transformed into 5-arylidene-5H-[1,2,3]triazolo[5,1-b][1,3,4]thiadiazines using sodium hydroxide. The reaction is accompanied by the saponification of the ethoxycarbonyl substituent. A number of the synthesized compounds were tested for fungicidal activity against four strains of phytopathogenic fungi and the oomycete P. infestans. Compound 5n exhibits a moderate inhibitory effect (56.31±0.13%) on the growth of P. infestans.