Enantiomeric excess, estimation

Carbonylative kinetic resolution of a racemic mixture of trans-2,3-epoxybutane was also investigated by using the enantiomerically pure cobalt complex [(J ,J )-salcy]Al(thf)2 [Co(CO)4] (4) [28]. The carbonylation of the substrate at 30 °C for 4h (49% conversion) gave the corresponding cis-/3-lactone in 44% enantiomeric excess, and the relative ratio (kre ) of the rate constants for the consumption of the two enantiomers was estimated to be 3.8, whereas at 0 °C, kte = 4.1 (Scheme 6). This successful kinetic resolution reaction supports the proposed mechanism where cationic chiral Lewis acid coordinates and activates an epoxide. 

Another phosphorus containing CDA (87) has also been described for the estimation of the enantiomeric excess of chiral amines80. The phosphorus CDA 87 is formed quantitatively and instantaneously in situ in an NMR tube by reaction of phosphorus trichloride (PCI3) with the chiral diamine 88. Addition to the NMR tube of the chiral amine (IC-NIIo) for which the ee is to be determined gives the diastereomeric phosphorous derivatives ... 

DNA catalyzes the racemization of [Fe(phen)3] " "—the rate is approximately doubled in the presence of 1,000 pM DNA. DNA also shifts the A A equilibrium towards a slight enantiomeric excess of the A form. The electrostatic and intercalative interactions of [Fe(phen)3] + and [Fe(phen)3] " " with DNA have been probed voltammetrically, and binding constants estimated. The [Fe(phen)3f+-DNA interaction appears to be primarily electrostatic, [Fe(4,7-Ph2phen)2(phen)] -DNA interaction to be intercalative. The discussions in these recent publications suggest that early work on the kinetics and mechanism of racemization of [M(bipy)3]" and [M(phen)3] " ", M = Ni, Cr as well as Fe, is in danger of being overlooked or forgotten. Comparison of k and AH values for racemization and dissociation shows that racemization of... 

Spectrophotometric assays can be used for the estimation of the enantiosel-ectivity of enzymatic reactions. Reetz and coworkers tested 48 mutants of a lipase produced by epPCR on a standard 96-well microtiter plate by incubating them in parallel with the pure R- and S-configured enantiomers of the substrate (R/S-4-nitrophenol esters) [10]. The proceeding of the enzyme catalyzed cleavage of the ester substrate was followed by UV absorption at 410 nm. Both reaction rates are then compared to estimate the enantiomeric excess (ee-value). They tested 1000 mutants in a first run, selecting 12 of them for development of a second generation. In this way they were able to increase the enantiomeric excess from 2% for the first mutants to 88% after four rounds of evolutive optimization. 

MIP assays can also be utilized in synthetic organic applications. For example, MIP-based assays have been used to measure the chiral purity of samples in organic solvents. An L-phenylalanine anilide (l-PAA) imprinted polymer was utilized as a recognition element to measure the enantiomeric excess (ee) of PAA samples (Chen and Shimizu 2002). The MIP displays greater capacity for l-PAA versus d-PAA samples of similar concentration, and this difference was used to estimate enantiomeric excess. The enantiomeric excess of an unknown solution was determined by comparing the UV absorbance of the PAA remaining in solution after equilibration against a calibration curve. This MIP assay was demonstrated to be rapid and accurate with a standard error of +5% ee. 

The vibrational circular dichroism(VCD) spectroscopy can be used to elucidate the stereochemistries of chiral molecules, including the accurate estimation of enantiomeric excess and their absolute configrations[20]. Optically pure samples as well as a racemic sample(c) were used as a reference to compare the VCD spectra. Three VCD spectra are shown in Fig. 7 a spectrum of 99 % ee R(-)-1-phenyl 1,2-ethanediol(a) and that of 99 % ee S(+ )-1-phenyl l,2-ethanediol(b) obtained from Aldrich Co., and the other is that of the product obtained on the Ti-MCM-41/chiral Co(HI) salen catalyst(d). 

Reduction of perfluoroalkyl ketones with chiral lithium alkoxides gave chiral a-perfluoroalkyl alcohols in high enantiomeric excesses. The order of steric effects on this reaction is estimated as C7F15 > substituted phenyl > CF3.388... 

The enantiomeric excess of several a-alkyl amino acids from the Murchison meteorite has been measured by Cronin and Pizzarello [69-71], who found significant values (1 -15% ee l). Conversely to previous estimates often biased by terrestrial contamination, these results can be considered as reliable since target AA are both non-biogenic and non-terrestrial in origin (the latter confirmed by D/H and 13C/12C isotopic ratios). The quaternary a-carbon prevents the racemization of the compounds, thus allowing the long-term preservation of the enantiomeric excess. 

It is expected that similar to method reported previously based on the use of cyclodextrins and sucrose the present method should have high sensitivity. Its sensitivity can be evaluated from two values the lowest enantiomeric excess (EE% which is defined as EE% = [(R-enantiomer - S-enantiomer)/(R-enantiomer + S-enantiomer)]) that can be determined at the lowest concentration of a sample. It should be noted that these two terms are interdependent to each other, namely, the limit of detection (LOD) on ee% can be improved by increasing sample concentration or vice versa. In an attempt to estimate the sensitivity of the method, we performed measurements on 10 samples of 10.0 mM or 2.66 mg/mL of atenolol with different cc% s in S- CHTA Tf2N. Results obtained are listed in Table 2. It is evident from the table that the method is not only effective but also very sensitive. It can accurately determine samples with concentration as low as micrograms having ee value as high as -90.00% (or +97.00% ) and as low as 0.6%. Furthermore, even at ee as low as 0.6%, the relative error was only 3.33%. 

Estimated from an experiment with optically enriched sarin (64% enantiomeric excess). 

Irradiation performed with racemic substrate at room temperature, unless noted otherwise. Anisotropy (g) factor at or around irradiation wavelength, if reported or estimated. Extent of destruction. Maximum observed rotation a of irradiated solution, or specific rotation [a] of isolated sample or of residue obtained upon evaporation. Maximum observed ellipticity of irradiated solution or molar ellipticity of isolated sample. Enantiomeric excess of isolated sample. Not reported. Compound (mp 113 C) of unknown structure, obtained in a reaction of humulene with sodium nitrite, according to the reported procedure Chapman, AC. J. Chem. Soc. 1895 67 780. A mixed case of asymmetric destruction and photoderacemization irradiation performed at 0 C. Enantiomerically enriched sample used. Estimated g factor enhanced by two-quantum excitation with high intensity picosecond laser pulse. High-inten-sity laser of indicated pulse duration used. "Irradiation performed at 77 K in a hydrocarbon glass matrix. Optically pure sample photolyzed only to evaluate the enhanced g factor. Estimated g factor enhanced by two—quantum excitation with high-intensity femtosecond laser pulse. 

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