Enantiomer separations have been one of the most important and, simultaneously, one of the most difficult to accomplish analytical (and technological) tasks, present at the top of separation scientists' agenda since the early sixties of the last century. Awareness of their importance has been awakened by an infamous case of the racemic drug thalidomide, a widely advertised sedative drug which had unexpected teratogenic activity in pregnant women that resulted in thousands of ‘flipper babies’ born in the late fifties and the early sixties in many countries around the world. Since that time, separation scientists have developed numerous methods for enantiomer separation, basically by use of gas chromatography (GC), high-performance liquid chromatography (HPLC), and capillary electrophoresis (CE). In this respect, planar chromatography has remained to a large extent an undervalued enantiomer separation technique, despite separation performance sufficient to separate a pair of enantiomers. The large number of GC, HPLC, and CE enantiomer separation strategies and methods developed are evidence that — once confronted with this particular and no doubt very important challenge — instrumental chromatographic techniques have lost if not face, then, to a large extent, their reputation as robust, universal, and efficient separation tools. In these circumstances, planar chromatography on silica gel seems a very promising and tempting alternative, basically because of the advantageous properties of microcrystalline silica gel and the 2D effective diffusion available only in planar chromatographic mode. Enhancement of the enantiomer separating power of the silica gel by simple mechanical impregnation with a properly chosen chiral selector, and additional coupling of this with efficient instrumental detection (e.g. densitometric, DAD, or mass spectrometric) can yield in a simple, robust, and universal tool for separation of enantiomers comparable with the long-established chromatographic enantiomer-separation techniques. In this mini review, favourable preconditions for silica-gel-based planar chromatographic separation of enantiomers which can elevate planar chromatography to the status of leading tool for separation of enantiomers are discussed. Further improvements which can enhance the enantiomer separation performance of chiral planar chromatography are also indicated.
Authors:Mieczysław Sajewicz, Dorota Kronenbach, Monika Gontarska, Magdalena Wróbel, Robert Piętka, and Teresa Kowalska
As a result of our earlier studies, we were the first research group to report the spontaneous oscillatory in-vitro chiral conversion of a considerable number of α-substituted propionic acids (i.e., selected profens, amino acids, and L
-lactic acid). It is noteworthy that TLC proved highly instrumental in collecting relevant confirmatory evidence on this issue. Besides, in two papers we introduced a theoretical model and with its aid, we could simulate basic physical and physicochemical features of the discussed process. Reflecting on the oscillatory chiral conversion, we realized that it seems to be a rather general phenomenon and we started contemplating its scope and limitations. It became our primary concern to check whether it is confined to α-substituted propionic acids only or can occur with other classes of chiral compounds also. To this effect, in this study we present empirical (thin-layer chromatographic and polarimetric) evidence on an ability of
-α-hydroxybutyric acids, and
-mandelic acids to undergo spontaneous oscillatory in-vitro chiral conversion when dissolved in 70% aqueous ethanol. From the obtained results, general conclusion can be drawn that the process of interest occurs not only with chiral carboxylic acids having three carbon atoms per molecule, but also with those having two and four carbon atoms.
Authors:D. Kozma, L. Poszáváca, Mária Ács, G. Pokol, and E. Fogassy
The diastereoisomeric salt pair formed between α-phenyl-ethyl-amine and R-1-phenylethylsuccinamic acid were investigated by
physico-chemical methods. Melting and solubility phase diagrams were determined, the coincidence of the eutectic points of
the two phase diagrams were demonstrated. The large difference in physico-chemical properties of the salt pair explains the
efficient enantiomer separation.
Authors:D. Kozma, Klára Tomor, C. Novák, G. Pokol, and E. Fogassy
Racemic malic acid (I) was resolved by R-α-phenylethylamine(II). The S-(−)-I.R-(+)-II diastereoisomer was in excess in the precipitated salt. DSC curves and X-ray powder diffractograms proved that the diastereoisomeric salt mixture precipitated during the resolution was isomorphous with the optically pure S-(−)-I.R-(+)-II salt. The diastereoisomeric salt mixture containing the R-(−)-I.R-(+)-II salt in abundance bound crystal solvate (water or methanol) when produced by the total evaporation of the mother liquor, while the optically pure R-(−)-I.R-(+)-II salt crystallized without solvate. It is generally assumed that solid solution formation takes place when the two diastereoisomers are alike and the high similarity results in less efficient enantiomer separation.
Authors:D. Kozma, H. Simon, Gy. Pokol, and E. Fogassy
The enantiomeric enrichment of N-methyl-amphetamine (MA) was tried by eight different preparative separation methods. The enrichment process was also studied by thermoanalitical methods (DSC,TG). There is no enantiomeric enrichment during fractional distillation and selective extraction of the liquid base. The enantiomeric mixture of MA·HCl can be enriched by crystallization or by sublimation. The most effective enrichment can be achieved by fractional crystallization or distillation of the reaction mixture after partial salt formation with hydrochloric acid. The separation is less effective in case of fractional steam distillation and selective extraction when the enantiomer and the racemate are distributed between two liquid phases. This supports the general experience that having a solid phase in the system makes enantiomer separation more efficient.
Authors:Antoine-Michel Siouffi, Patrick Piras, and Christian Roussel
The paper reviews the latest achievements in chiral separation by planar chromatography (PC) since 2001. The emphasis is on cellulose derivatives and, especially, microcrystalline cellulose triacetate (MCTA). A comparison is made with HPLC data retrieved from Chirbase. It is shown that TLC has some interesting features compared with HPLC. Some enantiomer separations have been successfully achieved by TLC whereas no data are available for HPLC. For tribenzoyl cellulose derivatives general trends for resolution by both TLC and HPLC are discussed. The last part of the paper discusses reasons for the scarcity of publications on chiral separations by either PC or overpressured layer chromatography (OPLC). The possibilities of PC for chiral separations are rather unexploited.
Authors:M. Sajewicz, E. John, D. Kronenbach, M. Gontarska, and T. Kowalska
Enantiomer separation by TLC is still much less frequent than with other, mostly instrumental, chromatographic techniques. From a literature survey it is apparent that separation of the enantiomers of d,l-lactic acid is primarily of interest to the diary industry and that this particular separation is less frequently performed by chromatographic than by membrane techniques. As far as we are aware, before our studies only one report of TLC separation of the enantiomers of d,l-lactic acid was available in the literature; this is dated 1991 and describes the use of non-instrumental TLC only. In this study, we started by reproducing the TLC procedure originating from 1991, for this purpose using TLC with automatic sample application and densitometric detection. We managed to repeat the earlier procedure and to achieve full, i.e. baseline, separation of the enantiomers, with a remarkable distance between the two antimers. However, we revealed a significant drawback of this separation procedure — d-(−)-lactic acid was transported almost with the mobile-phase front and its densitometric quantification was barely possible because of the relatively high UV absorption of the mobile-phase front line. The reference method for separation of the enantiomers of d,l-lactic acid consisted in preliminary impregnation of commercial silica gel TLC plates with copper(II) acetate. In-situ formation of bidentate complexes of the d,l-lactic acid antimers with the Cu2+ cation resulted in different mobilities of these complex cations in the planar chromatographic system. The objectives of this study were twofold — to investigate separation of the enantiomers of d,l-lactic acid with other transition metal cations (i.e., Co2+, Ni2+, and Mn2+) used to impregnate the silica gel (to achieve resolution that might enable quantification of the two lactic acid antimers and not only the l-(+) enantiomer) and to gain deeper insight into the mechanism of separation with these metal cations. For purposes of comparison, we chromatographed d,l-lactic acid on non-impregnated silica gel layers. As a result, we managed to establish efficient separation conditions with the Ni2+ and Co2+ cations that outperformed the earlier established procedure involving the Cu2+ cation, and — partially at least — to elucidate the mechanism of separation of the enantiomers of d,l-lactic acid by these TLC systems. The Mn2+ cation proved unsuitable for the purpose. Finally, we managed to separate the enantiomers of d, l-lactic acid on non-impregnated silica gel layer also, which seems yet more proof of the microcrystalline chirality of silica gel used as stationary phase and of its substantial contribution to the enantiomer separation investigated.