Novel Flavonol Derivatives Containing Quinoxaline: Insights into the Antifungal Mechanism against Sclerotinia sclerotiorum

In this study, 12 pairs of tautomeric flavonol derivatives containing quinoxaline were synthesized. The results of antifungal activity showed that in the enol-keto tautomerism, the target compounds containing keto (Y_B series) had better inhibitory activity against Sclerotinia sclerotiorum (S.s.) than compounds containing enol (Y_A series). Y_B9 showed the strongest antifungal activity against S.s., and the median effective concentration (EC₅₀) value was 1.0 μg/mL, which was better than azoxystrobin (Az, 35.3 μg/mL). In vivo fungal inhibition experiments showed that the protective activity of Y_B9 against rape leaves was 83.4% at 200 μg/mL, which was superior to that of Az (70.2%). The activity of succinate dehydrogenase and molecular docking results showed that Y_B9 had a stronger antifungal effect than Y_A9. The results of oxalic acid content determination showed that Y_B9 could reduce the pathogenic ability of S.s. Then, the inhibitory effect of Y_B9 against S.s. was further verified by scanning electron microscopy, fluorescence microscopy, cell membrane permeability, cell content leakage, and malondialdehyde content.