Heterocyclic compound

A heterocyclic compound or ring structure is a hydrocarbon-based cyclic compound that contains at least one heteroatom as member of its ring. Heterocyclic organic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and applications of organic heterocycles.
Examples of heterocyclic compounds include all of the nucleic acids, the majority of drugs, most biomass, and many natural and synthetic dyes. More than half of known compounds are heterocycles. 59% of US FDA-approved drugs contain nitrogen heterocycles.

Classification

The study of organic heterocyclic chemistry focuses especially on organic unsaturated derivatives, and the preponderance of work and applications involves unstrained organic 5- and 6-membered rings. Included are pyridine, thiophene, pyrrole, and furan. Another large class of organic heterocycles refers to those fused to benzene rings. For example, the fused benzene derivatives of pyridine, thiophene, pyrrole, and furan are quinoline, benzothiophene, indole, and benzofuran, respectively. The fusion of two benzene rings gives rise to a third large family of organic compounds. Analogs of the previously mentioned heterocycles for this third family of compounds are acridine, dibenzothiophene, carbazole, and dibenzofuran, respectively.
Heterocyclic organic compounds can be usefully classified based on their electronic structure. The saturated organic heterocycles behave like the acyclic derivatives. Thus, piperidine and tetrahydrofuran are conventional amines and ethers, with modified steric profiles. Therefore, the study of organic heterocyclic chemistry focuses on organic unsaturated rings.

Inorganic rings

Some heterocycles contain no carbon. Examples are borazine, hexachlorophosphazene, and trithiazyl trichloride. In comparison with organic heterocycles, which have numerous commercial applications, inorganic ring systems are mainly of theoretical interest. IUPAC recommends the Hantzsch-Widman nomenclature for naming heterocyclic compounds.

3-membered rings

Although subject to ring strain, 3-membered heterocyclic rings are well characterized.

4-membered rings

5-membered rings

The 5-membered ring compounds containing two heteroatoms, at least one of which is nitrogen, are collectively called the azoles. Thiazoles and isothiazoles contain a sulfur and a nitrogen atom in the ring. Dithioles have two sulfur atoms.
A large group of 5-membered ring compounds with three or more heteroatoms also exists. One example is the class of dithiazoles, which contain two sulfur atoms and one nitrogen atom.

6-membered rings

The 6-membered ring compounds containing two heteroatoms, at least one of which is nitrogen, are collectively called the azines. Thiazines contain a sulfur and a nitrogen atom in the ring. Dithiines have two sulfur atoms.
Six-membered rings with five heteroatoms
The hypothetical chemical compound with five nitrogen heteroatoms would be pentazine.
Six-membered rings with six heteroatoms
The hypothetical chemical compound with six nitrogen heteroatoms would be hexazine. Borazine is a six-membered ring with three nitrogen heteroatoms and three boron heteroatoms.

7-membered rings

In a 7-membered ring, the heteroatom must be able to provide an empty π-orbital for "normal" aromatic stabilization to be available; otherwise, homoaromaticity may be possible.

8-membered rings

is an eight-membered ring with four nitrogen heteroatoms and four boron heteroatoms.

9-membered rings

Images of rings with one heteroatom

Fused/condensed rings

Heterocyclic rings systems that are formally derived by fusion with other rings, either carbocyclic or heterocyclic, have a variety of common and systematic names. For example, with the benzo-fused unsaturated nitrogen heterocycles, pyrrole provides indole or isoindole depending on the orientation. The pyridine derivative is quinoline or isoquinoline, and the class of analogues with two nitrogen atoms is known as the benzodiazines. For the azepine derivative, benzazepine is the preferred name. Likewise, the compounds with two benzene rings fused to the central heterocycle are carbazole, acridine, and dibenzoazepine. Heptazine is a tricyclic nitrogen-containing heterocyclic system derived by fusion of three triazine rings, and analog of the carbocycle phenalene.

History of heterocyclic chemistry

The history of heterocyclic chemistry began in the 1800s, in step with the development of organic chemistry. Some noteworthy developments:

1818: Brugnatelli makes alloxan from uric acid.
1832: Dobereiner produces furfural by treating starch with sulfuric acid.
1834: Runge obtains pyrrole by dry distillation of bones.
1906: Friedlander synthesizes indigo dye, allowing synthetic chemistry to displace a large agricultural industry.
1936: Treibs isolates chlorophyll derivatives from crude oil, explaining the biological origin of petroleum.
1951: Chargaff's rules are described, highlighting the role of heterocyclic compounds in the genetic code.
Uses

Heterocyclic compounds are pervasive in many areas of life sciences and technology. Many drugs are heterocyclic compounds. Among the modifications to the family of antitumor compounds, heterocyclic organic compounds have been extensively applied by many groups in order to modify the reactivity profile. Pyrrole, pyrimidine, indole, quinoline and purine are few classes of heterocycles which showed interesting cytotoxicity profiles, which can be highly beneficial when developing cancer drugs.