Page 317 - Chiral Separation Techniques

12.4 Chiral Stationary Phases in SFC 309

12.4.3 Derivatized Polysaccharides

Polysaccharide-based CSPs incorporate derivatives of cellulose and amylose
adsorbed on silica gel. The selectivity of these CSPs depends upon the nature of the
substituents introduced during the derivatization process. The secondary structure of
the modified polysaccharide is believed to play a role in selectivity, but the chiral
recognition mechanisms have not been fully elucidated [55].
Several research groups have reported the separation of β-blocker enantiomers on
cellulose tris(3,5-dimethylphenylcarbamate) CSPs [56, 57]. Bargmann-Leyder et al.
performed a detailed comparison of LC and SFC on a cellulosic CSP (Chiralcel OD)
[58]. Although resolution was generally higher in SFC than in LC for this family of
compounds, differences in selectivity between the two techniques were observed,
and these discrepancies seemed to be compound specific. Examination of a series of
propranolol analogues provided additional insight into differences in the chiral
recognition mechanisms operative in LC and SFC.
Benzodiazepine enantiomers have also been resolved on the Chiralcel OD CSP.
Wang et al. utilized this CSP to determine the enantiomeric composition of
camazepam and its metabolites [59]. SFC provided improved resolution of the com-
pounds of interest in a shorter period of time than LC. Phinney et al. demonstrated
the separation of a series of achiral and chiral benzodiazepines. An amino column
was coupled in series with the Chiralcel OD CSP to achieve the desired separation
[41].
The selectivity of another cellulose-based CSP, Chiralcel OJ, has also been exam-
ined in SFC [60]. Separations of racemic drugs such as benoxaprofen, temazepam,
and mephobarbital were obtained. Acetonitrile proved to be a better modifier than
methanol for some of the compounds investigated. The four optical isomers of a cal-
cium channel blocker were resolved by Siret et al. on the Chiralcel OJ CSP [30]. In
LC, two CSPs were required to perform the same separation.
Derivatized amylose is the basis for the Chiralpak AD CSP. This CSP has been
utilized for the resolution of ibuprofen and flurbiprofen, as well as other members of
the family of nonsteroidal inflammatory drugs (NSAIDs) [39, 61]. Ibuprofen was
not resolved on the Chiralpak AD CSP in LC. Pressure-related effects on stereose-
lectivity were observed by Bargmann-Leyder et al. on a Chiralpak AD CSP [58]. No
corresponding effect of pressure on selectivity was observed with a Chiralcel OD
CSP. The authors speculated that the helical conformation of the amylose-based CSP
is more flexible than that of the cellulose-based CSP.

12.4.4 Macrocyclic Antibiotics

This relatively new class of CSPs incorporates glycopeptides attached covalently to
silica gel. These CSPs can be used in the normal phase, reversed phase, and polar
organic modes in LC [62]. Various functional groups on the macrocyclic antibiotic
molecule provide opportunities for π-π complexation, hydrogen bonding, and steric
interactions between the analyte and the chiral selector. Association of the analyte