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.^[1]

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:^[2]

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:

In contemporary chemical literature, the term benzannulation also means "construction of benzene rings from acyclic precursors".^[3]

A transannular interaction in chemistry is any chemical interaction (favorable or nonfavorable) between different non-bonding molecular groups in a large ring or macrocycle.^[5] See for example atranes.