Brønsted Acidic Ionic Liquid: An Efficient Organocatalyst for the Synthesis of Pyrrolo[1,2-a]indoles under Neat Conditions

• Published in: Synlett 2024-12, Vol.35 (20), p.2447-2452
• Main Authors: Pramanik, Tanmay, Pal, Satyajit, Santra, Sougata, Majee, Adinath
• Format: Article
• Language: English
• ISSN: 0936-5214; 1437-2096
• DOI: 10.1055/a-2412-9738

A new synthetic approach has emerged for constructing 9H-pyrrolo[1,2-a]indole scaffolds by the reactions between indoles and chalcones under metal- and solvent-free conditions at 80 °C. The reaction occurs smoothly in the presence of a Brønsted acidic ionic liquid, 1-methyl-3-(4-sulfobutyl)-1H-imidazol-3-ium tosylate, as a catalyst, permitting the synthesis of the desired products with satisfactory yields. The developed protocol is applicable to the construction of biologically important pyrrolo[1,2-a]indole derivatives from easily accessible chalcones having various substituents. The process commences with Michael addition to chalcones, followed by annulations induced by the elimination of a water molecule, yielding the 9H-pyrrolo[1,2-a]indole scaffolds. Several control experiments were carried out to achieve a better understanding of the reaction pathway. The feasibility of recycling the catalyst was also demonstrated. This method produces water as the sole byproduct and represents a green synthetic protocol. The clean reaction, easily accessible reactants, and the metal- and solvent-free and environmentally friendly reaction conditions are the notable advantages of this procedure.