Chiral ion-exchange chromatography

Sellergren, B. and Shea, K. J., Chiral ion-exchange chromatography Correlation between solute retention and a theoretical ion-exchange model using imprinted polymers, /. Chromatogr. A, 654, 17, 1993. 

TABLE 3 Chiral Compounds Tested for Separation by Chiral Ion-Exchange Chromatography... 

Although the racemization of the a-carbon can now be considered a potential problem, the synthesis of 32-peptides has been achieved in the same way as seen for 33-peptides. As the 32-amino acids cannot be prepared from the analogous a-amino acids, Seebach and co-workers 5,7 opted to use Evans oxazolidinone chemistry to produce enantiomerically pure 32-amino acids. Alkylation of 3-acyloxazolidin-2-ones 17 with A-(chloromethyl)benzamide yielded the products 18 with diastereomeric ratios between 93 7 and 99 1 (Scheme 8). Removal of the chiral auxiliary (Li0H/H202) and debenzoylation (refluxing acid) was followed by ion-exchange chromatography to yield the free 32-amino acids 20 which were converted by standard means into Boc 21 or benzyl ester 22 derivatives for peptide synthesis. 

Chiral ligand-exchange chromatography resolves enantiomers on the basis of their ability to complex with transition metal ions, such as copper, zinc, and cadmium, as illustrated by the separation of amino acid racemates using copper102 (Fig. 2.21). The principle of exchange is similar to that... 

In addition to these, there are many specialised reverse phase materials for specific applications, e.g., chiral materials for separating enantiomers, or charged groups for ion-exchange chromatography. 

Chiral ligand-exchange chromatography is based on the formation of diastereomeric ternary complexes that involve a transition metal ion (M), usually copper II a single enantiomer of a chiral molecule (L), usually an amino acid and the eitantiomers of the racemic solute R and S). The diastereomeric mixed chelate complexes formed in this system are represented by the formulas L-M-R and L-M-S. When these complexes have different stabilities, the less stable complex is eluted first, and the enantiomeric solutes are separated. 

Chiral hgand exchange chromatography can be performed either on an achiral stationary phase with a chiral mobile phase or on a chiral stationary phase IMAC is performed on the metal ion immobihzed stationary phase. The latter two stationary phases may be symbohzed by the same formula as... 

Ferrioxamines are the only known siderophores with no chiral center, so ferrioxa-mines are the only siderophores with no intrinsic optical activity of the metal complex. Ferrioxamine E, a cyclic ferrioxamine, crystallizes as a racemic mixture of A-cis and A-cis isomers as determined by X-ray crystallography 187). From an examination of molecular models of ferrioxamine B five enantiomeric pairs of non-redundant isomers are distinguishable. By ion exchange chromatography the more polar cis... 

Alternatively, ion exchange chromatography may be used with a suitable chiral counter ion for elution. 

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