Chapter 3 Three-membered ring systems

Abstract

This chapter provides information on three-membered ring systems. Three-membered ring systems contain a wealth of worthy synthetic targets, and compact and efficient intermediates and reagents. The chapter discusses the preparations and reactions of epoxides, aziridines, and azirines. Particular emphasis has been placed on practical pathways to enantiopure epoxides, whether derived from the chiral pool or by asymmetric methods. One of the most seminal protocols in this regard is the Sharpless asymmetric epoxidation (AE), a type of chemistry both fundamental and contemporary. Epoxides are highly reactive species, which provide products containing versatile oxygen functionality in the products, a combination highly coveted for synthetic applications. Alkenes undergo diastereoselective aziridination in the presence of chiral 3-acetoxyaminoquinazolinones, prepared in situ by acetoxylation of the corresponding 3-aminoquinazolinones. Azirines can be prepared in optically enriched form by the asymmetric Neber reaction mediated by Cinchona alkaloids. Azirines, which have pendant electron-withdrawing functionality, undergo an interesting reaction with aldehydes and acetone via a so-called “ 3-X mode ”, a reactivity arising from the pushing effect on the azirine ring by the active methylene center.