Development of a New Herbicide, Pyraflufen-ethyl

「INTRODUCTION」 A novel 3-phenylpyrazole herbicide, pyraflufen-ethyl, is a Protox inhibiting cereal herbicide that controls a wide range of annual broad-leaved weeds including Galium aparine at a rate of around 10 g a.i. /ha in post-emergence application. It was discovered and has been developed by Nihon Nohyaku Co., Ltd. It was registered in Japan in 1999 under the trade name of ECOPART(R) for use on cereals and also approved in combination with glyphosate-trimesium salt as a non-selective herbicide named THUNDERBOLT(R). This paper describes a short history of discovery, the synthesis, the structure-activity relationship, herbicidal characteristics and a safety evaluation of pyraflufen-ethyl. DISCOVERY AND SYNTHESIS Through a series of research projects on biologically active compounds, the lead compound, a 3-phenylpyrazole derivative, was synthesized by the utilization of ketene dithioacetal chemistry extended by Nihon Nohyaku. Many 3-phenylpyrazole derivatives with various substituents on the pyrazole ring and the benzene ring were synthesized, and their herbicidal activities and crop selectivities were evaluated. As a result of precise investigations, pyraflufenethyl was selected as the most potent candidate for controlling various broad-leaved weeds including Galium aparine in cereals. The synthetic routes of pyraflufen-ethyl are illustrated in scheme 3 and scheme 4. The key intermediats were (13) and (19) in a Heck and a Friedel-Crafts reaction route, respectively.