Synthesis and Antifungal Activity of Derivatives of the Natural Product Griseofulvin against Phytopathogenic Fungi

Natural products are important sources for the discovery of new pesticides. Chemical synthesis and structural modification can lead to pesticides. Despite abundant research in fungicide discovery for crop protection, there is an emerging need for the development of novel antifungal agrochemicals. Herein, 39 diversified griseofulvin derivatives were effectively synthesized from the natural product griseofulvin by diversity-oriented synthesis through the reactions of demethylation, ammonolysis, methylation, nitration, acylation, reduction, and chlorination. Among them, 31 derivatives were novel. All structures were characterized by 1 H NMR, 13 C NMR, and high-resolution mass spectrometry (HR-MS), and the antifungal activity was investigated against five phytopathogenic fungi. Compounds 5h and 5l had excellent activity against Botrytis cinerea ( 5h, IC 50 = 17.29 ± 0.64 μg/mL) and Alternaria solani ( 5l, IC 50 = 22.52 ± 0.79 μg/mL), respectively. Compound 9 exhibited the more promising activities against three target fungi, especially against Colletotrichum gloeosporioides (IC 50 = 7.24 ± 0.66 μg/mL), which is obviously better than positive control hymexazol, thifluzamide, and parent compound griseofulvin. In addition, compound 10 showed significant and extensive activities against four target fungi Cytospora sp. (IC 50 = 18.72 ± 0.35 μg/mL), C. gloeosporioides (IC 50 = 31.39 ± 1.48 μg/mL), A. solani (IC 50 = 40.82 ± 1.04 μg/mL), and Fusarium solani (IC 50 = 36.81 ± 0.82 μg/mL). Unexpectedly, 11 and 12, the chlorinated products of compound 9, exhibited the most promising activity against C. gloeosporioides (IC 50 = 4.48 ± 0.54 μg/mL for 11, 2.24 ± 0.76 μg/mL for 12 ). Furthermore, 12 showed remarkable activity against Cytospora sp. (IC 50 = 5.85 ± 0.72 μg/mL). Additionally, in vivo antifungal activity against C. gloeosporioides, homology modeling, and docking analysis of 11, 12, and griseofulvin were conducted. All results indicated that 11 and 12 had potency as antifungal agents against C. gloeosporioides, and the modifications of the 2′ and 4′ positions of griseofulvin should be further explored for higher-activity lead compounds or potential agricultural fungicides.