Type III Chiral Stationary Phases

Type III chiral stationary phases include cyclodextrin (CD), polymethacrylate and crown ether stationary phases. The solute enters into chiral cavities to form inclusion complexes and the relative stability constants of the resulting diastereomeric complexes are different. The cavities of CD and polymetha-crylate are hydrophobic, while those of crown ethers are hydrophilic.

a-, ft- and y-CD are cyclic oligosaccharides containing 6, 7 and 8 ft-1, 4-d-glucoside units, respectively. Because an aromatic portion of a molecule can enter into the chiral cavity, solutes having an aromatic moiety at, or adjacent to, the chiral centre are well resolved. Acetyl, hydroxypropyl, naphthylethyl car-bamate and phenyl carbamate derivatives of CDs have been prepared and used for chiral resolution of many solutes which cannot be separated using native CDs. Chiral stationary phases based on CDs and derivatized CDs have been predominantly used in reversed-phase mode.

Chiral stationary phases based on polymetha-crylate have been prepared using a chiral monomer such as (S)-acryloylphenylalanine, and achiral monomers such as triphenylmethyl methacrylate and diphenyl-2-pyridylmethyl methacrylate together with chiral cation catalysts. In normal-phase mode, chiral stationary phases based on these polymers can separate many racemic solutes that are difficult to resolve by other methods because of a lack of functionality.

Crown ethers, which are synthetic macrocyclic polyethers, can form selective complexes with various cations. Chiral crown ethers covalently bound or adsorbed on to silica supports can separate enan-tiomeric ammonium compounds such as amino acid esters, amines and amino alcohols. The multiple hydrogen-bonding interactions between the ammonium group and the ether oxygens play an important role in the chiral recognition.

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Solar Panel Basics

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