Quassinoids from Picrasma chinensis with Insecticidal Activity against Adults and Larvae of Diaphorina citri Kuwayama and Neuroprotective Effect

Eleven new tetracyclic quassinoids, picrachinensin A–K (<b>1–11</b>), along with six known congeners, were isolated from the stems and leaves of Picrasma chinensis. Their structures were elucidated by integrated multiple spectroscopic techniques, single-crystal X-ray diffraction analysis, and electronic circular dichroism. Notably, compounds <b>3</b> and <b>4</b> are a pair of undescribed epimers, and <b>8</b> and <b>9</b> are unusual quassinoids with a hydroxymethyl group at C-13. Biologically, compound <b>7</b> exhibited insecticidal activity on both adults and larvae of Diaphorina citri Kuwayama even more effectively than the positive control (abamectin), with an LD₅₀ of 55.69 mg/L for adults and a corrected mortality rate of 30.42 ± 2.78% for larvae (100 mg/L). According to preliminary structure–activity relationship investigations, the hydroxymethyl at the C-13 position of quassinoids was beneficial for their insecticidal activity. In addition, compounds <b>1</b>, <b>4</b>, and <b>12</b> exhibited excellent neuroprotective effect against H₂O₂-induced oxidative injury on SH-SY5Y cells, with more potent activity than the positive control (Trolox), and all the compounds exhibited no cytotoxicity to SH-SY5Y and BV-2 cells at the indicated concentrations.