Design, Synthesis, and Molecular Mechanism Studies of N -Phenylisoxazoline-thiadiazolo[3,4- a ]pyridazine Hybrids as Protoporphyrinogen IX Oxidase Inhibitors

Protoporphyrinogen oxidase (PPO, EC 1.3.3.4) is an important target for green agrochemical discovery. Herein, a novel N -phenylisoxazoline-thiadiazolo[3,4- a ]pyridazine herbicidal active scaffold was designed by the scaffold hybridization strategy. Systematic structural optimization enabled the discovery of a series of derivatives with excellent weed control at 9.375–150 g ai/ha by the post-emergent application. Some derivatives exhibited improved Nicotiana tabacum PPO (NtPPO)-inhibitory activity than fluthiacet-methyl. Of these, 2b, with K i = 21.8 nM, displayed higher weed control than fluthiacet-methyl at the rate of 12–75 g ai/ha, and selective to maize at 75 g ai/ha. In planta, 2b was converted into a bioactive metabolite 5 ( K i = 4.6 nM), which exhibited 4.6-fold more potency than 2b in inhibiting the activity of NtPPO. Molecular dynamics simulation explained that 5 formed stronger π–π interaction with Phe392 than that of 2b . This work not only provides a promising lead compound for weed control in maize fields but is also helpful to understand the molecular mechanism and basis of the designed hybrids.