Annulation

In organic chemistry, annulation is a chemical reaction in which a new ring is constructed on a molecule.
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:

Benzannulation

The term benzannulated compounds refers to derivatives of cyclic compounds 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	Pyridine
Chromene	Pyran
Isochromene	Pyran
Indole	Pyrrole
Isoindole	Pyrrole
Benzofuran	Furan
Isobenzofuran	Furan
Benzimidazole	Imidazole

In contemporary chemical literature, the term benzannulation also means "construction of benzene rings from acyclic precursors".image:VerkadeProtn.svg|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 between different non-bonding molecular groups in a large ring or macrocycle. See for example atranes.