Indole frameworks via transition-metal-free annulation: a current perspective

Abstract

Indole structural motifs are ubiquitous in natural products, pharmaceuticals, agrochemicals and functional materials. The immense importance of the indole moiety, especially in the field of drug discovery, has fuelled continued attention in indole chemistry for over a century. In the recent past, a large number of metal-free methodologies have been developed apart from metal-catalyzed annulation processes. The present review focuses on recent (2019–mid-2023) efforts towards the construction of the indole core via transition-metal-free methods, which include acid-catalyzed, base-catalyzed, photochemical, electrochemical, iodonium-salt promoted, SOCl₂/DMSO-mediated, uncatalyzed/thermally assisted and miscellaneous methods. The reactions described in this review mostly employed readily accessible starting materials, and inexpensive catalysts that are environmentally benign and easy to handle. In most cases, mechanistic aspects have also been highlighted for better understanding of the reaction pathways. Interestingly, some of the synthesized compounds mentioned in this review are potentially bioactive.