Polarimetry mixtures

The analytical capability of these matrices has been demonstrated for chiral amines [12, 13]. The procedure is illustrated in Fig. 8-4 for the separation of NapEtNH " CIO . Concentrated methanol/dichloromethane solutions of the racemic mixture were placed on a column containing the chiral macrocycle host. The enantiomers of the ammonium salts were resolved chromatographically with mixtures of methanol and dichloromethane as the mobile phase. The amounts of R and S salts in each fraction were determined by polarimetry. Because the chiral supported macrocycle interacts more strongly with S salts, the R salt passes through the column first and the S salt last, as seen in Fig. 8-4. 

Secondary isotope effects are small. In fact, most of the secondary deuterium KIEs that have been reported are less than 20% and many of them are only a few per cent. In spite of the small size, the same techniques that are used for other kinetic measurements are usually satisfactory for measuring these KIEs. Both competitive methods where both isotopic compounds are present in the same reaction mixture (Westaway and Ali, 1979) and absolute rate measurements, i.e. the separate determination of the rate constant for the single isotopic species (Fang and Westaway, 1991), are employed (Parkin, 1991). Most competitive methods (Melander and Saunders, 1980e) utilize isotope ratio measurements based on mass spectrometry (Shine et al., 1984) or radioactivity measurements by liquid scintillation (Ando et al., 1984 Axelsson et al., 1991). However, some special methods, which are particularly useful for the accurate determination of secondary KIEs, have been developed. These newer methods, which are based on polarimetry, nmr spectroscopy, chromatographic isotopic separation and liquid scintillation, respectively, are described in this section. The accurate measurement of small heavy-atom KIEs is discussed in a recent review by Paneth (1992). 

The kinetics of the reaction have been monitored350 polarimetri-cally, and by separation of the products, after various intervals, by g.l.c. of their per(trimethylsilyl) ethers. L-Fucose dissolved completely in boiling methanol within 30 min, even in the absence of the resin, and the initial solution contained 5% of L-fucofuranoses, 38% of a-L-fucopyranose, and 57% of /3-L-fucopyranose. Immediately upon adding the resin, glycoside formation occurred. After 1 min, all four isomers appeared, in approximately equimolar ratios and the furano-side content attained a maximum after 15 min (18% of a- and 34% of /3-furanoside, with 23% of a- and 21% of /3-pyranoside). The final equilibrium mixture (after 12 h at the b.p., with the resin) contained 6% of a- and 13% of /3-fucofuranoside, and 54% of a- and 27% of /3-fuco-pyranoside. 

Limits of detection for many amino acids were enhanced considerably by pre-column derivatization with the achiral reagent dansyl chloride whose function it was to increase their specific rotation [17]. Determinations of the enantiomeric purities for mixtures of D-and L-tryptophan [18] and of isomeric ratios for mixtures of pseudoephedrine and its diastereomer ephedrine [19], were effected using diode-laser polarimetry and using OR detection in series with UV absorbance detection respectively. 

Calculate specific rotations from polarimetry data, and use specific rotations to determine the optical purity and the enantiomeric excess of mixtures. 

Just as solvents can serve as CSRs, it is possible to use enantiomerically pure solvents to provide the asymmetric environment necessary to differentiate enantiomers. This then allows us to determine the relative amount of two enantiomers in a mixture of them, something that is otherwise difficult to accomplish without resorting to polarimetry. Enantiomerically pure alcohol (R)-10-11 has been used as a solvent to differentiate the enantiomers of aminoester 10-1412 ... 

In principle, any measurable property of a reacting system that is proportional to the extent of reaction may be used to monitor the progress of the reaction. The most common techniques are spectrophotometric (UV-visible, fluorescence, IR, polarimetry and NMR) or electrochemical (pH, ion-selective electrodes, conductivity and polarography). Either a "batch" method can be used, in which samples are withdrawn from the reaction mixture and analyzed, or the reaction may be monitored in situ. By far the most widely used technique involves UV-visible spectrophotometry. 

In early work on the methanolysis of tetra-O-methyl a-o-glucopyranosyl and -mannopyranosyl chlorides, it was shown that the reactions were cleanly first order whether followed by titration of the acid produced or by polarimetry. The gluco chloride gave exclusively the p-methyl glucoside, the manno an anomeric mixture. Reactions of the gluco but not manno chloride with thiophenoxide ion were cleanly bimolecular and gave exclusive inversion, as expected. 

Another measure of the asymmetric kinetic properties of the two bases in the alanine racemase mechanism is the qualitative behavior of the equilibrium overshoots observed. Overshoots are often observed in reaction progress curves run in deuterium oxide that are initiated with a single stereoisomer that is protiated at the Ca position (Fig. 7.3). The optical activity is monitored by polarimetry or circular dichroism (CD). At equilibrium, the signal is zero, since the product is a racemic mixture of d- and L-isomers. However, when there is a significant substrate-derived KIE on the reverse direction (product being fully deuterated in a two-base mecha-... 

It is apparent from the preceding chapter that the analysis of enantiomers (by whatever means) addresses only part of the problem often, a stereoselective reaction produces a mixture of diastereomers, and polarimetry is an inappropriate technique. Thus, asymmetric synthesis requires the means for the analysis of both enantiomeric and diastereomeric mixtures. Ultimately, the ratio of isomers and the cofifiguration of each new stereocenter should be determined. 

A final proof for the stereochemistry present in alapyridaine was performed by CD spectroscopy. As shown in Figure 8, the alap5a idaine sample isolated from heated Maillard mixtures did not show any CD effect, thus confirming the formation of racemic alap)o idaine during thermal treatment as suggested by the optical rotation of 0° measured by polarimetry. In contrast, the synthetic ( S)-(+)-enantionier showed a pronounced CD effect, in particular, when measured at pH 9, thus confirming the enantiopurity of that sample. 

Detection based on differences in chirality makes use of polarimetry or circular dichroism. A racemic mixture that has been separated in a chiral column vill then be seen as tv o peaks, opposite in sign but equal in magnitude. The sensitivity of chiral detectors is not very high 1-10 ng. 

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