Sharpless Epoxidation

Recall that epoxidation of alkenes by either peroxy acids or halohydrin cyclization follows syn addition and yields a racemic mixture of epoxides.

The formation of just one enantiomer in excess from alkenes can be achieved through a chiral catalyst, which favors epoxidation at only one facet of the alkene.

A chiral catalyst acts by lowering the activation energy for the formation of one enantiomer more than the other, leading to enantiomeric excess.

This forms the basis for Sharpless asymmetric epoxidation that converts allylic alcohols into chiral epoxy alcohols with very high enantioselectivity.

The reagents for Sharpless epoxidation include tert-butyl hydroperoxide or TBHP, titanium tetraisopropoxide, and a specific stereoisomer of diethyl tartrate or DET.

While TBHP acts as the oxidizing agent, titanium tetraisopropoxide and optically active DET combine and act as the chiral catalyst for the epoxidation.

The enantiomer of DET used in Sharpless epoxidation is either L-(+)-diethyl tartrate or D-(−)-diethyl tartrate.

For Sharpless epoxidation, the stereochemistry of the product depends on which enantiomer of DET used.

For instance, when trans-2-hexen-1-ol reacts with TBHP in the p