Enantioselective analytical methods

We might think that the Murchison meteorite would have been studied thoroughly enough in the years since its arrival on Earth. But the results obtained always depend on the performance of the technical resources available in this case, the analytical methods and the apparatus. Thus, it is not really surprising that a new class of amino acid has been discovered in Murchison material diamino acids, such as DL-2,3-diaminopropionic acid, DL-2,4-diaminobutanoic acid etc. These were identified using a new enantioselective GLPC/MS method, which is also being used in analysis of material from the Rosetta mission. 

The product of this preparation is the most enantioselective catalyst developed to date for asymmetric epoxidation of a broad range of unfunctionalized olefins.6 The procedure includes a highly efficient resolution of trans-1,2-diaminocyclohexane as well as a convenient analytical method for the determination of its enantiomeric purity. This method is general for the analysis of chiral 1,2-diamines. The Duff formylation described in Step B is a highly effective method for the preparation of 3,5-di-tert-... 

Authentication of genuine flavours is an important topic in view of quality assurance in the food industry and in consumer protection as well. Both isotope discrimination as well as enantioselectivity during biosynthesis may serve as inherent parameters of authenticity, provided that appropriate analytical methods and concise data from authentic samples are available. 

A rapid, sensitive, and enantioselective LC-MS-MS method using deuterium-labeled IS was developed and evaluated for the simultaneous quantitative determination of donepezil enantiomers in human plasma without interconversion during clean-up process and measurement [37]. The use of an avidin column allowed the separation of donepezil enantiomers, which were specifically detected by MS-MS without interference from its metabolites and plasma constituents. Evaluation of this assay method shows that samples can be assayed with acceptable accuracy and precision within the range from 0.0206 to 51.6 ng/ml for both R-donepezil and S-donepezil. This analytical method was applied to the simultaneous quantitation of donepezil enantiomers in human plasma. 

Conversion of summarized values (e.g. mean ER cr to the equivalent EF tr) can lead to substantial discrepancies, and should be avoided [109]. In addition, the conventions used in describing ERs and EFs may differ between studies and analytical methods. For example, enantioselective separations on different stationary phases may result in reversal of elution order and lead to different values if elution orders are not known. In this discussion, EFs are used to the extent possible, and both EF and ER are defined using using Equations (4.1) and (4.3), respectively, for those analytes for which the elution order is known unless otherwise indicated. Otherwise, these metrics are defined using Equations (4.2) and (4.4) on the specified column. 

The four isomers of 45 showed identical IR, H-NMR, and 13C-NMR spectra. In addition, all of them were equally bioactive as the sex pheromone. Accordingly, their derivatization to 52 followed by HPLC analysis was the only way to distinguish the stereoisomers. Finally, the natural pheromone component was shown to be (6i , 19Jfi )-45 by its derivatization to 52 followed by HPLC analysis.35 Enantioselective HPLC or gas chromatography (GC) and chromatographic analysis after derivatization with Ohrui s reagent seems to be the most sensitive method for discrimination of stereoisomers. Thus, it may be concluded that structural analysis must, from time to time, be carried out by employing various kinds of different analytical methods. Otherwise, mistakes are likely to occur. 

There are many other useful analytical methods. Chromatographic methods such as gas chromatography (GC) and high-performance liquid chromatography (HPLC) are used daily for identification and estimation of the purity of a synthetic product. Chiroptical methods, such as circular dichroism (CD) spectroscopy, are also important especially in studying the relationships between absolute configuration and bioactivity of biofunctional molecules. In later chapters I will give some examples of application of CD spectroscopy in enantioselective synthesis. 

There are three methods available for the enantioselective synthesis of pheromones (1) derivation from enantiopure natural products, (2) enantiomer separation (optical resolution), and (3) chemical or biochemical asymmetric synthesis. Practitioners of enantioselective synthesis must be familiar with the analytical methods for the determination of enantiomeric purity of an optically active compound. These basic methods will be explained briefly in this section, and discussed in depth through examples in the later sections of this chapter. 

Specificity is a property of monocomponent systems and it occurs when the method is free of interference. Selectivity is related to the complexity of the matrix and it occurs when not more than one ion (molecule) interferes in determination. Enantioselectivity is a relatively new term introduced for the assay of enantiomers.257258 An analytical method is enantioselective when it can discriminate between enantiomers. Enantiospecificity is an extreme case of enantioselectivity. It is possible to create the conditions for a highly enantioselective analysis, and in this case enantiospecificity can also occur. For example, a maltodextrin with dextrose equivalence (DE) of 4.0 to 7.0 was used in capillary zone electrophoresis as a stationary phase for the separation of the enantiomers,259 and also in the design of a potentiometric, enantioselective membrane electrode.260 The method for capillary zone electrophoresis is enantioselective, as it is for the potentiometric method. 

Noctor, T.A.G. Enantioselective binding of drugs to plasma proteins. In Drug Stereochemistry, Analytical Methods and Pharmacology, 2nd Ed. Wainer, I.W., Ed. Marcel Dekker New York, 1993 337-364. 

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... 

Many times an analyte must be derivatized to improve detection. When this derivatization takes place is incredibly important, especially in regards to chiral separations. Papers cited in this chapter employ both precolumn and postcolumn derivatization. Since postcolumn derivatization takes place after the enantiomeric separation it does not change the way the analyte separates on the chiral stationary phase. This prevents the need for development of a new chiral separation method for the derivatized analyte. A chiral analyte that has been derivatized before the enantiomeric separation may not interact with the chiral stationary phase in the same manner as the underivatized analyte. This change in interactions can cause a decrease or increase in the enantioselectivity. A decrease in enantioselectivity can result when precolumn derivatization modifies the same functional groups that contribute to enantioselectivity. For example, chiral crown ethers can no longer separate amino acids that have a derivatized amine group because the protonated primary amine is... 

The ability to design chiral ILs in which the cation and anion is of fixed chirality represents additional tuning features of ILs. Two approaches have incorporated ILs as new stationary phases for chiral GC. One method involves the use of chiral ILs as stationary phases in WCOT GC [37]. In the second approach, chiral selectors (e.g., cyclodextrins) were dissolved in an achiral IL and the mixture coated onto the wall of the capillary colunm [38]. Both approaches can separate a variety of different analytes, but the observed enantioselectivities and efficiencies do not rival those observed with commercially available chiral stationary phases (CSPs). 

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