A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s).^[1] Heterocyclic organic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and applications of organic heterocycles.^[2]

Examples of heterocyclic compounds include all of the nucleic acids, the majority of drugs, most biomass (cellulose and related materials), and many natural and synthetic dyes. More than half of known compounds are heterocycles.^[3] 59% of US FDA-approved drugs contain nitrogen heterocycles.^[4]

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.

• "Heteroatoms" are atoms in the ring other than carbon atoms.
• Names in italics are retained by IUPAC and do not follow the Hantzsch-Widman nomenclature
• Some of the names refer to classes of compounds rather than individual compounds.
• Also no attempt is made to list isomers.

Although subject to ring strain, 3-membered a heterocyclic rings are well characterized.^[6]

In a 7-membered ring, the heteroatom must be able to provide an empty π-orbital (e.g. boron) for "normal" aromatic stabilization to be available; otherwise, homoaromaticity may be possible. Compounds with one heteroatom include:

Those with two heteroatoms include:

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

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 analog is quinoline or isoquinoline. For azepine, benzazepine is the preferred name. Likewise, the compounds with two benzene rings fused to the central heterocycle are carbazole, acridine, and dibenzoazepine. Thienothiophene are the fusion of two thiophene rings. Phosphaphenalenes are a tricyclic phosphorus-containing heterocyclic system derived from the carbocycle phenalene.

The history of heterocyclic chemistry began in the 1800s, in step with the development of organic chemistry. Some noteworthy developments:^[10]

• 1818: Brugnatelli makes alloxan from uric acid
• 1832: Dobereiner produces furfural (a furan) by treating starch with sulfuric acid
• 1834: Runge obtains pyrrole ("fiery oil") 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 (purines and pyrimidines) in the genetic code.

Heterocyclic compounds are pervasive in many areas of life sciences and technology.^[2] Many drugs are heterocyclic compounds.^[11]

• Spiroketals