In organic chemistry, annulation (; occasionally annelation) is a chemical reaction in which a new ring is constructed on a molecule.
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: