Novel Flavonol Derivatives Containing Quinoxaline: Insights into the Antifungal Mechanism against <i>Sclerotinia sclerotiorum</i>

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 (<b>Y</b>_<b>B</b> series) had better inhibitory activity against <i>Sclerotinia sclerotiorum</i> (<i>S.s</i>.) than compounds containing enol (<b>Y</b>_<b>A</b> series). <b>Y</b>_<b>B9</b> showed the strongest antifungal activity against <i>S.s.,</i> and the median effective concentration (EC₅₀) value was 1.0 μg/mL, which was better than azoxystrobin (<b>Az</b>, 35.3 μg/mL). In vivo fungal inhibition experiments showed that the protective activity of <b>Y</b>_<b>B9</b> against rape leaves was 83.4% at 200 μg/mL, which was superior to that of <b>Az</b> (70.2%). The activity of succinate dehydrogenase and molecular docking results showed that <b>Y</b>_<b>B9</b> had a stronger antifungal effect than <b>Y</b>_<b>A9</b>. The results of oxalic acid content determination showed that <b>Y</b>_<b>B9</b> could reduce the pathogenic ability of <i>S.s.</i> Then, the inhibitory effect of <b>Y</b>_<b>B9</b> against <i>S.s.</i> was further verified by scanning electron microscopy, fluorescence microscopy, cell membrane permeability, cell content leakage, and malondialdehyde content.