Design, Synthesis, and Structure–Activity Relationship Studies of Spirotryprostatin A-Inspired Spirooxindole-Diketopiperazine Derivatives with Antifungal Activity

To address the escalating challenge of fungal resistance in agriculture and explore structurally diverse antifungal scaffolds, a series of novel spirotryprostatin A-inspired spirooxindole-diketopiperazine derivatives was designed and synthesized. Seven spirooxindole intermediates and fourteen target derivatives were successfully obtained and systematically evaluated for their antifungal activities against twelve phytopathogenic fungi. Structure–activity relationship (SAR) analyses indicated that incorporation of the diketopiperazine framework contributed to improved antifungal activity, while the electronic properties and substitution patterns of aromatic substituents influenced the biological profiles. Generally, compounds with electron-withdrawing groups (EWGs) exhibited relatively improved antifungal activity compared with those bearing electron-donating groups (EDGs), with ortho-trifluoromethyl substitution showing a favorable effect. Among the synthesized derivatives, compounds 4e and 4e′ exhibited promising broad-spectrum antifungal activity, with MIC values of 4 μg/mL observed against some tested phytopathogenic fungi. Isothermal microcalorimetry (IMC) experiments further supported the antifungal potential of 4e. Additionally, molecular docking studies suggested that the selected active derivative could interact with succinate dehydrogenase (SDH) and myosin through hydrogen-bonding and hydrophobic interactions, providing preliminary insights into possible binding modes. Overall, this study expands the structural diversity of spirotryprostatin A-inspired spirooxindole-diketopiperazines and provides valuable information for the discovery and optimization of novel antifungal compounds.