Design and Evaluation of Piperonyl Amidine Derivatives as Inhibitors of Insect Chitinolytic Enzymes

Chitinolytic enzymes play essential roles in insect molting and represent attractive targets for the development of insect growth regulators. A series of piperonyl amidine derivatives (7a–7x) was designed and synthesized by integrating a piperonyl scaffold with an amidine-containing fragment. These compounds were evaluated against four chitinolytic enzymes from Ostrinia furnacalis. Several analogues showed notable inhibition of OfChtI and OfChi-h at 10 μM. Compound 7i inhibited OfChtI and OfChi-h by 83.2% and 93.6%, respectively, with Ki values of 1.424 and 0.113 μM. Molecular docking suggested that differences in subsite occupancy and binding interactions may account for the differential inhibitory activity of 7i toward the four enzymes. In vivo, 7i showed insecticidal activity against O. furnacalis, Mythimna separata, and Plutella xylostella at 500 mg·L–1 and suppressed larval growth and development in O. furnacalis. These findings support the piperonyl amidine scaffold as a useful lead for further optimization.