Chiral organic stereochemistry: Chiral HPLC, chiral auxiliaries, CD spectroscopy, X-ray crystallography, and light-powered chiral molecular motors

In the studies of chiral organic stereochemistry, it is important to use enantiopure compounds. For this purpose, the chiral HPLC (High-Pressure Liquid Chromatography) columns containing chiral stationary phases were invented by Y. Okamoto and coworkers for enantio-separating various racemic compounds. In addition, the use of chiral auxiliaries is also useful for preparing enantiopure compounds and also for determining their absolute configurations, where covalent-bonded diastereomers are separated by HPLC on silica gel. In this review article, these HPLC methods will be discussed together with the applications to some interesting organic compounds including light-powered chiral molecular motors.

Optically active synthetic polymers as chiral stationary phases in HPLC

Synthetic, optically active polymers used as CSP are reviewed. The polymers are classified into three major categories, namely, addition polymers, condensation polymers, and cross-linked gels. The emphasis lies on polymethacrylates having helical conformation belonging to the first category. Helical polymethacrylates are synthesized using asymmetric anionic or radical polymerization techniques and show resolving ability towards a wide range of racemate.

Retention and selectivity of flavanones on homopolypeptide-bonded stationary phases in both normal- and reversed-phase liquid chromatography

Three linear polymers of repeating amino acid units, or homopolypeptides, have been individually covalently bonded to microparticulate silica and evaluated for liquid chromatographic separations. The retention and selectivity of seven flavanones were investigated on these stationary phases and a structurally similar, commercially available reference stationary phase, Chiraspher. All three of the homopolypeptide stationary phases retain solutes in the normal-phase mode. The aromatic-containing homopolypeptide stationary phases also retain solutes in the reversed-phase mode. Selectivity values for the flavanones were higher in the normal-phase mode; chiral selectivity was observed for the amphiphilic homopolypeptide stationary phase in the reversed-phase mode. The retention mechanism of each stationary phase is suggested based on the chemical nature and conformation of the corresponding homopolypeptide ligand.

Chromatographic resolution of dipeptide enantiomers and diastereomers on chiral stationary phases from poly(L-leucine) or poly(L-phenylalanine)

Two types of chiral stationary phase, polymer-bonded and non-bonded, containing poly(L-leucine) and poly(L-phenylalanine) were prepared. These stationary phases gave higher enantioselectivity for the optical resolution of various derivatives of leucylphenylalanine methyl ester by liquid chromatography than did poly(L-alanine) and poly(L-glutamate).

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.