Urea derivatives are highly active catalysts for the base-mediated generation of terminal epoxides from aldehydes and trimethylsulfonium iodide.

N,N'-Diarylureas have been shown to efficiently catalyse sulfonium ylide-mediated aldehyde epoxidation reactions for the first time. These processes are of broad scope and can be coupled with a subsequent Cu(II) ion-catalysed Meinwald rearrangement to give an efficient and convenient protocol for aldehyde homologation without intermediate purification.