Page 221 - Chiral Separation Techniques

7.2 Different Approaches for Derivatization Chromatography 199
Free amino acids can be derivatized with isothiocyanates to phenyl- or methyl-
thiohydantoin derivatives. The thiohydantoins can be separated on a CSP with
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poly-[N-acryloyl-L-phenylalanine ethylester] (Chiraspher ) as a chiral selector
[25]. This CSP offers a known selectivity for many five-membered heterocyclic
rings.
A derivatization with acid chlorides is also possible. Amino acids can be deriva-
tized with 9-fluorenylmethyl chloroformate (FMOC) and separated on a CSP with
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χ-cyclodextrin (ChiraDex gamma ), a cyclic oligosaccharide which consists of
eight glucose units.
A CSP with a smaller β-cyclodextrin moiety (seven glucose units) immobilized
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on silica gel (ChiraDex ) is able to separate the dansyl-derivatives [5-(dimethy-
lamino)-naphthalin-1-sulfonylchloride] of amino acids [26].

The separations described above can be used because of the known selectivities
with certain groups which, in the case of derivatized compounds, are mostly the
derivatizing groups. This has led towards the concept of rational CSP design. One of
the most successful methods was the introduction of the brush-type CSP by Pirkle.
The CSPs are designed to offer distinct attractive groups in a fixed stereochemical
orientation towards an analyte. These interactions can be polar functions, H-donor
and H-acceptor functions and π–π-interactions. When a CSP is designed which
shows recognition for a certain derivatizing group, this derivatizing agent may be
used for the separation of a whole range of compounds.
An example for this approach is the immobilization of (S)-(–)-α-N-(2-naph-
thyl)leucine, a π-donating group on silica. This chiral selector exhibits excellent
recognition for 3,5-dinitrobenzoyl (DNB)- and 3,5-dintroanilido (DNAn)-deriva-
tives. Amines and alcohols can be derivatized with DNB- or DNAn-chloride to the
esters or carbamates and separated on the CSP, as shown by Pirkle for a wide vari-
ety of compounds [27].

7.2.4 Type IV: Derivative with best Selectivity

It is important not only that a multiplicity of compounds in the sample mixture may
be selectively derivatized – as was shown for Type III reactions – but also that one
racemate may be derivatized with a multiplicity of derivatizing agents (Fig. 7-17).
Although this approach can be used to optimize the analogues of a compound [28,
29], it is of special interest when a compound is required to be separated on a prepar-
ative scale.
A major interest in the field of preparative enantioseparation has begun to emerge
from the pharmaceutical industry, in which the preparation of unichiral drugs on a
large scale is especially important. Since the introduction of simulated moving bed
(SMB) technology (which has been used in the petrochemical and sugar industries
for more than 30 years) to the field of chromatographic enantioseparation, the pro-
duction of several tons of pure enantiomer per year has become possible, and indeed
is currently performed on a regular basis [30, 31]. It is clear that for such a large-