Rhein–Amino Acid Ester Conjugates as Potential Antifungal Agents: Synthesis and Biological Evaluation

In the search for crop protectants, amino acid ester conjugates have been widely investigated as potential antifungal agents. In this study, a series of rhein–amino acid ester conjugates were designed and synthesized in good yields, and their structures were confirmed by (1)H-NMR, (13)C-NMR and HRMS. The bioassay results revealed that most of the conjugates exhibited potent inhibitory activity against R. solani and S. sclerotiorum. In particular, conjugate 3c had the highest antifungal activity against R. solani with an EC(50) value of 0.125 mM. For S. sclerotiorum, conjugate 3m showed the highest antifungal activity with an EC(50) value of 0.114 mM. Satisfactorily, conjugate 3c exhibited better protective effects than that of the positive control, physcion, against powdery mildew in wheat. This research supports the role of rhein–amino acid ester conjugates as potential antifungal agents for plant fungal diseases.