Iminium salt-catalysed asymmetric epoxidation
This account focuses on the iminium-salt mediated asymmetric epoxidation of olefins including recently developed ranges of novel chiral iminium salts that effect the asymmetric epoxidation of various olefins in good to excellent enantioselectivites. High enantioselectivities (>78% ee) were originally reported under homogeneous aqueous solvent systems utilising Oxone as the oxidant. More recent developments have led to the first examples of non-aqueous solvent systems utilising TPPP (tetraphenylphosphonium monoperoxysulfate) as the oxidant, affording ees of up to 98 percent.
INTRODUCTION: EPOXIDES, ORGANOCATALYTIC ASYMMETRIC EPOXIDATION
Enantiomerically enriched chiral epoxides are highly sought after, and have been identified as key building blocks in the biosynthesis of complex natural products (1). Natural products and drug molecules containing an epoxide moiety, such as Epithilone B and Ixempra (2, 3), have been shown to exert strong biological activities (Figure 1).
Many successful reactions have been developed to prepare chiral epoxides. K.B. Sharpless shared the Nobel prize for chemistry in 2001 for his work on the asymmetric epoxidation of allylic alcohols with chiral tartrate-titanium catalysts (4). Other metal catalysed epoxidations have been extensively reviewed in the literature (5), particularly the work of Jaco