Cycliseringar

Cycliseringar, or cyclizations, are chemical processes in which a linear or branched molecule closes to form a ring. The term is often used for intramolecular ring closure, which converts a chain into a cyclic product, but related processes also occur in enzymes and on polymers.

Cyclization can proceed through several mechanisms. Electrophilic cyclizations form a new bond by attack of an internal nucleophile on an electrophilic site; nucleophilic cyclizations involve internal attack of an anionic center; radical cyclizations use radical intermediates; and cycloadditions generate rings by concerted bonding. Ring-closing metathesis (RCM) is a widely used catalytic method for forming carbon–carbon rings, especially 5- to 6-membered rings.

Ring size and kinetics: Five- and six-membered rings are generally favored due to lower strain and favorable entropy. Larger rings can form but often require specific conditions. Cyclization efficiency depends on concentration (lower concentrations favor cyclization over intermolecular reactions) and on the presence of leaving groups or activating groups.

In biosynthesis, many natural products originate from enzyme-catalyzed cyclizations that assemble rings during polyketide, terpene, or peptide biosynthesis. In polymer science, cyclization can convert linear polymers into cyclic polymers, impacting properties such as viscosity and thermal stability.

Applications: Pharmaceuticals, materials, and chemical biology rely on cyclized motifs for activity and stability.