Novel Flavonol Derivatives Bearing a 1,2,3,4-Tetrahydroquinoline Moiety: Design, Synthesis, and Anti-Phytophthora capsici Activity.

Twenty-three novel flavonol derivatives bearing a 1,2,3,4-tetrahydroquinoline (THQ) scaffold were designed and synthesized, and their antifungal activities against eight plant pathogenic fungi were evaluated. Several compounds exhibited 100% inhibition against Sclerotinia sclerotiorum and Phytophthora capsici (Pc). Among them, Q8 showed the most potent activity against Pc with an EC50 value of 4.1 μg mL-1, superior to azoxystrobin (69.0 μg mL-1) and fluopyram (123.2 μg mL-1). In vivo tests on chili fruits demonstrated that Q8 at 200 μg mL-1 provided superior curative and protective effects (73.7 and 80.8%) compared to azoxystrobin (53.0 and 54.4%). Mechanistic studies revealed that Q8 disrupted cell membrane integrity and inhibited succinate dehydrogenase (SDH) activity, with computational simulations suggesting a higher binding affinity for SDH than fluopyram. Safety evaluation showed no obvious adverse effects on seed germination or earthworm survival. Collectively, these results indicate that Q8 represents a promising lead compound for fungicide development.