The Johnson–Corey–Chaykovsky reaction (sometimes referred to as the Corey–Chaykovsky reaction or CCR) is a chemical reaction used in organic chemistry for the synthesis of epoxides, aziridines, and cyclopropanes. It was discovered in 1961 by A. William Johnson and developed significantly by E. J. Corey and Michael Chaykovsky. The reaction involves addition of a sulfur ylide to a ketone, aldehyde, imine, or enone to produce the corresponding 3-membered ring. The reaction is diastereoselective favoring trans substitution in the product regardless of the initial stereochemistry. The synthesis of epoxides via this method serves as an important retrosynthetic alternative to the traditional epoxidation reactions of olefins. The reaction is most often employed for epoxidation via methylene transfer, and to this end has been used in several notable total syntheses (See Synthesis of epoxides below). Additionally detailed below are the history, mechanism, scope, and enantioselective variants of the reaction. Several reviews have been published. History The original publication by Johnson concerned the reaction of 9-dimethylsulfonium fluorenylide with substituted benzaldehyde derivatives. The attempted Wittig-like reaction failed and a benzalfluorene oxide was obtained instead, noting that "Reaction between the sulfur ylid and benzaldehydes did not afford benzalfluorenes as had the phosphorus and arsenic ylids." The subsequent development of (dimethyloxosulfaniumyl)methanide, (CH3)2SOCH2 and (dimethylsulfaniumyl)methanide, (CH3)2SCH2 (known as Corey–Chaykovsky reagents) by Corey and Chaykovsky as efficient methylene-transfer reagents established the reaction as a part of the organic canon. Mechanism The reaction mechanism for the Johnson–Corey–Chaykovsky reaction consists of nucleophilic addition of the ylide to the carbonyl or imine group. A negative charge is transferred to the heteroatom and because the sulfonium cation is a good leaving group it gets expelled forming the ring. In the related Wittig reaction, the formation of the much stronger phosphorus-oxygen double bond prevents oxirane formation and instead, olefination takes place through a 4-membered cyclic intermediate. The trans diastereoselectivity observed results from the reversibility of the initial addition, allowing equilibration to the favored anti betaine over the syn betaine. Initial addition of the ylide results in a betaine with adjacent charges; density functional theory calculations have shown that the rate-limiting step is rotation of the central bond into the conformer necessary for backside attack on the sulfonium. The degree of reversibility in the initial step (and therefore the diastereoselectivity) depends on four factors, with greater reversibility corresponding to higher selectivity: Stability of the substrate with higher stability leading to greater reversibility by favoring the starting