Page 54 - Chiral Separation Techniques

30 2 Method Development and Optimization of Enantiomeric Separations Using …

2.2.3 Predictability of Enantioselectivity

One of the characteristics of glycopeptide CSPs is the predictability of chiral sepa-
ration for racemates with the same stereogenic environment. If a compound is sepa-
rated on a glycopeptide column under certain mobile phase conditions, it is very
likely that the molecules with similar stereogenic configuration will be separated
under the same conditions. For example, when the compound albuterol was sepa-
rated in the new polar organic mode on a teicoplanin column, a number of other
β-blockers were tested and found to be baseline resolved as well by using the same
mobile phase on this stationary phase (Fig. 2-2). This indicates that further separa-
tion is likely to be achieved for analogous compounds with an aromatic moiety, a
hydroxyl group on the chiral center and a secondary amine group in the β-position
to the chiral center. This predictability is extremely beneficial to the combinatory
chemists who synthesize huge number of analogous chiral compounds. The pre-
dictability is further shown with enantiomeric separation of profens on vancomycin
in reversed-phase (Fig. 2-3) and α-hydroxyl/halogen carboxylic acids on ristocetin
A in the new polar organic mode (Fig. 2-4).

2.2.4 Complementary Separations

One thing that makes the macrocyclic glycopeptide CSPs unique and different from
other CSPs is the “principle of complementary separations” [17, 19]. This refers to
an empirical observation that an increase in selectivity is obtained when one gly-
copeptide phase is switched to another under the same or similar mobile phase. War-
farin, for example, is only partially separated on a teicoplanin column under opti-
mized reversed-phase conditions. When the same mobile phase is used with a van-
comycin column, a baseline resolution of warfarin enantiomers is achieved with
shorter analysis time. Some other examples are given in Fig. 2-5 to show the phe-
nomenon of complementary separations. One advantage of the complementary sep-
aration is that one can switch from one column to another to achieve better selectiv-
ity without changing the mobile phase. Another advantage is that two or three dif-
ferent glycopeptide columns can be coupled together in order to achieve broader
selectivity and to screen a larger number of analytes. The column coupling technique
will be discussed later in this chapter.