Discovery of Cholesterol-Type Oxime Ester/Ether Derivatives as Aphicidal Candidates against <i>Aphis citricola</i> Van der Goot and <i>Hyalopterus arundimis</i> Fabricius

The exploration of natural bioactive products as leads for high-value-added applications in crop protection has emerged as one of the prominent research focuses. Based on cholesterol as a lead molecule, herein oxime ester/ether derivatives of cholesterol at the C-3 position were semisynthesized via five successive chemical reactions. The steric configuration of derivative <b>Ip</b> was determined by a single-crystal analysis. Against <i>Aphis citricola</i> Van der Goot (Hemiptera: Aphididae), compounds <b>Id</b>, <b>Iq</b>-<b>Is</b>, <b>Iv</b>, <b>Iw</b>, <b>Iy</b>, <b>IIb</b>, <b>IIc</b>, <b>IIf</b>, and <b>IIi</b> (LC₅₀: 0.074-0.171 mg/mL) exhibited better aphicidal activity compared with cholesterol. Notably, derivative <b>IIc</b> displayed 130.2-fold aphicidal activity compared to cholesterol and good control efficiency on the fifth day in the field. Against <i>Hyalopterus arundimis</i> Fabricius (Homoptera: Aphididae), compounds <b>Ii</b>, <b>Im</b>, <b>IId</b>, and <b>IIg</b> depicted 59.2-66.7 times aphicidal activity compared to that of cholesterol. In addition, it demonstrated that the hydroxyl at the C-3 position of cholesterol is a vital modification site. Furthermore, the severe destruction of the superficial layers of treated <i>A. citricola</i> and <i>H. arundimis</i> was unveiled by scanning electron microscope (SEM). Hopefully, these results can lay the foundation for further optimization of cholesterol-type compounds as resultful aphicides.