Lignin-derived Brønsted acidic catalyst for the green synthesis of biologically relevant indole derivatives

Readily available, naturally derived lignin was transformed into a Brønsted acidic organocatalyst. The obtained catalyst was utilized in the environmentally benign synthesis of biologically relevant indole derivatives such as vibrindole, turbomycin, etc. The scope of the developed methodology was further extended for the synthesis of aniline-tethered indoles, 4H-chromene, and indolin-2-one derivatives. Further, vicinal difunctionalization was successfully carried out with the aid of the developed organocatalyst. Imperatively, all the aforementioned reactions were carried out in the aqueous medium. The reusability of the heterogeneous catalyst was also proved by carrying out the reaction with the recovered catalyst.

Graphical abstract

Lignin-based naturally derived Brønsted acidic organocatalyst was utilized in the environmentally benign synthesis of biologically relevant indole derivatives, 4H-chromene, and indolin-2-one derivatives. All the aforementioned transformations were carried out in an environmentally benign aqueous medium.