Department of Chemistry, Shri Durga Ji Post Graduate College, Chandeshwar, Azamgarh, U.P. (INDIA)

1,3,4-oxadiazolo [3,2-a]-s-triazine-5-7-diones (Va-l) and their thione (VIa-l) were synthesized from their corresponding N1-(5-aryl-1,3,4- oxadiazolo-2,-yl)-N3-(4-substituted phenyl) ureas (IVa-l) by cyclising with ethyl chloro formate and carbon disulphide respectively. Requisite N1-(5-aryl-1,3,4-oxadiozol-2yl)-N3-(4-subsituted phenyl) ureas were obtained from ethyl-N-(5-aryl-1,3,4-oxadiazole-2yl) carbamate (IIIa-d) by treating it with aromatic amine in ethanol solution. Ethyl-N-5-aryl-1,3,4- oxadizole-2yl) carbamate (IIIa-d) were prepared from starting material 2- amino-5-aryl-1,3,4-oxadiazoles as shown in scheme-I. Fungitoxicity of the synthesise compounds were evaluated for their fungitoxicity against Helminsthosporium oryzae and Phytophtora infestans. Structure, activity, relationship of the different substituents on phenyl ring withantifungal activity have been discussed.