Diastereomeric association complexes

Figure 5. Assumed structure of the diastereomeric association complex...

The geometry of the resulting diastereomeric association complexes is distinct and diastereotopic nuclei reside in different spatial environments. In this case the sense of nonequivalence would not necessarily be the same for all shifted resonances, but reversals could be expected. 

The equilibrium constants of the diastereomeric association complexes formed between the enantiomers (R and S) and the auxiliary compound (R ) are different, e.g., for binary... 

Direct separation by using chiral stationary phases, effected by the formation of diastereomeric association complexes... 

Equations [1] and [2] describe the interaction of enantiomeric solutes A and A with chiral solvating agent S. Solvates (or association complexes) are formed that are diastereomeric and thus can, in principle, have different properties. Relevant differences are those in... 

One of the main principles for chiral separation used in modern capillary GC is the bonding of the optically active compounds via hydrogen bridges to a stationary-phase material. Feibush and Gil-Av [8] suggested a rapid and reversible formation of association complexes between carbonyl and amide functions of selector and selectand. The formation of diastereomeric associates yields complexes of different stability, depending on the relative configuration. The introduction of dipeptide and diamide phases leads... 

Diastereomeric association between chiral molecules and chiral transition metal complexes was first described by Schurig in 1977. Since hydrogen bonding interaction is not essential for chiral recognition in such a system, a number of compounds could be separated, but this method was limited by the nonsufficient thermal stability of the applied metal complexes. 

Thus either the binding constant difference or the mobility difference of the corresponding diastereomeric complexes may result in enantioseparations in CE [2, 3, 6,7]. Rather common is the former case (Ki 7 K2) or the combination of both principles, although the possibility of enantioseparations solely based on the different mobilities of the diastereomeric associates has been unambiguously proven [7]. 

In theory, the two diastereomeric complexes will have different association constants. The evaluation of any chiral discrimination will depend upon measurement of the different proportions of the diastereoisomers formed. For example, nmr experiments have been successful in determining the degree of complex formation by each enantiomer. Alternatively, an extraction procedure has been employed this involves the interaction... 

The relevant AGB values, calculated by using Te = 970 are listed in Table 8. According to the reported values, the heterochiral Trp/Pro and Phe/Ala complexes are more stable than the homochiral ones. The reverse is true for the Phe/ Pro and PheA al complexes. By the same token, the chiral discrimination factor, AR measured by ESl-MS for 19 amino acids was found to vary between 0.3 and 3. ° The stereochemistry associated to the CID of diastereomeric peptides has been investigated using a similar approach. The results suggest that the secondary structure of protonated peptides may play an important role in their gas-phase behavior. ... 

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