Sulfur Ylide Mediated Catalytic Asymmetric Epoxidation and Aziridination

Abstract

The development of catalytic methods for the synthesis of non-racemic epoxides has been a long standing goal in asymmetric synthesis. For epoxide synthesis, attention has largely focused on the asymmetric oxidation of alkenes and good enantioselectivities are now beginning to emerge for an increasing range of substrates.^1-3 Alkenes are themselves commonly obtained by Wittig reaction from the corresponding aldehyde or ketone and so epoxidation is usually a two step process from carbonyl compounds. However, in terms of efficiency and atom economy this overall process is poor compared to direct epoxidation of carbonyl compounds using sulfur ylides. However, in terms of catalysis and asymmetric induction, oxidation of alkenes is still superior as epoxidation of carbonyl compounds using sulfur ylides usually requires stoichiometric amounts of sulfides/sulfur ylides and only gives moderate enantioselectivities.⁴ We have described a catalytic process for epoxidation involving sulfur ylides which now overcomes this limitation (Scheme 1).⁵ Whilst only low levels of enantioselectivity were originally achieved we now report significant improvements and describe chiral sulfides which provide high levels of asymmetric induction.⁶