Exploration of an Ancient Dye: Synthesis and Modeling an Indirubin-Inspired GSK-3β Inhibitor

Abstrak

Indirubin, a bis-indole alkaloid found in indigo-producing plants and Indigo Naturalis, exhibits a broad range of pharmacological activities and serves as a versatile scaffold for medicinal chemistry modification. In this study, 5-(2-oxoindolin-3-ylidene)imidazolidine-2,4-dione (4) was synthesized via Knoevenagel condensation of isatin and hydantoin under acetic acid reflux with sodium acetate catalysis, affording the product in 44% yield. The structure was confirmed by ¹H-NMR, ¹³C-NMR, and high-resolution ESI-TOF mass spectrometry. Density functional theory (DFT) calculations at the B3LYP-D4/def2-SVP level revealed a HOMO–LUMO gap of 3.27 eV, indicating moderate electronic stability alongside appreciable electron-accepting character, further supported by the calculated global reactivity descriptors. Electron localization function (ELF) and localized orbital locator (LOL) analyses indicated coexisting localized heteroatom lone-pair density and delocalized π-electron character across the conjugated framework. Molecular docking against GSK-3β showed that compound 4 occupies the ATP-binding site and reproduces key hinge hydrogen-bond interactions through its indole moiety. This combined synthetic, electronic, and docking results identify compound 4 as a promising indirubin-derived scaffold for further development as a GSK-3β inhibitor.