Determination of enantiomeric purity

Enantiomeric purity of an enantiomeric mixture is defined as below. 

Where A+ = % mole fraction of the dextrorotatory enantiomer A = % mole fraction of the levorotatory enantiomer ee = enantiomeric excess 

Accordingly, estimation of A+ and A of a sample by an appropriate physical method such as GC, HPLC and NMR enables the calculation of its enantiomeric purity.12 

Direct determination of the enantiomeric purity of an enantiomeric mixture is usually achieved by GC or HPLC on a chiral stationary phase derived from chiral materials such as cellulose and cyclodextrin. NMR analysis in the presence of a chiral shift reagent is also applicable, although with less accuracy than the chromatographic methods. 

If the above methods are unsuccessful, an enantiomeric mixture must be derivatized with an enantio-pure derivatizing agent to give a diastereomeric mixture. The diastereomeric ratio can then be estimated 

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Authenticity evaluation has recently received increased attention in a number of industries. The complex mixtures involved often require very high resolution analyses and, in the case of determining the authenticity of natural products, very accurate determination of enantiomeric purity. Juchelka et al. have described a method for the authenticity determination of natural products which uses a combination of enantioselective multidimensional gas chromatography with isotope ratio mass spectrometry (28). In isotope ratio mass spectrometry, combustion analysis is combined with mass spectrometry, and the ratio of the analyte is measured versus a... 

Racemic mixtures of sulfoxides have often been separated completely or partially into the enantiomers. Various resolution techniques have been used, but the most important method has been via diastereomeric salt formation. Recently, resolution via complex formation between sulfoxides and homochiral compounds has been demonstrated and will likely prove of increasing importance as a method of separating enantiomers. Preparative liquid chromatography on chiral columns may also prove increasingly important it already is very useful on an analytical scale for the determination of enantiomeric purity. 

SCHIFF BASES AS TOOLS FOR DETERMINATION OF ENANTIOMERIC PURITY AND ABSOLUTE CONFIGURATION... 

As was already mentioned, the phenomenon of nonequivalence of NMR spectra of enantiomers in chiral solvents is a basis for the determination of enantiomeric purity of a variety of chiral sulfur compounds. This method, developed by Pirkle, has the advantage over other methods of being absolute that is, the chemical shift difference between enantiotopic nuclei induced by the chiral solvent increases with increasing optical purity of the solvent, whereas the relative intensities of the signals that are used to measure the enantiomeric composition of the solute are not affected. 

Finally, one should note that the determination of enantiomeric purity by means of chiral shift reagents appears to be more advantageous than the method of Pirkle because the magnitude of nonequivalence A5 is generally greater, thus leading to a more accurate... 

Online CD detectors are now commercially available for use with HPLC that are inherently more sensitive than corresponding OR detectors and not affected by solvent changes to the same extent and are thus more gradient compatible [121]. Provided Ae and the concentration of an analyte are known with good precision/accuracy, the measurement of CD will allow the determination of enantiomeric purity. In addition, with CD-based detection systems, both chiroptical and ordinary absorbance can be determined simultaneously allowing the measurement of the g-factor (or dissymmetry factor), which is defined as the ratio of the CD to the absorbance (AA/A) [122]. The g-factor is concentration independent and its measurement allows a more reliable determination of enantiomeric purity (without using a CSP) with reference to standards of known enantiomeric composition irrespective of their concentration [123]. A small number of recent literature examples have suggested the potential use of achiral HPLC with online CD detection for the determination of extreme enantiomeric ratios [121, 124-126] however, chiral separation techniques currently provide a more reliable measurement of enantiomeric purity. 

GC methods for analytical purposes, i.e., the determination of enantiomeric purities in compounds obtained from asymmetric synthesis, from optical resolution or by isolation from... 

Thin-layer chromatography (TLC) is also a simple and rapid method for the determination of enantiomeric purity, especially for nonvolatile substances. Suitable plates (Chi-raplate) are commercially available273. 

NMR spectroscopy the NMR spectrum of the sample in the presence of a chiral solvating agent (see Section 3.1.4.1.) or a chiral paramagnetic lanthanide shift reagent (see Section 3.1.4.2.2.) is recorded. The determination of enantiomeric purity rests on the nonequivalence of externally enantiotopic nuclei which are rendered externally diastereotopic in a non-racemic chiral environment. 

Therefore, direct Group I methods for the determination of enantiomeric purity are generally preferred. A most important merit of the direct methods is the fact that the chiral auxiliary compounds, e.g., the chromatographic stationary phase, need not be enantiomerically pure. 

At present, polarimetry and allied techniques such as circular dichroism, NMR spectroscopy (employing nonracemic solvents or shift reagents) and various types of chromatography (employing nonracemic stationary or mobile phases) are the most widely used techniques for the direct determination of enantiomeric purities, and only these techniques are discussed further (see Sections 3.1.3. to 3.1.5.). 

The determination of enantiomeric purity (ee) by NMR spectroscopy is usually carried out with the help of a nonracemic chiral auxiliary compound. NMR methods not requiring a chiral auxiliary compound are also known and are based on self-association of the enantiomers or on their reaction with a bifunctional achiral compound (see Section 3.1.4.7.). 

NMR methods that do not require a chiral auxiliary compound for the determination of enantiomeric purities are based on... 

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