Tartaric chiral discrimination

Early examples of enantioselective extractions are the resolution of a-aminoalco-hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion) [184-186] by partition between aqueous and lipophilic phases containing esters of tartaric acid [184-188]. Alkyl derivatives of proline and hydroxyproline with cupric ions showed chiral discrimination abilities for the resolution of neutral amino acid enantiomers in n-butanol/water systems [121, 178, 189-192]. On the other hand, chiral crown ethers are classical selectors utilized for enantioseparations, due to their interesting recognition abilities [171, 178]. However, the large number of steps often required for their synthesis [182] and, consequently, their cost as well as their limited loadability makes them not very suitable for preparative purposes. Examples of ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino acid derivatives have also been described. 

Cinchona diimides have also been used to develop a simple colorimetric indicator displacement assay for the determination and chiral discrimination of tartaric acid. Thus, conjugation of 72 with bromophenol blue 73 resulted in the formation of a dark blue complex (A,m lx at 597 nm) that after treatment by an appropriate acid released the protonated form of the indicator with a well-separated absorption maximum at 426 nm. This assay allowed to determine (R,R)-tartaric acid with the detection limit as low as 0.02 mg/ml and efficiently discriminated the enantiomers of tartaric adds,... 

Ketone syntheses. Reactivity discrimination between thiol esters and ordinary esters is the basis of an approach to chiral yalkylated butyrolactones from tartaric acid. Morpholine amides are simple substrates for Grignard reaction to provide ketones. ... 

Examples of (a) are quite common and of (b) much less common. A few are shown in Table 1. It should be observed that there are numerous d-d and d-l pairs reported as melting as the same temperature, as for example the diethyl esters of d- and tartaric acid (M.P. 17 °C). Where the chiral centre is sequestered within the molecule, and has httle or no influence on the packing shape, differences of packing energy may well be too small to be measured except under the most refined conditions (see also Section 1.12). The effects of chemical contamination must in any case put in doubt the interpretation of small differences in the search for evidence of discrimination. 

Considerable work has been done in the field of chiral hosts capable of discriminating between enantiomeric guest molecules. A variety of chiral residues either synthetic or derived from natural products l (sugars, tartaric acid, amino acids etc.) have been fused to macrocyclic rings. In most cases, chiral primary ammonium cations have been the guest species of choice, although the complexation of chiral carboxylates has been examined to some extent 2]. 

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