Johnson–Corey–Chaykovsky reaction converts aldehydes and ketones into epoxides through sulfur ylide mediated one-carbon transfer. First introduced by A. William Johnson and later refined into a broadly useful synthetic method by E. J. Corey and Michael Chaykovsky, it established sulfur ylides as practical homologation reagents for carbonyl chemistry. The transformation proceeds by ylide addition to the carbonyl group followed by intramolecular ring closure with expulsion of sulfide or sulfoxide.