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Heterocycle

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Heterocycle

"Ring structure" redirects here. For the literary structure, see Chiastic structure.



A heterocyclic compound is a cyclic compound that has atoms of at least two different elements as members of its ring(s).[1] The counterparts of heterocyclic compounds are homocyclic compounds, the rings of which are made of a single element.

Although heterocyclic compounds may be inorganic, most contain at least one carbon. Since in organic chemistry non-carbons usually are considered to replace carbon atoms, they are called heteroatoms, meaning 'different from carbon and hydrogen' (rings of heteroatoms of the same element are homocyclic). The IUPAC recommends the Hantzsch-Widman nomenclature for naming heterocyclic compounds.

Heterocyclic chemistry is the branch of chemistry dealing with synthesis, properties, and applications of heterocycles.

Classification based on electronic structure

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

3-Membered rings

Heterocycles with three atoms in the ring are more reactive because of ring strain. Those containing one heteroatom are, in general, stable. Those with two heteroatoms are more likely to occur as reactive intermediates.
Common 3-membered heterocycles with one heteroatom are:

Heteroatom Saturated Unsaturated
Nitrogen Aziridine Azirine
Oxygen Oxirane (ethylene oxide, epoxides) Oxirene
Sulfur Thiirane (episulfides) Thiirene

Those with two heteroatoms include:

Heteroatom Saturated Unsaturated
Nitrogen Diazirine
Nitrogen/oxygen Oxaziridine
Oxygen Dioxirane

4-Membered rings

Compounds with one heteroatom:

Heteroatom Saturated Unsaturated
Nitrogen Azetidine Azete
Oxygen Oxetane Oxete
Sulfur Thietane Thiete

Compounds with two heteroatoms:

Heteroatom Saturated Unsaturated
Nitrogen Diazetidine
Oxygen Dioxetane Dioxete
Sulfur Dithietane Dithiete

5-Membered rings

With heterocycles containing five atoms, the unsaturated compounds are frequently more stable because of aromaticity.

Five-membered rings with one heteroatom:

Heteroatom Saturated Unsaturated
Nitrogen Pyrrolidine (Azolidine is not used) Pyrrole (Azole is not used)
Oxygen Tetrahydrofuran(Oxolane is rare) Furan (Oxole is not used)
Sulfur Thiolane Thiophene (Thiole is not used)
Boron Borolane Borole
Phosphorus Phospholane Phosphole
Arsenic Arsolane Arsole
Antimony Stibolane Stibole
Bismuth Bismolane Bismole
Silicon Silolane Silole
Tin Stannolane Stannole

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. Dithiolanes have two sulfur atoms.

Heteroatom Saturated Unsaturated (and partially unsaturated)
Nitrogen/nitrogen Imidazolidine
Pyrazolidine
Imidazole (Imidazoline)
Pyrazole (Pyrazoline)
Nitrogen/oxygen Oxazolidine
Isoxazolidine
Oxazole (Oxazoline)
Isoxazole
Nitrogen/sulfur Thiazolidine
Isothiazolidine
Thiazole (Thiazoline)
Isothiazole
Oxygen/oxygen Dioxolane
Sulfur/sulfur Dithiolane

A large group of 5-membered ring compounds with three heteroatoms also exists. One example is dithiazoles that contain two sulfur and a nitrogen atom.

Heteroatom Saturated Unsaturated
3 × Nitrogen Triazoles
2 × Nitrogen / 1 × oxygen Furazan
Oxadiazole
2 × Nitrogen / 1 × sulfur Thiadiazole
1 × Nitrogen / 2 × sulfur Dithiazole

Five-member ring compounds with four heteroatoms:

Heteroatom Saturated Unsaturated
4 × Nitrogen Tetrazole

With 5-heteroatoms, the compound may be considered inorganic rather than heterocyclic. Pentazole is the all nitrogen heteroatom unsaturated compound.

6-Membered rings

Six-membered rings with a single heteroatom:

Heteroatom Saturated Unsaturated
Nitrogen Piperidine (Azinane is not used) Pyridine (Azine is not used)
Oxygen Oxane Pyran (2H-Oxine is not used)
Sulfur Thiane Thiopyran (2H-Thiine is not used)
Silicon Salinane Siline
Germanium Germinane Germine
Tin Stanninane Stannine
Boron Borinane Borinine
Phosphorus Phosphinane Phosphinine
Arsenic Arsinane Arsinine

With two heteroatoms:

Heteroatom Saturated Unsaturated
Nitrogen / nitrogen Piperazine Diazines
Oxygen / nitrogen Morpholine Oxazine
Sulfur / nitrogen Thiomorpholine Thiazine
Oxygen / oxygen Dioxane Dioxine
Sulfur / sulfur Dithiane Dithiine

With three heteroatoms:

Heteroatom Saturated Unsaturated
Nitrogen Triazine
Oxygen Trioxane

With four heteroatoms:

Heteroatom Saturated Unsaturated
Nitrogen Tetrazine

The hypothetical compound with six nitrogen heteroatoms would be hexazine.

7-Membered rings

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

Heteroatom Saturated Unsaturated
Nitrogen Azepane Azepine
Oxygen Oxepane Oxepine
Sulfur Thiepane Thiepine

Those with two heteroatoms include:

Heteroatom Saturated Unsaturated
Nitrogen Homopiperazine Diazepine
Nitrogen/sulfur Thiazepine

8-Membered rings

Heteroatom Saturated Unsaturated
Nitrogen Azocane Azocine
Sulfur

Images

Names in italics are retained by IUPAC and they do not follow the Hantzsch-Widman nomenclature
Saturated Unsaturated
Heteroatom Nitrogen Oxygen Sulfur Nitrogen Oxygen Sulfur
3 Atom Ring Aziridine Oxirane Thiirane Azirine Oxirene Thiirene
4 Atom Ring Azetidine Oxetane Thietane Azete Oxete Thiete
5 Atom Ring Pyrrolidine Oxolane Thiolane Pyrrole Furan Thiophene
6 Atom Ring Piperidine Oxane Thiane Pyridine Pyran Thiopyran
7 Atom Ring Azepane Oxepane Thiepane Azepine Oxepine Thiepine

Fused 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 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.

History of heterocyclic chemistry

The history of heterocyclic chemistry began in the 1800s, in step with the development of organic chemistry. Some noteworthy developments:[2]
1818: Brugnatelli isolates 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 chlorophyl 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.

Commercial Exploitation

Leading companies with a vast amount of patents related to heterocyclic compounds are Bayer, Merck, Ciba-Geigy, Pfizer, Eli Lily, BASF, Hoffmann La Roche, ER Sqibb, Warner Lambert and Hoechst.[3]

References

External links

  • Hantzsch-Widman nomenclature, IUPAC
  • Heterocyclic amines in cooked meat, US CDC
  • List of known and probable carcinogens, American Cancer Society
  • (more comprehensive)

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