Cyclodextrins as Stationary Phases in Gas Chromatography

For the preparation of cyclodextrin-based capillary columns for gas chromatographic applications, two complementary methods have been developed and are commercially used. In the first method, alkylated cyclodextrins are diluted with polysiloxanes and immobilized on the inside wall of a fused silica capillary. In the other method, pentyl and hydroxyalkyl-dimethylcyclodextrins are coated on the inside wall of a fused silica capillary. Different well-known chro-matography companies offer cyclodextrin-based capillary columns:

• Chrompack: diluted cyclodextrins;

• Macherey-Nagel: undiluted n-pentylated or acylated cyclodextrins;

• Advanced Separation Technologies: a broad variety of undiluted cyclodextrin derivatives (per-methylated, hydroxypropyl, trifluoroacylated, butyrylated, dialkylated).

Different types of compounds (amines, epoxides, al-kanes, alcohols, lactones, sugars, etc.) can be separated on cyclodextrin-based capillary columns.

The usefulness of these materials is illustrated by means of the separation of the four isomers of a piperidine derivative. Gas chromatography on cyclo-dextrin-based columns was tried after acetylation with trifluoracetic anhydride using the following procedure:

An advantage of this dervatization procedure was that all types of salts (used to investigate the possibilities of diastereomeric salt formation as a stereoselec-tive synthesis method) could be acylated without prior liberation of the free base.

Figure 11 Preparative chromatographic separation on a P-cyclodextrin columns. Experimental condition: column: 80 mm ID dynamic axial compression column (Prochrom); stationary phase: 500 g 10 ^m chemically bonded hydroxypropyl-p-cyc-lodextrin (experimental phase Merck Darmstadt; packing pressure 80 bar; mobile phase: 50 mM triethylamine adjusted with 50 mM sulfuric acid to pH 2.5-methanol (80-20, v/v); flow rate: 150mLmin~1; detection: UV, wavelength 220 nm, range 2.56 AUFS; sample size: 1 g dissolved in 50 mL of concentrated sulfur-ic acid; solute:

Figure 11 Preparative chromatographic separation on a P-cyclodextrin columns. Experimental condition: column: 80 mm ID dynamic axial compression column (Prochrom); stationary phase: 500 g 10 ^m chemically bonded hydroxypropyl-p-cyc-lodextrin (experimental phase Merck Darmstadt; packing pressure 80 bar; mobile phase: 50 mM triethylamine adjusted with 50 mM sulfuric acid to pH 2.5-methanol (80-20, v/v); flow rate: 150mLmin~1; detection: UV, wavelength 220 nm, range 2.56 AUFS; sample size: 1 g dissolved in 50 mL of concentrated sulfur-ic acid; solute:

(trans)

The first experiments were done on a 15 mx 0.32 mm ID Chiralsil-Dex® column (Chrompack). On this type of column, it was only possible to separate the cis and trans isomers.

Thereafter, chromatographic experiments were performed on four different cyclodextrin phases from Advanced Separation Technologies:

• Chiraldex® B-PH: (S)-2-hydroxypropyl per-methylated P-cyclodextrin;

• Chiraldex® B-DA: (2,6-di-0-n-pentyl)-P-cyclodex-trin;

• Chiraldex® B-TA: (2,6-di-0-n-pentyl-3-0-tri-fluoroacetyl)-P-cyclodextrin;

• Chiraldex® G-TA: (2,6-di-0-n-pentyl-3-0-tri-fluoroacetyl)-y-cyclodextrin.

Chromatograms of the different experiments are illustrated in Figure 12 and Figure 13. The largest differentiation between cis and trans isomers was observed on the permethylated hydroxypropyl-P-cyclodextrin column (Chiraldex® B-PH). However, the best separation of all isomers individually was observed on the trifluoroacylated P-cyclodextrin column (Chiraldex® B-TA).

Noticeable is the time required to perform an analysis. Compared with the analysis method on the crown ether column, a GC analysis is more than six times faster, although one has to take into account that on the crown ether column the diastereomeric salt could be analysed as such, while for the gas chromatographic method the product has to be de-rivatized prior to chromatography.

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