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

Abstract

Protoporphyrinogen oxidase (PPO, EC 1.3.3.4) is an important target for green agrochemical discovery. Herein, a novel <i>N</i>-phenylisoxazoline-thiadiazolo[3,4-<i>a</i>]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 <i>Nicotiana tabacum</i> PPO (NtPPO)-inhibitory activity than fluthiacet-methyl. Of these, <b>2b</b>, with <i>K</i>_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. <i>In planta</i>, <b>2b</b> was converted into a bioactive metabolite <b>5</b> (<i>K</i>_i = 4.6 nM), which exhibited 4.6-fold more potency than <b>2b</b> in inhibiting the activity of NtPPO. Molecular dynamics simulation explained that <b>5</b> formed stronger π-π interaction with Phe392 than that of <b>2b</b>. 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.