The use of stereoselective reactions to produce stereospecific centres

Stereoselective reactions are those that result in the selective production of one of the stereoisomers of the product. The extent of the selectivity may be recorded as the enantiomeric excess (e.e.) when the reaction produces a mixture of enantiomers and the diastereoisomeric excess (d.e.) when it produces a mixture of diastereoisomers. These quantities are defined by the expression  

The values of e.e. and d.e. are obtained by measuring the yields of the individual stereoisomers. An e.e. or d.e. value of 0 per cent means that the stereoisomers are produced in equal amounts. In the case of enantiomeric mixtures the product is likely to be in the form of a racemic modification. Conversely, an e.e. or d.e. value of 100% indicates that only one product is formed. This rarely occurs in practice since most reactions yield a mixture of isomers. 

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