Chiral Dynamically Coated Stationary Phases

Ligand exchanging columns of an exceptional efficiency and enantioselectivity can be easily prepared from commercially available reversed-phase columns by dynamically coating the column with a hydropho-bic chiral selector. According to the technique suggested by Davankov and Unger in 1980, percolating a solution of N-alkyl-l-hydroxyproline in aqueous methanol through the column results in depositing the chiral selector on the bonded alkyl stationary phase. After a subsequent saturation of the retained ligand with copper ions, the column is ready for racemate resolutions in water-rich eluents which do not desorb the chiral selector. Depending on the length of the N-alkyl group (heptyl, decyl or hexa-decyl) and conditions of coating, the amount of the chiral selector deposited can be easily regulated over a very broad range. Such columns (also commercially available from Regis Technologies, USA, as 'Davankov columns') display a high resolving power toward numerous amino acids (Figure 2) and their derivatives, racemic glycyl-di- and tripeptides, 3-amino-s-caprolactam, as well as norephedrine and its analogous amino alcohols. Interestingly, the N-alkyl-l-hydroxyproline-coated columns, under severe overloading conditions, show a peculiar distortion of elution profiles of the enantiomers in that the first peak has no tail whereas the second peak shows a very sharp front, which enormously increased the column loadability, implying high preparative potential of the system.

Coating reversed-phase columns with N,N-dioctyl-l-alanine has proved especially efficient in resolving racemic a-hydroxy acids. Chiral coatings prepared with Na-decyl-l-histidine, N^-decyl-l-histidine, N2-octyl-(l)-phenylalaninamide, N,S-dioctyl-N-methyl-(d)-pencillamine, hydrophobic Schiff bases of chiral amino alcohols and N-dodecyl-N-carboxymethyl-(1S,2R)-norephedrine show high efficiency and selectivity. Reversed-phases coated with N,S-dioctyl-(d)-penicillamine and (R,R)-tartaric acid mono-(R)-1-(a-naphthyl)ethylamide are commercially available under the name Sumichiral 0A-5000 and 6000 (Sumica Chemical Analysis Service, Japan).

Figure 2 Enantiomeric resolution of a mixture of seven DL-amino acids with a chiral coating of A/-decyl-L-Hypro on reverse phase material. Conditions: column, 100 x4.2 mm, LiChrosorb RP 18, dp 5 |im; eluent, methanol-water (15 : 85), 0.1 mM Cu(II) acetate, pH 5.0; flow rate 2 mL min~1; temperature 20°C, UV 254 nm; elution sequence; 1, L-Ala; 2, D-Ala; 3, L-Val; 4, L-Arg; 5, D-Arg; 6, L-Leu; 7, L-Nleu; 8, D-Val; 9, L-Phe; 10, D-Leu; 11, L-Trp; 12, D-Nleu; 13, D-Phe; 14, D-Trp. (From Davankov VA, Bochkov AS, Kurganov AA, RoumeliotisP and Unger KK(1980) Separation of unmodified a-amino acid enantiomersby reverse phase HPLC. Chromatographia 13: 677-685, Figure 2, p. 680.)

Figure 2 Enantiomeric resolution of a mixture of seven DL-amino acids with a chiral coating of A/-decyl-L-Hypro on reverse phase material. Conditions: column, 100 x4.2 mm, LiChrosorb RP 18, dp 5 |im; eluent, methanol-water (15 : 85), 0.1 mM Cu(II) acetate, pH 5.0; flow rate 2 mL min~1; temperature 20°C, UV 254 nm; elution sequence; 1, L-Ala; 2, D-Ala; 3, L-Val; 4, L-Arg; 5, D-Arg; 6, L-Leu; 7, L-Nleu; 8, D-Val; 9, L-Phe; 10, D-Leu; 11, L-Trp; 12, D-Nleu; 13, D-Phe; 14, D-Trp. (From Davankov VA, Bochkov AS, Kurganov AA, RoumeliotisP and Unger KK(1980) Separation of unmodified a-amino acid enantiomersby reverse phase HPLC. Chromatographia 13: 677-685, Figure 2, p. 680.)

Whereas long N-alkyl substituents of the above chiral selectors are best suitable to bring about the intercalation of the latter between the surface alkyl groups of the reversed phase support, porous graphite offers a flat and rigid surface for the chiral selector to stick to. In this case, polyaromatic substituents show better retention and resolving power of the coated chiral selector, as is the case with N-naphthylmethyl-l-proline or N-(2-naphthalenesulfonyl)phenylalanine (see Figure 3).

Similarly, hydroxylated macroporous silica gel can be coated with hydrophilic chiral selectors such as silver(I)-^-camphor-10-sulfonate or (+ )-cobalt(III)-tris-ethylenediamine trichloride. In non-aqueous media, these phases easily resolve racemates of some chiral diene and cyclopentadienylrhodium(I)-nor-bornadiene complexes. Since in the latter case ethylenediamine ligands of the inert cobalt complex are not exchanged for any new ligands, the separation should be ascribed to ligand exchange in the outer, solvation shell of this complex.

Finally, polymeric chains of polyacrylamide grafted with l-proline can also be permanently adsorbed onto the surface of silica gel, to give a chiral ligand exchanger which resolves amino acids in polar media.

The ease of preparation of chiral-coated ligand exchanging phases, their exceptional efficiency, durability, wide application range and the opportunity of regulating the elution order of the enantiomers of interest by simply changing the configuration of the selector applied, make the technique increasingly popular.

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