Antifungal Agents Against Phytopathogenic Fungi: Discovery Of Benzofuroxan And Benzofurazan Design And Synthesis Of Novel Strobilurin Analogues

Plant pathogenic fungi remain a serious problem for food security and human health. Fungus infection can cause severe yield reduction for crops and quality decline in agricultural product. Moreover, the threat of fungus-produced mycotoxins can last for many years. Although fungus-resistant crop varieties and other biological methods have been employed to deal with the fungus problem, fungicidal chemicals remain to be the principal method for control of pathogenic fungi. At present, in addition to strobilurins, other fungicides have been applied for a long time. More virulent and fungicide-resistant pathogenic strains continually emerged and are reported to be no longer under control by chemicals that were once effective. As a result, it is necessary to develop novel and effective fungicidal agents to effectively control those agricultural diseases.By screening of our in house compound library, the benzofuroxan scaffold was identified to exhibit good in vitro activity against four important phytopathogenic fungi. A series of benzofuroxan and benzofurazan derivatives were prepared and evaluated for their biological activities against four important phytopathogenic fungi, namely, Rhizoctonia solani, Sclerotinia sclerotiorum, Fusarium graminearum and Phytophthora capsici, using the mycelium growth inhibition method. Compound A17 exhibited high antifungal effect against Sclerotinia sclerotiorum, Fusarium graminearum and Phytophthora capsici, with IC50 values of 0.81, 0.77 and 4.88 μg/mL respectively, which is similar to that of carbendazim(0.15 μg/mL, 0.50 μg/mL) or chlorothalonil(4.48 μg/mL). Compared with benzofuroxans, benzofurazan derivatives still kept the antifungal activity. It is noteworthy that compound D10 displayed a broad spectrum of antifungal activity against all four tested fungi phytopathogens, and result from activity evaluation against the human normal liver cell line HL-7702 demonstrated the compound good selectivity against phytopathogenic fungi. Significantly, A5 was demonstrated to successfully suppress disease development in Sclerotinia sclerotiorum infected cole in vivo.Strobilurin derivatives is one of the most important classes of agricultural fungicide due to their novel mode of action, broad fungicidal spectrum, low toxicity toward mammalian cells, and environmentally benign characteristics. Strobilurins act through inhibition of mitochondrial respiration by blocking electron transfer at the ubiquinol oxidation center(Qo site) of the cytochrome bc1-enzyme complex(complex III). Several types of strobilurin analogues were synthesized and their biological activities were tested against several fungal species. Compound 56 f, 56 k and 56 s, which are the pyraclostrobin analogues with pyridine on the bridged chain, displayed good antifungal activity against the phytopathogenic fungi. And the inhibitory activity of 56 s against porcine bc1 complex at 100 μM is 99.9%, which is better than that of penthiopyrad(95%) and similar to pyraclostrobin(99.9%).The benzofuroxan and benzofurazan structural scaffolds with variations at different phenyl positions could be further explored for the discovery of more potent antifungal agents against different pyhtopathogenic fungi. And the IC50 of pyraclostrobin analogues against the enzyme(Porcine bc1 Complex) is under going.