Structure-Activity Relationships of Acetolactate Synthase Inhibition among New Benzenesulfonylureas in Rice (Oryza sativa) and Barnyardgrass (Echinochloa crus-galli var. oryzicola)

Structure-activity relationships of 15 new benzenesulfonylurea derivatives were investigated using acetolactate synthase (ALS) assays in vitro and in vivo in rice (Oryza sativa) and barnyardgrass (Echinochloa crus-galli var. oryzicola). In vitro ALS activities of both rice and barnyardgrass were significantly inhibited by the compounds with smaller numbers of carbon atoms and alpha-fluorine substituents in the R radical of the new benzenesulfonylurea structure. Among the stereoisomers, erythro forms showed 10 times higher ALS inhibition than threo forms. However, the in vitro ALS assay did not clearly account for the selectivity of the compounds tested in rice and barnyardgrass. The selectivity could, however, be explained by the results of the in vivo ALS assay using an inhibitor of ketol-acid reductoisomerase, 1,1-cyclopropanedicarboxylic acid. Among the analogues tested, the compound K-8 was the most potent ALS inhibitor in vitro, and it has a good selectivity in in vivo ALS assays in rice and barnyardgrass. These results should lead to an assay model for structure-activity relationships for the synthesis and development of new ALS-inhibiting herbicides.