Chiral Recognition by Crown Ethers

One of the most important aims of molecular recognition is chiral recognition, because it is commonly achieved in biological systems. Receptors in our body 

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Chiral recognition by crown ethers with asymmetric carbon atoms 406... 

CHIRAL RECOGNITION BY CROWN ETHERS WITH ASYMMETRIC CARBON ATOMS... 

With regard to the chiral recognition by crown ethers D. J. Cram kindly informed us that the EDC value of 38 (footnote b, Table 67) proved to be in error, and that the reported RR-S configuration in Table 68, footnote d and page 403, is still uncertain. Recent work (Peacock et al., 1980) has shown that the chiral recognition of amino acids (page 397 and Table 69) is comparable to that of amino-acid esters. The peculiar optimum in EDC values as a function of acetonitrile concentration (page 401 and Table 72) could not be duplicated. 

Chiral Recognition by Crown Ethers Chiral recognition is one of the most important topics in host-guest chemistry. Crown ethers with axis chirality result in chiral guest molecules. 

With amino-acid salts the effect of the medium is considerably larger. Peacock and Cram (1976) reported that the degree of chiral recognition of DL-phenylglycine perchlorate by crown ether [285] depends on the ratio of acetonitrile and chloroform. The observed EDC values vary from 6 to 52, which corresponds to a difference in free energy of —1.15 kcal mol-1 (Table 72). The optimum is very sharply defined (23.1% of acetonitrile) and is... 

Chiral recognition of (f S)-a-phenylethylammonium and ( S)-phenylglycine methyl ester salts by crown ethers in which chiral carbohydrate structure elements are incorporated has been reported by Stoddart et al. (Curtis et al.,... 

H and 13C NMR have also been widely used to study the binding of alkylammonium cations to crown ethers. The shift in substrate proton resonances can give detailed information of the preferred structure of the complex. This is particularly important in the study of chiral recognition by asymmetric receptors (79CSR85) (Section 5.21.3.2.2) and also the development of bioorganic models and catalysts (Section 5.21.5.1.1). 

Crown ethers have been used for improving the detection of fullerenes by electrospray mass spectrometry [13]. Sawada et al. have developed a FABMS methodology for the determination of chiral recognition of amino acid esters by crown ethers [14]. This method requires that the racemic mixture of the guests contains one enantiomer in its isotoplcally labeled form. Mass spectral analysis of the molecular ion peaks for H + and H + G(5) allows a direct comparison of their relative abundances, where H and G are host and guest, respectively. 

The previous study showed that chiral recognition by the chiral crown ethers occurs not in the complex formation but in the intra-complex thiolysis, and that... 

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. 

The recognition of barium containing crown ether bridged chiral Schiff base zinc complex [44] with the rigid bidendate guest 1,4-diazobicyclo-[2,2,2]octane (DABCO) was studied by aH NMR titration.107... 

A different concept of chiral recognition was used by Lehn et al. (1978) for the differentiation between pairs of enantiomeric anions. Following the terminology used for metallo-enzymes, the chiral crown ether [309] acts as an apo-receptor, complexing a metal cation and thus becoming a chiral metal receptor that may discriminate between enantiomeric anions (cascade-type complexation). Extraction experiments with racemic mandelic acid dissolved in... 

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