The 2001 Nobel Prize in Chemistry was awarded to three
organic chemists who have developed methods for
catalytic asymmetric syntheses. An asymmetric
(or enantioselective) synthesis is one that converts an achiral
starting material into mostly one enantiomer of a chiral product.
K. Barry Sharpless (The Scripps Research Institute) developed
an asymmetric epoxidation of allylic alcohols that gives
excellent chemical yields and greater than 90%
enantiomeric excess.
The Sharpless epoxidation uses tert-butyl hydroperoxide,
titanium(IV) isopropoxide, and a dialkyl tartrate ester
as the reagents. The following epoxidation of geraniol is typical.
<
of reactions>
(a) Which of these reagents is most likely to be the actual
oxidizing agent? That is, which reagent is reduced
in the reaction? What is the likely function of
the other reagents?
(b) When achiral reagents react to give a chiral product,
that product is normally formed as a racemic
mixture of enantiomers. How can the Sharpless epoxidation
give just one nearly pure enantiomer of the product?
(c) Draw the other enantiomer of the product.
What reagents would you use if you wanted to epoxidize
geraniol to give this other enantiomer?
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