Chromatographic Chiral Stationary Phase Research Articles

Resolution of methoxyphenamine and its analogues on a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid.

A liquid chromatographic chiral stationary phase, which contains two N-CH3 amide connecting groups, based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was first applied to the resolution of methoxyphenamine (2-methoxy-N-methylamphetamine, a β-adrenergic receptor agonist used as a bronchodilator). The resolution of methoxyphenamine on the chiral stationary phase containing two N-CH3 amide connecting groups was quite successful with the separation factor (α) of 1.42 and resolution (RS ) of 4.22 compared with those (α of 1.09 and RS of 1.55) on the chiral stationary phase containing two N-H amide connecting groups. In addition, the chiral stationary phase containing two N-CH3 amide connecting groups was applied to the resolution of methoxyphenamine analogues. From the comparison of the resolution results of methoxyphenamine with those of methoxyphenamine analogues on the chiral stationary phase containing two N-CH3 amide connecting groups, the N-methyl group and the 2-methoxyphenyl group of methoxyphenamine were elucidated to be the structurally essential parts for the resolution on the chiral stationary phase.

Liquid chromatographic resolution of proline and pipecolic acid derivatives on chiral stationary phases based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid

Two liquid chromatographic chiral stationary phases based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid were applied to the resolution of the amide derivatives of cyclic α-amino acids including proline and pipecolic acid. Among the five amide derivatives of proline, aniline amide was resolved best on the first chiral stationary phase, which contains two N-H tethering amide groups, with the separation factor of 1.31 and the resolution of 2.60, and on the second chiral stationary phase, which contains two N-CH3 tethering amide groups, with the separation factor of 1.57 and the resolution of 5.50. Among the five amide derivatives of pipecolic acid, 2-naphthyl amide was resolved best on the first chiral stationary phase with the separation factor of 1.30 and the resolution of 1.75, but 1-naphthylmethyl amide was resolved best on the second chiral stationary phase with the separation factor of 1.30 and the resolution of 2.26. In general, the second chiral stationary phase was found to be better than the first chiral stationary phase in the resolution of the amide derivatives of cyclic α-amino acids. In this study, the second chiral stationary phase was first demonstrated to be useful for the resolution of secondary amino compounds.

Superficially porous particles vs. fully porous particles for bonded high performance liquid chromatographic chiral stationary phases: Isopropyl cyclofructan 6

This work reports a comparison of HPLC separations of enantiomers with chiral stationary phases (CSPs) prepared by chemically bonding cyclofructan-6, functionalized with isopropyl carbamate groups on fully and superficially porous particles (SPPs). The chromatographic performance of the superficially porous CSP based column was compared with columns packed with 5μm and 3μm fully porous particles (FPPs). At a flow rate of 3.0mL/min the number of plates on column afforded by the SPP column was ∼7× greater than the number of plates on column (same length) obtained when using the 5μm FPP based column. The flow rate providing the highest efficiency separation was ∼1.0mL/min for the SPP column while it was ∼0.5mL/min for both FPP columns. It was found that the selectivity and resolution of the separations were comparable between fully porous and superficially porous based columns (under constant mobile phase conditions), even though the SPP column contained lower absolute amounts of chiral selector. When tested under constant retention conditions, the SPP based CSP greatly improved resolution compared to the FPP based columns. At high flow rates the efficiency gained by using superficially porous CSP was accentuated. The advantages of columns based on SPPs become more obvious from the viewpoint of plate numbers and resolution per analysis time.

Enantioseparation of Methyl 2-Hydroxypropionate with Two Peracylated β-Cyclodextrin Derivatives as CGC Chiral Stationary Phases

Two peracylated β-cyclodextrin derivatives, 2,3,6-tri-O-valeryl-β-cyclodextrin and 2,3,6-tri-O-octanoyl-β-cyclodextrin, were synthesized and used as capillary gas chromatography (CGC) chiral stationary phases. The two peracylated β-cyclodextrin derivatives showed enantioseparation abilities to some chiral compounds tested, such as enantiomers of methyl 2-chloropropionate, 5-hydroxy-4, 4-dimethyl-dihydrofuran-2-one, and methyl 2-hydroxypropionate. As CGC chiral stationary phases, the two peracylated β-cyclodextrin derivatives showed excellent enantioseparation abilities to methyl 2-hydroxypropionate (i.e., methyl lactate). On 2,3,6-tri-O-valeryl-β-cyclodextrin, the enantiomer separation factor and resolution of methyl lactate were 1.21 and 6.08, respectively, and on 2,3,6-tri-O-octanoyl-β-cyclodextrin, 1.22 and 5.49, respectively. The enantioseparation results of methyl lactate on the two peracylated β-cyclodextrin derivatives were better than on most reported β-cyclodextrin derivatives, including a commercial chiral column of permethylated β-cyclodextrin tested in this study. Results suggest that CGC methods using 2,3,6-tri-O-valeryl-β-cyclodextrin or 2,3,6-tri-O-octanoyl-β-cyclodextrin as chiral stationary phase could be used for enantioseparation and evaluating the enantiomeric optical purity of methyl lactate.

Liquid chromatographic resolution of racemic rasagiline and its analogues on a chiral stationary phase based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid

A liquid chromatographic chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was applied to the resolution of 15 analytes, including racemic rasagiline, a chiral drug for the treatment of Parkinson's disease, and its analogues. The composition of mobile phase was optimized to be ethanol/acetonitrile/acetic acid/triethylamine (80:20:0.2:0.3, v/v/v/v) by evaluating the chromatographic results for the resolution of five selected analytes under various mobile phase conditions. Under the optimized mobile phase conditions, racemic rasagiline was resolved quite well with a separation factor of 1.48 and resolution of 2.71 and its 14 analogues were also resolved reasonably well with separation factors of 1.06-1.54 and resolutions of 0.54-2.11. Among 15 analytes, racemic rasagiline was resolved best except for just one analyte. The analyte structure-enantioselectivity relationship indicated that racemic rasagiline has the most appropriate structural characteristics for resolution on the chiral stationary phase.

Comparison of polarimetry and crown ether-based HPLC chiral stationary phase method to determine (l)-amino acid optical purity

Although various pharmacopoeias provide titration methods to assay (l)-amino acid content, none of these methods distinguish between (l)- and (d)-amino acids and do not consider the presence of enantiomeric impurities. Consequently, these methods are limited in scope to describe the relationship between content and specific rotation, [α]. In this study, the US Pharmacopoeia method was compared with the crown ether-based high performance liquid chromatographic (HPLC) chiral stationary phase (CSP) method to determine (l)-amino acid content and specific rotation. The (l)-amino acid content specified by the US Pharmacopoeia method was not consistent with the specific rotation in the presence of enantiomeric impurities, whereas the HPLC-CSP method was very effective for determining the (l)-amino acid content and the optical purity. The other advantage is that the HPLC-CSP method requires amino acid samples of quite low concentration (as low as 1μg/mL), whereas the pharmacopoeia method requires higher concentrations (20–110mg/mL).

Resolution of Aryl α-Aminoalkyl Ketones on a Doubly Tethered Liquid Chromatographic Chiral Stationary Phase Based on (+)-(18-Crown-6)-2,3,11,12-tetracarboxylic Acid

A doubly tethered chiral stationary phase (CSP) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was applied to the resolution of various aryl <TEX>${\alpha}$</TEX>-aminoalkyl ketones with the use of 80% ethanol in water containing 10 mM sulfuric acid as a mobile phase. The chiral resolution was quite successful, the separation factors (<TEX>${\alpha}$</TEX>) and the resolutions (<TEX>$R_S$</TEX>) being in the range of 1.39-2.05 and 3.18-5.22, respectively. The separation factors (<TEX>${\alpha}$</TEX>) on the doubly tethered CSP were slightly worse than those on the corresponding singly tethered CSP. However, the resolutions (<TEX>$R_S$</TEX>) on the doubly tethered CSP were generally greater than those on the corresponding singly tethered CSP. The chromatographic behaviors for the resolution of aryl <TEX>${\alpha}$</TEX>-aminoalkyl ketones on the doubly tethered CSP were demonstrated to be dependent on the type and the content of the organic and acidic modifiers in aqueous mobile phase and the column temperature.

Evaluations of poly(methyl methacrylate) bearing glucose pendant or cellobiose pendant in high performance liquid chromatography as polymeric chiral stationary phases

The glucopyranos units consisting of cyclodextrin and polysaccharide derivatives have been extensively used as chromatographic chiral stationary phases. The synthetic polymeric chiral stationary phases based on methyl methacrylate bearing glucose or cellobiose pendants for high performance liquid chromatography (HPLC) were prepared by radical polymerization. Fifteen racemic chiral compounds including alcohols, amines, amides and ketones, were resolved on the two chiral stationary phases using hexane-isopropanol (90/10, v/v), hexane-isopropanol-triethylamine (90/10/0.2, v/v/v) or hexane-isopropanol-trifluoroacetic acid (90/ 10/0.2, v/v/v) as mobile phase, separately. The results showed that both chiral stationary phases possessed good enantioselectivity for most of the racemic alcohols and amines. A few amides and ketones could also be separated. There was some compensation of the chiral discriminating abilities between the two chiral stationary phases. This work indicates that polymers of mono- and disaccharide derivatives can soon become a very attractive new chiral stationary phase for HPLC.

Liquid chromatographic resolution of 1-aryl-1,2,3,4-tetrahydroisoquinolines on a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

A liquid chromatographic chiral stationary phase (CSP) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was applied for the first time to the resolution of biologically important 1-aryl-1,2,3,4-tetrahydroisoquinolines. The unusual resolution of cyclic secondary amino compounds on a chiral crown ether-based CSP was quite successful with the use of a mixture of methanol–acetonitrile–triethylamine at a ratio of 30/70/0.5 (v/v/v) as a mobile phase. From the chromatographic behaviours for the resolution of seven 1-aryl-1,2,3,4-tetrahydroisoquinolines, the steric bulkiness of the 1-phenyl ring at the chiral center of analytes was concluded to play an important role in the chiral recognition.

Ionic cyclodextrins in ionic liquid matrices as chiral stationary phases for gas chromatography

Ionic liquids (ILs) are used to dissolve ionic cyclodextrin (CD) derivatives to produce a new type of gas chromatographic chiral stationary phase. Compared to a previous study with neutral cyclodextrin chiral selectors, the new ionic liquid-based stationary phase exhibits broader enantioselectivities, up to seven times higher efficiencies, and greater thermal stabilities. When compared to the analogous commercial column with polysiloxane matrix, it exhibits different enantioselectivities, more symmetric peak shapes and some complementary enantioseparations. The most profound separation enhancements are usually found for more polar analytes.

Liquid chromatographic direct resolution of flecainide and its analogs on a chiral stationary phase based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid

Flecainide, an antiarrythmic agent, and its analogs were resolved on a high performance liquid chromatographic chiral stationary phase (CSP) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid with the use of a mobile phase consisting of methanol-acetonitrile-trifluoroacetic acid-triethylamine (80/20/0.1/0.3, v/v/v/v). The chiral resolution was quite successful, the separation factors (alpha) and the resolutions (R(S)) for 20 analytes including flecainide being in the range of 1.19-1.82 and 1.73-6.80, respectively. The ortho-substituent of the benzoyl group of analytes was found to cause decrease in the retention times of analytes probably because of the conformational deformation of analytes originated from the steric hindrance exerted by the ortho-substituent.

Preparation and enantioseparation characteristics of three chiral stationary phases based on modified β-cyclodextrin for liquid chromatography

In order to study the effect of the nature and the length of the spacer, three mixed 10-undecenoate/phenylcarbamate derivatives of beta-cyclodextrin have been prepared and linked to allylsilica gel by means of a radical reaction. The chiral recognition ability of the resulting materials, when used as liquid chromatography chiral stationary phases (CSPs), was evaluated using heptane and either 2-propanol or chloroform as organic mobile-phase modifiers. A large variety of racemic compounds have been separated successfully on these CSPs (mainly pharmaceuticals and herbicides). Optimization of these separations was discussed in terms of mobile-phase composition and structural patterns of the injected analytes. The efficiencies of the three prepared materials were compared to those of previously described perphenylated-beta-cyclodextrin column and to analogous cellulose derivative-based CSPs.

Preparation of a New π-Basic Chiral Stationary Phase Based on Cefaclor and the Liquid Chromatographic Resolution of π-Acidic Analytes

A new liquid chromatographic chiral stationary phase (CSP) was prepared by bonding N-(3,5-dimethoxybenzoyl)cefaclor to spherical silica gel. The new CSP was successfully utilized for the resolution of racemic N-(3,5-dinitrobenzoyl)-α-amino acid derivatives and racemic 3-substituted 1,4-benzodiazepin-2-ones. For the resolution of racemic N-(3,5-dinitrobenzoyl)-α-amino acid derivatives on the CSP, a chiral recognition mechanism utilizing the enantioselective π−π donor–acceptor interaction between the π-basic 3,5-dimethoxybenzoyl group of the CSP and the π-acidic 3,5-dinitrobenzoyl group of analytes was proposed.

Preparation and Application of a New Liquid Chromatographic Chiral Stationary Phase Based on Cefaclor

A new HPLC chiral stationary phase (CSP) was prepared by bonding N-(3,5-dinitrobenzoyl)cefaclor to silica gel. The effective π-π donor-acceptor interaction between the CSP and analytes was demonstrated to be important for the chiral recognition by applying the new CSP to the resolution of N-(substituted benzoyl)leucine ethyl esters and N-(substituted benzoyl)leucine propyl amides. The new CSP was also successfully applied to the resolution of anilide derivatives of N-acetyl-α-amino acids, anilide derivatives of Oethoxycarbonyl-2-hydroxycarboxylic acids and 3-substituted 1,4-benzodiazepin-2-ones.

Preparation and Application of a New Ion-Pairing Chiral Stationary Phase for the Liquid Chromatographic Resolution of N-(3,5-Dinitrobenzoyl)-α-amino Acids

A liquid chromatographic chiral stationary phase (CSP) was prepared starting from (1S, 2S)-1,2-diaminocyclohexane. The CSP containing two simple functional groups, such as a primary amino group and an ureide tethering linkage on a cyclohexane ring, was quite successful with the use of 60% 2-propanol in hexane containing 0.5% acetic acid as a mobile phase for the resolution of various N-(3,5-dinitrobenzoyl)-α-amino acids, the separation (α) and the resolution factors (R S ) being in the range of 1.12–1.23 and 1.33–2.99, respectively. In contrast, N-(3,5-dinitrobenzoyl)-α-amino acid ethyl esters and N-(3,5-dinitrobenzoyl)-α-amino acid amides were not resolved at all, or resolved very poorly on the CSP. From these results, it was concluded that the ionic or ion-paring interaction between the CSP and analytes plays a very important role for the chiral recognition. The chromatographic behaviors for the resolution of N-(3,5-dinitrobenzoyl)-α-amino acids on the CSP were found to be controlled with the variation of the type and the content of alcohol and/or acid in hexane.

Liquid chromatographic resolution of vigabatrin and its analogue γ-amino acids on chiral stationary phases based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

Racemic vigabatrin, an anticonvulsant drug used for the treatment of epilepsy, and its analogue γ-amino acids were resolved without derivatization on liquid chromatographic chiral stationary phases (CSPs) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid. Between the two CSPs which contain 3 methylene-unit or 11 methylene-unit spacer group, the latter was found to be greater than the former in the resolution of vigabatrin and its analogue γ-amino acids, the separation, α, and the resolution factor, R S, for the resolution of vigabatrin on the latter being 1.91 and 4.57, respectively. The chromatographic behaviors for the resolution of vigabatrin and its analogue γ-amino acids on the two CSPs were found to be dependent on the type and the content of organic and acidic modifiers in aqueous mobile phase.

Synthesis of New Chiral Crown Ethers Incorporating Two Different Chiral Units and 'Matched/Mismatched' Effect of the Two Chiral Units on the Chiral Recognition

Chiral crown ethers have been widely applied as chiral selectors in differentiating the two enantiomers of racemic compounds containing a primary amino group. Especially chiral crown ethers have been quite useful as chiral selectors of liquid chromatographic chiral stationary phases (CSPs) for the recognition of the two enantiomers of racemic primary amino compounds. Among a large number of chiral crown ethers, two types have been most successfully utilized as liquid chromatographic CSPs. The first type is chiral crown ethers incorporating chiral 1,1'-binaphthyl unit, which were first developed by Cram. Especially, chiral crown ethers such as bis-(1,1'-binaphthyl)-22-crown-6 immobilized on silica gel or polystyrene were used as CSPs in separating the enantiomers of racemic α-amino acids and their derivatives in the late 1970s by Cram and coworkers. Afterward, chiral crown ethers related to (3,3'-diphenyl-1,1'binaphthyl)-20-crown-6 (1, Fig. 1) dynamically coated on octadecyl silica gel or covalently bonded to silica gel were developed as liquid chromatographic CSPs and successfully utilized in the resolution of racemic α-amino acids and other racemic compounds containing a primary amino group. The second type of chiral crown ether successfully utilized as a chiral selector of CSPs is the one incorporating two tartaric acid units, (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (2, Fig. 1), which was first synthesized by Lehn and coworkers. Initially, (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (2) was utilized as a chiral selector for the chiral separation by capillary electrophoresis. The use of (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (2) was then extended as a chiral selector of liquid chromatographic CSPs. Liquid chromatographic CSPs based on (+)-(18crown-6)-2,3,11,12-tetracarboxylic acid (2) were very successful in the resolution of various racemic primary amino compounds or racemic non-primary amino compounds. In addition, (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (2) was utilized as an NMR chiral solvating agent. However, chiral crown ethers incorporating both chiral 3,3'-diphenyl-1,1'-binaphthyl and tartaric acid unit have never been known to the best of our knowledge. Chiral crown ethers incorporating both chiral 3,3'-diphenyl-1,1'binaphthyl and tartaric acid unit are expected to show characteristics somewhat different from chiral crown ether 1 or 2 in the chiral recognition. In this study, we wish to report the synthesis of novel chiral crown ethers incorporating both chiral 3,3'-diphenyl-1,1'-binaphthyl and tartaric acid unit and their applications as chiral selectors of liquid chromatographic CSPs. We prepared actually two diastereomeric chiral crown ethers, (R,R,R)-3 and (R,S,S)-3 (Fig. 2), incorporating both (R)-3,3'-diphenyl-1,1'-binaphthyl and (R,R)or (S,S)-tartaric acid unit as chiral barriers. Combination of the two different chiral units as chiral barriers is expected to endow the two diastereomeric chiral crown ethers with characteristic chiral recognition ability according to the stereochemistry of the two chiral units. Synthesis of the two diastereomeric chiral crown ethers was successfully performed as shown in Scheme 1. Diols (R,R)-4 and (S,S)-4 prepared from commercially available (R,R)and (S,S)-N,N,N',N'-tetramethyltartaramide respectively via the known procedure were converted into their tosyl derivatives, (R,R)-5 and (S,S)-5. Tosylation of (R,R)-4 or (S,S)-4 was most successful when a small amount of 4(dimethylamino)pyridine (DMAP) was added to the reaction mixture. The cyclization of tosyl derivative (R,R)-5 or (S,S)-

Preparation of two new liquid chromatographic chiral stationary phases based on diastereomeric chiral crown ethers incorporating two different chiral units and their applications

Two new liquid chromatographic chiral stationary phases based on diastereomeric chiral crown ethers incorporating two different chiral units such as optically active 3,3′-diphenyl-1,1′-binaphthyl and tartaric acid unit were prepared. Between the two CSPs, one was much superior to the other especially in the resolution of tocainide and its analogues (for example, in the resolution of tocainide the separation factor, α, was 4.26 vs. 1.00 on the two CSPs). From these results, the two chiral units composing the two diastereomeric chiral crown ether moieties of the stationary phases were expected to show “matched” or “mismatched” effect on the chiral recognition according to their stereochemistry. The different chiral recognition abilities of the two CSPs were rationalized by the different three-dimensional structures of the two diastereomeric chiral crown ethers.

An NMR Chiral Solvating Agent for the Chiral Recognition of the Two Enantiomers of N-(3,5-Dinitrobenzoyl)-α-amino Acids

Two enantiomers of chiral drugs often show different pharmaceutical activities. Consequently, the study on the stereochemical dependence of the pharmacokinetics and the pharmaceutical activities of chiral drugs is important. Determination of the enantiomeric composition of chiral drugs is, therefore, essential. Enantiomeric composition of chiral compounds has been determined most widely by highperformance liquid chromatographic chiral separations on chiral stationary phases (CSPs). Another important method of determining the enantiomeric composition of chiral compounds is the NMR techniques utilizing chiral solvating agent (CSA). The two transient diastereomeric adducts formed between a CSA and each of the two enantiomers of chiral analytes might induce anisochronous NMR resonances and consequently the enantiomeric composition of chiral analytes can be easily assessed from the NMR spectrum. Most CSAs, which have been developed and successfully utilized in the determination of the enantiomeric composition of chiral compounds by NMR spectroscopy, contain aromatic functional group(s) to induce magnetic anisotropic influence and to invoke π-π interaction with analytes. Derivatives of optically active trans-1,2-diaminocyclohexane has also been utilized as NMR CSAs and these CSAs have also been designed to contain aromatic functional group(s) in order to induce magnetic anisotropic influence and to invoke π-π interaction with analytes. Consequently, CSAs derived from optically active trans-1,2diaminocyclohexane have been utilized for the determination of enantiomeric composition of chiral compounds containing aromatic functional group(s). For example, CSA 1 and CSA 2 derived from (1R,2R)-1,2-diaminocyclohexane shown in Figure 1 have been successfully applied to the determination of enantiomeric composition of chiral carboxylic acids by NMR spectroscopy. Recently, we found that a liquid chromatographic chiral stationary phase (CSP 3, Figure 1) derived from optically active (1S,2S)-1,2-diaminocyclohexane is quite successful for the resolution of N-(3,5-dinitrobenzoyl)-α-amino acids even though the CSP does not contain any aromatic functional group, but contains only a simple primary amino group and a ureide tethering group. From these results, we inferred that CSA 4, which has the same structure as the chiral selector of CSP 3, might show nonequivalences for the two enantiomers of N-(3,5-dinitrobenzoyl)-α-amino acids by NMR spectroscopy. CSA 4 was prepared simply by treating (1S,2S)-1,2diaminocyclohexane with ethyl isocyanate. CSA 4 thus prepared was used to see the H NMR chemical shift nonequivalence of the two enantiomers of N-(3,5-dinitrobenzoyl)phenylglycine. As shown in Figure 2, the chemical shift nonequivalence of the two enantiomers of N-(3,5-dinitrobenzoyl)phenylglycine was observed at the NMR peaks corresponding to the protons on the 3,5-dinitrophenyl ring (denoted by a in Figure 1). In addition the chemical shift nonequivalence of the two enantiomers of N-(3,5-dinitrobenzoyl)phenylglycine was observed to be greatest when the ratio of CSA 4 and N-(3,5-dinitrobenzoyl)phenylglycine is 2:1 at ambient temperature (22 ± 0.5 C). The H NMR experimental results for the chemical shift nonequivalences of the two enantiomers of five N-(3,5dinitrobenzoyl)-α-amino acids in the presence of two equivalents of CSA 4 are summarized in Table 1. As shown

Liquid chromatographic resolution of secondary amino alcohols on a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid: Dependence of temperature effect on analyte structure

Two types of secondary amino alcohols were successfully resolved on a liquid chromatographic chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid. The secondary amino alcohols containing a secondary amino group connected directly to the stereogenic center were resolved much better than those containing a hydroxyl group attached directly to the stereogenic center. In addition, the dependence of the separation factor ( α) on column temperature was exactly opposite for the two different types of secondary amino alcohols. From the thermodynamic calculations based on Van’t Hoff plots, we conclude that the enantioselectivity for the resolution of the secondary amino alcohols containing a secondary amino group connected directly to the stereogenic center is enthalpically controlled while that for the resolution of the secondary amino alcohols containing a hydroxyl group attached directly to the stereogenic center is entropically controlled.