The bleaching herbicide flurtamone shows a concentration-dependent inhibition of phytoene desaturase activity in a cell-free assay. Structure-activity studies were carried out with 26 derivatives. For this purpose, I50 values for inhibition of formation of colored carotenoids in cultures of Synechococcus and with some selected compounds in vitro inhibition data were measured. For meta-substituted 4-phenyl derivatives a quantitative structure-activity correlation was calculated. Regression analysis show ed a significant contribution of lipophilicity π and steric parameter B2 of the substituents to inhibitory activity of these derivatives. In addition, the stericrequirement of 4-phenyl substituents was further revealed by the substitution pattern at different positions. In general, mono substituents at the meta position were most favorable and only the very small fluoro group was allowed as a second substituent. When the ligands at position 2 of the furanone heterocycle were modified, phenyl or para-fluorophenyl groups were most advantageous. Among the available compounds with different alkyl substitutions of the 5-amino group, the ethyl derivative show ed the highest inhibitory activity whereas the one with the branched isopropyl group w as quite inactive.
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