Organic chemistry enantiomeric excess

Very few enzyme-catalysed reactions involving the reduction of alkenes have achieved any degree of recognition in synthetic organic chemistry. Indeed the only transformation of note involves the reduction of a, (3-unsaturated aldehydes and ketones. For example, bakers yeast reduction of (Z)-2-bromo-3-phenylprop-2-enal yields (S)-2-bromo-3-phenylpropanol in practically quantitative yield (99 % ee) when a resin is employed to control substrate concen-tration[50]. Similarly (Z)-3-bromo-4-phenylbut-3-en-2-one yields 2(5), 3(,S)-3-bromo-4-phenylbutan-2-ol (80% yield, >95% ee)[51]. Carbon-carbon double bond reductases can be isolated one such enzyme from bakers yeast catalyses the reduction of enones of the type Ar—CH = C(CH3)—COCH3 to the corresponding (S)-ketones in almost quantitative yields and very high enantiomeric excesses[52]. 

Commenting their discovery of the first case of chiral autocatalysis in organic chemistry, Soai et al. stated that it seems conceivable that the [Soai] reaction. .. may be an example of the scheme proposed by Frank [9]. Indeed, the so-called Frank model that has been developed in 1953 predicts the spontaneous amplification of an initial enantiomeric excess by means of a simple set of coupled differential equations [1] ... 

Historically, it is interesting to note that this was the first example reported of a catalytic asymmetric aldol reaction.31 The diastereomeric ratio of oxazole isomers was ca. 9 1 with an enantiomeric excess of up to 97%. This reaction has had a significant impact on organic chemistry, and already several summarising reviews have appeared on the reaction and its application in organic synthesis.32,33... 

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. 

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