Enantioselective Response of Rice and Barnyard Millet on Root Growth Inhibition by Optically Active α-Methylbenzyl Phenylureas

Optical active α-methylbenzyl p -tolylureas ( 7, 8 ) show diverse plant physiological properties. Experiments were conducted to evaluate the response of the optically active α-methylbenzyl phenylureas on root growth inhibition against rice and barnyard millet and to assess differential chiral responses of both plants from the viewpoints of activity, enantioselectivity, and cross intergenus response. A preference configuration to inhibit the root growth was not necessarily the same, and the toxicity and enantioselectivity were dependent on the substituent at the phenyl ring. To rice, the (R) -4-Pr ( 15 ) and (R) -4-Bu ( 23 ) derivatives exhibited the most potent activity with an I 50 value below 2 μM. To barnyard millet, similar to rice, the (R) -enantiomers with a lower alkyl group (2- tert Bu ( 25 ), 4-Pr ( 15 ), 2-isoPr ( 17 ), 3-Et ( 11 ), 4-Bu ( 23 ), 3-isoPr ( 19 ) and 4-Et ( 13 )) exhibited significant activity. The (R) -2-F-5-CF 3 ( 119 ) derivative was the most potent inhibitor, showing an I 50 value of 0.58 μM. The (S) -2-F ( 60 ), (S) -2,3-F 2 ( 90 ), (S) -2,5-F 2 ( 94 ) and (S) -4-COOEt ( 52 ) derivatives also inhibited the root growth of barnyard millet. Enantioselectivity and selective toxicity were quite high. Only chiral pairs of the 3-Me ( 5, 6 ) and 4-Me ( 7, 8 ) derivatives exhibited a cross intergenus response between rice and barnyard millet; rice was more affected by the (R) -enantiomers and barnyard millet by the (S) -enantiomers. Differential chiral responses including enantioselectivity and cross intergenus response between Oryza and Echinochloa plants against optically active α-methylbenzyl phenylureas were indicated in this study.