Structure Activity Relationship of o-Substituted Phenylacetic Acids in Plant Growth Regulation

概要

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The factors affecting growth regulating activity of 2-(3′-phenylpropyl)phenylacetic acids and benzoic acids were investigated by conformational analyses and their steric parameters estimated from molecular mechanistic calculation. The rotational barrier of the bond (∅3) between 3′-phenylpropyl moiety and the first phenyl ring (Ar1) was existed around 180° in the less biologically active phenylacetic acids with a thiazole 1, 3-dioxopropyl or 1-oxopropenyl (the bridging system), while the rotational barrier disappeared in the more active phenylacetic acids with a pyrazole or oxazole in the bridging system. Planar conformation with ∅3=180°, which can be superposed on the highly active fixed ring phenylacetic acid derivative, 2-phenyl-5H-pyrazolo[5, 1-a]isoquinolin-5-one, was postulated as the biologically active conformation. The larger bond angle (∅) between An1 and Ar2, which diminished the rotational barrier of ∅3 caused by steric repulsion between the bridging system and methylene protons of the carboxymethyl group, was prerequisite for the higher plant growth regulating activity of the phenylacetic acid. While it was not prerequisite for benzoic acids without methylene protons.
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