Isothiocyanates (2), obtained fromaza-Wittig reaction of vinyliminophosphoranes (1) with CS₂, reacted with phenylhydrazine to give thiosemicarbazides (3).2-Alkylthio-4-arylmethylene-5-hydro-1-phenylamino-1H-imidazol-5-ones (4) were directly prepared by reaction of 3 with alkyl halide in the presence of K₂CO₃. The reaction conditions for cyclization and biological activities of the new heterocycles synthesized were investigated. The results showed that some of the compounds exhibited good fungicidal activities. For example, 4e exhibited 87% inhibition of Botrytis Cinerea Pers. in 50 mg/L.

Key words: 1H-imidazol-5-one, aza-Wittig reaction, isothiocyanate, synthesis, fungicidal activity