The Synthesis and Biological Activity of 1-Alkyl-4-(3-azacyclobenzoyl)-5-hydroxypyrazole Herbicides

The benzoylpyrazoles belong to a class of herbicides that inhibit the enzyme 4-hydroxy-phenylpyruvate dioxygenase (HPPD). This mode of action is characterized by bleaching due to the disruption of plastoquinone and ?-tocopherol biosynthesis. Early studies indicated that the C(3) position of benzoylpyrazoles can accommodate a wide range of functionality. This paper describes synthetic efforts to improve cool season grass weed activity and wheat selectivity by incorporating cyclic moieties attached through nitrogen at this position. The aza substituents were generally installed by nucleophilic aromatic substitution, however, an efficient four-step method was developed for constructing substituted morpholino moieties directly on pre-formed benzoylpyrazoles. The structure–activity relationships revealed that certain piperidino moieties provided good activity on wild oat, while exhibiting selectivity toward wheat. They also showed that excellent levels of activity on wild oat and blackgrass can be achieved with morpholino substituents.