Annulation (derived from annular, occasionally annelation) in organic chemistry is a chemical reaction in which a new ring is constructed on another molecule (often another ring).

Examples are the Robinson 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) chloride induced ketone - alkene cyclization of 5-methylenecyclooctanone which proceeds through a ketyl intermediate :

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

References