Group III

Chiralcel-OD CSP tris(3,5-dimethylphenyl carba-mate cellulose) has been used successfully for the SFC enantioseparation of ft-blockers, potassium channel activator analogues and other compounds. A Chiral-pak-AD column, tris(3,5-dimethylphenyl carbamate amylose), has been used to resolve enantiomeric mixtures of nonsteroidal antiinflammatories.

Other CSPs derived from cellulose have been successfully applied to the SFC enantioseparation of compounds of pharmaceutical interest. For example, an intermediate in the synthesis of a drug targeted for cardiac arrhythmia was separated on Chiralcel-OB; the four optical isomers of a new calcium channel blocker, LF 2.0254, were resolved on Chiralcel-OJ; and some CSPs have been applied to the SFC separation of various frequently used drug racemates such as profens and barbiturate derivatives, benzo-diazepines, etc.

A Chiralcel-OD-H column and an achiral amino-propyl column have been employed for the analysis of products formed in rat liver microsomal metabolism

Figure 9 Comparison of the separation of the 2-naphthyl and o-anisyl phosphine oxides on Cyclobond I using LC and SFC. LC conditions: mobile phase, hexane/ethanol (85 :15, v/v); flow rate, 1 mLmin~1; UV detection at 234 nm. SFC conditions: mobile phase, carbon dioxide/methanol (94:6, w/w); flow rate, 4.5 mL min "1; temperature 25°C; pressure, 150 bar; UV detection at 234 nm. (Reproduced from Macaudiere P, Caude M, Rosset R and Tambute A (1987) Resolution of racemic amides and phosphine oxides on a ß-cyclodextrin-bonded stationary phase by subcritical fluid chromatography. Journal of Chromatography 405:135-143, with permission from Elsevier Science.)

Figure 10 SFC separation of the cromakalim enantiomers on the (S)-naphthylethylcarbamoylated-ß-cyclodextrinCSP. Operating conditions: mobile phase, carbon dioxide/methanol (96 : 4); flow rate, 2 mL min"1; temperature 30°C; pressure, 15MPa; UV detection at 254 nm. (Reproduced from Williams KL, Sander LC, and Wise SA (1996) Comparison of liquid and supercritical fluid chromatography using naphthylethylcarbamoylated-ß-cyclodex-trin chiral stationary phases. Journal ofChromatographyA 746: 91-101, with permission from Elsevier Science.)

Figure 10 SFC separation of the cromakalim enantiomers on the (S)-naphthylethylcarbamoylated-ß-cyclodextrinCSP. Operating conditions: mobile phase, carbon dioxide/methanol (96 : 4); flow rate, 2 mL min"1; temperature 30°C; pressure, 15MPa; UV detection at 254 nm. (Reproduced from Williams KL, Sander LC, and Wise SA (1996) Comparison of liquid and supercritical fluid chromatography using naphthylethylcarbamoylated-ß-cyclodex-trin chiral stationary phases. Journal ofChromatographyA 746: 91-101, with permission from Elsevier Science.)

characteristics)). This coupling allowed the authors to achieve baseline separations with all solutes investigated, basic (^-blockers, benzodiazepines)

Figure 9 Comparison of the separation of the 2-naphthyl and o-anisyl phosphine oxides on Cyclobond I using LC and SFC. LC conditions: mobile phase, hexane/ethanol (85 :15, v/v); flow rate, 1 mLmin~1; UV detection at 234 nm. SFC conditions: mobile phase, carbon dioxide/methanol (94:6, w/w); flow rate, 4.5 mL min "1; temperature 25°C; pressure, 150 bar; UV detection at 234 nm. (Reproduced from Macaudiere P, Caude M, Rosset R and Tambute A (1987) Resolution of racemic amides and phosphine oxides on a ß-cyclodextrin-bonded stationary phase by subcritical fluid chromatography. Journal of Chromatography 405:135-143, with permission from Elsevier Science.)

of racemic camazepam (a hypnotic/anxiolytic drug in clinical use) and the fast chiral separation of different compounds (oxprenolol, pindolol, warfarin) has been achieved by microbore SFC using a Chiralcel-OD type stationary phase.

Kot and co-workers proposed the serial coupling of different CSP columns (Chiralpak-AD, Chiralcel-OD and Chirex 3022 (brush-type with rn-donor

Figure 11 SFC separation of ibuprofen (1), fenoprofen (2), clenbuterol (3), propranolol (4) and lorazepam (5) using the serial coupling of different CSP columns. Operating conditions: columns, Chiralpak AD-Chiralcel OD-Chirex 3022; mobile phase, carbon dioxide/methanol (0.5% triethylamine # 0.5% trifluoro-acetic acid) with methanol programmed from 4% (5 min) to 30% at 5% min~1; flow rate 2 mL min~1; temperature, 25°C; pressure, 200 bar. (Reproduced with permission from Kot A, Sandra P and Venema A (1994) Sub- and supercritical fluid chromatography on packed columns a versatile tool for the enantioselective separation of basic and acidic drugs. Journal of Chromatographic Science 32: 423-448.)

Figure 11 SFC separation of ibuprofen (1), fenoprofen (2), clenbuterol (3), propranolol (4) and lorazepam (5) using the serial coupling of different CSP columns. Operating conditions: columns, Chiralpak AD-Chiralcel OD-Chirex 3022; mobile phase, carbon dioxide/methanol (0.5% triethylamine # 0.5% trifluoro-acetic acid) with methanol programmed from 4% (5 min) to 30% at 5% min~1; flow rate 2 mL min~1; temperature, 25°C; pressure, 200 bar. (Reproduced with permission from Kot A, Sandra P and Venema A (1994) Sub- and supercritical fluid chromatography on packed columns a versatile tool for the enantioselective separation of basic and acidic drugs. Journal of Chromatographic Science 32: 423-448.)

and acidic (nonsteroidal anti-inflammatory drugs, ^-agonists). As an example, Figure 11 shows the separation of ibuprofen, fenoprofen, clenbuterol, prop-ranolol and lorazepam in a modifier-programmed run.

Systematic comparison of the chiral recognition mechanisms in LC and SFC for type III CSPs has been performed. It appears that, contrary to what occurs for type I CSPs, important discrepancies in selectivity values may exist between LC and SFC. The systematic comparison of LC and SFC for Chiralcel-OD and Chiralpak-AD CSPs demonstrates clearly that the presence of polar functional groups such as primary or secondary hydroxyl or amine functions may cause

Figure 12 Comparison of LC and SFC for the separation of mefloquine (A), viloxazine (B) and temazepam (C) using Chiralcel OD CSP. Operating conditions: column, Chiralcel OD. LC, mobile phase hexane/ethanol containing 1 % (v/v) of n-propylamine (90 :10, v/v) for (A) and (C), 50 : 50 (v/v) for (B); flow rate 1 mL min-1; room temperature. SFC: mobile phase, carbon dioxide/ethanol containing 1 % (v/v) of n-propylamine 90 :10 (v/v) for (A) and (B) 95 : 5 (v/v) for (C); flow rate, 2 mL min-1; temperature, 25°C; pressure: 200 bar. UV detection. Separations are optimized for selectivity. (Reproduced with permission from Bargmann-Leyder N, Tambute A and Caude M (1995) A comparison LC-SFC for cellulose and amylose-derived chiral stationary phases. Chirality 7: 311-325.)

Figure 12 Comparison of LC and SFC for the separation of mefloquine (A), viloxazine (B) and temazepam (C) using Chiralcel OD CSP. Operating conditions: column, Chiralcel OD. LC, mobile phase hexane/ethanol containing 1 % (v/v) of n-propylamine (90 :10, v/v) for (A) and (C), 50 : 50 (v/v) for (B); flow rate 1 mL min-1; room temperature. SFC: mobile phase, carbon dioxide/ethanol containing 1 % (v/v) of n-propylamine 90 :10 (v/v) for (A) and (B) 95 : 5 (v/v) for (C); flow rate, 2 mL min-1; temperature, 25°C; pressure: 200 bar. UV detection. Separations are optimized for selectivity. (Reproduced with permission from Bargmann-Leyder N, Tambute A and Caude M (1995) A comparison LC-SFC for cellulose and amylose-derived chiral stationary phases. Chirality 7: 311-325.)

large discrepancies in selectivity between LC and SFC. This result is peculiar to cellulose and amylose-derived CSPs, for which the interactions involved in chiral recognition are not always well balanced. Therefore, in the case of chiral resolution of polar solutes, the analyst should try both LC and SFC so that the more stereoselective one can be chosen. Figure 12A-C show some examples of the different selectivities that may exist between LC and SFC for polymer-type CSPs.

Other polymer-type CSPs have been used in SFC, such as those based on polymethacrylates of helical conformation and a polysiloxane CSP (polyWhelk-O), the 'polymeric version' of the commercially available brush-type CSP, Whelk-O 1. For the latter, the comparison was performed between the polymeric CSPs and its brush-type analogue, and it appeared that the polyWhelk-O CSP affords greater enantioselectiv-ity and shorter retention under the same conditions.

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