Discrimination of enantiomers

NMR spectra of oxaziridine enantiomers may be different from each other in chiral media. In the presence of chiral arylperfluoroalkylcarbinols, shift differences of up to 0.35 p.p.m. are observed, which may be used for discrimination of enantiomers (77JOC3217). 

Enantioseparation is typically achieved as a result of the differences in interaction energies A(AG) between each enantiomer and a selector. This difference does not need to be very large, a modest A(AG) = 0.24 kcal/mol is sufficient to achieve a separation factor a of 1.5. Another mechanism of discrimination of enantiomers involves the preferential inclusion of one into a cavity or within the helical structure of a polymer. The selectivity of a selector is most often expressed in terms of retention of both enantiomers using the separation factor a that is defined as ... 

Stretched gelatin gels as chiral alignment media for the discrimination of enantiomers by NM R spectroscopy. Angew. Chem. Int. Ed. 2005, 44, 3145-3147. 

In the following section covalently bound derivatives are mainly discussed. Complexes are described only if they have a defined structure. Other methods of NMR discrimination of enantiomers are described elsewhere in this series. They involve mostly chiral solvents7,8 or chiral lanthanide shift reagents9,10. 

In this section an overview of the numerous methods and principles for the discrimination of enantiomers is given. First, the interaction principles of the polymer-based methods adapted from chromatographic procedures are illustrated. The discrimination of enantiomers was achieved some decades ago by using different types of stationary materials, like cyclodextrins or polymer-bonded amide selectors. These stationary-phase materials have successfully been appointed for label-free optical sensing methods like surface plasmon resonance (SPR) or reflectometric interference spectroscopy (RIfS). Furthermore, various successful applications to optical spectroscopy of the well-established method of fluorescence measurements for the discrimination of enantiomers are described. 

I. Alkorta, J. Elguero, Self-discrimination of enantiomers in hydrogen-bonded dimers. J. Am. Chem. Soc. 124, 1488-1493 (2002)... 

Biological Discrimination of Enantiomers 189 5-6 Racemic Mixtures 191 5-7 Enantiomeric Excess and Optical Purity 192 5-8 Chirality of Conformationally Mobile Systems 193 5-9 Chiral Compounds without Asymmetric Atoms 195 5-10 Fischer Projections 197... 

Further helical structures in chemistry are formed, e.g. by urea, which in the course of the crystallization from suitable solvents builds up a hexagonal lattice. It contains long channels, surrounded by urea molecules, which either form a left- or right-handed helix. If urea is crystallized in the presence of a suitable racemic guest, the right-handed lattice preferably encloses the one enantiomer of the guest, the left-handed lattice the other one This discrimination of enantiomers can be applied for the separation of racemates. 

Yamamoto reported the discrimination of enantiomers of the racemic organoa-luminium reagent 172 through reaction with terpene-derived chiral ketones [98]. The resulting mixture served as a catalyst for the hetero-Diels-Alder reaction of benzaldehyde and the diene 173. ds-174 was obtained as the major stereoisomer in... 

FIGURE 9.1 The difference in relative Gibbs free energy (AG) of association, for the formation of diastereomeric complexes, provides the fnndamental basis for thermodynamic discrimination of enantiomers based on their complexation with a chiral selector. 

FIGURE 9.5 Measurement and treatment of data for a host-guest screening experiment. The chiral selector, ferf-butylcarbamoyl quinidine (tBuCQD) is shown to provide a high degree of enantioselectivity toward the discrimination of enantiomers of N-(3,5)-dinitrobenzoyl-leucine (DNB-Leu) using ESI-MS [87]. 

Mustaparta, H. (1979) Chemoreception in bark beetles of the genus Ips synergism, inhibition and discrimination of enantiomers. In Chemical Ecology Odor Communication in Animals (Ritter, F. J., ed.) Elsevier/N. Holland, Amsterdam. 

K., Vite, J. P. and Blum, M. S. (1982) Southern pine beetle olfactory receptor and behavior discrimination of enantiomers of the attractant pheromone frontalin. J. Chem, EcoL (in press). 

Although many optical techniques can be used for the discrimination of enantiomers in a simple read-out format like UV-measurements, colorimetric, or even naked eye detection [26], the use of luminescence (mostly fluorimetric) detection has several advantages in terms of sensitivity and selectivity of the... 

Su WC, Zhang WG, Zhang S et al (2009) A novel strategy for rapid real-time chiral discrimination of enantiomers using serum albumin functionalized QCM biosensor. Biosens Bioelectron 25 488-492... 

In this chapter we have, in our opinion, presented the most important motifs that have been used to construct anion receptors effective in chiral recognition. The carbohydrates have been successfully applied for modification of binding pocket of anion receptors as well as chiral barriers modulating discrimination of enantiomers. The works described in this chapter illustrate that in the design of new enantioselective receptors, important are not only individual anion binding motifs but also their arrangement in the space. 

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