Synthesis and Reactivity of Indole Derivatives: A Comprehensive Review

Indole derivatives, a versatile class of N,O-heterocyclic compounds, due to their biochemical processes, demonstrate a variety of pharmacological characteristics. The indole structure is fundamental in both natural and synthetic chemistry, which contributes enormous opportunities to develop novel synthetic drug molecules. Furthermore, indole derivatives are essential in different applications, as flavoring ingredients, fragrances, and pharmaceutical formulations. From a synthetic perspective, several methods have been developed for indole production. In this review, we report the classical strategies to prepare indole derivatives via Fischer indole synthesis and Skraup synthesis, both of which remain widely employed in the design of complex indole-based heterocyclic compounds. Some green methods and efficient protocols have been discussed to increase yield, selectivity, and sustainability.