Novel Pyrazole-Quinoxaline Conjugates as Potent HPPD-Inhibiting Herbicides with Notable Crop Safety.
4-Hydroxyphenylpyruvate dioxygenase (HPPD) is a vital target for herbicide development. A series of pyrazole-quinoxaline conjugates were designed and synthesized via scaffold hopping and structural optimization. Bioassays revealed that B9 exhibited potent inhibitory activity against Arabidopsis thaliana HPPD (AtHPPD) with an IC50 of 0.11 μM, outperforming mesotrione (0.22 μM) and topramezone (0.48 μM). B18 achieved 100% control over six weeds at 75 g a.i./ha and retained full efficacy against three broadleaf weeds even at 18.75 g a.i./ha. Furthermore, B18 produced less than 20% injury to rice, wheat, cotton, and peanut at 150 g a.i./ha, exhibiting superior crop safety to topramezone (47%-72%). Molecular docking elucidated the binding mechanism of B18 with AtHPPD via Fe2+ coordination, π-π interactions, and hydrogen bonds. Fluorescence titration and microscale thermophoresis assays further confirmed strong binding of B9 and B18 to AtHPPD. This study demonstrated that B18 is a promising, safe, and efficient HPPD-inhibiting herbicide candidate.