Synthesis and antifungal activity of 1-methyl-5-indolecarboxamide derivatives

Indole derivatives possess excellent agricultural antifungal activity, and the introduction of alkyl groups at the N-atom of the indole ring may contribute to the enhancement of activities. Based on our previous research on indole and amide compounds, the amide group was introduced into the indole scaffold to discover compounds with high antifungal activity. Using 1-methyl-5-indolecarboxylic acid as the raw material, 26 1-methyl-5-indolecarboxamide derivatives were designed and synthesized, and their structures were characterized by ¹H NMR, ¹³C NMR and HRMS. The mycelial growth rate method was employed to evaluate their in vitro antifungal activities against Valsa mali, Botrytis cinerea and Rhizoctonia solani at a concentration of 50 μg/mL. Compounds with an inhibition rate greater than 80% were selected for further EC₅₀ values determination. The antifungal activity results showed that the target compounds exhibited certain inhibitory activities against the three test pathogens, among which 3o and 3q showed good inhibitory activities against V. mali, with EC₅₀ values of 1.62 and 5.04 μg/mL, respectively. In particular, compound 3o (N-(3-bromophenyl)-1-methyl-1H-indole-5-carboxamide) demonstrated a protective activity of 52.9% and a therapeutic activity of 63.6% against V. mali at a concentration of 100 μg/mL. The protective activity of compound 3o showed comparable effects to the commercial fungicide boscalid. It has the potential as a fungicide lead compound for in-depth structural optimization research.