The mechanism of nucleophilic epoxidation begins with conjugate addition of the peroxide (or other O-nucleophilic species) to the enone. Metal ions or conjugate acids present in solution coordinate both the peroxide oxygen and the enolate oxygen. Attack of the enolate on the peroxide oxygen generates the epoxide product and releases a leaving group.
Because the process is stepwise, the configuration of the carbon-carbon double bond is not necessarily preserved. Both cis and trans enones form trans epoxides under nearly all nucleophilic epoxidation conditions (methods employing lanthanide-BINOL systems are the exception).