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Ring-closing metathesis (RCM) is an intramolecular olefin metathesis in which two alkene units of the same molecule exchange alkylidene fragments at a metal–carbene catalyst to produce a cyclic alkene and a volatile olefin, typically ethylene ^[1]. In advanced synthetic organic chemistry the concept is bounded by the intramolecular pairing of the two alkenes and by the metathesis elementary steps of (2+2) cycloaddition and cycloreversion. Essential features are a diene (or enyne) substrate whose tether length sets the ring size, a well-defined ruthenium, molybdenum, or tungsten alkylidene catalyst, and an equilibrium that is driven toward the cycle by removal of the small-olefin byproduct ^[2]. RCM constructs five- to medium-sized rings and macrocycles, including oxygen- and nitrogen-containing heterocycles, when the tether geometry permits productive approach of the two olefins ^[1][3]. The transformation is distinguished from ring-opening metathesis by the direction of the equilibrium (cycle-forming versus cycle-opening) and from intermolecular cross-metathesis by the unimolecular connection of the two reacting alkenes. Conceptual limits exclude metal-catalyzed cyclizations that proceed by oxidative cyclization or radical addition rather than by alkylidene exchange.