Heterocyclic compounds – definition, classification and characterization. Five–membered heterocycles with one heteroatom. Pyrrole, indole, and their derivatives.
Organic compounds whose molecules contain a ring composed of carbon and other atoms are called heterocyclic compounds.
Depending on the types of the bonds between the atoms in the ring, the heterocyclic compounds may be divided into saturated, unsaturated and aromatic Heterocyclic compounds.
The saturated heterocyclic compounds include lactones, lactams, cyclic ethers, cyclic amines and cyclic anhydrides.
Five–membered heterocycles with one heteroatom.
The parent aromatic compounds of this family - pyrrole, furan, and thiophene - have the structures shown. They are building units of compounds of biological importance.
These three heterocyclic compounds can interconvert between each other at high temperature and in the presence of catalyst, using source of heteroatom – water, ammonia or hydrogen sulfide.
Pyrrole
Nitrogen atoms in heterocyclic compounds can be described as “pyrrole-like” and “pyridine-like”. Both pyrrole and pyridine contain a nitrogen atom inside their ring structure and each one of these nitrogen's contains a lone pair of electrons, but these atoms are in different state. In the molecule of pyrrole nitrogen atom is in its excited state.
Furan
Furan exhibits aromatic properties like benzene, but due to the greater electronegativity of oxygen, the delocalization is hampered, and aromatic properties are less pronounced.
Hydrogenation of furan: An important furan derivative is furfural, which may be produced by the acid catalyzed dehydration of pentoses.
Thiophene
Consisting of a planar five-membered ring, it is the simplest sulfur-containing aromatic compound. In most of its reactions, it resembles benzene. Thiophene structure can be found in certain natural products and is also incorporated in several pharmacologically active compounds. Biotin (Vitamin H) is a reduced thiophene derivative.
Indole (benzo pyrrole) and its derivatives
Indole is one of the many heterocyclic compounds having fused rings; i.e. rings that share two adjacent ring atoms and the bond between them.
Indole has a benzene ring fused to pyrrole ring:
Tryptophan is naturally found in animal and plant proteins. It is an essential L-amino acid, meaning that humans and certain other animals cannot synthesize it and must obtain it from their diets. Tryptophan is used by the body to manufacture several important substances, including neurotransmitter serotonin.
Serotonin (5-hydroxytryptamine) is a hormone that transmits signals between nerve cells. It also causes blood vessels to narrow. Changes in the level of serotonin in the brain can alter mood. Many natural products and some drugs possess indole structures.
The last compounds in tryptophan degradation are indolylacetic acid and skatole.