Percent enantiomer excess

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS Percent enantiomer excess,... 

Enantiomer excess, ee (percent enantiomer excess, % ee) For a mixture of a pure enantiomer and its racemate, the percent excess of the pure enantiomer over the racemate. % ee is given by ... 

For a chiral compound, percent enantiomer excess (ee) is defined as ... 

Mixtures containing equal quantities of enantiomers are called racemic mixtures Racemic mixtures are optically inactive Conversely when one enantiomer is present m excess a net rotation of the plane of polarization is observed At the limit where all the molecules are of the same handedness we say the substance is optically pure Optical purity or percent enantiomeric excess is defined as... 

For a nonracemic mixture of enantiomers prepared by resolution or asymmetric synthesis, the composition of the mixture was given earlier as percent optical purity (equation 1), an operational term, which is determined by dividing the observed specific rotation (Mobs) of a particular sample of enantiomer with that of the pure enantiomer ( max), both of which were measured under identical conditions. Since at the present, the amount of enantiomers in a mixture is often measured by nonpolarimetric methods, use of the term percent optical purity is obsolete, and in general has been replaced by the term percent enantiomeric excess (ee) (equation 2) introduced in 197163, usually equal to the percent optical purity, [/ ] and [5] representing the relative amounts of the respective enantiomers in the sample. 

If one enantiomer is present in excess over the other, then the solution will have a net rotation corresponding in sign (+ or —) to that of the more abundant enantiomer. The composition of the mixture is denoted by the optical purity or the percent enantiomeric excess (ee%). The enantiomeric excess is defined as ee = % major enantiomer — % minor enantiomer and is a measure of the optical purity of the sample. Values range from 100% (pure enantiomer, ee = 100% — 0%) to 0% (racemic mixture or ee = 50% — 50%). A sample which has an optical purity of 92% is thus a mixture of 96% of one enantiomer and 4% of the other enantiomer. 

The purity of a mixture of enantiomers can be expressed as a simple ratio or, more commonly, as a percent enantiomeric excess, or e.e. The formula for converting a known ratio to a percent e.e. is shown in Equation 13.1. 

Enantiomeric excess or % ee (or % optical purity) = [observed specific rotation] divided by [specific rotation of pure enantiomer] X 100. Note that there are examples where the linear relationship between enantiomeric excess and optical rotation fails. A percent enantiomeric excess (% ee) of less than 100% indicates that the compound is contaminated with the other enantiomer. The ratio of enantiomers in a sample of known (measured) optical purity may be calculated as follows fraction of the major isomer = [(% ee) + 0.5 (100 - % ee)]. Thus,... 

Optical yield For a chemical reaction, the enantiomer excess of the products relative to that of the starting material, expressed as a percent. In asymmetric synthesis, the denominator may be the ee of the chiral reagent or catalyst. 

Entiomer Excess The proportion of one enantiomer in a given mixture of both enantiomers usually expressed as a percent ee=100(xR-xs)/(X.R + Xs) where XR and XS are the mole or weight fraction of the R- and S-enantiomers, respectively, and ee the enantiomer excess. 

Optical purity = percent enantiomeric excess = percent of major enantiomer... 

To understand the literature in this field, several terms need to be defined. An important practical property of these systems is their stereoselectivity or enantioselectivity, which is their ability to produce one enantiomer in preference to the other. This property is commonly given as the percent enantiomeric excess, ee, determined from an analysis of the products. If R and S are the percentages of the major and minor... 

Data shown by circles in the figure correspond to percent conversion, as measured by liquid chromatography (LC). Data points marked by squares correspond to percent enantiomeric excess, as measured by LC of the areas under the peaks for the 1 -enantiomer of sitagbptin free base. Since solutions of ammonium chloride are... 

It is common to express the relative amounts of enantiomers as percent enantiomeric excess (ee). A similar diastereometic excess (de) or diaste-reomeric ratio (dr) can be calculated for those compounds. In the NMR-resolved examples given earlier, enantiometic excess is calculated from the areas of the NMR signals for each enantiomer as in Equation 3.1, where and represent the areas of the larger and smaller signals. The remaining percentage is considered as racemic. A sample of 94% ee contains 97% of one and 3% of the other enantiomer. 

A molecule, or an arrangement of molecules, is chiral if it is not superimposable on its mirror image. A chiral molecule possesses a handedness and can exist in either one form, an enantiomer, or its mirror image, the opposite enantiomer. A sample of chiral molecules can have varying chiral purity, referred to as enantiomeric excess. The percent enantiomeric excess (%ee) is defined as the percent excess of one enantiomer relative to the total sample. The %ee of a pure sample of only one enantiomer is 100%. If a sample is composed of an equal mixture of both... 

Now consider a circumstance in which the percent ratio of a mixture of enantiomers is not 50 50. The percent enantiomeric excess of the enantiomer present in the larger amount is calculated as follows. 

Optical purity is usually reported in percent enantiomeric excess (% cc) because it gives the difference between the % of one pure enantiomer and the % of the racemic mixture. If the amounts of each enantiomer are known (e.g., measured by chiral chromatography), then the enantiomeric excess can be determined in another way ... 

Induction of chiraUty to obtain enantiomerically or diastereomericaUy enriched materials is achieved by several approaches (1) absorption of circularly polarized light by a racemate, causing different reaction rates for the enantiomers (2) reactions in chiral medium such as chiral solvent (3) the use of an enantiomerically pure sensitizer and (4) the use of enantiomerically pure materials, that is, photochem-ically reactive components attached to an enantiomerically pure auxfliary. The first two methods have not yielded product with more than a few percent enantiomeric excess (ee < 5%). Chiral sensitization has yielded, in a few select examples, moderate ee ( 15-65%), while the chiral auxiliary approach has yielded high diastereomeric excess (de). In this approach, because there is a chiral center in the reactant, the products are formed as diastereomers. Removal of the chiral auxfliary results in an enantiomerically enriched product. 

Assuming a linear relationship between [a] and concentration, which is true for most cases, the optical purity is equal to the percent excess of one enantiomer over the other ... 

This definition refers to an enantiomeric mixture produced in an asymmetric synthesis. In some cases where a diastereomeric mixture is produced, the definition has to be altered accordingly. Percent optical purity is an operational term that depends on optical rotation measurements. It is not necessarily equal to the percent enantiomeric purity (13), which is a more meaningful term and is the extent to which one enantiomer is formed in excess over the other ... 

Enantiomeric excess (% ee) is the percentage of the major enantiomer minus that of the min or enantiomer optical purity (% op) is the ratio, in percent, of the optical rotation of a mixture of en antiomers to that of the pure enantiomer. 

We next synthesized the pyridoxal-bound fl-CD catalyst 44 (Scheme 2.8) [43], which produced 3-5 times more tryptophan when incubated with indole, [f-chloroalanine, and A12(S04)3 (pH 5.2 and 100°C) than the reaction in which the pyridoxal derivative was replaced by simple pyridoxal. However, tryptophan yield was still only a few percent. As expected, this kinetic advantage disappeared at higher indole concentrations due to saturation of the binding site. Furthermore, L-tryptophan was produced in ca. 10% excess relative to the D-enantiomer. 

Specific rotation data may assist in the identification of a specific enantiomer, or may be used to determine the optical purity (enantiomeric purity) of a mixture of enantiomers. Optical purity is defined as the percent excess of one enantiomer over another in a mixture and is expressed in Eq. (3) ... 

Enantioselectivity (percent enantioselectivity, % es) In a reaction or reaction sequence in which one enantiomer (Ei) is produced in excess, the enantioselectivity is the mole fraction formed of the major enantiomer, expressed as a percent ... 

The final definition concerns formation of enantiomers, and the terms enantioselective and enantiospecific are used. If a reaction produces an unequal mixture of enantiomers it is enantioselective. If it generates only one enantiomer of two possibilities, it is enantiospecific. The baker s yeast reduction (see sec. 4.10.F) of 134 gave 135 with >99% ee (S). (Here % ee means percent of enantiomeric excess.) A 0% ee means a 50 50 mixture (racemic mixture), 50% ee means a 75 25 mixture and 90% ee means a 95 5 mixture. The predominance of the (S) enantiomer makes this reaction highly enantioselective. 

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