Page 82 - Chiral Separation Techniques

58 3 Combinatorial Approaches to Recognition of Chirality: Preparation …

where t and t are the retention times for the analyte and the unretained compound
r o
(void volume marker), respectively.
Chiral selectors are the most important part of the separation system. This is why
most attention during the development of new chiral separation media has always
been devoted to selectors. As a result of the growing interest in chiral chromatogra-
phy, a large number of phases and additives have emerged to meet the challenge of
enantiomer separations [5, 15, 16]. For example, more than 90 CSPs were commer-
cially available for separations in the liquid chromatographic mode in the early
1990s [17].
The majority of currently known selectors can be divided into the following cat-
egories:
1. Proteins. A chiral stationary phase with immobilized α -acid glycoprotein on sil-
1
ica beads was introduced by Hermansson in 1983 [18, 19]. Several other proteins
such as chicken egg albumin (ovalbumin), human serum albumin, and cellohy-
drolase were also used later for the preparation of commercial CSPs. Their selec-
tivity is believed to occur as a result of excess of dispersive forces acting on the
more retained enantiomer [17]. These separation media often exhibit only modest
loading capacity.
2. Modified polysaccharides. Although derivatives of microcrystalline cellulose
have been used for chiral separations since the 1970s [20], materials useful in
high-performance liquid chromatography (HPLC) were only developed by
Okamoto in the mid-1980s. CSPs involving various esters and carbamates of cel-
lulose and amylose coated on wide-pore silica are currently the most frequently
used chiral media for chromatographic separations in both analytical and prepar-
ative scales [21, 22]. Although they often do not exhibit very high selectivities,
they separate an extremely broad range of different racemates.
3. Synthetic polymers. In the 1970s, Blaschke prepared several crosslinked gels from
N-acryloylated L-amino acids and a small percentage of ethylene dimethacrylate
or divinylbenzene and used them for the low-pressure chromatographic resolution
of racemic amino acid derivatives and mandelic acid [23]. Another polymer-based
CSP was later prepared by Okamoto from isotactic poly(triphenylmethyl
methacrylate). This material is the prototypical polymeric selector with a well-
defined one-handed helical structure [24]. This polymer was prepared by anionic
polymerization using a chiral organolithium initiator, and then coated onto porous
silica beads. While these columns were successful in the separation of a broad
variety of racemates, their relative lack of chemical stability and high cost make
them less suitable for large-scale applications.
4. Macrocyclic glycopeptides. The first of these CSPs – based on the “cavity” of the
antibiotic vancomycin bound to silica – was introduced by Armstrong [25]. Two
more polycyclic antibiotics teicoplanin and ristocetin A, were also demonstrated
later. These selectors are quite rugged and operate adequately in both normal-
phase and reversed-phase chromatographic modes. However, only a limited num-
ber of such selectors is available, and their cost is rather high.
5. Cyclic low molecular weight compounds. Chiral separations using chiral crown
ethers immobilized on silica or porous polymer resins were first reported in the