Design, Synthesis, and Synergistic Activity of Eight-Membered Oxabridge Neonicotinoid Analogues

Insecticide synergists are sought-after due to their potential in improving the pesticide control efficacy with a reduced dose of an active ingredient. We previously reported that a cis-configuration neonicotinoid (IPPA08) exhibited specific synergistic activity toward neonicotinoid insecticides. In this study, we synthesized a series of structural analogues of IPPA08 by converting the pyridyl moiety of IPPA08 into phenyl groups, via facile double-Mannich condensation reactions between nitromethylene compounds and glutaraldehyde. All of the oxabridged neonicotinoid compounds were found to increase the toxicity of imidacloprid against Aphis craccivora. Notably, compound 25 at 0.75 mg/L lowered the LC₅₀ value of imidacloprid against A. craccivora by 6.54-fold, while a 3.50-fold reduction of the LC₅₀ value was observed for IPPA08. The results of bee toxicity test showed that compound 25 display selectivity in its effects on imidacloprid toxicity against the honey bee (Apis mellifera L.). In summary, replacing the pyridyl ring with a phenyl ring was a viable approach to obtain a novel synergist with oxabridged moiety for neonicotinoid insecticides.