Enzymatic and chromatographic chiral discrimination of racemic (thio)glycidyl esters: Part II. Optical resolution of racemic (thio)glycidyl esters on modified cellulose chiral stationary phases

Chiral chromatography on cellulose tris(3,5-dimethylphenyl carbamate) (Chiralcel OD) and cellulose tribenzoate (Chiralcel OB) coated stationary phases has been successfully used for the optical resolution of rac-(thio)glycidyl esters (acetate, propionate, butyrate). Glycidyl esters could sufficiently be resolved on the OD column whereas for the thio analogues baseline resolution is obtained on CSP OB using hexane/2-propanol mobile phases. The separation factor (α) and resolution (R_S) depend on column temperature, eluent composition, and flow rate, respectively. Best results were obtained for the butyrates and at low temperatures in general. © 1993 Wiley-Liss, Inc.     

Comparative HPLC enantioseparation of ferrocenylalcohols on two cellulose-based chiral stationary phases

The direct HPLC enantiomeric separation of several ferrocenylalcohols on the commercially available Chiralcel OD and Chiralcel OJ columns has been evaluated in normal-phase mode. Almost all the compounds were resolved on one or both chiral stationary phases (CSPs) with separation factor (alpha) ranging from 1.06 to 2.88 while the resolution (R(s)) varied from 0.63 to 12.70 In the separation of the alpha-ferrocenylalcohols 1a-e and the phenyl analogues 2a-e, which were all resolved except 1c, a similar trend in the retention behavior for the two series of alcohols was evidenced and the selectivity was roughly complementary on the two investigated CSP. For three ferrocenylacohols, chosen as model compounds, the influence of the mobile phase composition and temperature on the enantioseparation were investigated and additional information on the chiral recognition mechanism were deduced from the chromatographic behavior of their acetylderivatives.     

Investigation of the enantioselectivity and enantiomeric elution order of some piperidine-2,6-dione drugs on chiralcel OD column

The enantioselectivity and enantiomeric separation of five racemic piperidine-2,6-dione compounds, on the cellulose tris(3,5-dimethylphenyl carbamate) chiral stationary phase Chiralcel OD-CSP were investigated under the same chromatographic conditions. This class of drugs includes glutethimide, aminoglutethimide, cyclohexylaminoglutethimide, pyridoglutethimide, and phenglutarimide. The results revealed that chiral recognition and the binding sites of these drugs on the Chiralcel OD column are similar, regardless of the absolute configuration of the individual enantiomers. A possible chiral recognition mechanism(s) for this class of drugs and the CSP is presented. © 1994 Wiley-Liss, Inc.     

High‐performance liquid chromatographic enantioseparation of N‐aryl‐thioureidoalkylphosphonates and thiourylenedi(alkylphosphonates) on polysaccharide‐based chiral stationary phases

The first successful enantioseparation of representative O,O‐diphenyl‐N‐arylthioureidoalkylphosphonates, (±)‐Ptc‐Val^P(OPh)₂ & (±)‐Ptc‐Leu^P(OPh)₂ and thiourylenedi(isobutyl phosphonate), Tcm[Val^P(OPh)₂]₂ on analytical and semipreparative scale was achieved by high‐performance liquid chromatography using polysaccharide‐based chiral stationary phases (CPs). Atc‐AA^P(OPh)₂ was obtained using modified tricomponent condensations of the corresponding aldehydes, N‐arylthiourea and triphenyl phosphite whereas Tcm[Val^P(OPh)₂]₂ by the condensations of aldehydes, thiourea, and triphenyl phosphite. The prepared, racemic (±)‐Atc‐AA^P(OPh)₂ [(±)‐Ptc‐Val^P(OPh)₂, (±)‐Ptc‐Leu^P(OPh)₂, (±)‐Ptc‐Pgly^P(OPh)₂ and (±)‐Ntc‐Pgly^P(OPh)₂] and racemic (±)‐Tcm[AA^P(OPh)₂]₂ [(±)‐Tcm[Nva^P(OPh)₂]₂ & (±)‐Tcm[Val^P(OPh)₂]₂] were adequately characterized and used for chromatographic separations on high‐performance liquid chromatography–chiral stationary phases. The best results were obtained for (±)‐Ptc‐Val^P(OPh)₂, (±)‐Ptc‐Leu^P(OPh)₂ and (±)‐Tcm[Val^P(OPh)₂]₂.     

Preparative scale enantioseparation of 1-cyclohexyl-1-phenylethyl hydroperoxide and 1,2,3,4-tetrahydro-1-naphthyl hydroperoxide on a modified cellulose stationary phase

The analytical and preparative scale optical resolution of 1-cyclohexyl-1-phenylethyl hydroperoxide and 1,2,3,4-tetrahydro-1-napthyl hydroperoxide has been achieved by chiral HPLC on a cellulose tris(3,5-dimethylphenyl carbamate) stationary phase coated on silica gel. The method has been used to obtain several hundred milligrams of highly enriched enantiomers (%ee >98) which were characterized by [α]_D and circular dichroism spectra, respectively. Configurational assignments were achieved for 1,2,3,4-tetrahydro-1-naphthyl hydroperoxide enantiomers. © 1995 Wiley-Liss, Inc.     

Mechanism of chiral recognition in the enantioseparation of 2-aryloxypropionic acids on new brush-type chiral stationary phases

New brush-type chiral stationary phases (CSP I-IV) comprising N-3,5,6-trichloro-2,4-dicyanophenyl-L-alpha-amino acids (1-4) were prepared by binding of chiral selectors 1-4 to gamma-aminopropyl silica gel. To check the role of excess free aminopropyl groups, CSP V was prepared by binding N-3,5,6-trichloro-2,4-dicyanophenyl-L-alanyl-(3-triethoxysilyl)propylamide to unmodified silica gel. The best separation of racemic 2-aryloxypropionic acids (TR-1-13) was obtained with CSP I; the -(-)-S enantiomer were regularly eluted first, as determined by a CD detector. The mechanism of chiral recognition implies a synergistic interaction of carboxylic acid analyte with the chiral selector and achiral free gamma-aminopropyl units on silica. In fact, CSP V, which is lacking an achiral aminopropyl spacer, shows a lower separation ability for 2-aryloxypropionic acids, but a similar enantioselective discrimination of esters TR-19-20, in comparison with CSP I. CSP I-IV retain unaltered separation ability after a few months of continuous work using a large number of various mobile phases.     

Structure and substituent effect on chiral separation of some 4a-methyl-2,3,4,4a-tetrahydro-1H-fluorene derivatives and 4a-methyl-1,2,3,4,4a,9a-hexahydro-fluoren-9-one derivatives on CTA-I and chiralcel OJ chiral stationary phases

The chromatographic parameters for 12 structurally related compounds in the 4a-methyl-2,3,4,4a-tetrahydro-1H-fluorene and 4a-methyl-1,2,3,4,4a,9a-hexahydro-fluoren-9-one series are reported on CTA-I and Chiralcel OJ chiral stationary phases. Arrangement of the k' values according to configurationally related enantiomer series (Class I and Class II) and not according to the actual order of elution, allows the treatment of the data by linear correlation with structure and substituent effect. A detailed analysis of the capacity factor variation with respect to the structural changes shows clearly that the framework and substitution effects do not result in the same response on the two cellulose ester chiral stationary phases. More interestingly, it emerges that chiral discimination may be attributed to certain areas of the molecule, these areas being different in the interaction within CTA-I and Chiralcel OJ. Furthermore, our analysis points out the relevance of attempting to develop quantitative relationships for configurationally related series of enantiomers (in our case Class I and Class II), the main effort being devoted to the understanding of the capacity factor variation in each class rather than of the α values, which are derived entities. Chirality 10:522–527, 1998. © 1998 Wiley-Liss, Inc.     

Enantiomeric discrimination of pyrethroic acid esters on polysaccharide derived chiral stationary phases

Enantiomeric separation of pyrethroic acid methyl and ethyl esters was examined on cellulose-based chiral stationary phases (CSPs): chiralcel OD (cellulose tris(3,5-dimethylphenyl carbamate)) and chiralcel OF (cellulose tris(4-chlorophenyl carbamate)). The good resolution of pyrethroic acid esters was achieved on chiralcel OD and OF. Separation factors ranged from 1.19-5.12 for Chiralcel OD and 1.00-1.59 for chiralcel OF. Hexane/2-propanol (100:0.15, v/v %) was used as the eluent. The resolution capability of CSPs was greater chiralcel OD than chiralcel OF in the case of the pyrethroic acid esters. The flow rate was 0.8 ml/min and detection was set at 230 nm. The results of the chromatographic data and molecular mechanics suggest that steric effect was a major factor in the enantioseparation. Furthermore, the hydrogen bond between analytes and CSP played an important role in the chiral recognition.     

High-performance liquid chromatography of imine stereoisomers on cellulose-based chiral stationary phases

A high-performance liquid chromatographic method has been developed for the analysis of the intermediate imines and end products in an asymmetric isomerization route toward optically active amines. Separation of the imine enantiomers was performed on commercially available Chiralcel OD-H, Chiralcel OJ, and Chiralpak AD chiral stationary phases. All substituted imine enantiomers could be readily resolved with selectivities (α) higher than 1.10 using the Chiralpak AD column. By derivatization with ring-substituted benzaldehydes, aromatic amines were converted into Schiff base derivatives and the enantiopurity of these amines was determined. Chirality 9:727–731, 1997. © 1997 Wiley-Liss, Inc.     

Steric hindrance influence on the enantiorecognition ability of tyrosine-derived chiral stationary phases

Four chiral stationary phases (CSPs) derived from N-(3,5-dinitrobenzoyl)tyrosine have been synthesized. They differ by the substituent nature (methyl, ethyl, isopropyl, tert-butyl) of the aliphatic amide function. The enantiorecognition ability of these CSPs was evaluated with 10 racemates. For the majority of them, the stereoselectivity increases with the steric hindrance of the substituent. The chiral selector enantiomeric separation on the resulting CSPs has evidenced a reversal of elution order only for CS 4 on CSP 4 (tert-butyl substituent), suggesting a change in its conformation.     

Direct enantioselective separation of some propranolol analogs by HPLC on normal and reversed cellulose chiral stationary phases

The enantiomeric separation of several racemic aryloxyaminopropan-2-ol derivatives related to propranolol on normal and reversed phase of cellulose tris (3,5-dimethylphenylcarbamate) chiral stationary phases known as Chiralcel OD and Chiralcel OD-R were studied. It was observed that the chiral separation depends on the substitution pattern of the aryl group, i.e., 1-naphthyl, 2-naphthyl, and phenyl group and polarity on the basic nitrogen in the side chain. In both normal and reversed phase modes the (+)-R-enantiomer eluted first in all of the analogs resolved. It can be concluded that: (1) substituents on the side chain did affect the interaction of the enantiomers with the polar carbamate moiety in the CSP; and (2) the dipole-dipole stacking between the π-donor 3,5-dimethylphenyl carbamate group pending from the glucose rings of the CSP and π-acceptor aryl group of the analyte is crucial for the efficient chiral discrimination. The chiral recognition mechanism(s) between these analogs and the chiral stationary phases are proposed. © 1996 Wiley-Liss, Inc.     

Chiral HPLC analysis of milnacipran and its FMOC-derivative on cellulose-based stationary phases

The HPLC enantioseparation of the last generation antidepressive drug milnacipran (+/-)-1 was investigated on different cellulose-based chiral stationary phases (CSPs). On carbamate-type columns, Chiralcel OD and OD-H (+/-)-1 could be separated with alpha value about 1.20 but the resolution was quite low because of the tailing of the peaks. Direct determination of (+/-)-1 with high selectivity and resolution was obtained on Chiralcel OJ in normal phase mode elution. Precolumn derivatization of milnacipran with Fmoc-Cl gave compound (+/-)-2 which was enantioseparated on all the investigated CSPs and allowed enhanced UV or fluorimetric detection. The Chiralpak IB, that could be considered the immobilized version of Chiralcel OD-H, was found completely ineffective in the chiral recognition of (+/-)-1 and moderately efficient in the separation of (+/-)-2.     

Direct HPLC enantioseparation of chiral aptazepine derivatives on coated and immobilized polysaccharide-based chiral stationary phases

The direct HPLC enantioseparation of Mianserin and a series of aptazepine derivatives is accomplished on polysaccharide-based chiral stationary phases (CSPs). The resolutions are performed on the coated-type Chiralcel OD and Chiralpak AD CSPs and on the first commercially available immobilized-type Chiralpak IA CSP, in normal-phase and polar-organic modes. The complete separation of enantiomers of all racemates investigated was successfully achieved under at least one of CSP/eluent combinations employed. Pure alcohols such ethanol or 2-propanol, with a fixed percentage of DEA added, serve as valuable alternatives to the more common n-hexane-based normal-phase eluents in resolution of Mianserin on the AD CSP. In order to study the chiroptical properties of aptazepine derivatives, chromatographic resolutions are carried out at semipreparative scale using Chiralpak AD and Chiralpak IA as CSPs. Nonconventional dichloromethane-based eluents have permitted to expand the chiral resolving ability of the immobilized Chiralpak IA CSP and to perform mg-scale enantioseparations with an analytical-size column. Assignment of the absolute configuration of the separated enantiomers is empirically established by comparing their chiroptical data with those of structurally related Mianserin.     

Enantiomeric resolution,thermodynamic parameters,and modeling of clausenamidone and neoclausenamidone on polysaccharide‐based chiral stationary phases

The aim of the paper is to describe a new synthesis route to obtain synthetic optically active clausenamidone and neoclausenamidone and then use high‐performance liquid chromatography (HPLC) to determine the optical purities of these isomers. In the process, we investigated the different chromatographic conditions so as to provide the best separation method. At the same time, a thermodynamic study and molecular simulations were also carried out to validate the experimental results; a brief probe into the separation mechanism was also performed. Two chiral stationary phases (CSPs) were compared with separate the enantiomers. Elution was conducted in the organic mode with n‐hexane and iso‐propanol (IPA) (80/20 v/v) as the mobile phases; the enantiomeric excess (ee) values of the synthetic R‐clausenamidone and S‐clausenamidone and R‐neoclausenamidone and S‐ neoclausenamidone were higher than 99.9%, and the enantiomeric ratio (er) values of these isomers were 100:0. Enantioselectivity and resolution (α and Rs, respectively) levels with values ranging from 1.03 to 1.99 and from 1.54 to 17.51, respectively, were achieved. The limits of detection and quantitation were 3.6 to 12.0 and 12.0 to 40.0 ug/mL, respectively. In addition, the thermodynamics study showed that the result of the mechanism of chiral separation was enthalpically controlled at a temperature ranging from 288.15 to 308.15 K. Furthermore, docking modeling showed that the hydrogen bonds and π‐π interactions were the major forces for chiral separation. The present chiral HPLC method will be used for the enantiomeric resolution of the clausenamidone derivatives.     

Enantioselective potential of polysaccharide‐based chiral stationary phases in supercritical fluid chromatography

The enantioselective potential of two polysaccharide‐based chiral stationary phases for analysis of chiral structurally diverse biologically active compounds was evaluated in supercritical fluid chromatography using a set of 52 analytes. The chiral selectors immobilized on 2.5 μm silica particles were tris‐(3,5‐dimethylphenylcarmabate) derivatives of cellulose or amylose. The influence of the polysaccharide backbone, different organic modifiers, and different mobile phase additives on retention and enantioseparation was monitored. Conditions for fast baseline enantioseparation were found for the majority of the compounds. The success rate of baseline and partial enantioseparation with cellulose‐based chiral stationary phase was 51.9% and 15.4%, respectively. Using amylose‐based chiral stationary phase we obtained 76.9% of baseline enantioseparations and 9.6% of partial enantioseparations of the tested compounds. The best results on cellulose‐based chiral stationary phase were achieved particularly with propane‐2‐ol and a mixture of isopropylamine and trifluoroacetic acid as organic modifier and additive to CO₂, respectively. Methanol and basic additive isopropylamine were preferred on amylose‐based chiral stationary phase. The complementary enantioselectivity of the cellulose‐ and amylose‐based chiral stationary phases allows separation of the majority of the tested structurally different compounds. Separation systems were found to be directly applicable for analyses of biologically active compounds of interest.     

High-performance liquid chromatographic enantioseparation of 1-(aminoalkyl)-2-naphthol analogs on polysaccharide-based chiral stationary phases

The enantiomers of various 1-(alpha-aminobenzyl)-2-naphthol and 1-(aminoalkyl)-2-naphthol analogs were separated on cellulose-tris-3,5-dimethylphenyl carbamate-based chiral stationary phases (Chiralcel OD-H and Chiralcel OD-RH), using n-hexane/2-propanol/diethylamine or phosphate buffer/organic modifier mobile phases. The 3,5-dimethylphenyl carbamoylated cellulose columns were effective in both normal and rev ersed-phase modes. The effects of the mobile phase composition, the pH, the buffer concentration, and the structures of the substituents on the 2-naphthol on the enantioseparations were studied. The absolute configuration and elution sequence were determined for 1-(1-amino-2-methylpropyl)-2-naphthol: the elution sequence was S < R.     

Gas chromatographic separation of PCB atropisomers on cyclodextrin stationary phases

Gas chromatographic study on chiral separation of PCBs was performed in a series of capillary columns coated with 0.1-μm film of modified cyclodextrin (CD) stationary phases. The preparation of columns included the investigation into the effect of the content of cyclodextrin derivative in polysiloxane, the type of polysiloxane and temperature of analysis on the quality of separation and retention of atropisomers of 15 selected PCB congeners. The separation properties towards PCBs of stationary phase heptakis(2,3-di-O-methyl-6-O-tert-butyl-dimethylsilyl)-β-CD dissolved in SE-30, SE-54, and OV-1701, were compared with those of 6-monokis-octamethylene-permethyl-β-CD anchored to polydimethylsiloxane polymer (ChirasilDex column, Chrompack, Middelburg, The Netherlands) and octakis(2,6-di-O-methyl-3-O-pentyl)-γ-CD in OV-1701 (MEGA, Legnano (MI), Italy). The correctness of quantitative enantiomer ratio determination was assesed by splitless analysis of PCBs reference solutions in concentration of 1.25–125 ng/ml (PCBs 45 and 91) and 2.5–250 ng/ml (PCB 95) (the PCB congeners are numbered according to IUPAC). Chirality 10:540–547, 1998. © 1998 Wiley-Liss, Inc.     

Separation of nicotine and nornicotine enantiomers via normal phase HPLC on derivatized cellulose chiral stationary phases

This paper describes the enantiorecognition of (±)nicotine and (±)nornicotine by high-performance liquid chromatography using two derivatized cellulose chiral stationary phases (CSPs) operated in the normal phase mode. It was found that different substituents linked to the cellulose backbone significantly influence the chiral selectivity of the derivatized CSP. The results showed that, in general, the tris(4-methylbenzoyl) cellulose CSP (Chiralcel OJ) surpasses tris(3,5-dimethylphenyl carbamoyl) cellulose CSP (Chiralcel OD). On the former column, the resolution (±)nicotine and (±)nornicotine enantiomers depended largely on mobile phase compositions. For the separation of the nicotine enantiomers, the addition of trifluoroacetic acid to a 95:5 hexane/alcohol mobile phase greatly improved the enantioresolution, probably due to enhanced hydrogen bonding interactions between the protonated analytes and the CSP. For (±)nornicotine separation, a reduction in the concentration of alcohol in the mobile phase was more effective than the addition of trifluoroacetic acid. Possible solute-mobile phase-stationary phase interactions are discussed to explain how different additives in the mobile phase and different substituents on the cellulose glucose units of the CSPs affect the separation of both pairs of enantiomers. Chirality 10:364–369, 1998. Published 1998 Wiley-Liss, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Normal phase chiral HPLC of methylphenidate: comparison of different polysaccharide-based chiral stationary phases.

A comparison of the enantiomeric resolution of (+/-)-threo-methylphenidate (MPH) (Ritalin) was achieved on different polysaccharide based chiral stationary phases. The mobile phase used was hexane-ethanol-methanol-trifluoroacetic acid (480:9.75:9.75:0.5, v/v/v/v). Benzoic acid and phenol were used as the mobile phase additives for the enantiomeric resolution of MPH on Chiralcel OB column only. The alpha values for the resolved enantiomers were 1.34, 1.29, 1.30, and 1.24 on Chiralpak AD, Chiralcel OD, Chiralcel OB (containing 0.2 mM benzoic acid in mobile phase), and Chiralcel OB (containing 0.2 mM phenol in mobile phase) columns, respectively. The R(s) values were 1.82, 1.53, 1.19, and 1.10 on Chiralpak AD, Chiralcel OD, Chiralcel OB (containing 0.2 mM benzoic acid in mobile phase), and Chiralcel OB (containing 0.2 mM phenol in mobile phase), respectively. The role of benzoic acid and phenol as mobile phase additives is discussed.