Enantiomeric purity, definition

This definition refers to an enantiomeric mixture produced in an asymmetric synthesis. In some cases where a diastereomeric mixture is produced, the definition has to be altered accordingly. Percent optical purity is an operational term that depends on optical rotation measurements. It is not necessarily equal to the percent enantiomeric purity (13), which is a more meaningful term and is the extent to which one enantiomer is formed in excess over the other ... 

Of course, during processing of fruit juices hydrolysis effects may occur, leading to decreased amounts of ethyl 2-methylbutanoate. However, its enantiomeric purity remains unchanged, whilst the corresponding 2-methylbutanoic acid is found as the (S)-enantiomer (99.5% or more) [33-37]. Consequently, the detection of racemic 2-methybutanoic acid (or the corresponding esters) definitely proves the addition of a synthetic (so called nature-identical) flavour compound. 

In its broadest terms the discussion of HPLC detection for chiral species must include the analysis of mixtures with achiral substances as well as the quality testing of, for example, the enantiomeric purity of a chemically pure drug form. The distinction between the definitions of chemical purity versus optical purity can not be overemphasized. In an efficient chiral HPLC system the latter problem is trivial, and if retention times are significantly different then any conventional detector such as RI, electrochemical, absorption, etc., could be used. Co-elutions are a major experimental concern in separations of mixtures and at this juncture it is not only prudent but absolutely necessary to involve a chiroptical detector to preferentially identify the chiral analyte. 

In 1974 Marumo and coworkers synthesized the enantiomers of JH II by microbial asymmetric hydrolysis of the epoxy ring of ( )-JH II (prepared by Mori) with a fungus Helminthosporium sativum 23 The hydrolysed diol was converted to (+)-JH II, while the epoxide remained intact was (—)-JH II. Their enantiomeric purities, however, were rather low (66-73% ee), and no definite biological data could be obtained. 

The simplicity of the Marckwald definition has been its most enduring feature, but our understanding of structure and mechanism has evolved since Marckwald s time, and spectroscopic and chromatographic techniques have displaced polari-metry as the primary determinant of enantiomeric purity. In light of these... 

The optical purity is usually, but not always, equal to enantiomer excess. In order for the two to be equal, it is necessary that there be no aggregation. It is possible, for example, that a homochiral or heterochiral dimer (see Glossary, Section 1.6, for definitions) would refract the circularly polarized light differently than the monomer (or each other). In 1968 [19] Krow and Hill showed that the specific rotation of (S)-2-ethyl-2-methylsuccinic acid (85% ee) varies markedly with concentration, and even changes from levorotatory to dextrorotatory upon dilution. In 1969 [20], Horeau followed up on Krow and Hill s observation, and showed that the optical purity (at constant concentration) and enantiomer excess of (5)-2-ethyl-2-methylsuccinic acid were unequal except when enantiomerically pure or completely racemic. This deviation from linearity is known as the Horeau effect, and its possible occurence should be remembered when determining enantiomeric purity by polarimetry. 

In 1980 Seebach1 introduced the collective name EPC-synthesis for "all approaches leading to enantiomerically pure compounds or according to a later, and more precise, definition any type of synthesis leading to enantiomerically pure compounds 2. Seebach also realized that a definition of "enantiomerically pure was required and therefore added The purity can be defined according to the analytic methods available and according to practical requirements . EPC-syntheses can be carried out via three approaches (see Table 12) ... 

At this point, it is important to describe the various ways the enantiomeric content of a mixture of isomers is defined and measured. There are three definitions that the chiral chemist uses to describe a product purity. 

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