Amino alcohol-derived chiral stationary phases.

An updated minireview of chiral stationary phases (CSPs) based on amino alcohols is presented. In this minireview, we focused on amino alcohols as starting materials in preparation of chiral catalysts for asymmetric organic synthesis and CSPs for chiral separations. Among the various CSPs, we summarized the important developments and applications of the amino alcohol-based Pirkle-type CSPs, ligand exchange CSPs, α-amino acid-derived amino alcohol CSPs, and symmetric CSPs from their first appearance to the present day to propose ideas for the development of new CSPs with improved performance.

Synthesis and application of N-3,5-dinitrobenzoyl and C3 symmetric diastereomeric chiral stationary phases.

Three diastereomeric chiral compounds, namely, (R,R)-(+)-2-amino-1,2-diphenylethanol, (1S,2R)-(+)-2-amino-1,2-diphenylethanol, and (1R,2R)-(+)-1,2-diphenylethylenediamine were used as starting materials for preparing three N-3,5-dinitrobenzoyl derivative chiral stationary phases (CSPs) (CSP 1, 2, and 3) and three C3 symmetric CSPs (CSP 4, 5, and 6). The six newly prepared CSPs were applied to the chiral separation of 44 chiral samples by HPLC. Most samples were isolated on CSP 6, with the highest average separation factor among the six newly prepared CSPs.

Improvement of chiral separation efficiency through temperature control during one time high performance liquid chromatography analysis.

This study examined how manual temperature control affects the separation of two enantiomers of five racemic mixtures using high-performance liquid chromatography (HPLC) in terms of separation factor (α), resolution (Rs), and the number of theoretical plates (N). The results showed that heating/cooling during one time HPLC analysis improved the separation factor and resolution.

Synthesis of new C3 symmetric amino acid- and aminoalcohol-containing chiral stationary phases and application to HPLC enantioseparations.

We recently reported a new C3-symmetric (R)-phenylglycinol N-1,3,5-benzenetricarboxylic acid-derived chiral high-performance liquid chromatography (HPLC) stationary phase (CSP 1) that demonstrated better results as compared to a previously described N-3,5-dintrobenzoyl (DNB) (R)-phenylglycinol-derived CSP. Over a decade ago, (S)-leucinol, (R)-phenylglycine, and (S)-leucine derivatives were used as the starting materials of 3,5-DNB-based Pirkle-type CSPs for chiral separation. In this study, three new C3-symmetric CSPs (CSP 2, 3, and 4) were prepared by combining the ideas and results mentioned above. Here we describe the synthetic procedures and applications of the new C3-symmetric CSPs (CSP 2-CSP 4).

Chiral separation on various modified amino alcohol-derived HPLC chiral stationary phases.

3,5-Dinitrobenzoyl chloride was previously used for the preparation of (R)-phenylglycinol- and (S)-leucinol-derived chiral stationary phases. In this study, 3,5-bis(trifluoromethyl)benzoyl chloride, 2-furoyl chloride, 2-theonyl chloride, 10,11-dihydro-5H-dibenzo[b,f]azepine-5-carbonyl chloride, diphenylcarbamoyl chloride, and 1-adamantanecarbonyl chloride were used to prepare six new phenylglycinol-derived chiral stationary phases (CSPs) and five new leucinol-derived CSPs. Using these 11 CSPs, chiral separation of nine π-acidic amino acid derivatives and five π-basic compounds was performed, and the separation results were compared. An adamantyl-derived CSP showed good separation.

Synthesis and Application of C2 and C3 Symmetric (R)-Phenylglycinol-Derived Chiral Stationary Phases.

A C3 symmetric (R)-phenylglycinol N-1,3,5-benzenetricarboxylic acid-derived chiral stationary phase (CSP) and three C2 symmetric (R)-phenylglycinol CSPs were newly synthesized using o-, m-, and p-phthaloyl dichlorides. These CSPs were used to compare the resolution of 25 chiral samples using a previously reported 3,5-dinitrobenzoyl (R)-phenylglycinol-derived CSP. Even though all CSPs have the same chiral moiety, the C3 symmetric CSP showed the best resolution.

Synthesis and chiral recognition of nickel(II) macrocyclic complex with (R)-naphthylethyleneamine pendant groups and its self-assembled framework.

A novel nickel(II) hexaaza macrocyclic complex, [Ni(L(R,R))](ClO(4))(2) (1), containing chiral pendant groups was synthesized by an efficient one-pot template condensation and characterized (L(R,R) ═1,8-di((R)-α-methylnaphthyl)-1,3,6,8,10,13-hexaazacyclotetradecane). The crystal structure of compound 1 was determined by single-crystal X-ray analysis. The complex was found to have a square-planar coordination environment for the nickel(II) ion. Open framework [Ni(L(R,R))](3) [C(6)H(3)(COO)(3)](2) (2) was constructed from the self-assembly of compound 1 with deprotonated 1,3,5-benzenetricarboxylic acid, BTC(3-). Chiral discrimination of rac-1,1'-bi-2-naphthol and rac-2,2,2-trifluoro-1-(9-anthryl)ethanol was performed to determine the chiral recognition ability of the chiral complex (1) and its self-assembled framework (2). Binaphthol showed a good chiral discrimination on the framework (2). The optimum experimental conditions for the chiral discrimination were examined by changing the weight ratio between the macrocyclic complex 1 or self-assembled framework 2 and racemates. The detailed synthetic procedures, spectroscopic data including single-crystal X-ray analysis, and the results of the chiral recognition for the compounds are described.

Use of sulfated cyclofructan 6 and sulfated cyclodextrins for the chiral separation of four basic pharmaceuticals by capillary electrophoresis.

Sulfated cyclofructan 6 (S-CF6) and sulfated cyclodextrins (S-α-, ß-, γ-CDs) are highly selective chiral selectors for the enantioseparation of basic solutes. In this study, S-CF6 was introduced for the enantiomeric separation of four basic pharmaceuticals (including tamsulosin, tiropramide, bupivacaine, and norephedrine) by capillary electrophoresis (CE), and the enantiomeric separation performance was compared with S-α-, ß-, γ-CDs. The effects of the chiral selector type, chiral selector concentration, operating voltage, and column temperature were examined and optimized. Excellent resolutions were obtained for all solutes on these chiral selectors.

Mechanistic study of ultrasound-assisted chromatography using plastic and stainless steel columns.

Based on the previous ultrasound-assisted chromatography (UAC) studies on plastic and stainless steel (SS) columns, this study explores the UAC mechanism by comparatively analyzing ultrasound effects on plastic and SS columns with C18 stationary phase when separating a mixture of polycyclic aromatic hydrocarbons (PAHs) under various ultrasound intensities. The results showed a substantial difference in H values between the PEEK and SS columns under the influence of ultrasound agitation. Specifically, for the pyrene peak, as the ultrasonic intensity increased from 0% to 100% of 900 W, the H values of the SS column slightly rose from 8.82 µm to 9.86 µm. Conversely, the corresponding values for the PEEK column exhibited a significant 12-fold increase from 11.5 µm to 134 µm. The findings demonstrated poor penetration of ultrasound energy through the SS column, and the temperature rise of the medium induced by the ultrasound was the primary contributing factor to PAH separation. However, ultrasound easily penetrated through the plastic column, resulting in acoustic cavitation within the C18 polyether ether ketone (PEEK) column. Cavitation induced heat generation and contributed to a decrease in retention time and the magnitude of peak broadening or distortion, depending on the specific ultrasonic energy. Based on the estimated change in inlet temperature of the PEEK column due to an acoustic effect, the comparison with temperature effects under non-sonic conditions consistently demonstrated a stronger acoustic effect in reducing the retention time, by 2-9%, depending on specific peaks and pairs. We revisited the previously described separation mechanism of ultrasound-assisted ion chromatography and conjoined with our findings to infer and establish a thorough explanation for the previously unexplained separation mechanism of chiral separation and size exclusion chromatography by UAC using SS columns.

Comparison between the use of polyether ether ketone and stainless steel columns for ultrasonic-assisted extraction under various ultrasonic conditions.

The ultrasound-assisted extraction (UAE) was conducted using the stainless steel (SS) and polyether ether ketone (PEEK) columns and analyzed with high-performance liquid chromatography (HPLC) to understand the mechanism of ultrasound-assisted chromatography (UAC). Empty SS and PEEK columns were used to extract dyes from a fabric under identical conditions with several parameters including the initial ultrasonic bath temperatures (30 °C and 40 °C), ultrasound power intensities (0, 20, 40, 60, 80, and 100 %), ultrasound operation modes (normal and sweep), and ultrasound frequencies (25 kHz, 40 kHz, and 132 kHz) to compare their extraction capabilities. After 30 min of extraction, the amount of extract was determined by HPLC. The PEEK material was significantly affected by ultrasonic radiation compared to the SS material, especially at a higher temperature (40 °C), power intensity (100 %), and frequency (132 kHz) with sweep mode. At a maximum power density of 45 W/L, the extraction effectiveness ratio of PEEK to SS was in the range of 1.8 - 3.9 depending on the specific frequency, initial temperature, and with or without temperature control. The most optimal ultrasound frequencies, in terms of enhancing extraction effectiveness, are in the order of 132 kHz, 40 kHz, and 25 kHz. Unlike the SS material, the PEEK material was more affected by temperature and acoustic effects under identical conditions, especially at 132 kHz ultrasound frequency. In contrast, at lower frequencies of 40 kHz and 25 kHz, no significant differences in the acoustic effects were observed between the PEEK and SS materials. The findings of this study contribute to elucidating the roles of column materials in UAE and UAC.

Determination of optical purity of 3,5-dimethoxybenzoyl-leucine diethylamide by chiral chromatography and 1H and 13C NMR spectroscopy.

(R)-N-3,5-dinitrobenzoyl (DNB) leucine derived chiral selector was used as an HPLC chiral stationary phase for the resolution of various racemic amino acids derivatives. In this study, determination of optical purity of an amino acid derivative was performed by chiral high performance liquid chromatography and 1H and 13C NMR spectroscopy by using the DNB leucine derived chiral selector. The accuracy and precision of each optical purity value are calculated and the data are compared to each other.

Self-assembly and chiral recognition of a two-dimensional coordination polymer from a chiral nickel(II) macrocyclic complex and trimesic acid.

A two-dimensional chiral open framework, [Ni(L(R,R))](3)[C(6)H(3)(COO)(3)](2).12H(2)O.CH(3)CN (3), is prepared from a nickel(II) macrocyclic complex with chiral pendants and benzenetricarboxylate in MeCN/H(2)O and characterized [L(R,R) = 1,8-bis[(R)-alpha-methylbenzyl]-1,3,6,8,10,13-hexaazacyclotetradecane]. The open framework shows selective chiral recognition by interactions between the solid 3 and rac-1,1'-bi-2-naphthol.

trans-{1,8-Bis[(S)-1-phenyl-eth-yl]-1,3,6,8,10,13-hexa-aza-cyclo-tetra-deca-ne}bis(thio-cyanato--κN)copper(II).

In the title thio-cyanate-coordinated aza-macrocyclic copper(II) complex, [Cu(NCS)(2)(C(24)H(38)N(6))], the Cu(II) atom is coordinated by the four secondary N atoms of the aza-macrocyclic ligand and by the two N atoms of the thio-cyanate ions in a tetra-gonally distorted octa-hedral geometry. The average equatorial Cu-N bond length is shorter than the average axial Cu-N bond length [2.010 (2) and 2.528 (4) Å, respectively]. An N-H⋯N hydrogen-bonding inter-action between the secondary amine N atom and the adjacent thio-cyanate ion leads to a polymeric chain along the a axis.

Comparison of monomeric and polymeric chiral stationary phases.

Two-type polymeric chiral stationary phases (pCSPs) were prepared by surface grafting of chiral acryl-type monomers on a silica gel surface modified with 3-(trimethoxysilyl)propylmethacrylate. The prepared pCSPs were characterized by IR, Fr-Raman, scanning electron microscopy, and elemental analysis. In addition, two-type monomeric chiral stationary phases (mCSPs) were also prepared. The racemic analytes were separated using the prepared mCSPs and pCSPs. The separation factor (alpha) and capacity factor (k1) of the racemic analytes for the pCSP and mCSP were compared. The alpha and k1 values of the mCSP were higher than those of the pCSP.

Enantioseparation of racemic N-acylarylalkylamines on various amino alcohol derived tau-acidic chiral stationary phases.

Five tau-acidic chiral stationary phases (CSPs), CSP 4, CSP 5, CSP 6, CSP 7 and CSP 8, were prepared by connecting the N-(3,5-dimethylbenzoyl) derivative of (R)-alaninol, (S)-leucinol, (1S,2R)-ephedrine and (S)-tert-leucinol and the O-(3,5-dinitrobenzoyl) derivative of (R)-phenylglycinol to silica gel through a carbamate or urea linkage. The CSPs were applied to the resolution of various racemic N-acyl-1-naphthylaminoalkanes by chiral HPLC, and the chromatographic resolution results were compared with those of previously reported CSPs (CSP 2, CSP 3), which are derived from N-(3,5-dinitrobenzoyl)-(1S,2R)-norephedrine and N-(3,5-dinitrobenzoyl-(R)-phenylglycinol. Based on a comparison of the resolution results for each CSP, the role of each functional group on the five chiral selectors is explained.

Retention behavior of C1-C6 aliphatic monoamines on anion-exchange and polymethacrylate resins with heptylamine as eluent.

Retention behavior of C1-C6, aliphatic monoamines (methylamine, ethylamine, propylamine, butylamine, amylamine and hexylamine) on columns (150 mm x 6 mm i.d.) packed with various anion-exchange resins (styrene-divinylbenzene (PS-DVB) copolymer-based strongly basic anion-exchange resin: TSKgel SAX, polymethacrylate-based strongly basic anion-exchange resin: TSKgel SuperQ-5PW and polymethacrylate-based weakly basic anion-exchange resin: TSKgel DEAE-5PW) and unfunctionized polymethacrylate resins (TSKgel G5000PW and TSKgel G3000PWXL) was investigated with basic solutions (sodium hydroxide and heptylamine) as the eluents. Due to strongly electrostatic repulsion (ion-exclusion effect) between these anion-exchange resins and these amines, peak resolution between these amines on these anion-exchange resin columns was unsatisfactory with both sodium hydroxide and heptylamine as the eluents. In contrast, these polymethacrylate resins were successfully applied as the stationary phases for the separation of these C1-C6 amines with heptylamine as eluent, because of both small hydrophobicity and small cation-exchange ability of these resins. Excellent simultaneous separation, highly sensitive conductimetric detection and symmetrical peaks for these C1-C6 amines were achieved on the TSKgel G3000PWXL column in 35 min with 5 mM heptylamine at pH 11.1 as the eluent.

Separation of aliphatic carboxylic acids and benzenecarboxylic acids by ion-exclusion chromatography with various cation-exchange resin columns and sulfuric acid as eluent.

The application of various hydrophilic cation-exchange resins for high-performance liquid chromatography (sulfonated silica gel: TSKgel SP-2SW, carboxylated silica gel: TSKgel CM-2SW, sulfonated polymethacrylate resin: TSKgel SP-5PW, carboxylated polymethacrylate resins: TSKgel CM-5PW and TSKgel OA-Pak A) as stationary phases in ion-exclusion chromatography for C1-C7 aliphatic carboxylic acids (formic, acetic, propionic, butyric, isovaleric, valeric, isocaproic, caproic, 2-methylhexanoic and heptanoic acids) and benzenecarboxylic acids (pyromellitic, trimellitic, hemimellitic, o-phthalic, m-phthalic, p-phthalic, benzoic, salicylic acids and phenol) was carried out using diluted sulfuric acid as the eluent. Silica-based cation-exchange resins (TSKgel SP-2SW and TSKgel CM-2SW) were very suitable for the ion-exclusion chromatographic separation of these benzenecarboxylic acids. Excellent simultaneous separation of these benzenecarboxylic acids was achieved on a TSKgel SP-2SW column (150 x 6 mm I.D.) in 17 min using a 2.5 mM sulfuric acid at pH 2.4 as the eluent. Polymethacrylate-based cation-exchange resins (TSKgel SP-5PW, TSKgel CM-5PW and TSKgel OA-Pak A) acted as advanced stationary phases for the ion-exclusion chromatographic separation of these C1-C7 aliphatic carboxylic acids. Excellent simultaneous separation of these C1-C7 acids was achieved on a TSKgel CM-5PW column (150 x 6 mm I.D.) in 32 min using a 0.05 mM sulfuric acid at pH 4.0 as the eluent.

Preparation and characterization of cyclodextrin polymer and its high-performance liquid-chromatography stationary phase.

Cyclodextrin (CD) polymers were synthesized from the reaction of native CDs with a hexamethylene diisocyanate (HDI) compound in a dried DMF solution. The obtained CD polymer contained a range of 8-14% N due to HDI by elemental analysis. The physical and chemical properties of the CD polymers were characterized by IR, solid state 13C NMR, TGA, and DSC, respectively. An HPLC column was prepared using the CD polymer with a carbamate linker by a slurry method. Separation of the phenol isomers was conducted using the CD polymer stationary phase and CD by HPLC. Furthermore, an inclusion complex of the phenol isomer was studied by FT-Raman spectroscopy. From the HPLC and FT-Raman results, inclusion phenomena of o-, m-, and p-nitrophenol onto CD and CD polymers were analyzed.

Synthesis and characterization of enantiopure copper(II) complexes using chiral bidentate ligands.

Chiral copper(ii) complexes of the types [Cu(II)(R/S-ppme)(3)](ClO(4))(2) and [Cu(II)(R/S-ppme)(2)(SO(4))] have been synthesized by reactions of Cu(ClO(4))(2).6H(2)O and CuSO(4).5H(2)O with R/S-ppme in methanol under aerobic conditions (R/S-ppme = (R/S)-(+/-)-1-phenyl-N-(pyridine-2-ylmethylene)ethanamine). All complexes were isolated and characterized by X-ray crystallography and circular dichroism (CD): (Lambda)-[Cu(II)(R-ppme)(3)](ClO(4))(2) (1), (Delta)-[Cu(S-ppme)(3)](ClO(4))(2).H(2)O (2), (Delta)-[Cu(II)(R-ppme)(2)(SO(4))].4CH(3)OH (3), and (Lambda)-[Cu(II)(S-ppme)(2)(SO(4))].4CH(3)OH (4). 1 and 2 with ClO(4)(-) anions are formed as tris(R/S-ppme) copper(ii) complexes, while 3 and 4 with SO(4)(2-) are formed as bis(R/S-ppme) copper(ii) complexes. The structures of 1 with R-ppmes and 4 with S-ppmes show the same absolute configurations (Lambda). In contrast, 2 with S-ppmes and 3 with R-ppmes exhibit Delta configurations. In the electron paramagnetic resonance (EPR) spectra of 1 and 3, both compounds demonstrate z-axis elongated structures. The CD spectra of 1 and 2 show Cotton effects in an enantiomeric pattern; those of 3 and 4 also display the same effects.

Characterization of sodium laureth sulfate by reversed-phase liquid chromatography with evaporative light scattering detection and 1H nuclear magnetic resonance spectroscopy.

A direct and effective method utilizing reversed-phase liquid chromatography combined with evaporative light scattering detection was developed to determine the relative ratio of different alkyl chain lengths, to quantify the average ethylene oxide (EO) level and to identify EO distribution in the presence of sodium laureth sulfates. A C8 bonded silica gel column and an acetonitrile-water gradient mobile phase containing ammonium acetate were used as the best stationary and mobile phase, respectively. The results were confirmed by nuclear magnetic resonance spectroscopy. The detection limit was 80 microg/mL and the calibration curve, i.e., the log-log plots (peak area vs. concentration), was linear in the working range of 80-4200 microg/mL with R(2) values of above 0.999 (in the case of 3 mol sodium laureth sulfates). Furthermore, the application of the chromatographic method to a commercial product without pretreatment was presented. The raw material was identified by fast atom bombardment mass spectrometry.