The indole subunit is a near-ubiquitous component of biologically active natural products, and its study has been a major focus of research for generations. Substituted indoles have frequently been referred to as privileged structures since they are capable of binding to multiple receptors with high affinity, and thus have applications across multiple therapeutic areas. Because of this activity, it is not surprising that the indole ring system has become an important building block or intermediate in the synthesis of many pharmaceutical agents.
Indol has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring，which is an aromatic heterocyclic organic compound. It can be produced by bacteria as a degradation product of the amino acid tryptophan. It happens naturally in human feces and has an intense fecal smell. At very low concentrations, however, with a flowery smell, it is a constituent of many flower scents (such as orange blossoms) and perfumes.