Heterocyclic compound

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structures and names of common and not so common heterocycle compounds
Pyridine, a heterocyclic compound
cyclo-Octasulfur, a homocyclic compound

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][2][3][4][5][6][7][8] Heterocyclic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and applications of these heterocycles.[9]

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.


Although heterocyclic chemical compounds may be inorganic compounds or organic compounds, most contain at least one carbon. While atoms that are neither carbon nor hydrogen are normally referred to in organic chemistry as heteroatoms, this is usually in comparison to the all-carbon backbone. But this does not prevent a compound such as borazine (which has no carbon atoms) from being labelled "heterocyclic". IUPAC recommends the Hantzsch-Widman nomenclature for naming heterocyclic compounds.

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 conjugated system, pi system.

3-membered rings[edit]

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
Boron Borirane Borirene
Nitrogen Aziridine Azirine
Oxygen Oxirane (ethylene oxide, epoxides) Oxirene
Phosphorus Phosphirane Phosphirene
Sulfur Thiirane (episulfides) Thiirene

Those with two heteroatoms include:

Heteroatom Saturated Unsaturated
Nitrogen Diaziridine Diazirine
Nitrogen/oxygen Oxaziridine
Oxygen Dioxirane

4-membered rings[edit]

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 Diazete
Oxygen Dioxetane Dioxete
Sulfur Dithietane Dithiete

5-membered rings[edit]

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

Five-membered rings with one heteroatom[edit]

Heteroatom Saturated Unsaturated
Antimony Stibolane Stibole
Arsenic Arsolane Arsole
Bismuth Bismolane Bismole
Boron Borolane Borole
Nitrogen Pyrrolidine (Azolidine is not used) Pyrrole (Azole is not used)
Oxygen Tetrahydrofuran Furan
Phosphorus Phospholane Phosphole
Selenium Selenolane Selenophene
Silicon Silacyclopentane Silole
Sulfur Tetrahydrothiophene Thiophene
Tin Stannolane Stannole

Five-membered rings with two heteroatoms[edit]

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
Imidazole (Imidazoline)
Pyrazole (Pyrazoline)
Nitrogen/oxygen Oxazolidine
Oxazole (Oxazoline)
Nitrogen/sulfur Thiazolidine
Thiazole (Thiazoline)
Oxygen/oxygen Dioxolane
Sulfur/sulfur Dithiolane

Five-membered rings with at least three heteroatoms[edit]

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
2 × Nitrogen / 1 × sulfur Thiadiazole
1 × Nitrogen / 2 × oxygen Dioxazole
1 × Nitrogen / 2 × sulfur Dithiazole
4 × Nitrogen Tetrazole
4 × Nitrogen/1 × Oxygen Oxatetrazole
4 × Nitrogen/1 × Sulfur Thiatetrazole
5 × Nitrogen Pentazole

6-membered rings[edit]

Six-membered rings with a single heteroatom:

Heteroatom Saturated Unsaturated
Antimony Stibinin[10]
Arsenic Arsinane Arsinine
Bismuth Bismin[11]
Boron Borinane Borinine
Germanium Germinane Germine
Nitrogen Piperidine (Azinane is not used) Pyridine (Azine is not used)
Oxygen Tetrahydropyran Pyran (2H-Oxine is not used)
Phosphorus Phosphinane Phosphinine
Silicon Silinane Siline
Sulfur Thiane Thiopyran (2H-Thiine is not used)
Tin Stanninane Stannine

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 Dithiin

With three heteroatoms:

Heteroatom Saturated Unsaturated
Nitrogen Hexahydro-1,3,5-triazine Triazine
Oxygen Trioxane
Sulfur Trithiane

With four heteroatoms:

Heteroatom Saturated Unsaturated
Nitrogen Tetrazine

With five heteroatoms:

Heteroatom Saturated Unsaturated
Nitrogen Pentazine

The hypothetical compound with six nitrogen heteroatoms would be hexazine.

7-membered rings[edit]

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
Boron Borepin
Nitrogen Azepane Azepine
Oxygen Oxepane Oxepine
Sulfur Thiepane Thiepine

Those with two heteroatoms include:

Heteroatom Saturated Unsaturated
Nitrogen Diazepane Diazepine
Nitrogen/sulfur Thiazepine

8-membered rings[edit]

Heteroatom Saturated Unsaturated
Nitrogen Azocane Azocine
Oxygen Oxocane Oxocine
Sulfur Thiocane Thiocine

9-membered rings[edit]

Heteroatom Saturated Unsaturated
Nitrogen Azonane Azonine
Oxygen Oxonane Oxonine
Sulfur Thionane Thionine


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
Structure of Aziridine Structure of Oxirane Structure of Thiirane Structure of Azirine Structure of Oxirene Structure of Thiirene
4-Atom Ring Azetidine Oxetane Thietane Azete Oxete Thiete
Structure of Acetidine Structure of Oxetane Structure of Thietane Structure of Azete Structure of Oxete Structure of Thiete
5-Atom Ring Pyrrolidine Oxolane Thiolane Pyrrole Furan Thiophene
Structure of Pyrrolidine Structure of Oxolane Structure of Thiolane Structure of Pyrrole Structure of Furan Structure of Thiophene
6-Atom Ring Piperidine Oxane Thiane Pyridine Pyran Thiopyran
Structure of Piperidine Structure of Oxane Structure of Thiane Structure of Pyridine Structure of Pyran Structure of Thiopyran
7-Atom Ring Azepane Oxepane Thiepane Azepine Oxepine Thiepine
Structure of Azepane Structure of Oxepane Structure of Thiepane Structure of Azepine Structure of Oxepine Structure of Thiepine

Fused rings[edit]

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.

History of heterocyclic chemistry[edit]

The history of heterocyclic chemistry began in the 1800s, in step with the development of organic chemistry. Some noteworthy developments:[12]
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.


Heterocyclic compounds are pervasive in many areas of life sciences and technology.[9] Many drugs are heterocyclic compounds.[13]


  1. ^ IUPAC Gold Book heterocyclic compounds
  2. ^ Kumar, Achutha Dileep; Prabhudeva, Malledevarapura Gurumurthy; Bharath, Srinivasan; Kumara, Karthik; Lokanath, Neratur Krishnappagowda; Kumar, Kariyappa Ajay (October 2018). "Design and Amberlyst-15 mediated synthesis of novel thienyl-pyrazole carboxamides that potently inhibit Phospholipase A2 by binding to an allosteric site on the enzyme". Bioorganic Chemistry. 80: 444–452. doi:10.1016/j.bioorg.2018.06.023. ISSN 0045-2068.
  3. ^ Dileep Kumar, Achutha; Bharath, Srinivasan; Dharmappa, Rekha N.; Naveen, Shivalingegowda; Lokanath, Neratur Krishnappagowda; Ajay Kumar, Kariyappa (2018-04-24). "Design, synthesis and spectroscopic and crystallographic characterisation of novel functionalized pyrazole derivatives: biological evaluation for their cytotoxic, angiogenic and antioxidant activities". Research on Chemical Intermediates. 44 (9): 5635–5652. doi:10.1007/s11164-018-3445-6. ISSN 0922-6168.
  4. ^ Prabhudeva, Malledevarapura Gurumurthy; Bharath, Srinivasan; Kumar, Achutha Dileep; Naveen, Shivalingegowda; Lokanath, Neratur Krishnappagowda; Mylarappa, Bantaganahalli Ningappa; Kumar, Kariyappa Ajay (August 2017). "Design and environmentally benign synthesis of novel thiophene appended pyrazole analogues as anti-inflammatory and radical scavenging agents: Crystallographic , in silico modeling, docking and SAR characterization". Bioorganic Chemistry. 73: 109–120. doi:10.1016/j.bioorg.2017.06.004. ISSN 0045-2068.
  5. ^ Lokeshwari, Devirammanahalli Mahadevaswamy; Achutha, Dileep Kumar; Srinivasan, Bharath; Shivalingegowda, Naveen; Krishnappagowda, Lokanath Neratur; Kariyappa, Ajay Kumar (August 2017). "Synthesis of novel 2-pyrazoline analogues with potent anti-inflammatory effect mediated by inhibition of phospholipase A2: Crystallographic, in silico docking and QSAR analysis". Bioorganic & Medicinal Chemistry Letters. 27 (16): 3806–3811. doi:10.1016/j.bmcl.2017.06.063. ISSN 0960-894X.
  6. ^ Lokeshwari, Devirammanahalli Mahadevaswamy; Rekha, Nanjappagowda Dharmappa; Srinivasan, Bharath; Vivek, Hamse Kameshwar; Kariyappa, Ajay Kumar (July 2017). "Design, synthesis of novel furan appended benzothiazepine derivatives and in vitro biological evaluation as potent VRV-PL-8a and H + /K + ATPase inhibitors". Bioorganic & Medicinal Chemistry Letters. 27 (14): 3048–3054. doi:10.1016/j.bmcl.2017.05.059. ISSN 0960-894X.
  7. ^ Raghavendra, Kanchipura Ramachandrappa; Renuka, Nagamallu; Kameshwar, Vivek H.; Srinivasan, Bharath; Ajay Kumar, Kariyappa; Shashikanth, Sheena (August 2016). "Synthesis of lignan conjugates via cyclopropanation: Antimicrobial and antioxidant studies". Bioorganic & Medicinal Chemistry Letters. 26 (15): 3621–3625. doi:10.1016/j.bmcl.2016.06.005. ISSN 0960-894X.
  8. ^ Nagamallu, Renuka; Srinivasan, Bharath; Ningappa, Mylarappa B.; Kariyappa, Ajay Kumar (January 2016). "Synthesis of novel coumarin appended bis(formylpyrazole) derivatives: Studies on their antimicrobial and antioxidant activities". Bioorganic & Medicinal Chemistry Letters. 26 (2): 690–694. doi:10.1016/j.bmcl.2015.11.038. ISSN 0960-894X.
  9. ^ a b Thomas L. Gilchrist "Heterocyclic Chemistry" 3rd ed. Addison Wesley: Essex, England, 1997. 414 pp. ISBN 0-582-27843-0.
  10. ^ "Stibinin". chemspider. Royal Society of Chemistry. Retrieved 11 June 2018.
  11. ^ "Bismin". ChemSpider. Royal Society of Chemistry. Retrieved 11 June 2018.
  12. ^ E. Campaigne "Adrien Albert and the Rationalization of Heterocyclic Chemistry" J. Chemical Education 1986, Volume 6, 860. doi:10.1021/ed063p860
  13. ^ Companies with the highest number of patents related to heterocyclic compounds.

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