Pseudo-enantiomeric pairs

To overcome this limitation, readily available and cheap pseudo-enantiomeric carbohydrates can be used. Scheme 4 shows the pseudo-enantiomeric pairs D-ga-lactose 3/D-arabinose 4 [19] and D-2-deoxyglucose 5/L-2-deoxyrhamnose 6 [20]. 

The cinchona alkaloids are particularly valuable ligands in asymmetric addition of diethylzinc to a N-diphenyl phosphinoylimine (228) leading to enantiomerically enriched (R)- and (S)-N-(l-phenylpropyl-diphenylphosphinic amide) (229). Cinchonine and cinchonidine were found to be the pseudo-enantiomeric pair which gave the adduct in highest enantiomeric excess (up to 93% ee) (Scheme 62)." ... 

A similar approach was reported by Lygo and co-workers who applied comparable anthracenylmethyl-based ammonium salts of type 26 in combination with 50% aqueous potassium hydroxide as a basic system at room temperature [26, 27a], Under these conditions the required O-alkylation at the alkaloid catalyst s hydroxyl group occurs in situ. The enantioselective alkylation reactions proceeded with somewhat lower enantioselectivity (up to 91% ee) compared with the results obtained with the Corey catalyst 25. The overall yields of esters of type 27 (obtained after imine hydrolysis) were in the range 40 to 86% [26]. A selected example is shown in Scheme 3.7. Because the pseudo-enantiomeric catalyst pairs 25 and 26 led to opposite enantiomers with comparable enantioselectivity, this procedure enables convenient access to both enantiomers. Recently, the Lygo group reported an in situ-preparation of the alkaloid-based phase transfer catalyst [27b] as well as the application of a new, highly effective phase-transfer catalyst derived from a-methyl-naphthylamine, which was found by screening of a catalyst library [27c],... 

It was mentioned at the beginning of this chapter that alkaloids were among the first catalysts to be used for asymmetric hydrocyanation of aldehydes. More recent work by Tian and Deng has shown that the pseudo-enantiomeric alkaloid derivatives 5/6 and 7/8 catalyze the asymmetric addition of ethyl cyanoformate to aliphatic ketones (Scheme 6.6) [50]. It is believed that the catalytic cycle is initiated by the alkaloid tertiary amine reacting with ethyl cyanoformate to form a chiral cyanide/acylammonium ion pair, followed by addition of cyanide to the ketone and acylation of the resulting cyanoalkoxide. Potentially, the latter reaction step occurs with dynamic kinetic resolution of the cyano alkoxide intermediate... 

As summarized in Scheme 6.6, the cyanohydrins of a,a-dialkylated and a-acetal ketones were obtained with quite remarkable enantiomeric excess. Clearly the pseudo-enantiomeric catalyst pairs 5/6 and 7/8 afford products of opposite configuration. Catalyst loadings were in the range 10-35 mol%. 

An ionic chiral micelle is used as a pseudo-stationary phase it works as a chiral selector. When a pair of enantiomers is injected to the MEKC system, each enantiomer is incorporated into the chiral micelle at a certain extent determined by the micellar solubilization equilibrium. The equilibrium constant for each enantiomer is expected to be different more or less among the enantiomeric pair that is, the degree of solubilization of each enantiomer into the chiral micelle would be different for each. Thus, the difference in the retention factor would be obtained and different migration times would occur. 

Analogs and metabolites of DES are of Interest as additional probes of the structure-activity relationships of estrogens and because of uncertainty concerning the form responsible for the carcinogenic properties of DES. Pseudo-DES differs from DES in the location of the double bond and exists as E and Z isomers, EPD and ZPD (Figures 14.8 b and 14.8 c), each as an enantiomeric pair. While both forms bind to the estrogen receptor, only the Z form has appreciable activity. X-ray analysis reveals that both EPD and ZPD have bent conformations (Figures 14.9 c and 14.9d),... 

An ionic achiral micelle [e.g., sodium dodecyl sulfate (SDS)] and a neutral CD are typically used as a pseudo-stationary phase and a chiral selector, respectively. When a pair of enantiomers is injected into this system, two major distribution equilibria can be considered for the solutes or enantiomers (a) the equilibrium between the aqueous phase and the micelle (i.e., micellar solubilization) and (b) the equilibrium between the aqueous phase and CD (i.e., inclusion complex formation). Each enantiomer may have a different equilibrium constant for the inclusion complex formation among the enantiomeric pairs due to the enantioselectivity of the CD. As a result, each enantiomer exists in the aqueous phase at a different time among... 

QN and QD consist of a planar quinoline and a rigid quinuclidine ring and are different in the configuration of Cs and C9 chiral centers, thus diastereomers to each other (Fig. 9a). Interestingly, QN and QD CSPs display pseudo-enantiomeric property in most cases. It means that an opposite elution order can be obtained when the same enantiomeric pair is separated on both CSPs. This property is likely due to... 

Geselowitz et al. (200) showed that the oxidation of A,A-[Co(en)3] by A[Co(edta) ]" results in the production of A[Co(en)3] in excess of the other enantiomer, and vice versa for A[Co(edta)] . The enantiomeric excess was around 9% in H2O but rose with decreasing solvent polarity to 40% in sulfolane. The substitution of the en backbone of edta with alkyl groups has a negligible effect on the enantiomeric excess of [Co-(enlal or on the rate of the reaction. This seems to be evidence for the view that the selectivity arises from the pre-equilibrium step of the reaction and that [Co(edta)] and analogues were oriented with a carboxylate face toward the [Co(en)3] ion. The same conclusion was reached (221) in the study of the ion pair formation between [Co(edta) ] and [Co(en)3] + that is, that the stronger ion pairs were formed between complexes of opposite absolute configuration and that [Co-(edta) ] associated via its pseudo-C3 carboxylate face. 

In order to pin-point the properties of order-disorder we concentrated on molecules whose racemates are centrosymmetric and the resolved enantiomorphs, therefore, quasi-centrosymmetric. These systems are advantageous since the enantiomeric crystal contains two independent molecules related to each other by a pseudo centre of inversion, and replaced in the racemate by a centrosymmetric pair. 

Heterodimers and heterotrimers are formed in water if planar cations and anions are mixed and if polymerization is impeded by ethyl groups or larger alkyl substituents (Fig. 1.5.4). Other possibilities are the isolation of hydrophobic C-G or A-T pairs in small micelles (Fig. 2.5.4), formation of racemates (Figs. 4.2.7, 8.6.1, and 9.5.3), and formation of pseudo-racemates (Fig. 4.5.10) between enantiomeric amphiphiles with chiral head groups and the same or similar hydrophobic skeletons. 

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