Page 30 - Chiral Separation Techniques

1.3 Chromatographie Techniques 5

mobile phase [34]; hence evaporation of the solution derived from the preparative
separation leaves a pure product. Cyclodextrin-based CSPs [35, 36], antibiotics [34,
37–39] such as the above-mentioned teicoplanin, and certain types of Pirkle phases
have been utilized with preparative purposes. In those cases, although the loading
capacity is not high, other advantages, such as a broad application domain for antibi-
otics, or the ease of preparation and the chemical stability, for multiple interaction
supports, can balance this limitation. Polyacrylamides [40, 41] have also been used
extensively, often in nonreported separations. Nevertheless, polysaccharide-derived
CSPs are the most commonly applied for preparative chromatography. This is due
both to their substantial loading capacity and broad enantiodiscrimination scope.
They are the best adapted supports for this purpose either in HPLC, or in medium-
pressure liquid chromatography (MPLC) [19].
Several types of polysaccharide-derived CSPs can be considered (Table 1-1). The
support with the highest number of applications described, either at high or medium
pressure, is microcrystalline cellulose triacetate (CTA). Other polysaccharide deriva-
tives, mainly some benzoates, are used in their pure form, as beads which are
directly packed [42, 43]. Coated CSPs, consisting of a polysaccharide derivative,
benzoate or aryl carbamate of cellulose or amylose, on a matrix of aminopropylsi-
lanized silica gel, are also among the most extensively utilized CSPs [44–49]. How-
ever, these supports have the limitation of the choice in the mobile phase composi-
tion. CSPs whose chiral selectors are bonded to the chromatographic matrix (usually
silica gel) can perform in a number of different conditions and compositions in the
mobile phase. This is the case of the already mentioned Pirkle phases, cyclodextrin,
antibiotic or polyacrylamide-derived CSPs. Unfortunately, polysaccharide deriva-
tives in coated CSPs often swell, or even dissolve, in a number of solvents. Thus, the
compatible mobile phases with these supports are mixtures of a hydrocarbon (hex-
ane or heptane) with an alcohol (ethanol or isopropanol), though many compounds
have a reduced solubility in these mixtures. This feature, which is not a problem
when these CSPs are used for analytical purposes, can be a major disadvantage at
preparative scale. The low solubility limits the amount of product that can be
injected in a single run and, therefore, the maximum loadability of the column can-
not be attained [45]. This limitation can be overcome when the chiral selector is
bonded to the chromatographic matrix [50–57]. In this case, a broader choice of sol-
vents can be considered as mobile phase or simply to dissolve the racemate to be
separated [58, 59]. It must be taken into account that, changes in selectivity as well
as in the loading capacity of the CSP occur when solvents are changed [58].
In this context, the enantiomeric pair containing the eutomer of cyclothiazide can
be resolved by HPLC on cellulose-derived coated CSPs. Nevertheless, the poor solu-
bility of this compound in solvents compatible with this type of support makes this
separation difficult at preparative scale. This operation was achieved with a cellulose
carbamate fixed on allylsilica gel using a mixture of toluene/acetone as a mobile
phase [59].
On occasion, the broad choice of existing phases is not enough to resolve a par-
ticular problem successfully. Derivatization with achiral reagents can be useful to
introduce additional interacting groups in a poorly functionalized substract, or to