Direct gas chromatographic resolution of enantiomers on optically active mesophases

Recent advances in methods of direct optical resolution

Very great advances have been made in the field of direct optical resolution of organic compounds by chromatographic techniques. Chiral capillary gas chromatography now permits a determination of the enantiomeric composition of a few nanograms of a compound present in a mixture of many others. Coupled with high resolution mass spectrometry the technique will additionally permit structural elucidation; of great interest in pheromone research and related areas. Analytical separations of enantiomers are now also carried out by high-performance liquid chromatography (HPLC) methods based on a variety of principles. Basically, two main types are used, differing as to whether the mobile phase has to be a chiral medium or not. Two-dimensional HPLC, whereby compounds separated on a non-chiral column are progressively and automatically transferred to a chiral column for optical resolution, has been used successsfully for chiral amino acid separations. Many different chiral sorbents for preparative LC and HPLC resolutions have been prepared; some of these are now used in columns capable of producing pure enantiomers from a given racemate at a rate of the order of one gram/hour in continuous, automatic HPLC procedures. Apart from all important applications of these results of optical resolution technology, an increased knowledge of the underlying chiral recognition phenomena responsible for enantioselection has also been achieved.

Gas chromatographic resolution of enantiomeric amphetamines and related amines. II. Effects of cyclic structures on diastereomer and enantiomer resolution

Proline and other cyclic amino acids were used as acylated acid chlorides for the resolution of enantiomers of amphentamine and related amines as diastereomers by gas chromatography. Variation of ring size or incorporation of a heteroatom into the ring of the cyclic resolving agent changed retention times and separation factors, depending on the racemic amine derivatized. Separations were achieved on a short, packed Carbowax 20M column. Because N-trifluoroacetyl L-prolyl chloride has proved especially effective for derivatization of amine enantiomers to give diastereomers which often may be resolved by gas chromatography, the proline ring was incorporated into a new optically active stationary phase. For the enantiomeric derivatives examined, the new proline ester phase gave separations essentially equivalent to those obtained on the commercially available phase, carbonyl bis-(L-valine isopropyl ester). The fact that resolution was achieved on the new phase is evidence that, as has been previously suggested, the ester carbonyls of this type of phase are the significant sites of interaction with the antipodes undergoing separation.

Chromatographic resolution of enantiomers selective review

Recent research has produced a notable increase in knowledge useful for chromatographic enantiomer resolution. Some major advances have emerged in the understanding of asymmetric solute-solvent interactions, and many successful separations by gas and by liquid chromatography have been reported. This review presents a selective discussion of the major advances, with primary emphasis on the use of chiral stationary phases. The latter offer advantages (over indirect techniques) such as generally easier sample preparation, decreased analysis time, and simultaneous chemical as well as optical purity analysis.

Gas chromatographic resolution of enantiomeric amphetamines and related amines. I. Structural effects on some diastereomer separations

Eight structurally related enantiomeric primary amines including several amphetamines were resolved as distereomers by gas chromatography. Optically active acylated amino acid chlorides were reacted with the amine racemates to yield volative diastereomers which were chromatographed on polar and on nonpolar columns. Selectivity of diastereomeric pair separations was found to be dependent on substitution at the chiral centers of both the amino acids and the amines. Baseline resolution was achieved on a 6-ft. diethylene glycol succinate column at 200 degrees for seven of the eight amines. The polar stationary phases yielded higher relative separation factors than the nonpolar phase in all cases for the same chromatographic conditions. The effects of varying the perfluoroacyl group of the amino acid resolving agent from trifluoroacetyl to pentafluoropropionyl to heptafluorobutyryl was to significantly decrease the retention times without significantly decreasing these separation factors.