Enantiomers determination

Jirovsky D, Lemr K, Sevcik J, Smysl B, Stransky Z. 1998. Methamphetamine — properties and analytical methods of enantiomer determination. Forensic Sci Int 96 61. 

Fig. 6.30. Column efficiencies for (S) enantiomer determined from DNZ-(R,S)-Leu separations on quinidine-functionalized monoliths as a function of pore diameter. (Reprinted with permission from [60]. Copyright 2000 American Chemical Society). Conditions polymerization mixture, chiral monomer 8 wt%, 2-hydroxyethyl methacrylate 16 wt%, ethylene dimethacrylate 16 wt%, porogenic solvent 60 wt% (consisting of 1-dodecanol and cyclohexanol in different proportions), UV initiated polymerization for 16 h at room temperature ( ) and thermally initiated polymerization for 20 h at 60°C ( ), capillary columns 335 mm (250 mm active length) x 0.1 mm i.d., mobile phase 0.4 mol/L acetic acid and 4 mmol/L triethylamine in 80 20 acetonitrile-methanol, separation temperature 50°C, voltage -25 kV.

Dobler, J. Peters, N. Larsson, C. Bergman, A Geidel, E. Hiihnerfuss, H., The absolute structures of separated PCB-methylsulfone enantiomers determined by vibrational circular dichroism and quantum mechanical calculations J. Molec. Struct. (Theochem) 2002, 586, 159-166. 

As more receptors are identified and isolated, the importance of RRA wiU likely increase, and they will be applied to enantiomer determination. As with enantioselective immunoassays, careful attention must be paid to the optical purity of standards and radioligands. The potential for interference by the inactive enantiomer should be considered. When possible, results should be obtained by use of a standard curve of the enantiomer and reported as concentration of the enantiomer. A disadvantage of RRA is its lack of ability to measure the pharmacologically inactive enantiomer, which may be important for toxicological reasons. The ability of RRA to... 

Ensure that the inhibition constants of the test systems refer to a single compound. In the case of chiral compounds, separate the enantiomers. Check the activity of both enantiomers. Determine the absolute configuration of the active enantiomer. 

Percent area of each enantiomer determined by chiral high-performance liquid chromatography (HPLC) on the crude reaction mixture. 

Unlike tertiary amines the inversion barriers of simple tertiary phosphines of the type PRMePh are high enough to allow isolation, if not distillation of pure enantiomers. Determinations of... 

Murphy-Poulton SF, Boyle F, Gu XQ, Mater LE, Thalidomide enantiomers Determination in biological samples by HPLC and vancomycin-CSP, J. Chromatogr. B 2006 831 48-56. 

Optical purity Measure of the composition of a mixture of enantiomers determined... 

N is the total number of lanthanide complexes present, and is the probability that an initial excitation of an A (or A) complex remains on an A (or A) molecule at time r. In the derivation of this result, it was also assumed that the energy transfer rate between identical enantiomers was the same as between opposite enantiomers. Determination of the form of eA(0 depends on the particular energy transfer mechanism, and various assirmptions concerning intermolecular orientation and distances. 

Kinetic resolution involving the reaction of Ps a racemic substrate, achiral reagent, and a resolved catalyst. The relative rate of reaction of the enantiomers determines p the efficiency of the KR. 

In a classic paper on optically active coordination compounds, Mills and Quibell reported the synthesis of a Pt complex containing one molecule of meso-stilbenediamine (H2N-CHPh-CHPh-NH2) and one molecule of isobutylenedi-amine (H2N-CH2-CMe2-NH2). Also, the complex was successfully resolved into its enantiomers. Determine whether the coordination around the metal ion is tetrahedral or square planar. Also illustrate these modes pictorially. 

As mentioned, a chiral environment is required to induce differences between enantiomers. In chromatography, a chiral molecule, the so-called chiral selector (CS), is responsible for producing this environment. The mechanism of enantioseparation involves the formation of transient diastereomeric adsorbates between CS and enantiomers, which are based on weak noncovalent interactions. Differences in stability for the two adsorbates CS/enantiomer determine separation ... 

Being an enantiomer does not provide specific qualities other than optical to conventional organic compounds. Therefore, their analysis does not impose any particular requirements with respect to equipment. Any HPLC apparatus, provided with the appropriate chiral column, can be used for enantiomer determination. Only the availability of some accessories, such as multiple port valves to permit the programmed test of several columns and mobile phases, can be of help to speed up the search and optimization of analytical chromatographic conditions. 

The. same rcfcTcncc molecule s now reflected at a plane. spanned by the Cl, Br, and C atoms, to give the other enantiomer, (n the same manner as previously, we write down the permutation matrices of the two structures, and then determine the transpositions (Figure 2-82). 

Figure 2-83. EKample of the process to decide whether two structures are enantiomers by determining the permutation descriptor.

The number of discrete values of/cocc(i ) determines the resolution of the chirality code. Again, is a smoothing factor. An example with the conformation-dependent chirality codes for the enantiomers of 4 in two different conformations is shown in Eigurc 8-1 f. 

The enantiomeric excess of 3.10c has been determined by HPLC analysis using a Daicel Chiracel OD column and eluting with a 60 / 1 (v/v) hexane(HPLC-grade) / 2-propanol(p.a.) mixture. At a flow of 1 ml per minute the rentention times for the different isomers of 3.10c were 6.3 min. (exo, major enantiomer) 7.1 min. (exo, minor enantiomer) 7.7 min. (endo, major enantiomer) 10.7 min. (endo, minor enantiomer). 

Techniques for determining the absolute configuration of chiral molecules were not developed until the 1950s and so it was not possible for Eischer and his contemporaries to relate the sign of rotation of any substance to its absolute configuration A system evolved based on the arbitrary assumption later shown to be correct that the enantiomers... 

Enzyme-Catalyzed Asymmetric Synthesis. The extent of kinetic resolution of racemates is determined by differences in the reaction rates for the two enantiomers. At the end of the reaction the faster reacting enantiomer is transformed, leaving the slower reacting enantiomer unchanged. It is apparent that the maximum product yield of any kinetic resolution caimot exceed 50%. 

DETERMINATION OF POLYPHENOLIC ENANTIOMERS IN GREEN TEA EXTRACT BY CAPILLARY ZONE ELECTROPHORESIS... 

Catechin and epicatechin are two flavanols of the catechin family. They are enantiomers. The capillary zone electrophoresis (CE) methods with UV-detection were developed for quantitative determination of this flavanols in green tea extracts. For this purpose following conditions were varied mnning buffers, pH and concentration of chiral additive (P-cyclodextrin was chosen as a chiral selector). Borate buffers improve selectivity of separation because borate can make complexes with ortho-dihydroxy groups on the flavanoid nucleus. 

The presence of asymmetric C atoms in a molecule may, of course, be indicated by diastereotopic shifts and absolute configurations may, as already shown, be determined empirically by comparison of diastereotopic shifts However, enantiomers are not differentiated in the NMR spectrum. The spectrum gives no indication as to whether a chiral compound exists in a racemic form or as a pure enantiomer. 

Figure 2.24, Determination of the enantiomeric excess of 1-phenylethanol [30, 0.1 mmol in 0.3 ml CDCI3, 25 °C] by addition of the chiral praseodymium chelate 29b (0.1 mmol), (a, b) H NMR spectra (400 MHz), (a) without and (b) with the shift reagent 29b. (c, d) C NMR spectra (100 MHz), (c) without and (d) with the shift reagent 29b. In the C NMR spectrum (d) only the C-a atoms of enantiomers 30R and 30S are resolved. The H and C signals of the phenyl residues are not shifted these are not shown for reasons of space. The evaluation of the integrals gives 73 % R and 27 % S, i.e. an enantiomeric excess (ee) of 46 %...

The property of chirality is determined by overall molecular topology, and there are many molecules that are chiral even though they do not possess an asymmetrically substituted atom. The examples in Scheme 2.2 include allenes (entries 1 and 2) and spiranes (entries 7 and 8). Entries 3 and 4 are examples of separable chiral atropisomers in which the barrier to rotation results from steric restriction of rotation of the bond between the aiyl rings. The chirality of -cyclooctene and Z, -cyclooctadiene is also dependent on restricted rotation. Manipulation of a molecular model will illustrate that each of these molecules can be converted into its enantiomer by a rotational process by which the ring is turned inside-out. ... 

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