Isoxazole-Based Optimization of Pyrido[1,2-a]pyrimidine Mesoionic Compounds for Discovering Aphicidal Molecules Tolerated by Honeybees

For a long time, the poor selectivity of insecticides between pollinating insects and pests has posed challenges to enabling biodiversity. Currently, there have been relatively few successes involving the development of agrochemicals that are safe for bees. We here disclose an achievement in this context: an isoxazole-based modification of pyrido[1,2-a]pyrimidine structures is implemented to provide 26 new mesoionics (Y1-Y26) for aphicidal bioassays. The resulting derivatives Y2 and Y21 show an outstanding lethal effect on Aphis craccivora (A. craccivora), with LC₅₀ values of 1.68 and 1.46 mg/L, respectively, lower in contrast to triflumezopyrim (TFM) (LC₅₀ = 2.66 mg/L). Interestingly, structurally similar compounds Y2 and Y21 exhibit significant differences in acute oral experiments with Apis mellifera, with Y21 (LD₅₀ = 0.87 μg a.i./bee) demonstrating toxicity comparable to that of TFM (LD₅₀ = 0.72 μg a.i./bee), whereas Y2 (LD₅₀ = 14.65 μg a.i./bee) is found to be slightly level. Comprehensive analyses of proteomics, reverse transcription quantitative polymerase chain reaction (RT-qPCR), enzyme activity assays, and molecular docking results suggest that Y21 exerts its insecticidal function by modulating pest nicotinic acetylcholine receptors, specifically subunits β1 and β4. The observed tolerance of bees to Y2 may be attributed to their metabolic advantages. This study is expected to contribute valuable insights into developing mesoionic insecticides with better selectivity.