Biological importance of the indole nucleus in recent years: A comprehensive review

Heterocyclic compounds are those cyclic compounds in which one or more of the ring carbons are replaced by another atom. The non-carbon atoms in such rings are referred to as ‘‘heteroatoms.’’ Such bicyclic heterocyclic compounds containing pyrrole ring with benzene ring fused to a,b-position are known as Indoles. Indole has a benzene ring and pyrrole ring sharing one double bond. It is a heterocyclic system with 10 electrons from four double bonds and the lone pair from the nitrogen atom. Indole is an important heterocyclic system because it is built into proteins in the form of amino acid tryptophan, because it is the basis of drugs like indomethacin and because it provides the skeleton of indole alkaloids—biologically active compounds from plants including strychnine and LSD. The incorporation of indole nucleus, a biologically accepted pharmacophore in medicinal compounds (Table 1), has made it versatile heterocyclic possessing wide spectrum of biological activities (Table 2). In the present study, we have made an attempt to collect biological properties of imidazole nucleus reported in the new millennium.