Discovery and development of pyriminobac-methyl: A novel herbicide

In 1989, Pyriminobac-methyl (Code KIH-6127:Fig. 1, 1) was found to beparticularly effective against barnyard grass (Echinochloa spp.) over a wide range or growth stages from pre-emergence through to the 4.0. leaf stage, with excellent crop safity in both direct-seeded and transplanted rice. It was registered in late 1996 in Japanese as an active ingredient of the one-shot mixture herbicide, Prosper sup (TM) (registered trade name or Kumiai Chemical) and was commercially launched in early 1997, followed soon after by the commercialization or mixtures such as Pat-ful, ASat-ful and Topgun which all contain this com-pound as an active ingredient. Sulfonylurea herbicides like bensulfuron-methyl and pyrazosulfuro-ethyl control sedge and broad-leaf weeds at very low application rates in transplanted rice. These herbicides, however, have only moderate levels of activity against barnyard grass, the most problematic weed in paddy rice, at practical application rates. Since ALS (plant enzyme: acetolactate synthase) inhib-ltory low-dose herbicides including sulfonylurea were not commercially available for the control of barnyard-grass when our studies were initiated, we focused on low-dose herbicides particularly those effective in controlling barnyardgrass in paddy fields. Studies of pyrimidinyl salicylates (Fig. 1, 2) bearing common functional groups, which act by inhibiting ALS, had already been made by the authors. Although most of the compounds had good herbicidal activity, only a few had a broad spectrum of activity and acceptable level of crop safety for commercial use. Therefore, a study of 6-substituted compounds (Fig. 1, 2) was initiated. For the preparation of a variety of 6-substitutions, certain key synthetic intermediates containing a highly reactive functional group linked to the 6-position of the salicylate moiety were required. Our previous studies of 2 had provided the following findings: substitution at the 6-position of the salicylate moiety was preferable for herbicidal and ALS inhibitory