Page 41 - Chiral Separation Techniques

16 1 Techniques in preparative chiral separations

have been brought about at a reduced scale, though the potential of the extraction
techniques is very promising. In principle, the type of chiral additives used can be
the same as the selectors applied to supported-liquid membranes or CCC. Neverthe-
less, as all the chiral recognition process occurs in solution, and an aqueous phase is
often involved, the solvation of selector and racemate molecules competes with the
chiral interactions selector-enantiomers, especially those implying hydrogen bonds.
Therefore, it is very often the case that chiral selectors with very high chiral recog-
nition abilities are needed [171, 182, 183]. The main disadvantage of liquid–liquid
extraction in the separation of enantiomers is the need for an additional treatment to
separate the chiral selector from the phase containing one of the enantiomers of the
resolved racemate.
Early examples of enantioselective extractions are the resolution of α-aminoalco-
hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion)
[184–186] by partition between aqueous and lipophilic phases containing esters of
tartaric acid [184–188]. Alkyl derivatives of proline and hydroxyproline with cupric
ions showed chiral discrimination abilities for the resolution of neutral amino acid
enantiomers in n-butanol/water systems [121, 178, 189–192]. On the other hand,
chiral crown ethers are classical selectors utilized for enantioseparations, due to their
interesting recognition abilities [171, 178]. However, the large number of steps often
required for their synthesis [182] and, consequently, their cost as well as their lim-
ited loadability makes them not very suitable for preparative purposes. Examples of
ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino
acid derivatives have also been described.
Proteins (BSA or ovomucoid, OVM) have also been successful in the preparative
resolution of enantiomers by liquid–liquid extraction, either between aqueous and
lipophilic phases [181] or in aqueous two-phase systems (ATPS) [123, 180]. The res-
olution of d,l-kynurenine [180] and ofloxacin and carvediol [123] were performed
using a countercurrent extraction process with eight separatory funnels. The signi-
ficant number of stages needed for these complete resolutions in the mentioned ref-
erences and others [123, 180, 189], can be overcome with more efficient techniques.
Thus, the resolution of d,l-kynurenine performed by Sellergren et al. in 1988 by
extraction experiments was improved with CCC technologies 10 years later [128].
It is worth noting that the extractive process can be performed continuously. Thus,
the separation of (±)-mandelic acid into its enantiomers was achieved with a liquid
particle extractor described by Abe et al. [190–192] using N-docecyl-L-proline as
chiral selector.

1.5.2 Preparative Gel Electrophoresis and Thin-Layer
Chromatography

Recently, the separation of some milligram quantities of terbutaline by classical gel
electrophoresis has been reported [194]. A sulfated cyclodextrin impregnated on the
agarose gel was used as a chiral selector and the complete resolution was achieved
in 5 h. Analogously, small amounts of enantiomers can be isolated using thin-layer