BACKGROUND: Arylpyrazole insecticides display broad-spectrum insecticidal activity against insect pests. However, the high toxicity toward honeybees associated with fipronil prohibits its agronomic utility. To explore reducing the toxicity of aryl pyrazole analogs to bees, a series of new spiro-pyrazolo[1,5-a]quinazoline derivatives were designed and synthesized. RESULTS: Bioassay results showed that these compounds exhibited good insecticidal activity. In particular, the insecticidal activity of compound 5f against Plutella xylostella larvae (median lethal contentration, LC 50 = 1.43 mg L −1 ) was equivalent to that of fipronil. Moreover, some compounds also showed good insecticidal activity against Solenopsis invicta. Importantly, the bee toxicity study confirmed that compound 5f had much lower acute oral toxicity, with a median lethal dose (LD 50 ) = 1.15 ∼g bee −1 that was three to four orders of magnitude greater than that of fipronil (0.0012 ∼g bee −1 ). Electrophysiological studies were conducted using honeybee γ-aminobutyric acid receptor heterologously expressed in Xenopus oocytes to explain the reduced bee toxicity of compound 5f. The inhibitory effect of compound 5f (16.29 ∼mol L −1 ) was determined to be approximately 700-fold lower than that of fipronil (0.023 ∼mol L −1 ).CONCLUSION: These spiro-pyrazolo[1,5-a]quinazoline derivatives could be potential candidates and lead structures for the discovery of novel insecticides with low bee toxicity.

To develop effective insecticides against Lepidoptera pests, 25 novel N-pyridylpyrazole derivatives containing thiazole moiety were designed and synthesized based on the intermediate derivatization method (IDM). The insecticidal activities of these target compounds against Plutella xylostella (P. xylostella), Spodoptera exigua (S. exigua), and Spodoptera frugiperda (S. frugiperda) were evaluated. Bioassays indicated that compound 7g−7j exhibited good insecticidal activities. Compound 7g showed especially excellent insecticidal activities against P. xylostella, S. exigua, and S. frugiperda with LC50 values of 5.32 mg/L, 6.75 mg/L, and 7.64 mg/L, respectively, which were adequate for that of commercial insecticide indoxacarb. A preliminary structure-activity relationship analysis showed that the insecticidal activities of thiazole amides were better than that of thiazole esters, and the amides with electron-withdrawing groups on the benzene ring were better than the ones with electron-donating groups. This work provides important information for designing novel N-pyridylpyrazole thiazole candidate compounds and suggests that the 7g is a promising insecticide lead for further studies.

Based on our previous work, using the strategies of “scaffold hopping” and “intermediate derivatization method” (IDM), a total of 44 novel pyridylpyrazolo carboxylate derivatives were designed and synthesized. The structures of these compounds were identified by 1H NMR and 13C NMR, and the insecticidal activities of the target compounds against Plutella xylostella and Spodoptera frugiperda were tested. Compound G35 showed the best insecticidal activities against P. xylostella (LC50 = 33.65 mg/L) and S. frugiperda (mortality rate = 61.21% at 100 mg/L). Preliminary structure–activity relationship analysis showed that the introduction of amino group and acetamide on the benzene ring helped to improve the insecticidal activity of the scaffold. Molecular docking model between compounds G34 or G35 and Ryanodine receptors (RyRs) of P. xylostella showed that RyRs may be a potential target of this series compounds and explained the difference in insecticidal activities. These findings provided guidance for further optimization of the pyridylpyrazolo carboxylate scaffold as potential insecticide.

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