Enantiomeric purity and

The hydrolysis of seven alkyl arenesulfinylalkanoates by the bacterium Corynebacterium equi IFO 3730 studied by Ohta and coworkers34 are recent examples of kinetic resolutions which give sulfoxides of high enantiomeric purity and in reasonable yield. Compounds 16a, 16b and 16c were recovered in 30 to 43% yield and in 90 to 97% e.e. The S enantiomers underwent hydrolysis more rapidly than the R isomers. Sulfoxide 17 was isolated in 22% yield and 96% e.e., but sulfoxide 18 was completely metabolized. Esters other than methyl gave inferior results. The acids formed upon hydrolysis, although detected, were for the most part further metabolized by the bacterium. 

Thus, the enantiomeric contents in a pair of sulphoxides can be determined by the NMR chemical shifts in the methine or methylene protons in the two diastereomeric complexes which are stabilized by the hydrogen bond between the hydroxyl and the sulphinyl groups147-151 (Scheme 13). Similarly, the enantiomeric purity and absolute configurations of chiral sulphinate ester can be determined by measuring the H NMR shifts in the presence of the optically active alcohols152. 

Three excellent reviews cover this subject through the early 1980s12-14. The second review summarizes through 1981 the methods used for determining the enantiomeric purity and the absolute configuration of sulfoxides13, and the third review summarizes this area through 198314. 

Because of the instability of cyanohydrins, the characterization of cyanohydrins mostly should be hydroxyl protected. In 2001, Gerrits et al. [26] investigated the influence of solvent composition on the stability of unprotected cyanohydrins and then described a method to analyze unprotected cyanohydrins (with regard to enantiomeric purity and conversion) via chiral high-performance liquid chromatography (HPLC). Hernandez et al. [27] and the groups... 

Enantiometrically pure alcohols are important and valuable intermediates in the synthesis of pharmaceuticals and other fine chemicals. A variety of synthetic methods have been developed to obtain optically pure alcohols. Among these methods, a straightforward approach is the reduction of prochiral ketones to chiral alcohols. In this context, varieties of chiral metal complexes have been developed as catalysts in asymmetric ketone reductions [ 1-3]. However, in many cases, difficulties remain in the process operation, and in obtaining sufficient enantiomeric purity and productivity [2,3]. In addition, residual metal in the products originating from the metal catalyst presents another challenge because of the ever more stringent regulatory restrictions on the level of metals allowed in pharmaceutical products [4]. An alternative to the chemical asymmetric reduction processes is biocatalytic transformation, which offers... 

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

This idea is elegant for its simplicity and also for its usefulness. While often in phenomenological theories of materials, control of parameters with molecular structure would provide useful properties, but the parameters are not related in any obvious way to controllable molecular structural features. Meyer s idea, however, is just the opposite. Chemists have the ability to control enantiomeric purity and thus can easily create an LC phase lacking reflection symmetry. In the case of the SmC, the macroscopic polar symmetry of this fluid phase can lead to a macroscopic electric dipole, and such a dipole was indeed detected by Meyer and his collaborators in a SmC material, as reported in 1975.2... 

CSAs have found stereochemical applications in areas other than those of enantiomeric purity and absolute configuration determination. 

In another report of Singh and Han [61], Ir-catalyzed decarboxylative amidations of benzyl allyl imidodicarboxylates derived from enantiomerically enriched branched allylic alcohols are described. This reaction proceeded with complete stereospecificity-that is, with complete conservation of enantiomeric purity and retention of configuration. This result underlines once again (cf. Section 9.2.2) that the isomerization of intermediary (allyl) Ir complexes is a slow process in comparison with nucleophilic substitution. 

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.). 

From other approaches to optically active [2.2]metacyclophanes the following are noteworthy as just mentioned for 64 (medium pressure) liquid chromatography on microcrystalline triacetylcellulose (cf. Ref. 82 ) in ethanol or ether (practicable also at lower temperatures) is a very efficient and successful method for the optical resolution of many axial and planar chiral (aromatic) compounds 83). In many cases baseline-separations can be achieved and thereby both enantiomers obtained with known enantiomeric purity and in amounts sufficient for further investigations, especially for studying their chiroptical properties (see also 3.2 and 3.3). The disub-stituted [2.2]metacyclophanes 57 and 59 (which had been previously correlated to many other derivatives) 78- 79) were first resolved by this method83). 

Unfortunately, enantiomeric purities and total percentages of a-pinene, ]3-pinene, limonene and a-terpineol from tea tree oils more or less overlap with those of Eucalyptus oils (Table 17.3). Only enantiomeric purities and total percentages of terpinen-4-ol and a-phellandrene are significantly different, when Melaleuca and Eucalyptus oils are compared with regard to authenticity assessment [45]. 

H-mnr is commonly used to determine enantiomeric purity and is reliable to above 98% ee. 

Tanaka, K., Ootani, M., and Toda, F. (1992) Optically Active trans-Bis-(hydroxydiphenylmethyl)-2,2-dimethyl-l,3-dioxacyclopentane and Its Derivatives as Chiral Shift Reagents for the Determination of Enantiomeric Purity and Absolute Configuration, Tetrahedron Asymm., 3, 709-712. 

A novel asymmetric synthesis of a-amino acids via electrophilic amination has been demonstrated by Zheng and Armstrong and co-workers.94 No +NHBoc was observed when lithium tert-butyl-A-tosyloxycarbamate (LiBTOC) was reacted with zinc and lithium enolates of 48. Transmetallation of the lithium enolate with copper cyanide was necessary to generate a reactive amide cuprate, which then added efficiently to the electrophile. The electrophilic amination of chiral cuprates with LiBTOC provided an expedient approach to a-amino acids with predictable absolute configuration in high enantiomeric purity and good yield (Scheme 24.23). 

The article is a brief review of the applications of CD as a detector in both preparative and analytical liquid chromatography. The objectives are to identify elution orders for enantiomers, to measure enantiomeric purities and enantiomeric excesses, to analytically determine diastereoisomers, and to selectively determine chiral analytes when present as components in mixtures with achiral substances. 

The enzymatic reduction of a thiocarbonyl compound has been investigated [159] for the first time, in order to provide a new route for enan-tiopure thiols, molecules which are currently needed for asymmetric synthesis. Reaction of easily available /1-thioxoesters with baker s yeast under classical conditions did furnish the expected thiols, but with lower enantiomeric purity and moderate conversion rate, due to the competitive hydrolysis of the thioxo group into a carbonyl leading to an alcohol. However, conditions (ethyl acrylate, dry yeast) were found to improve the production of (S)-ethyl 3-mercaptobutanoate. Cyclic thioxo esters led to high stereoselectivity of cis (1S,2S) products, but with moderate chemical yields. 

Nafie LA, Long F, Freedman TB et al (1998) The determination of enantiomeric purity and absolute configuration by vibrational circular dichroism spectroscopy. In de Haseth JA (ed) Fourier transform spectroscopy 11th International Conference, vol. 430, American Institute of Physics. Woodbury, New York, pp 432-434... 

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