Cyclisierungsreaktionen

Cyclization, also spelled cyclisation in British English, is the chemical process by which a linear or branched molecule forms a ring. Cyclization can occur intramolecularly, when a single molecule closes on itself, or intermolecularly, when two molecular units join to form a cyclic product. In organic synthesis it is a key strategic step used to construct ring systems found in natural products, pharmaceuticals, and materials.

Common mechanisms include radical cyclizations (where a radical generated at one end adds to another part of the same molecule to close a ring), ionic cyclizations (nucleophilic or electrophilic attack leading to ring formation), and pericyclic processes (such as electrocyclizations and cycloadditions). Ring-closing metathesis (RCM) is an important catalyzed method that forms cyclic alkenes from dienes; intramolecular Diels–Alder or enyne cyclizations are other examples. Enzyme-catalyzed cyclizations also occur in biosynthetic pathways to produce cyclic peptides, terpenoids, and alkaloids.

Ring size and geometry influence cyclization outcomes. Five- and six-membered rings are typically favored due to lower ring strain and favorable enthalpy, while three- and four-membered rings are more strained and larger rings face entropic barriers. Reaction conditions, tether length, substituents, and the presence of catalysts or directing groups all affect feasibility and selectivity.

Applications include the construction of natural products and pharmaceuticals, macrocyclic ligands, and polymer precursors. Cyclization strategies are essential in cascade or tandem reactions that form multiple bonds or rings in a single operation, contributing to efficient synthesis of complex structures.