{{short description|Chemical reaction constructing a new ring on a molecule}} {{distinguish|Annulment}}
In organic chemistry, '''annulation''' ({{ety|la|anellus|little ring}}; occasionally '''annelation''') is a chemical reaction in which a new ring is constructed on a molecule.<ref>{{GoldBookRef|title=annulation|file=A00367 }}{{GoldBookRef|title=annelation|file=A00365 }}</ref>
:[[Image:AnnulationStrategies.png|400px|Annulation: <br /> '''A''') intramolecular ring closing <br /> '''B''') transannulation<br /> '''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:<ref>''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}}</ref>
: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: {| class="wikitable" style="text-align:center" ! Benzannulated derivative !! Source of cyclic compound |- | Benzopyrene || Pyrene |- | Quinoline |rowspan=2| Pyridine |- | Isoquinoline |- | Chromene |rowspan=2| Pyran |- | Isochromene |- | Indole |rowspan=2| Pyrrole |- | Isoindole |- | Benzofuran |rowspan=2| Furan |- | Isobenzofuran |- | Benzimidazole || Imidazole |} In contemporary chemical literature, the term benzannulation also means "construction of benzene rings from acyclic precursors".<ref>{{Cite journal |last1=Swami |first1=Bhawna |last2=Yadav |first2=Deepak |last3=Menon |first3=Rajeev S. |date=January 2022 |title=Benzannulation Reactions: A Case for Perspective Change From Arene Decoration to Arene Construction |url=https://onlinelibrary.wiley.com/doi/10.1002/tcr.202100249 |journal=The Chemical Record |language=en |volume=22 |issue=1 |article-number=e202100249 |doi=10.1002/tcr.202100249 |pmid=34796605 |s2cid=244403005 |issn=1527-8999|url-access=subscription }}</ref>[[image:VerkadeProtn.svg|thumb|upright=1.4|Protonation of Verkade base induces a transannular bonding, giving an atrane.<ref>{{cite book|doi=10.1002/047084289X.rn00702.pub2|chapter=Proazaphosphatrane|title=Encyclopedia of Reagents for Organic Synthesis|year=2012|last1=Verkade|first1=John G.|last2=Urgaonkar|first2=Sameer|isbn=978-0-471-93623-7}}</ref>]]
==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.<ref> ''Experimental evidence in support of transannular interactions in diketones'' Kata Mlinaric-Majerski, Marijana Vinkovic, Danko Škare, Alan P. Marchand Arkivoc DS-339E '''2002''' [http://www.arkat-usa.org/ark/journal/2002/I04_Sunko/DS-339E/339E.asp Online Article] {{Webarchive|url=https://web.archive.org/web/20060504182220/http://www.arkat-usa.org/ark/journal/2002/I04_Sunko/DS-339E/339E.asp |date=2006-05-04 }}</ref> See for example atranes.
==References== {{reflist}}
Category:Ring forming reactions