Novel isoxazoline insecticides featuring a "1-Pyrazole-2-cyano" motif and acting on the PxRDL receptor

The escalating problem of pest resistance drives the critical need for insecticides with novel modes of action and chemical structures. Owing to low cross-resistance and novel action mode, isoxazoline derivatives have been a leading focus in current insecticide development. Given that most existing derivatives retain the "1-amide-2-methyl" motif, we therefore embarked on a strategy to replace this core substructure with a novel "1-pyrazole-2-cyano" unit, leading to the design and synthesis of a new series of compounds. Bioassay results revealed that several synthesized compounds exhibited promising insecticidal activity. The representative compound A18 exhibited potent activity against Plutella xylostella (LC₅₀ = 1.06 μg/mL) and Spodoptera exigua (LC₅₀ = 2.41 μg/mL), with LC₅₀ values approximately 13- and 21-fold lower than those of ethiprole (13.67 and 51.11 μg/mL, respectively). Structure-activity relationship (SAR) analysis established that the steric bulk of the R-group is a critical determinant of insecticidal potency. This was mechanistically corroborated by two-electrode voltage clamp (TEVC) experiments, which revealed that a phenyl substituent at the R position sterically impeded the compound's binding to the PxRDL target, thereby diminishing inhibitory activity. Additionally, compound A18 showed reduced bee oral-toxicity than ethiprole or fluxametamide. Beyond the structural modification itself, the present work provides a rare integration of SAR, TEVC electrophysiology, and molecular dynamics simulations, allowing direct correlation between steric effects at the R-position and PxRDL inhibition.