The indole skeletons have been extensively utilized due to their significant potential and broad applications as pharmaceutical agents, synthetic scaffolds, and chelating agents. Additionally, numerous natural products exhibiting biological activities or medicinal properties encompass the indole scaffold. Therefore, there is a growing focus on the synthesis and modification of indole derivatives, particularly on the asymmetric catalytic functionalization of indole scaffolds. The characteristics of electron cloud distribution in indole framework have led to a predominant focus on the functionalization of indole within the five-membered azole ring, particularly at C-3, C-2, and N-1 positions. In light of this observation, the focus of research on the synthesis and alteration of the indole backbone primarily centers on the azole ring. The functionalization of the indole in the carbocyclic ring is relatively uncommon, primarily attributed to its lower reactivity. Over the past decades, advancements in transition-metal catalysis, particularly the introduction of C—H activation strategy, have led to the development of various methods for achieving C—H functionalization of indoles within the carbocyclic ring. However, these approaches often necessitated the incorporation of directing or blocking groups in the azole ring, the application of harsh reaction conditions, or the utilization of transition metals as catalysts. In comparison to well-established metal catalysis, organocatalysis exhibits certain inherent limitations, including low catalytic activity, the requirement for high catalyst dosages, and moderate tolerance levels. However, these limitations have not hindered organocatalysis from emerging as a prominent method for functionalizing indole in the carbocyclic ring. This is attributed to its potential cost savings, time efficiency, energy conservation, simplified experimental procedures, excellent ability on control of chirality and reduced chemical waste generation. In this review, the functionalization strategy of indole in the carbocyclic ring through organocatalysis has been outlined. This strategy encompasses modifications at C-4, C-5, C-6, and C-7 positions, offering a novel avenue for the creation and advancement of techniques for organocatalytic functionalization of indole in the carbocyclic ring.

Key words: functionalization of indole in the carbocyclic ring, organocatalysis, asymmetric synthesis, cascade reaction, axial chirality