Annulation

Not to be confused with Annulment.

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

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:

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:

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".

File:VerkadeProtn.svg

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. See for example atranes.