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:[1]
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 | |
|---|---|---|
| Pyrene | ||
| Pyridine | ||
| Isoquinoline | ||
| Chromene | Pyran | |
| Isochromene | ||
| Pyrrole | ||
| Isoindole | ||
| Furan | ||
| Isobenzofuran | ||
| Imidazole |
A transannular interaction in chemistry is any chemical interaction (favorable or nonfavorable) between different non-bonding molecular groups in a large ring or macrocycle.[4] See for example atranes.