Novel Flavonol Derivatives Containing Quinoxaline:\nInsights into the Antifungal Mechanism against <i>Sclerotinia\nsclerotiorum</i>

In\nthis study, 12 pairs of tautomeric flavonol derivatives containing\nquinoxaline were synthesized. The results of antifungal activity showed\nthat in the enol–keto tautomerism, the target compounds containing\nketo (<b>Y</b><sub><b>B</b></sub> series) had better inhibitory\nactivity against Sclerotinia sclerotiorum (<i>S.s</i>.) than compounds containing enol (<b>Y</b><sub><b>A</b></sub> series). <b>Y</b><sub><b>B9</b></sub> showed the strongest antifungal activity against <i>S.s.,</i> and the median effective concentration (EC<sub>50</sub>) value was\n1.0 μg/mL, which was better than azoxystrobin (<b>Az</b>, 35.3 μg/mL). In vivo fungal inhibition experiments showed\nthat the protective activity of <b>Y</b><sub><b>B9</b></sub> against rape leaves was 83.4% at 200 μg/mL, which was\nsuperior to that of <b>Az</b> (70.2%). The activity of succinate\ndehydrogenase and molecular docking results showed that <b>Y</b><sub><b>B9</b></sub> had a stronger antifungal effect than <b>Y</b><sub><b>A9</b></sub>. The results of oxalic acid content\ndetermination showed that <b>Y</b><sub><b>B9</b></sub> could reduce the pathogenic ability of <i>S.s.</i> Then,\nthe inhibitory effect of <b>Y</b><sub><b>B9</b></sub> against <i>S.s.</i> was further verified by scanning electron microscopy,\nfluorescence microscopy, cell membrane permeability, cell content\nleakage, and malondialdehyde content.