The variations in the light-dependent herbicidal activity against Echinochloa oryzicola of a number of 4-pyridone-3-carboxanilides with various substituents on the anilide and pyridone rings have been examined by use of physicochemical substituent parameters and regression analysis. The effects of substituents of the anilide ring were such that the activity was related parabolically not only with the substituent hydrophobicity (n) but also with the steric bulk parameter (E,) of the bulkier ortho substituent. The effect of substituents at the Iposition of the pyridone ring was analysed with steric(STERIM0L L and B,) and electronic(cr,) parameters. That of substituents at the 5-position of the pyridone ring was explained by a parabolic function of a steric (MR) parameter. Analyses of the activity of 105 analogs confirmed that our previous selection of a candidate compound in this series for field trials was indeed appropriate in terms of physicochemical substituent effects. 27 4-pyridone-)-carbox-TABLE 1 Structures and Herbicidal Activity of 4-Pyridone-3-carboxanilides A, No. R, R2 R, Xn m.p.("C) Substituenr parameters PI50 x,, R , R, Obsd E P ( 1 )

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Using a set of representative members selected from 4‐pyridone‐3‐carboxanilides and ortho‐chlorinated diphenyl ethers exhibiting light‐dependent herbicidal activity, we examined the three‐dimensional structure‐activity relationships quantitatively. The most stable conformation of each compound, regarded as the ‘active conformation’, was determined by either semi‐empirical or ab initio molecular orbital calculations. With a hypothesis defining a common substructural skeleton between the two different compound series. each molecule was superimposed. We first analysed the structure‐activity relationship using an index for the substructure shape similarity according to the superimposed conformations. After finding the relevance of the hypothesis, we examined the three‐dimensional structure‐activity relationship using the comparative molecular field analysis procedure. The results suggested that the two different series of compounds have a common region of the receptor site. The variations in the light‐dependent herbicidal potency were governed by hydrophobicty and three‐dimensional steric and electronic field parameters of molecules participating in the transport process and the interaction with the receptor site. The result was consistent with that derived from our previous quantitative analysis with the use of free‐energy‐related substituent parameters and the traditional regression procedure for a large number of pyridone‐carboxanilides.

A number of substituted 4-pyridone-3-carboxanilide derivatives show various degrees of light-dependent herbicidal activity. The effects of substituents at the anilide moiety on the activity against Echinochloa orpicola were analysed quantitatively with physicochemical substituent parameters. The activity was shown to vary parabolically with total hydrophobicity (Zn) of the anilide ring substituents as well as with the steric parameter (E,) of the ortho-substituents. The E, parameter for only the bulkier ortho-substituent, with the more negative value, but not for the two, was significant. The effect of substituents at the I-position was expressible by indicator variables assigned to each I-substituted series, the physicochemical rationalization being left for future analyses. 17Pesfic. Sci. 0031-613X/92/$05.00 0 1992 SCI. Printed in Great Britain ..