Biological Activities of 1,3,4-Thiadiazolo[3,2-a]pyrimidines and s-Triazolo[1,5-a]pyrimidines

In 1,3,4-thiadiazolo[3,2-alpyrimidine series, 5-methyl-7H-1,3,4-thiadiazolo[3,2-a] pyrimidin-7-one exhibited the larvicidal activity to housefly larvae, while the isomer, 7- methyl-5H-1,3,4-thiadiazolo [3,2-a] pyrimidin-5-one was very weak. 4-Methyl-2-thiocyanato-6(1H)pyrimidine, which is a common product of the two isomers by mild hydrolysis, was inactive on the development of the larvae. s-Triazolo[1,5-a]pyrimidinealso disrupted morphogenesis to induce larva-pupa intermediates which were incapable of survival. The protective activity of 7-methyl-6-nitro-5H-1,3,4-thiadiazolo[ 3,2-a] pyrimidin-5-one against bacterial leaf blight disease of rice plant was greater than that of nickel dimethyldithiocarbamate. 5,7-Dimethyl-1,3,4-thiadiazolo[3,2-a]pyrimidin-4-ium perchlorate showed a good protectivity to Helminthosporium leaf spot and rice blast. The selective herbicidal activity for barnyard grass was found in 2,7-dimethyl-5H-1,3,4-thiadiazolo[3,2-alpyrimidin-5-one and 2-benzylthio-7-methyl-5H-l,3,4-thiadiazolo [3,2-alpyrimidin-5-one.