Annulation

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.

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.

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