Ethers, crown structural types

It can be concluded that stereoselective (and particularly enantioselective) separation can proceed by the simple apphcation of a chiral organic phase or in most cases by incorporation of carriers in the membrane phase. Sometimes, their structure is very complex and these molecules can act as real receptors for the enantiomers. Different types of transport mechanisms are involved in the separation and the most popular one is cotransport. This is a result of the fact that most frequently used carriers are based on crown ethers structure. The stereoselectivities by apphcation of carrier-mediated SLM separation are very different and depend on the structure of the guest and host molecules. The magnitudes of the stereoselectivity are in most cases moderate but similar to other membrane-based separation techniques for stereoisomers. 

The first three-dimensional macrobicyclic TTF derivatives have been obtained <96CC615> and work on TTF-containing crown ethers and thioethers has continued <96LA551, 96JCS(P1)1995> with structures of this type being used to obtain the first TTF-containing... 

If the photoequilibrium concentrations of the cis and trans isomers of the photoswitchable ionophore in the membrane bulk and their complexation stability constants for primary cations are known, the photoinduced change in the concentration of the complex cation in the membrane bulk can be estimated. If the same amount of change is assumed to occur for the concentration of the complex cation at the very surface of the membrane, the photoinduced change in the phase boundary potential may be correlated quantitatively to the amount of the primary cation permeated to or released from the membrane side of the interface under otherwise identical conditions. In such a manner, this type of photoswitchable ionophore may serve as a molecular probe to quantitatively correlate between the photoinduced changes in the phase boundary potential and the number of the primary cations permselectively extracted into the membrane side of the interface. Highly lipophilic derivatives of azobis(benzo-15-crown-5), 1 and 2, as well as reference compound 3 were used for this purpose (see Fig. 9 for the structures) [43]. Compared to azobenzene-modified crown ethers reported earlier [39 2], more distinct structural difference between the cis... 

In phase transfer catalysis of the solid/liquid type, the organic phase (containing dissolved organic reactant and a small amount of the crown) is mixed directly with the solid inorganic salt. Such a procedure enables the reaction to proceed under anhydrous conditions this is a distinct advantage, for example, when hydrolysis is a possible competing reaction. Because of their open structure, crown ethers are readily able to abstract cations from a crystalline solid and are often the catalysts of choice for many solid/liquid phase transfer reactions. 

In complexes of crown ethers [303] to [305] (types IV and V) both large groups are located in the larger of the two non-equivalent chiral cavities (Kyba et al., 1978). This was concluded from H nmr spectra of the diastereomeric complexes of [303] with a-phenylethylammonium salts. In the spectra the position of the methyl protons in both diastereomeric complexes is the same, in contrast to the methyl protons in complexes of [284] or [285] with the same salt. In the latter the upheld shift of the methyl protons in the more stable SS-(R) enantiomeric complex differs from that in the less stable SS-(S) enantiomer. Taking into account the available data summarized above, the tentative conclusion seems to be that the simple steric model (see structure... 

Different classifications for the chiral CSPs have been described. They are based on the chemical structure of the chiral selectors and on the chiral recognition mechanism involved. In this chapter we will use a classification based mainly on the chemical structure of the selectors. The selectors are classified in three groups (i) CSPs with low-molecular-weight selectors, such as Pirkle type CSPs, ionic and ligand exchange CSPs, (ii) CSPs with macrocyclic selectors, such as CDs, crown-ethers and macrocyclic antibiotics, and (iii) CSPs with macromolecular selectors, such as polysaccharides, synthetic polymers, molecular imprinted polymers and proteins. These different types of CSPs, frequently used for the analysis of chiral pharmaceuticals, are discussed in more detail later. 

Synthesis of zeolite Y in the presence of Gd(III) complexes of 18-crown-6 resulted not only in the encapsulation of the complex but the complex also served as a template for EMT polytype zeolite Y (Fig. 22b) (86). Feijen et al. described how the two different polytypes (the cubic FAU and the hexagonal EMT) can be formed (87). In the absence of an organic template, the FAU structure will form. If Na" "-18-crown-6 is present, it can be absorbed on the surface of the growing zeolite layer. This will influence the interconnection of the layers and, therefore, in the presence of this crown ether, the formation of the EMT framework may be favored. The difference between the pore window sizes is that in the EMT there are two different types 7.3 x 7.3 A in the [001] direction and 7.5 x 6.5 A perpendicular to the [001] direction. (The FAU has pore windows with 7.4 x 7.4 A in the [111] direction.)... 

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