Enantiomeric differentiation

Modest chiral recognition in ground-state complexation has also been achieved [5,12,58] with crown ethers incorporating carbohydrate residues as their source of chirality. However, no attempt has been made to optimise their enantiomer-differentiating ability. 

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Having established that pure enantiomer ( S,ZR)-77 was capable of undergoing remarkably regioselective and diastereoselective C-H activation, it followed that highly efficient enantiomeric differentiation of rac-77 could be accomplished.199 Hence, the Rh2(5Y-MEPY)4-catalyzed reaction of rac-77 effectively gave close to a 1 1 mixture of enantioenriched (lY)-78 (91% ee) and ( R)-79 (98% ee) (Equation (68)). Other equally spectacular examples of diastereo- and regiocontrol via chiral rhodium carboxamide catalysts in cyclic and acyclic diazoacetate systems have been reported.152 199 200 203-205... 

A substantial increase in the enantiomeric differentiation is observed when the 3- and 3 -positions of the 1,1 -dinaphthyl or 1,1 -ditetralyl groups are substituted (Helgeson et al., 1974 Peacock and Cram, 1976 Peacock et al.,... 

Aryl groups generally give rise to higher enantiomeric differentiation. 

Most of the work on chiral recognition has been carried out with amino-acid ester salts. The limited number of available data of similar experiments with amino-acid salts point to an increased enantiomeric differentiation (Table 69 Peacock and Cram, 1976). The reason might be the higher -acidity of the carboxyl group (vide ir fra). In solid-liquid chromatography (see Table 68) the difference between amino-acid salts and amino-acid ester salts is not consistent and not always in favour of the amino-acid salts. However, it should be... 

Enantiomeric differentiation of amino acid and the corresponding amino-acid methyl ester salts RCH(COOR )NHjX by crown ether [285] ... 

The influence of the counterion on the stability of crown-ether complexes in general was reviewed in detail in one of the preceding sections. There it was shown to be an important parameter. The nature of the counterion in diastereomeric complexes of chiral crown ethers with primary ammonium salts also influences the chiral recognition. First of all it greatly determines whether salt can be extracted into the organic phase where the chiral discrimination takes place. In a series of experiments (Kyba et al., 1978) it was shown that when S,S -6zs(dinaphthyl)-22-crown-6 [284] in chloroform was equilibrated with racemic er-phenylethylammonium salts the type of anion also influences the degree of enantiomeric differentiation (Table 70). The highest... 

Enantiomeric differentiation between (7 S)-cr-phenylethylamine (R = CHj) and (RS)-phenylglycine methyl ester salts (R = COOCH,)°... 

With the larger racemic cr-hydroxy-l-naphthaleneacetic acid too, extraction takes place in the presence of 1309] and the appropriate cations but enantiomeric differentiation is not observed. These results were confirmed in transport experiments in which the alkali mandelate is carried through a liquid membrane of [309] dissolved in chloroform. Lehn et al. (1978) explain these observations in terms of an ion pair included in the cavity of the crown ether. The reversal of chiral recognitions between potassium and cesium mandelate of 25% indicates that the structures of the two complexes are different. 

Lehn and Sirlin found a very high degree of enantiomeric differentiation in the reaction of [327] with racemic dipeptide p-nitrophenyl esters [Gly-(DL)-Phe-OPNP]. Gly-(L)-Phe-OPNP reacts at approximately the same rate as the achiral Gly-Gly-OPNP, but the D-enantiomeric dipeptide ester is converted at a... 

Decay time, and enantiomeric differentiation, 184 Dehalogenation reactions, 96/... 

The enantiomeric differentiation of linalool is useful in the quality control of essential oils and oleoresins, as it was found to provide an important indication of the authenticity of many herbs and spices. The enantiomeric composition of linalool has been determined in many essential oils, including basil, bergamot, rosemary, lavandin, lavender, balm, coriander, mace. Pelargonium, rose, Cymbopogon, lemon, mandarin, Osman-thus, davana, jasmine, Lippia alba and orange, as well as in many fruit... 

The reaction has been applied to many systems an impressive example is the enantiomeric differentiation and kinetic resolution of a-pinene 199 (Tab. 14.15) [135]. The Rh2(S-DOSP)4-catalyzed reaction with (-t)-a-pinene is the matched reaction, in which 200 is formed in 93% yield and with 96% diastereomeric excess. The corresponding re-... 

A Aumatell, RJ Wells. Enantiomeric differentiation of a wide range of pharmacologically active substances by cyclodextrin-modified micellar electrokinetic capillary chromatography using a bile salt. J Chromatogr A 688 329—337, 1994. 

Racemic N-methylimines derived from 4-substituted 1-tetralones were ki-netically resolved by asymmetric hydrosilylation with phenylsilane (1 equivalent) as a reducing agent using the titanocene catalyst (R)-ll (substrate Ti= 100 1) at 13 °C, followed by a workup procedure to afford the corresponding chiral ketones and chiral cis amines with very high enantio- and diastere-oselectivity (Scheme 12) [28], The extent of the enantiomeric differentiation, kfast/kslow was calculated to be up to 114. The ris-selectivity of this reaction was... 

Fig. 11.5. Os-catalyzed asymmetric dihydroxylation of olefins by enantiomeric differentiation (chiral induction). (Reprinted from S. Torii, Electrochemical Asymmetric Syntheses of Chiral Diols and Epoxides Interface, Winter 1997, p. 46, Scheme 1. Reproduced by permission of the Electrochemical Society.)...

RDCs are commonly used for the structure elucidation of proteins and nucleic acids nowadays. Only recently the approach was transferred back to also obtain structural information of small- to medium-sized organic molecules. The central application in this case is the determination of relative configurations of distant chiral and prochiral centres, and also conformational studies of biologically active molecules, for example the enantiomeric differentiation of small molecules in chiral alignment media can be achieved. 

A variety of chiral alignment media for organic solvents is known with the most widely used and best characterized being the poly(amino acids) PBLG and PCBLL. Both polymers form lyotropic mesophases and possess ot-helical structures for which many examples of enantiomeric differentiation have been shown. In addition to chiral poly(amino acids), it was demonstrated that achiral media with chiral cages like cyclodextrines serve as alignment media with the potential of chiral discrimination.121... 

The chiral 18-crown-6 derivative 1 (Scheme 4.1), and similar structures derived from it, are capable of exhibiting enantiomeric differentiation in the complexation of racemic primary ammonium salts. Compound ll-1 has been... 

Cryptands are macro-bi- or -poly-cycles able to encapsulate an ion by providing it higher protection because of their cagelike structures, as in (147) and (148). For these ligands the correspondence between cavity size and complex stability is more pronounced than for simple crown ethers. Recent approaches to improve the metal-ion selectivity of cryptands, as for example by replacement of ethylene units between each donor atoms with propylene units, or by insertion of several substituents into the macrocycles, have been reviewed.245 A new, interesting family of cryptands is constituted by borocryptands (149), which are useful receptors for chiral substrates, where enantiomeric differentiation can be achieved by using NMR spectroscopy.246... 

Keto derivatives (esters, thioesters, amides) are hydrogenated to the secondary alcohol with 93% to 99% e.e. Ketones having a heteroatom a to the carbonyl (—OH, —NRj, —halide) also undergo clean, stereochemically directed reduction. Coordination of the carbonyl oxygen and the heteroatom of the other functional group to the metal forms a chelate ring responsible for the enantiomeric differentiation. This clean process favorably competes with the use of bakers yeast, and is applied to the synthesis of natural products like carnitine and compactine" ... 

The same model is also applied to enantiomer discriminating hydroformylations of racemic chiral alkenes. Here four transition states are possible with two diastereomeric forms for each enantiomer. Results of enantioface differentiation and enantiomeric differentiation for a reference case is used to predict enantiomeric differentiations of other racemic alkenes with good agreement. 

Enantiomeric differentiation during the thiolysis of a-amino acid ester salts by two thiol-bearing 18-crown-6 derivatives prepared from (1/ , 2/ , 35, 45)- and R, 2S, 3R, 45)-camphane-2,3-diols has also been demonstrated [74]. Discrimination by factors of 1.7-1.9 in the rates of p-nitrophenol release from the enantiomers of alanine-p-nitrophenyl ester salts has been observed. By contrast, the tetrakis-L-cysteinyl methyl ester receptor molecule l,l-l,l,l,l-(60) exhibits [75] extremely high... 

Deber and Blout have reported enantiomeric differentiation between D-and L-amino acid salts in complexes with cyclo (L-Pro-Gly) peptides n = 3, 4). 

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