Enantiomers resolving agent

In case the racemate is a true racemic mixture, this cannot be separated by preferential crystallization, but can be resolved using the diastereomer crystallization developed by Pasteur in 1848. A solution of the racemic mixture in water or methanol is allowed to react with a pure enantiomer (resolving agent), thereby forming a mixture of diastereomers that can be separated by crystallization. 

Dissociate diastereomer to single enantiomer recover resolving agent... 

This procedure is restricted mainly to aminodicarboxyhc acids or diaminocarboxyhc acids. In the case of neutral amino acids, the amino group or carboxyl group must be protected, eg, by A/-acylation, esterification, or amidation. This protection of the racemic amino acid and deprotection of the separated enantiomers add stages to the overall process. Furthermore, this procedure requires a stoichiometric quantity of the resolving agent, which is then difficult to recover efficiendy. Practical examples of resolution by this method have been pubUshed (50,51). 

By far the eommonest adduetion method is via an aeid-base reaetion, in whieh an enantiomer pair eontaining an aeid funetion (e.g. a earboxylie aeid) is reaeted with a base resolving agent, or viee versa. 

The most direct method is the separation of the enantiomers by HPLC using chiral columns. It has the advantage that there is no risk of contamination from chiral resolving agents. The formation of diastereoisomers... 

Williams, B.A. and Vigh, G., Dry look at the CHARM (charged resolving agent migration) model of enantiomer separations by capillary electrophoresis,. Chromatogr. A, 777, 295, 1997. 

We have seen that the spectra of enantiomers, acquired under normal conditions, are identical. The NMR spectrometer does not differentiate between optically pure samples, and racemic ones. The wording is carefully chosen, particularly normal conditions , because it is often possible to distinguish enantiomers, by running their spectra in abnormal conditions - in the presence of a chiral resolving agent. Perhaps the best known of these is (-)2,2,2,trifluoro-l-(9-anthryl) ethanol, abbreviated understandably to TFAE. (W.H. Pirkle and D.J. Hoover, Top. Stereochem., 1982,13, 263). Structure 7.4 shows its structure. 

CR)-Phenylethylamine-2-CR)-methyl-l-boraadamantane (R,R) 57 (92% de) was obtained in a similar manner using the opposite enantiomer of the resolving agent, (i )-(+)-phenylethylamine (Scheme 21) <2003MC121, B-2003MI97>. 

The method presented is based on the procedure of Theilaker and Winkler." It makes use of (+)-tartaric acid, an inexpensive and readily available material, as the resolving agent and provides optically pure samples of both enantiomers of a-phenyl-cthylamine. 

JB Vincent, G Vigh. Nonaqueous capillary electrophoretic separation of enantiomers using the single isomer heptakis(2,3-diacetyl-6-sulfato)-cyclodex-trin as chiral-resolving agent. J Chromatogr A 816 233—241, 1998. 

A highly versatile method for enantiomer analysis is based on the direct separation of enantiomeric mixtures on nonraceinic chiral stationary phases by gas chromatography (GC)6 123-12s. When a linearly responding achiral detection system is employed, comparison of the relative peak areas provides a precise measurement of the enantiomeric ratio from which the enantiomeric purity ee can be calculated. The enantiomeric ratio measured is independent of the enantiomeric purity of the chiral stationary phase. A low enantiomeric purity of the resolving agent, however, results in small separation factors a, while a racemic auxiliary will obviously not be able to distinguish enantiomers. 

Scheme 9.6 Simultaneous preparation of both enantiomers using a half equivalent of resolving agent.

A is correct. Strychnine is the only chiral molecule and thus the only possibility. The passage states that the only chemical difference between enantiomers is their reactions with chiral compounds. Strychnine is often employed as a resolving agent for racemic acids. 

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