Recent Advances on the C2-Functionalization of Indole via Umpolung.

Heterocyclic compounds having a nitrogen atom in the ring exhibit very interesting biological activities. Indole is the core structure of many bioactive compounds owing to its high affinity to bind with most biological targets. Indole is an electron-rich compound and generally prefers electrophilic rather than nucleophilic substitution. Hence, many important indole derivatives are difficult to synthesize through the conventional reactivity of indole. This limitation can be avoided by using the umpolung, from the German word meaning polarity inversion. In umpolung, the indole molecule, especially the C2 and C3 positions, behave as an electrophile. As C2-functionalized indoles have substantial importance in synthetic and pharmaceutical chemistry, this review focuses on the C2 umpolung of indoles via the indirect approach which is less explored. Unlike direct approaches of indole umpolung, indirect methods have several advantages and therefore a number of research articles have been published in this field. But no review is available up till now. This is the first review on this topic and we believe that it will surely motivate the readers to work in this area further.