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 ¹H-NMR, ¹³C-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 EC₅₀ value of 0.125 mM. For S. sclerotiorum, conjugate 3m showed the highest antifungal activity with an EC₅₀ 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.

To develop fungicidal candidates with both high antifungal activity and phloem mobility, a total of 34 2-[2-(trifluoromethoxy)phenyl]thiazole-4-carbonyl amino acid and amino acid ester conjugates were designed and synthesized. Their structures were characterized by ¹H NMR, ¹³C NMR, and HRMS. Their in vitro antifungal activities against five phytopathogenic fungi and their phloem and xylem mobility in castor bean seedlings were subsequently evaluated. The results showed that the amino acid conjugates 9a-9p generally exhibited higher antifungal activity than the corresponding amino acid ester conjugates. Compounds 9m and 9n showed good inhibitory activity against Rhizoctonia solani, Sclerotinia sclerotiorum, Phoma citricarpa, and Phytophthora parasitica. Notably, compounds 9f, 9g, 9l, and 9n showed strong inhibitory activity against P. citricarpa at 200 μmol/L, with inhibition rates of 82.82%, 89.86%, 80.45%, and 91.55%, respectively, all higher than that of thifluzamide (29.30%). Mobility assays revealed configuration-related differences in the conjugate-derived material detected in phloem and xylem exudates, with differential hydrolysis to compound 3 contributing substantially to the observed L/D profiles In addition, some L-amino acid conjugates were hydrolyzed in castor bean seedlings to release the unconjugated carboxylic acid precursor (compound 3). These results indicate that appropriate amino acid conjugation can enhance antifungal activity and modulate systemic mobility in plants, providing a basis for the structural design of systemic fungicides.