Annulation

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title: "Annulation" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["ring-forming-reactions"] description: "Chemical reaction constructing a new ring on a molecule" topic_path: "general/ring-forming-reactions" source: "https://en.wikipedia.org/wiki/Annulation" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Chemical reaction constructing a new ring on a molecule ::

In organic chemistry, annulation (; occasionally annelation) is a chemical reaction in which a new ring is constructed on a molecule.

:[[Image:AnnulationStrategies.png|400px|Annulation:

A) intramolecular ring closing

B) transannulation

C) cycloaddition ]]

Examples are the Robinson annulation, Danheiser annulation and certain cycloadditions. Annular molecules are constructed from side-on condensed cyclic segments, for example helicenes and acenes. In transannulation a bicyclic molecule is created by intramolecular carbon-carbon bond formation in a large monocyclic ring. An example is the samarium(II) iodide induced ketone - alkene cyclization of 5-methylenecyclooctanone which proceeds through a ketyl intermediate:

:[[Image:KetoneOlefinCyclization.png|380px|Ketone olefin cyclization]]

Benzannulation

The term benzannulated compounds refers to derivatives of cyclic compounds (usually aromatic) which are fused to a benzene ring. Examples are listed in the table below: ::data[format=table]

Benzannulated derivative	Source of cyclic compound
Benzopyrene	Pyrene
Quinoline	Pyridine
Isoquinoline
Chromene	Pyran
Isochromene
Indole	Pyrrole
Isoindole
Benzofuran	Furan
Isobenzofuran
Benzimidazole	Imidazole
::

In contemporary chemical literature, the term benzannulation also means "construction of benzene rings from acyclic precursors".thumb|upright=1.4|Protonation of [[Verkade base]] induces a transannular bonding, giving an [[atrane]].

Transannular interaction

A transannular interaction in chemistry is any chemical interaction (favorable or nonfavorable) between different non-bonding molecular groups in a large ring or macrocycle. Experimental evidence in support of transannular interactions in diketones Kata Mlinaric-Majerski, Marijana Vinkovic, Danko Škare, Alan P. Marchand Arkivoc DS-339E 2002 Online Article See for example atranes.

References

References

1. "annulation".
2. ''Construction of Bicyclic Ring Systems via a Transannular SmI2-Mediated Ketone-Olefin Cyclization Strategy''Gary A. Molander, Barbara Czakó, and Michael Rheam [[J. Org. Chem.]]; '''2007'''; 72(5) pp 1755 - 1764; (Article) {{doi. 10.1021/jo062292d
3. (January 2022). "Benzannulation Reactions: A Case for Perspective Change From Arene Decoration to Arene Construction". The Chemical Record.
4. (2012). "Encyclopedia of Reagents for Organic Synthesis".

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