Page 35 - Chiral Separation Techniques

10 1 Techniques in preparative chiral separations

are partitioned on the basis of their different affinity for one or the other phase. The
chromatographic process occurs between them without any solid support. The CCC
instruments maintain one of the liquid phases as stationary by means of the cen-
trifugal force, while the other is pumped through as mobile phase [113–115]. Inter-
est in the technique has favored the development of improved devices based on the
same principle, namely the retention of the liquid phases by means of a centrifugal
field, but with slight technical modifications. Thus, classical CCC devices use a vari-
able-gravity field produced by a two-axis gyration mechanism, while centrifugal
partition chromatography (CPC) devices are based on the use of a constant-gravity
field produced by a single-axis rotation mechanism [113–115]. Both CCC and CPC
preparative-scale instruments are available commercially [116].
The technique has some advantages relating to the traditional liquid–solid sepa-
ration methods. The most important of these is that all the stationary phase takes part
in the separation process, whereas the activity of a solid phase is mainly concen-
trated in the surface of the support, an important part of this being completely inert.
This fact increases the loading capacity of the phase, and this is the reason why CCC
is especially suited for preparative purposes. Therefore, modern CCC overcomes the
disadvantages of direct preparative chromatography by HPLC with regard to the
high cost of the chiral solid stationary phase and its relatively limited loadability.
From the pioneering studies of Ito et al. [117], CCC has been mainly used for the
separation and purification of natural products, where it has found a large number of
applications [114, 116, 118, 119]. Moreover, the potential of this technique for
preparative purposes can be also applied to chiral separations. The resolution of
enantiomers can be simply envisaged by addition of a chiral selector to the station-
ary liquid phase. The mixture of enantiomers would come into contact with this li-
quid CSP, and enantiodiscrimination might be achieved. However, as yet few exam-
ples have been described in the literature.
The first partial chiral resolution reported in CCC dates from 1982 [120]. The sep-
aration of the two enantiomers of norephedrine was partially achieved, in almost 4
days, using (R,R)-di-5-nonyltartrate as a chiral selector in the organic stationary
phase. In 1984, the complete resolution of d,l-isoleucine was described, with N-
dodecyl-L-proline as a selector in a two-phase buffered n-butanol/water system con-
taining a copper (II) salt, in approximately 2 days [121]. A few partial resolutions of
amino acids and drug enantiomers with proteic selectors were also published [122,
123].
However, it was not until the beginning of 1994 that a rapid (<1.5 h) total resolu-
tion of two pairs of racemic amino acid derivatives with a CPC device was published
[124]. The chiral selector was N-dodecanoyl-L-proline-3,5-dimethylanilide (1) and
the system of solvents used was constituted by a mixture of heptane/ethyl
acetate/methanol/water (3:1:3:1). Although the amounts of sample resolved were
small (2 ml of a 10 mM solution of the amino acid derivatives), this separation
demonstrated the feasibility and the potential of the technique for chiral separations.
Thus, a number of publications appeared subsequently. Firstly, the same chiral selec-
tor was utilized for the resolution of 1 g of (±)-N-(3,5-dinitrobenzoyl)leucine with a
modified system of solvents, where the substitution of water by an acidified solution