Synthesis and Biological Evaluation of Hexahydropyrrolo[2,3-b]Indole Derivatives as Fungicides against Phytopathogenic Fungi

Eighteen hexahydropyrrolo[2,3-b]indole derivatives were synthesized and evaluated their in vitro antifungal activities against five phytopathogenic fungal strains through the mycelium growth rate method. Analysis of the structure-activity relationship on these synthesized compounds revealed that the introduction of benzyl or substituted benzyl group at the C-3a or N-8 position of the pyrroloindoline scaffold conferred higher antifungal activity against all tested phytopathogenic fungi than compound 4a (both C-3a and N-8 positions are prenyl groups). Especially, compound 4r, among all the tested compounds, showed the most effective antifungal activity against Fusarium coeruleum, and Fusarium graminearum with IC50 values of 4.61 and 5.02 μg/mL, respectively. Moreover, all synthesized compounds 4a-4r displayed higher activities against Curvularia lunata than the positive control thiabendazole, a commercial agricultural fungicide.