anellation

Anellation refers to the process of forming an additional ring structure onto an existing ring system in organic chemistry. This is a common method for constructing polycyclic molecules, which are prevalent in natural products and pharmaceuticals. The term itself derives from the Latin "anellus," meaning "little ring."

The formation of an anellated ring typically involves a cyclization reaction. This can occur through various mechanisms, including intramolecular addition, substitution, or rearrangement reactions. The starting material usually possesses a functional group on a side chain that can react with the existing ring or a functional group within the ring itself. The specificity of the reaction and the regiochemistry of the newly formed ring are often dictated by the electronic and steric properties of the substrate and the reaction conditions employed.

Anellation is a fundamental concept in synthetic organic chemistry, enabling chemists to build complex molecular architectures with precise control. It plays a crucial role in the synthesis of many important compounds, including steroids, prostaglandins, and various alkaloids. Understanding anellation is key to designing efficient synthetic routes for these and other polycyclic systems.