Enantiomeric mixture separation

Group I,2,b (enantiomeric mixture, separated, auxiliary compound employed)... 

All enantioselective separation techniques are based on submitting the enantiomeric mixture to be resolved to a chiral environment. This environment is usually created by the presence of a chiral selector able to interact with both enantiomers of the mixture, albeit with different affinities. These differences in the enantiomer-selector association will finally result in the separation that is sought. 

One of the most powerful methods for determining enantiomer composition is gas or liquid chromatography, as it allows direct separation of the enantiomers of a chiral substance. Early chromatographic methods required the conversion of an enantiomeric mixture to a diastereomeric mixture, followed by analysis of the mixture by either GC or HPLC. A more convenient chromatographic approach for determining enantiomer compositions involves the application of a chiral environment without derivatization of the enantiomer mixture. Such a separation may be achieved using a chiral solvent as the mobile phase, but applications are limited because the method consumes large quantities of costly chiral solvents. The direct separation of enantiomers on a chiral stationary phase has been used extensively for the determination of enantiomer composition. Materials for the chiral stationary phase are commercially available for both GC and HPLC. 

The chiral reagent 122 was proposed for derivatization of enantiomeric mixtures of amino acids. Good HPLC separations were obtained for the diasteroisomer derivatives of a series of amino acids, including some unusual a-amino acids with long or bulky side chains, aryl and hetaryl groups, and -substituted /J-amino acids297. 

An alternative approach is provided by NMR spectroscopy. Separate NMR signals can be in principle obtained for stable or short-lived diastereomeric derivatives of the enantiomeric mixtures, the intensities of which are correlated with the enantiomeric composition and their relative stereochemistry to the absolute configuration. For this reason, great effort has been continually devoted to the development of new chiral auxiliaries for NMR spectroscopy. The majority of these are dedicated to the chiral assay of molecules having polar functional groups. 

This cycloaddition-reduction-hydrolysis sequence was also used in an approach to butyrolactones related to ribonolactone (71). These compounds are inducing agents of hunger and satiety in mammalians. Here, a subsequent aldol 1,3-diol reduction was used, and the required carboxy function was established by oxidation of the aromatic ring with ruthenium tetroxide. Cycloaddition of benzonitrile oxide to allyl alcohol afforded an enantiomeric mixture of isoxazolines 55 and 56, which were treated with sodium hydride and methyl iodide to achieve separation by chromatography on cellulose triacetate (71). 0-Demethylation, followed by... 

Group 1,1,a (enantiomeric mixture, not separated, no auxiliary compound)... 

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. 

P,y-Diamino analogues 49 of statine are prepared stereoselectively starting from the O-methyl hydroxamate derivative of N-protected statine. The reaction sequence involves the formation of a p-lactam intermediate obtained by internal cyclization under Mitsunobu conditions.184 Alternatively, direct amination of either a p-oxo ester 31 followed by reduction of the resulting enamine 50, 85 or by reduction of the corresponding ,p-unsaturated ester, 88 gives an enantiomeric mixture of the corresponding unprotected p-amine, which is protected by a carbamate prior to chromatographic separation (Scheme 20). 

Both techniques can be applied in two ways. In the first method the enantiomeric mixture (or racemate) is converted into a diastereoisomeric mixture with a suitable optically pure reagent, and this mixture chromatographed on a column having an achiral stationary phase. Separation then depends on the differential molecular interactions of the diastereoisomers with the stationary phase. In the second method, the stationary phase on the support material (usually chemically bonded) contains a chiral, optically pure residue. In this case the mixture of enantiomers which is loaded directly on to the column is separated by virtue of differential diastereoisomeric molecular interactions between each enantiomer and the optically pure stationary phase. 

Another way to obtain pure enantiomers is the separation of racemates through preparative chromatography on chiral stationary phases. In fact, the most significant developments over the last 20 years have been the application of GLC and HPLC techniques to the effective resolution of enantiomeric mixtures and to determining the enantiomeric ratio [7,8],... 

Practical advantage of this resolution method is that most of the investigated racemates could be solved in hexane but DBTA and the complex was insoluble in it. Consequently, complete separation of the solid diastereoisomeric complex and the remained free enantiomeric mixture of the alcohol is simple and chemists can avoid the exhausting work of solvent selection and concentration optimization. 

The following table provides information on the properties and applications of some of the more specialized stationary phases used to carry out the separation of enantiomeric mixtures. In many cases, the phases are not commercially available. Refer to the appropriate literature citation for details on the synthesis. 

Cellulose was the first sorbent for which the resolution of racemic amino acids was demonstrated [23]. From this beginning, derivatives such as microcrystalline triacetylcellulose and /3-cyclodextrin bonded to silica were developed. The most popular sorbent for the control of optical purity is a reversed-phase silica gel impregnated with a chiral selector (a proline derivative) and copper (II) ions. Separations are possible if the analytes of interest form chelate complexes with the copper ions such as D,L-Dopa and D.L-penicillamine [24], Silica gel has also been impregnated with (-) brucine for resolving enantiomeric mixtures of amino acids [25] and a number of amino alcohol adrenergic blockers were resolved with another chiral selector [26]. A worthwhile review on enantiomer separations by TLC has been published [27],... 

The technique of boxcar injections (not to be confused with boxcar chromatography) can be extremely productive for iso-cratic elution in any mode of chromatography and should always be considered when scaling up a separation. The preparative HPLC of an enantiomeric mixture utilising a chiral stationary phase is described here to demonstrate the approach for separation of a binary mixture. 

Even nowadays, particularly in industrial processes, the separation of enantiomers of racemic acids and bases is based on this molecular chiral recognition. The less soluble, i.e. the more stable of these diastereomer salts crystallizes even if the chiral agent in the better soluble salt is replaced by an achiral reagent of similar chemical character, or eventually eliminated, or substituated by a solvent. In this case, a mixture enriched with the more stable diastereomer can be isolated by filtration from the solution of the achiral salt of the enantiomeric mixture or the free enantiomers [2,3]... 

Preparative Uses of MTPA Derivatives. Resolution of racemic compounds on a preparative scale is always a challenging endeavor. Conversion of the enantiomeric mixture into a mixture of diastereomers, each with unique physical properties, makes it possible to separate the components by a variety of physical methods, such as fractional recrystallization, distillation, or chromatography. One of the earliest uses of MTPA was the resolution of racemic alcohols via the separation of diastereomeric MTPA esters by preparative gas-liquid chromatography, followed by alcohol regeneration with Lithium Aluminum Hydride (eq 2). More frequently, diastereomeric MTPA esters have been separated by high performance liquid chromatography (HPLC), followed by al-... 

Similarly, enantiomeric mixtures of carboxylic acid esters can be separated using transesterification (Fig. 22). An n-butanol/water biphasic medium aryloxypropionic acid methyl ester vras transesterified stereoselec-tively, yielding the butylester of the R enantiomer (69). 

We envisaged, therefore, a process whereby enantiomerically enriched [e.g., (R) > (S)] chiral compounds such as amines, alcohols, and acids are attached to a photodimerizable handle and the mixture separates on crystallization into a racemic portion crystallizing in racemic photodimerizable crystals, with the chiral excess crystallizing in chiral light-stable crystals. Irradiation of these solid mixtures will yield racemic (RS) dimer and chiral unreacted monomer as depicted in Scheme 9. 

Troger s base (Fig. B.2) was the first example of a chromatographic separation of an enantiomeric mixture. Since then it has been the subject of several publications. 

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