Enantiomer-specific purity

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) ... 

When dealing with reactions leading to stereoisomeric products we have the additional complication that descriptors such as enantiomeric (diastereomeric) excess and enantiomeric (diastereomeric) ratio are used to describe product purities. The evaluation of RME for a specific stereoisomer, say the R enantiomer, is exactly as above using the connecting relationships for the fraction of each product shown below. 

Specification in Table 9. b Yield of a mixture of 75 and 76 which was separated from the crude reaction product by a silica gel chromatography. All [a]D values were measured in CHC13 at c 1.0.d Optical purity was determined by HPLC on Chiralcel.e Since optically active 75 a and 76 a could not be separated, it is not clear whether both enantiomers are (—)-ones or not. Therefore, both are tentatively shown as (—)-enantiomers. Since [a]D value of each enantiomer is also not clear, [otJD value of the mixture is shown.f Compound is inert to irradiation. Optical purity was not determined. h When an acetone solution of the mixture of (+)— 75c and 76c was kept, racemic 76c crystallized out, mp 135-137 °C. 

One of the terms for describing enantiomer composition is optical purity. It refers to the ratio of observed specific rotation to the maximum or absolute specific rotation of a pure enantiomer sample. For any compound for which the optical rotation of its pure enantiomer is known, the ee value may be determined directly from the observed optical rotation. 

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amount of enantiomers in a mixture is often measured by nonpolarimetric methods, use of the term percent optical purity is obsolete, and in general has been replaced by the term percent enantiomeric excess (ee) (equation 2) introduced in 197163, usually equal to the percent optical purity, [/ ] and [5] representing the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. 

As in the case of other chiral compounds, the optical and enantiomeric purity of chiral organosulfur compounds can be determined by various methods (241). The simplest and most common method for the determination of optical purity of a mixture of enantiomers is based on polarimetric measurements. However, this method requires a knowledge of the specific rotation of the pure enantiomer. In 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. 

The nitroaldol-lipase DCR process could not only amplify specific (3-nitroalcohol derivatives, but also lead to their asymmetric discrimination. HPLC analysis proved that the enantioselectivity of the process is very high, resulting in products of very high optical purity. The R-enantiomer of the ester 45 was resolved to 99% ee, and the R-enantiomer of the ester 46 to 98% ee. 

Although the majority of current pharmacopoeial methods describe the use of specific optical rotation ([a]) for the determination of stereoisomer identity and/or content, separation techniques such as HPLC and CE are now the most frequently applied technologies during drug development and for new pharmaceutical products entering the market. Indeed, Supplementary Chapter K in Volume IV of the 2004 British Pharmacopoeia (BP) deals with this specific issue [22]. It states that in future when a monograph describes an enantiomer it will include both a test for identity ([a]) and a test to control stereoisomeric purity... 

Since the early times of stereochemistry, the phenomena related to chirality ( dis-symetrie moleculaire, as originally stated by Pasteur) have been treated or referred to as enantiomericaUy pure compounds. For a long time the measurement of specific rotations has been the only tool to evaluate the enantiomer distribution of an enantioimpure sample hence the expressions optical purity and optical antipodes. The usefulness of chiral assistance (natural products, circularly polarized light, etc.) for the preparation of optically active compounds, by either resolution or asymmetric synthesis, has been recognized by Pasteur, Le Bel, and van t Hoff. The first chiral auxiliaries selected for asymmetric synthesis were alkaloids such as quinine or some terpenes. Natural products with several asymmetric centers are usually enantiopure or close to 100% ee. With the necessity to devise new routes to enantiopure compounds, many simple or complex auxiliaries have been prepared from natural products or from resolved materials. Often the authors tried to get the highest enantiomeric excess values possible for the chiral auxiliaries before using them for asymmetric reactions. When a chiral reagent or catalyst could not be prepared enantiomericaUy pure, the enantiomeric excess (ee) of the product was assumed to be a minimum value or was corrected by the ee of the chiral auxiliary. The experimental data measured by polarimetry or spectroscopic methods are conveniently expressed by enantiomeric excess and enantiomeric... 

The optical purity (P. synonymous with optical yield) is defined as the specific rotation ([a]) of an enantiomeric mixture, divided by the specific rotation ([amax], maximum specific rotation) of the pure enantiomer (either enantiomer the sign of the rotation is ignored for convenience)... 

A requirement for determination of optical purity (P) is that the specific rotation of the pure enantiomer, [a]max, is known with certainty. This maximum rotation can be established by calculation, e.g., via competitive reaction methods41,42, or by direct determination employing an enantiomerically pure sample. Enantiomerically pure samples are generally believed to arise from enzymatic reactions performed on biogenic substrates, an assumption which in some instances is incorrect31, or are obtained in most cases by crystallization2. As many direct, nonchiroptical methods are available for determining enantiomeric purities, maximum rotations can also be extrapolated from the specific rotation of a sample of known ee. 

It is important to note that the measured optical purity (P) is not necessarily linearly related to the true enantiomeric purity (ee) when nonideal conditions prevail, e.g., when the enantiomers interact between themselves. Thus, the optical purity may markedly deviate from the true enanliomeric purity if the enantiomers undergo molecular self-association51. The oligomers formed in solution display their individual optical rotation and, depending on their concentration, contribute in a specific way to the overall optical rotation. The nonequivalence between the optical and enantiomeric purities has been experimentally demonstrated for 2-ethyl-2-methylbutanedioic acid in dichloromethane (Figure 4)51. 

Thus a racemic mixture (n1 = n2) has an enantiomeric purity of zero. Any other enantiomeric composition in principle can be determined provided the mixture has a measurable rotation and the rotation of the pure enantiomer, a0, is known. Unfortunately, there is no simple method of calculating a0 in advance. In fact, specific rotations of optically pure compounds are determined most reliably from Equation 19-4 after measurement of enantiomeric purity by independent methods. 

Values for the enantiomer ratio found in natural products can range from 0% to 100%. If the enantiomeric purity is 100%, care must be taken to determine which isomer is present by comparison of retention times with a known standard. Samples from different geographic or growing regions may show some variation in the ratio of a specific compound, while others will not deviate from a known value. 

Sensors for the detection of enantiomers are of great interest, as so far the on-line monitoring of production processes and medical diagnostics using standard chemical analytical methods is not possible. Quite often only one enantiomer of a chiral compound is actually a bioactive therapeutic. Therefore a proper analysis of the final product is essential. Currently, this involves separation techniques like liquid chromatography, GC and capillary electrophoresis, and determination of enantiomeric purity with circular dichro-ism and specific rotation. These are all off-line procedures and therefore no real-time analysis can be performed. Sensing devices for the distinction of different enantiomers would be a much cheaper, faster and easier-to-use alternative for this task, amenable to automation. 

The principle of the optical resolution of racemic pantolactone is shown in Fig. 13. If racemic pantolactone is used as a substrate for the hydrolysis reaction by the stereospecific lactonase, only the d- or L-pantolactone might be converted to d- or L-pantoic acid and the l- or D-enantiomer might remain intact, respectively. Consequently, the racemic mixture could be resolved into D-pan-toic acid and L-pantolactone, or D-pantolactone and L-pantoic acid. In the case of L-pantolactone-specific lactonase, the optical purity of the remaining d-pantolactone might be low, except when the hydrolysis of L-pantolactone is complete. On the other hand, using the D-pantolactone-specific lactonase, d-pantoic acid with high optical purity could be constantly obtained independently of the hydrolysis yield. Therefore, the enzymatic resolution of racemic pantolactone with D-pantolactone-specific lactonase was investigated [138 140]. 

Enzymatic treatment with Penicillin G acylase of the phenylacetamide of a RSMA leads to the specific hydrolysis of one enantiomer forming the amine in good yield and high enantiomeric purity (92%).236 Absolute configuration (R) and enantiomeric purity are determined by H1 NMR, after derivatization with (S)-a-trifluoromethylphenylacetic acid (Mosher s salt).302... 

---