Design, Synthesis, and Herbicidal Evaluation of Novel Synthetic Auxin Herbicides Containing 6-Indolylpyridine Oxime Ester/Amine

In this study, a series of 6-indolylpyridine oxime ester/amide derivatives were synthesized as novel synthetic auxin herbicides (SAHs) for postemergence herbicidal applications. At 30 g ai/ha, compounds 9q, 9u, 9v and 9w demonstrated inhibition rates of 90 to 100% against weeds Echinochloa crus-galli (EC) and Digitaria sanguinalis (DS). Even at a reduced 7.5 g ai/ha, these compounds maintained over 90% inhibition against four broadleaf weeds, with effects comparable to those of commercial herbicides halauxifen-methyl (HAM), indolauxipyr (IND) and indolauxipyr-cyanomethyl (INC). Crop sensitivity tests confirmed the suitability of compounds 9w and 9u for application in wheat and rice fields at 30 g ai/ha. Molecular docking analysis revealed that compound 9w formed significant hydrogen bonding and π–π stacking interactions with key amino acid residues. Additionally, half maximal inhibitory concentration (IC50) values of compounds 9u and 9w for IAA in vitro inhibition activity were 6.153 and 4.389 μM, respectively, outperforming HAM (9.061 μM). Compounds 9u and 9w show promising potential as lead candidates for novel SAHs.