Pharmacophore Reorganization-Based Design, Synthesis, and Safener Activity of Novel Isoxazole–Piperazinone Derivatives

Bensulfuron, an acetolactate synthase (ALS)-inhibiting herbicide, is widely used in rice cultivation for the effective control of sedge and broadleaf weeds. However, japonica varieties are highly sensitive to this herbicide and can develop phytotoxicity when it is improperly applied. Herbicide safeners enhance crop tolerance to herbicides without reducing their weed control efficacy, offering a significant strategy for the safe use of herbicides. To alleviate bensulfuron-induced phytotoxicity in rice, a series of novel isoxazole-piperazinone derivatives were designed through pharmacophore recombination. The structures of the target compounds were confirmed by infrared spectroscopy, ¹H and ¹³C nuclear magnetic resonance spectroscopy, and high-resolution mass spectrometry. The spatial configuration of compound <b>II-1</b> was further determined by single-crystal X-ray diffraction. Bioactivity assays demonstrated that most target compounds effectively reduced bensulfuron-induced phytotoxicity in rice. Evaluation of key physiological parameters, including plant growth indices, ALS enzyme activity, and detoxification enzyme activity, revealed that compounds <b>II-10</b> and <b>II-40</b> exhibited superior safener activity compared to isoxadifen-ethyl. Molecular structure analysis and absorption, distribution, metabolism, excretion, and toxicity (ADMET) predictions indicated that compound <b>II-10</b> possesses good pharmacokinetic properties comparable to those of isoxadifen-ethyl. Molecular docking analysis revealed that compound <b>II-10</b> exerts its protective effect by competitively binding to the ALS active site with bensulfuron. Furthermore, ecological risk predictions suggested that compound <b>II-10</b> poses low or negligible toxicity risks to nontarget organisms such as mammals, birds, and algae. Overall, these results highlight compound <b>II-10</b> as a promising structure for the development of novel herbicide safeners.