Other Epoxidation Methods
Other Epoxidation Methods: Sharpless Epoxidation
The Sharpless epoxidation, also known as asymmetric epoxidation, is an important method for the preparation of enantiomerically pure epoxy alcohols. The reaction is specifically designed for allylic alcohols.
The Sharpless epoxidation with allylic alcohols is carried out in the presence of catalytic amounts of tetraisopropoxy titanium and one of the two enantiomers of diethyl tartrate using tert-butyl hydroperoxide as the oxidizing agent. Stereochemical control is achieved by forming a complex of the titanium compound with the alkene, the tartrate and the peroxide. Differences in the spatial arrangements of the two enantiomeric tartrates in the complex allow for the formation of stereochemically different epoxides.
The advantage of the Sharpless epoxidation lies in the fact that the chiral auxiliary (tartrate) does not react per se and can be recovered after completion of the reaction. Additionally, both forms of diethyl tartrate are inexpensive and commercially available. The reaction is also very attractive because, under otherwise identical reaction conditions, either of the stereoisomeric epoxides can be obtained depending on the tartrate used ((2R, 3R) oder (2S, 3S)).