Design, Synthesis, and Bioactivity of Triketone-quinoxalin-2-ones as a Novel HPPD Inhibition Herbicide

4-Hydroxyphenylpyruvate dioxygenase (HPPD) is recognized as one of the most promising herbicide targets for sustainable weed control in modern agricultural practices. To address agricultural demands, we designed and synthesized a novel series of triketone-quinoxalin-2-ones as potent HPPD inhibitors. In vitro evaluation revealed that the newly synthesized compounds demonstrated remarkable <i>Arabidopsis thaliana</i> HPPD (<i>At</i>HPPD) inhibitory activity. Significantly, compound <b>23</b>, 3-(4-chloro-2-fluorophenyl)-6-(2-hydroxy-6-oxocyclohex-1-ene-1-carbonyl)-1,5-dimethylquinoxalin-2(1<i>H</i>)-one, showed the strongest <i>At</i>HPPD inhibition with an IC₅₀ value of 0.034 μM, 10-fold more potent than mesotrione (IC₅₀ = 0.350 μM). Furthermore, the postemergence herbicidal activity evaluation showed that compound <b>35</b> exhibited 100% inhibition of <i>Digitaria sanguinalis</i>, <i>Amaranthus retroflexus</i>, <i>Chenopodium serotinum</i>, and <i>Abutilon theophrasti</i> at 150 g ai/ha, and 90% inhibition of <i>Setaria viridis</i>, showing enhanced activity compared to mesotrione. The crystal structure of the <i>At</i>HPPD<b>-35</b> complex demonstrated that compound <b>35</b> engaged in a key bidentate chelating interaction with the metal ion in the catalytic active site and a π-π interaction with Phe381 and Phe424. Moreover, <b>35</b> established hydrophobic interactions with Leu427, Leu368, and Met335. These results indicate that the triketone-quinoxalin-2-one hybrid is a promising scaffold and <b>35</b> can be considered a viable lead compound for the development of HPPD inhibitors.