Design, Synthesis, and Antifungal Evaluation of Cryptolepine Derivatives against Phytopathogenic Fungi

Inspired by the widely antiphytopathogenic application of diversified derivatives from natural sources, cryptolepine and its derivatives were subsequently designed, synthesized, and evaluated for their antifungal activities against four agriculturally important fungi Rhizoctonia solani, Botrytis cinerea, Fusarium graminearum, and Sclerotinia sclerotiorum. The results obtained from in vitro assay indicated that compounds a1-a24 showed great fungicidal property against B. cinerea (EC₅₀ < 4 μg/mL); especially, a3 presented significantly prominent inhibitory activity with an EC₅₀ of 0.027 μg/mL. In the pursuit of further expanding the antifungal spectrum of cryptolepine, ring-opened compound f1 produced better activity with an EC₅₀ of 3.632 μg/mL against R. solani and an EC₅₀ of 5.599 μg/mL against F. graminearum. Furthermore, a3 was selected to be a candidate to investigate its preliminary antifungal mechanism to B. cinerea, revealing that not only spore germination was effectively inhibited and the normal physiological structure of mycelium was severely undermined but also detrimental reactive oxygen was obviously accumulated and the normal function of the nucleus was fairly disordered. Besides, in vivo curative experiment against B. cinerea found that the therapeutic action of a3 was comparable to that of the positive control azoxystrobin. These results suggested that compound a3 could be regarded as a novel and promising agent against B. cinerea for its valuable potency.