Ratio of enantiomers

The enantiomeric compositions of the titanium reagents are monitored easily by the reaction with enantiomerically pure chiral aldehydes, such as 2-(fer/-butyldimethylsilyloxy)propanal104. Here, the ratio of diastereomeric products reflects the ratio of enantiomers of the reagent, although a small error arises from double stereodifferentiation95 104. 

Alcohols will serve as hydrogen donors for the reduction of ketones and imi-nium salts, but not imines. Isopropanol is frequently used, and during the process is oxidized into acetone. The reaction is reversible and the products are in equilibrium with the starting materials. To enhance formation of the product, isopropanol is used in large excess and conveniently becomes the solvent. Initially, the reaction is controlled kinetically and the selectivity is high. As the concentration of the product and acetone increase, the rate of the reverse reaction also increases, and the ratio of enantiomers comes under thermodynamic control, with the result that the optical purity of the product falls. The rhodium and iridium CATHy catalysts are more active than the ruthenium arenes not only in the forward transfer hydrogenation but also in the reverse dehydrogenation. As a consequence, the optical purity of the product can fall faster with the... 

An interesting method for the estimation of optical purity of sulfoxides, which consists of the combination of chemical methods with NMR spectroscopy, was elaborated by Mislow and Raban (241). The optical purity is usually determined by the conversion of a mixture of enantiomers into a mixture of diastereomers, the ratio of which may be easily determined by NMR spectroscopy. In contrast to this, Mislow and Raban used as starting material for the synthesis of enantiomeric sulfoxides a diastereomeric mixture of pinacolyl p-toluenesulfinates 210. The ratio of the starting sulfinates 210 was 60.5 39.5, as evidenced by the H NMR spectrum. Since the Grignard reaction occurs with full stereospecificity, the ratio of enantiomers of the sulfoxide formed is expected to be almost identical to that of 210. This corresponds to a calculated optical purity of the sulfoxide of 20%. In this way the specific rotations of other alkyl or aryl p-tolyl sulfoxides can conveniently be determined. 

A crude enzyme preparation from Vinca rosea catalyzed the enantioselective coupling of coniferyl alcohol to give rise to an optically active C8-C5 neolignan, dehydrodiconiferyl alcohol, with a ratio of (—)-enantiomer to (+)-enantiomer of 2 1 (Fig. 12.10) [66],... 

This method amounts to a complete resolution of the type described in Section 19-3D, but on an analytical scale. For example, assume that you have a partially resolved compound, A, consisting of unequal amounts of the enantiomers Aand A. By reaction with a second chiral enantiomerically pure substance, B+, A is converted to a mixture of diastereomers A+B+ and A B+. Because these diastereomers are chemically and physically different, the mixture usually can be analyzed by gas-liquid chromatography (Section 9-2A). If the reaction of B+ with A+ and A was quantitative, the relative areas of the two peaks eluting from the column correspond to the ratio of the diastereomers A,.B+/AJB+, and thus to the ratio of enantiomers A+/A, from which the enantiomeric purity of the partially resolved mixture can be calculated. 

Alternatively, with a pure chiral reagent the enantiomeric mixture may be converted (quantitatively) to diastereomers. The nuclei of the diastereomeric compounds are expected to have small differences in chemical shifts, even in achiral solvents, and integration of their respective signal intensities should correspond to the ratio of diastereomers, and hence to the ratio of enantiomers in the original mixture. 

When 1 is added to a solution of a mixture of enantiomers, A and A, it associates differently with each of the two components to produce the diastereo-meric complexes A+ 1 and A 1. The nmr spectrum of the mixture then shows shift differences that are large compared to the uncomplexed enantiomers (because of the paramagnetic effect of the europium) and normally the resonances of the A+ 1 complex will be distinct from those of the A 1 complex. An example of the behavior to be expected is shown in the proton nmr spectrum (Figure 19-4) of the enantiomers of 1-phenylethanamine in the presence of 1. Although not all of the resonances are separated equally, the resolution is good for the resonances of nuclei closest to the metal atom and permits an estimate of the ratio of enantiomers as about 2 1 and the enantiomeric purity as 33%. 

The Z product is enantiomerically pure, but the E isomer is formed in a 90 10 ratio of enantiomers. One reason for this was revealed by considering the relevant conformations of the allylsilane, shown in equation 96. 

The E-values for the enzymes that generated the data in Figure 13.12 are not mentioned in the text, but the E-values can quickly be determined based on the graphs. The ratio of enantiomers in the initial product mixture is equivalent to the E-value of the enzyme. Estimate the E-values of both enzymes in Figure 13.12 based on their respective graphs. 

The optical purity is a measure of enantiomeric purity of a compound and is given in terms of its enantiomeric excess (ee). A pure enantiomer would have an optical purity and enantiomeric excess of 100 per cent. A fully racemised compound would have an optical purity of 0 per cent. If the enantiomeric excess is 90 per cent, it signifies that 90 per cent of the sample is pure enantiomer and the remaining 10 per cent is a racemate containing equal amounts of each enantiomer (i.e. 5% + 5%). Therefore the ratio of enantiomers in a sample having 90 per cent optical purity is 95.5. 

If the transition states are enantiotopic, they have the same energy. The addition proceeds through each of them to the same extent and we thus obtain a 50 50 ratio of enantiomers. This means that there is no enantioselectivity, i.e., ee = 0%.). If the transition states are diastereotopic, they may have different energies. The addition preferentially takes place via the lower energy transition state and preferentially results in more of one stereoisomer. It is thus diastereoselective (i.e., ds 50 50) or enantioselective (i.e., ee 0%). 

The table in the margin shows the ratio of di a stereoisomers produced by this reaction for a few alkylating agents. As you can see, none of these reactions is truly 100% diastereoselective and, indeed, only the best chiral auxiliaries (of which this is certainly one) give >98% of a single di aster eo isomer. The problem with less than perfect dia stereoselectivity is that, when the chiral auxiliary is removed, the final product is contaminated with some of the other enantiomer. A 98 2 ratio of diastereoisomers will result in a 98 2 ratio of enantiomers. 

When talking about compounds that are neither racemic nor enantiomerically pure (usually called enantiomerically enriched or, occasionally, scalemic) chemists talk not about ratios of enantiomers but about enantiomeric excess. Enantiomeric excess (or ee) is defined as the excess of one enantiomer over the other, expressed as a percentage of the whole. So a 98 2 mixture of enantiomers consists of one enantiomer in 96% excess over the other, and we call it an enantiomerically enriched mixture with 96% ee. Why not just say that we have 98% of one enantiomer Enantiomers are not like other isomers because they are simply mirror images. The 2% of the wrong enantiomer makes a racemate of 2% of the right isomer so the mixture contains 4% racemate and 96% of one enantiomer. 96% ee. 

Attack by acetate can occur on either side of the planar, achiral carbocation intermediate, resulting in a mixture of both the R and S enantiomeric acetates. The ratio of enantiomers is probably close to 50 50. 

Mossner, S. Spraker, T.R. Becker, P.R. BaUschmiter, K., Ratios of enantiomers of alpha-HCH and determination of alpha-, beta-, and gamma-HCH in brain and other tissues of neonatal Northern fur seals (Callorhinus ursinus) Chemosphere 1992, 24, 1171-1180. 

Capillary gas chromatographic determination of optical purities, investigation of the conversion of potential precursors, and characterization of enzymes catalyzing these reactions were applied to study the biogenesis of chiral volatiles in plants and microorganisms. Major pineapple constituents are present as mixtures of enantiomers. Reductions, chain elongation, and hydration were shown to be involved in the biosynthesis of hydroxy acid esters and lactones. Reduction of methyl ketones and subsequent enantioselective metabolization by Penicillium citrinum were studied as model reactions to rationalize ratios of enantiomers of secondary alcohols in natural systems. The formation of optically pure enantiomers of aliphatic secondary alcohols and hydroxy acid esters using oxidoreductases from baker s yeast was demonstrated. 

Capillary gas chromatographic investigation of diastereoisomeric derivatives revealed that in analogy to results obtained without precursors the chiral metabolites are present as mixtures of enantiomers. However for only a few of these compounds the ratios of enantiomers are identical with those determined in pineapple without precursors. The enantiomeric compositions of ethyl 3-hydroxyhexanoate and ethyl 3-acetoxyhexanoate are almost opposite to those determined for the naturally occurring methyl esters. 6-Octalactone obtained after addition of 5-oxooctanoic acid to pineapple tissue is almost optically pure (92% S) on the other hand -octalactone is naturally present in pineapple tissue as nearly racemic mixture (Table 1,8). 

Quantitative distribution and optical purities of the secondary alcohols obtained after fermentation with Penicillium citrinum are very similar to those isolated from coconut or corn (Figure 4). A combination of stereospecific reduction and following enantioselec-tive metabolization may be one of the keys to explain the ratios of enantiomers of aliphatic secondary alcohols observed in natural systems. 

Enantiomer-based methods exploit the fact that some pesticide compounds are applied in known ratios of enantiomers, most commonly as racemic mixtures, i.e., 1 1 ratios (Buser et al., 2000 Monkiedje et al., 2003). Although most abiotic transformation and partitioning processes are not affected by the structural differences between enantiomers (Bidleman, 1999), the biotransformation of some pesticide compounds has been found to be an enantioselective process, i.e., one that exhibits a preference for one enantiomer over the other (e.g., Harner et al., 1999 Monkiedje et al., 2003). The measurement of enantiomer concentration ratios for a pesticide compound that is applied as a racemic mixture but may undergo enantioselective biotransformation in the environment can thus provide an indication of whether or not the compound has undergone biotransformation since it was applied—and thus a rough... 

Chiral inversion yields fixed ratio of enantiomers... 

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