Quantitative structure–activity relationship of 2,6-dimethoxy-N-(3-(4-substituted phenyl)isoxazol-5-yl)benzamide for the inhibition of chitin synthesis

Previously, we found that 5-(2,6-dimethoxybenzoylamino)-3-phenylisoxazoles (IOXs) inhibit chitin synthesis in the cultured integument of Chilo suppressalis. In this study, IOXs with various substituents at the para-position of the 3-phenyl ring were synthesized, and the concentrations required to inhibit chitin synthesis to 50% (IC₅₀) were determined for all compounds. The introduction of halogens—such as F, Cl, and Br—and small alkyls—such as Me, Et, Pr, and n-Bu—at the 3-phenyl ring slightly enhanced the activity. However, the activity decreased drastically with the introduction of NO₂, CF₃, and t-Bu. The quantitative analysis of the substituent effect at the 3-phenyl ring on chitin-synthesis inhibition using the Hansch-Fujita method showed that the hydrophobic substituent with the optimum value was favored for the activity, but the bulky substituent in terms of E_s was detrimental to the activity.

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