Enantioselective relieving activity of α-methylbenzylphenylureas toward bensulfuron-methyl injury to rice (Oryza sativa)

Optically active alpha-methylbenzylphenylureas were tested for their relieving activities toward just-germinated rice seedlings injured by bensulfuron-methyl (methyl 2-[[[[[(4,6-dimethoxy-2-pyrimidinyl)amino sulfonyl]methyl]benzoate) in an agar test to evaluate the chiral requirement and enantioselectivity. Many kinds of derivatives of the alpha-methylbenzylphenylureas exhibited strong relieving activity without any affect on root growth at the highest concentration tested. Six compounds with an (S)-configuration were more active than daimuron. The log k' values of the most potent derivatives ranged from 0.42 to 0.65. A relatively strong parabolic relationship between the log k' value and the activity has only been found in the case of the (S)-enantiomers containing halogen atoms. The enantioselectivity of the chiral pairs was very high, and the chirality-activity function followed a Pfeiffer relationship, increasing the selectivity with increasing potency. Among them, the 2,3-Cl-2, 2-F-4-CH3, 4-COOEt, 2-Cl and 2,5-F-2 derivatives were highly enantioselective with a significantly high relieving activity, These results suggest that while hydrophobicity performed an important role, chirality and the mode of substitution essentially contributed to the activity.