Indole-2-carboxamide: a versatile synthetic handle for the synthesis of diversely substituted polycyclic indole structures.

Indoles are among the most important N-heterocycles found in natural products, with a wide range of biological and pharmacological properties. The synthesis of naturally occurring complex indole derivatives as well as bioactive synthetic molecules has progressed tremendously. Since many of the bioactive, naturally occurring indole molecules possess polycyclic frameworks, it became essential to develop these scaffolds. In this context, indole derivatives have been utilized as precursors for the synthesis of many important polycyclic indole frameworks. These polycyclic indole molecules also include a fused indole motif. In this regard, there are many fused indole ring systems which can be accessed by indole-2-carboxamide as a key precursor through intramolecular and intermolecular cyclization reactions. This review summarizes the synthetic advancements for the construction of polycyclic fused indole molecules, which have indole-2-carboxamide as a synthetic precursor, and also discusses the synthetic potential and perspective of this essential scaffold. Furthermore, the reaction mechanism of the formation of various fused polycyclic indoles has also been discussed.