Polycyclic compound

A chemical compound whose molecular structure contains two or more rings. Its rings may be fused, bridged, or linked, and may include carbon atoms or other elements.

Cyclic compound: A compound whose molecular structure contains at least one closed ring of bonded atoms. Polycyclic compounds are cyclic compounds with more than one ring.
Ring-closing reaction: A chemical reaction that forms a new bond within a molecule to create a ring. Repeated ring closures are a common route to building polycyclic frameworks.
Polycyclic aromatic hydrocarbon: An organic compound made of two or more fused aromatic rings composed chiefly of carbon and hydrogen. This class links fused-ring structures to combustion pollution and environmental persistence.
Spiro compound: A compound whose two rings share exactly one atom. Its rings meet at a single atom rather than sharing a bond or bridge path.
Molecular graph: A graph representing a molecule, with atoms as vertices and bonds as edges. Its cycles provide a precise way to count and compare a compound’s rings.
Intramolecular reaction: A reaction in which reacting groups are part of the same molecule. Many polycyclic syntheses form rings when groups within one molecule react.
Steroid: A lipid characterized by a core of four fused rings, with varied attached groups. The steroid framework underlies hormones, signaling molecules, and many medicines.
Catenane: A molecule composed of two or more mechanically interlocked rings. Its rings are linked topologically but are not necessarily joined by covalent bonds.
Ring fusion: A structural relationship in which two rings share a bond and its two atoms. Fused rings share edges, unlike rings connected only by a bridge or single bond.
Ring strain: An increase in molecular energy caused by geometric constraints in a ring. Small or awkwardly fused rings can store strain that affects stability and reactivity.

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