HPLC with cellulose Tris (3,5‐DimethylPhenylcarbamate) chiral stationary phase: Influence of coating times and coating amount on chiral discrimination

Coating cellulose tris (3,5‐dimethylphenylcarbamate) (CDMPC) on silica gels with large pores have been demonstrated as an efficient way for the preparation of chiral stationary phase (CSP) for high‐performance liquid chromatography (HPLC). During the process, a number of parameters, including the type of coating solvent, amount of coating, and the method for subsequent solvent removing, have been proved to affect the performance of the resultant CSPs. Coating times and the concentration of coating solution, however, also makes a difference to CSPs' performance by changing the arrangement of cellulose derivatives while remaining the coating amount constant, have much less been studied before, and thereby, were systematically investigated in this work. Results showed that CSPs with more coating times exhibited higher chiral recognition and column efficiency, suggesting that resolution was determined by column efficiency herein. Afterwards, we also investigated the effect of coating amount on the performance of CSPs, and it was shown that the ability of enantio‐recognition did not increase all the time as the coating amount; and four of seven racemates achieved best resolution when the coating amount reached to 18.37%. At the end, the reproducibility of CDMPC‐coated CSPs were further confirmed by two methods, ie, reprepared the CSP‐0.15‐3 and reevaluated the effect of coating times.     

Enantioseparation by HPLC using phenylcarbonate, benzoylformate, p-toluenesulfonylcarbamate, and benzoylcarbamates of cellulose and amylose as chiral stationary phases

Phenylcarbonate, benzoylformate, and p-toluenesulfonylcarbamate of cellulose and five new benzoylcarbamate derivatives of both cellulose and amylose were synthesized and their chiral recognition abilities were evaluated as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). Cellulose benzoylcarbamate has a higher chiral recognition ability compared to phenylcarbonate, p-toluenesulfonylcarbamate, and benzoylformate of cellulose. The benzoylcarbamate derivatives exhibited a characteristic chiral recognition for the racemates, which bear a hydrogen atom capable of hydrogen bonding to the carbonyl group of the benzoylcarbamates. The structures of the benzoylcarbamates were investigated by CD spectroscopy.     

Immobilization and chromatographic evaluation of novel regioselectively substituted amylose‐based chiral packing materials for HPLC

The regioselectively substituted amylose derivatives bearing a 4‐tert‐butylbenzoate or 4‐chlorobenzoate group at 2‐position, and 3,5‐dichlorophenylcarbamate and a small amount of 3‐(triethoxysilyl)propylcarbamate groups at 3‐ and 6‐positions were synthesized by a two‐step process based on the esterification of 2‐position of a glucose unit. The obtained derivatives were effectively immobilized onto macroporous silica gel by intermolecular polycondensation of triethoxysilyl groups. Their chiral recognition abilities were evaluated as chiral packing materials (CPMs) for high‐performance liquid chromatography. These CPMs showed high chiral recognition as well as the conventional coated‐type CPM, and can be used with the eluents‐containing chloroform and tetrahydrofuran. With the extended use of these eluents, improvement of chiral recognition and reversed elution orders were realized. For some racemates, the immobilized CPM exhibited ability comparable or better to the commercial immobilized amylose‐ or cellulose‐based columns, Chiralpak IA, IB, and IC. Chirality, 2011. © 2011 Wiley‐Liss, Inc.     

Preparation and evaluation of regioselectively substituted amylose derivatives for chiral separations

Six novel regioselectively substituted amylose derivatives with a benzoate at 2‐position and two different phenylcarbamates at 3‐ and 6‐positions were synthesized and their structures were characterized by ¹H nuclear magnetic resonance (NMR) spectroscopy. Their enantioseparation abilities were then examined as chiral stationary phases (CSPs) for high‐performance liquid chromatography (HPLC) after they were coated on 3‐aminopropyl silica gels. Investigations indicated that the substituents at the 3‐ and 6‐positions played an important role in chiral recognition of these amylose 2‐benzoate serial derivatives. The derivatives demonstrated characteristic enantioseparation and some racemates were better resolved on these derivatives than on Chiralpak AD, which is one of the most efficient CSPs, utilizing coated amylose tris(3,5‐dimethylphenylcarbamate) as the chiral selector. Among the derivatives prepared, amylose 2‐benzoate‐3‐(phenylcarbamate/4‐methylphenylcarbamate)‐6‐(3,5‐dimethylphenylcarbamate) exhibited chiral recognition abilities comparable to that of Chiralpak AD and may be useful CSPs in the future. The effect of mobile phase on chiral recognition was also studied. In general, with the decreased concentration of 2‐propanol, better resolutions were obtained with longer retention times. Moreover, when ethanol was used instead of 2‐propanol, poorer resolutions were often achieved. However, in some cases, improved enantioselectivity was achieved with ethanol rather than 2‐propanol as the mobile phase modifier.     

Chromatographic enantioseparation by cycloalkylcarbamate derivatives of cellulose and amylose

Cyclopentyl and (+/-)-exo-2-norbornylcarbamates of cellulose and amylose were prepared and their chiral recognition abilities as chiral stationary phases for high-performance liquid chromatography (HPLC) were evaluated. Among these carbamates, cellulose tris(cyclopentylcarbamate) and amylose tris((+/-)-exo-2-norbornylcarbamate) showed particularly high chiral recognition, which is comparable to that of several well-known phenylcarbamate derivatives. The chiral recognition mechanism of cellulose tris(cyclohexylcarbamate), which was previously found to be an effective chiral stationary phase for HPLC, was investigated using NMR spectroscopy. The derivative dissolved in chloroform exhibited the chiral discrimination of several enantiomers in NMR as well as in HPLC. For example, the 1,1'-bi-2-naphthol enantiomers were distinctly discriminated in the (1)H, (13)C, and 2D-NOESY spectra.     

Synthesis and HPLC chiral recognition of regioselectively carbamoylated cellulose derivatives

Four regioselective-carbamoylated cellulose derivatives having two different substituents at 2-, 3-, and 6-position were prepared and evaluated as chiral stationary phases (CSPs) for high-performance liquid chromatography. Investigations showed that the nature and arrangement of the substituents significantly influenced the chiral recognition abilities of the heterosubstituted cellulose derivatives and each derivative exhibited characteristic enantioseparation. Some racemates were better resolved on these derivatives than the corresponding homogeneously substituted cellulose derivatives including a commercial CSP, Chiralcel OD. Racemic compounds shown in this study were most effectively discriminated on cellulose 2,3-(3-chloro-4-methylphenylcarbamate)-6-(3,5-dimethylphenylcarbamate) and 2,3-(3,5-dimethylphenylcarbamate)-6-(3-chloro-4-methylphenylcarbamate).     

HPLC and SFC enantioseparation of (±)‐Corey lactone diol: Impact of the amylose tris‐(3,5‐dimethylphenylcarbamate) coating amount on chiral preparation

As an important intermediate of prostaglandins and entecavir, optically pure Corey lactone diol (CLD) has great value in the pharmaceutical industry. In this work, the enantioseparation of (±)‐CLD was evaluated using high‐performance liquid (HPLC) and supercritical fluid chromatography (SFC). In HPLC, the separations of CLD enantiomers on polysaccharide‐based chiral stationary phases with both normal phase and polar organic phase were screened. And the conditions for the enantioseparation were optimized in HPLC and SFC, including the selection of mobile phase, temperature, back‐pressure, and other conditions. More important, it was found that the chiral resolutions were greatly enhanced by the increase of the coating amount of ADMPC (amylose tris‐(3,5‐dimethylphenylcarbamate)) under both HPLC and SFC conditions, which can lead to the increase of the productivity and the decrease of the solvent consumption. The preparations of optically pure CLD were evaluated on a semi‐preparative (2 × 25 cm) column packed with 30% ADMPC‐coated CSP under HPLC and SFC conditions. Preparative performances in terms of kkd are 1.536 kg racemate/kg CSP/day and 1.248 kg racemate/kg CSP/day in HPLC and SFC, respectively.     

Evaluation of the chiral recognition properties and the column performances of three chiral stationary phases based on cellulose for the enantioseparation of six dihydropyridines by high‐performance liquid chromatography

Separations of six dihydropyridine enantiomers on three commercially available cellulose‐based chiral stationary phases (Chiralcel OD‐RH, Chiralpak IB, and Chiralpak IC) were evaluated with high‐performance liquid chromatography (HPLC). The best enantioseparation of the six chiral drugs was obtained with a Chiralpak IC (250 × 4.6 mm i.d., 5 μm) column. Then the influence of the mobile phase including an alcohol‐modifying agent and alkaline additive on the enantioseparation were investigated and optimized. The optimal mobile phase conditions and maximum resolution for every analyte were as follows respectively: n‐hexane/isopropanol (85:15, v /v) for nimodipine (R  = 5.80) and cinildilpine (R  = 5.65); n‐hexane/isopropanol (92:8, v /v) for nicardipine (R  = 1.76) and nisoldipine (R  = 1.92); and n‐hexane/isopropanol/ethanol (97:2:1, v /v/v) for felodipine (R  = 1.84) and lercanidipine (R  = 1.47). Relative separation mechanisms are discussed based on the separation results, and indicate that the achiral parts in the analytes' structure showed an important influence on the separation of the chiral column.     

Reversed-phase liquid chromatographic enantioseparation by cycloalkylcarboxylates of cellulose and amylose

Three novel cycloalkylcarboxylates, cyclopentyl, cyclohexyl, and 1-adamantylcarboxylates of cellulose and amylose were prepared and their chiral recognition abilities as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC) were evaluated using a methanol-water mobile phase. Among these esters, cellulose tris(cyclohexylcarboxylate) showed a relatively high chiral recognition ability. The 1-adamantylcarboxylates of cellulose and amylose showed dissimilar chiral recognition abilities from the other two, probably due to the low degree of substitution and the high hydrophobicity of this group.     

Preparation and chromatographic performance of a multifunctional immobilized chiral stationary phase based on dialdehyde microcrystalline cellulose derivatives

A novel high‐performance liquid chromatography (HPLC) multifunctional immobilized chiral stationary phase was prepared by bonding dialdehyde microcrystalline cellulose to aminosilica via Schiff base reaction and then derivatized with 3,5‐dimethylphenyl isocyanate. The HPLC multifunctional immobilized chiral stationary phase could not only achieve chiral separation but also achieve achiral separation. Chiral separation evaluation showed that 1‐(1‐naphthyl)ethanol and mandelonitrile got separation in normal phase (NP) mode. Ranolazine, benzoin ethyl ether, metalaxyl, and diclofop were successfully separated in reversed phase (RP) mode. Aromatic compounds such as polycyclic aromatic hydrocarbons (PAHs), anilines, and aromatic acids were selected as analytes to investigate the achiral separation performance of the multifunctional immobilized chiral stationary phase in NP and RP modes. The achiral separation evaluation showed that six PAHs could get good separation within 10 minutes in NP mode. Four aromatic acids were well separated in RP mode. The retention mechanism of aromatic compounds on the stationary phase was discussed, founding that π‐π interaction, π‐π electron‐donor‐acceptor (EDA) interaction, and hydrogen bonding interaction played important roles during the achiral separation process. This multifunctional immobilized chiral stationary phase had the advantages of simple bonding steps, short reaction time, and no need for space arm.     

Functionalities tuned enantioselectivity of phenylcarbamate cyclodextrin clicked chiral stationary phases in HPLC

The mixed chloro‐ and methyl‐ functionalities can greatly modulate the enantioselectivities of phenylcarbamate cyclodextrin (CD) clicked chiral stationary phases (CSPs). A comparison study is herein reported for per(4‐chloro‐3‐methyl)phenylcarbamate and per(2‐chloro‐5‐methyl)phenylcarbamate β‐CD clicked CSPs (i.e., CCC4M3‐CSP and CCC2M5‐CSP). The enantioselectivity dependence on column temperature was studied in both normal‐phase and reversed‐phase mode high performance liquid chromatography (HPLC). The thermodynamic study revealed that the stronger intermolecular interactions can be formed between CCC4M3‐CSP and chiral solutes to drive the chiral separation. The higher enantioselectivities of CCC4M3‐CSP were further demonstrated with the enantioseparation of 17 model racemates in HPLC.     

Crystallographic studies for the chiral recognition of the novel chiral stationary phase derived from (+)‐(R)‐18‐crown‐6 tetracarboxylic acid

Chiral discrimination observed in high‐performance liquid chromatography (HPLC) with the novel chiral stationary phase (CSP‐18C6I) derived from (+)‐(R)‐18‐crown‐6 tetracarboxylic acid [(+)‐18C6H₄] was investigated by X‐ray crystallographic analysis of the complex composed of the R‐enantiomer of 1‐(1‐naphthyl)ethylamine (1‐NEA) and (+)‐18C6H₄. Mixtures of 1‐NEA (the R‐ or S‐enantiomer) and (+)‐18C6H₄ were dissolved in methanol‐water (1:1) solution and allowed to stand for crystallization. The R‐enantiomer crystallized with (+)‐18C6H₄ as a co‐crystal, although the S‐enantiomer did not. This result was in good agreement with the enantiomer elution order of 1‐NEA in CSP‐18C6I. The apparent binding constants (K_a) of the enantiomers to the (+)‐18C6H₄ obtained from ¹H‐NMR experiments also supported the above‐mentioned result. The X‐ray crystal structure of the 1:1 complex of the R‐enantiomer and (+)‐18C6H₄ indicated the four sets of hydrogen bond association between the naphthylethylammonium cation and oxygen of polyether ring or carbonyl group of (+)‐18C6H₄. Chirality 11:173–178, 1999. © 1999 Wiley‐Liss, Inc.     

Optical resolution of glycerin sulfides and glycerin selenides on the chiral stationary phase of cellulose‐tris(3,5‐dimethylphenylcarbamate)

Cellulose‐tris(3,5‐dimethylphenylcarbamate) was prepared after a reported method and was coated onto an aminopropylated mesopore spherical silica gel. The final product was used as a chiral stationary phase of high performance liquid chromatography for the enantioseparation of a series of glycerin sulfides and glycerin selenides. Mixtures of hexane and 2‐propanol were used as mobile phases. The effects of 2‐propanol concentration in the mobile phase on the retention and resolution were investigated. Some enantiomers of the glycerin monosulfides and monoselenides could be separated satisfactorily, but none of the disulfides could be separated. The structural features of the solutes that influence chiral separation were discussed. Chirality 11:598–601, 1999. © 1999 Wiley‐Liss, Inc.     

Enantioseparation and molecular modeling study of chiral amines as three naphthaldimine derivatives using amylose or cellulose trisphenylcarbamate chiral stationary phases

This study describes the enantioseparation of three chiral amines as naphthaldimine derivatives, using normal phase HPLC with amylose and cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phases (CSPs). Three chiral amines were derivatized using three structurally similar naphthaldehyde derivatizing agents, and the enantioselectivity of the CSPs toward the derivatives was examined. The degree of enantioseparation and resolution was affected by the amylose or cellulose-derived CSPs and aromatic moieties as well as a kind of chiral amine. Especially, efficient enantiomer separation was observed for 2-hydroxynapthaldimine derivatives on cellulose-derived CSPs. Molecular docking studies of three naphthaldimine derivatives of leucinol on cellulose tris(3,5-dimethylphenylcarbamate) were performed to estimate the binding energies and conformations of the CSP–analyte complexes. The obtained binding energies were in good agreement with the experimentally determined enantioseparation and elution order.     

Preparation and enantioseparation of a mixed selector chiral stationary phase derived from benzoylated tartaric acid and 1,2‐diphenylethylenediamine

L ‐Dibenzoyl tartaric acid was mono‐esterified with benzyl alcohol, and then chlorinated with SOCl₂ to give (2S,3S)‐1‐(benzyloxy)‐4‐chloro‐1,4‐dioxobutane‐2,3‐diyl dibenzoate (Selector 1 ). (1R,2R)‐1,2‐Diphenylethylenediamine was mono‐functionalized with phenyl isocyanate and phenylene diisocyanate in sequence to give (1R,2R)‐1,2‐diphenyl‐2‐(3‐phenylureido)ethyl 4‐ isocyanatophenylurea (Selector 2 ). Two brush‐type chiral stationary phases (CSPs) of single selector were prepared by separately immobilizing selectors 1 and 2 on aminated silica gel. Selectors 1 and 2 were simultaneously immobilized on aminated silica gel to give a mixed selector CSP. The enantioseparation ability of these CSPs was studied. The CSP of selector 1 has strongest separation ability, while the enantioseparation ability of the mixed selector CSP is relatively lower. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

A 3D Homochiral MOF [Cd2(d‐cam)3]•2Hdma•4dma for HPLC Chromatographic Enantioseparation

Up to now, some chiral metal‐organic frameworks (MOFs) have been reported for enantioseparation in liquid chromatography. Here we report a homochiral MOF, [Cd2(d‐cam)3]·2Hdma·4dma, used as a new chiral stationary phase for high‐performance liquid chromatographic enantioseparation. Nine racemates of alcohol, naphthol, ketone, and base compounds were used as analytes for evaluating the separation properties of the chiral MOF packed column. Moreover, some effects such as mobile phase composition, column temperature, and analytes mass for separations on this chiral column also were investigated. The relative standard deviations for the resolution values of run‐to‐run and column‐to‐column were less than 2.1% and 3.2%, respectively. The experimental results indicate that the homochiral MOF offered good recognition ability, which promotes the application of chiral MOFs use as stationary phase for enantioseparation. Chirality 28:340–346, 2016. © 2016 Wiley Periodicals, Inc.     

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 (α) 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. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

New chiral stationary phases for liquid chromatography based on small molecules: Development,enantioresolution evaluation and chiral recognition mechanisms

Recently, we reported the development of new chiral stationary phases (CSPs) for liquid chromatography (LC) based on chiral derivatives of xanthones (CDXs). Based on the most promising CDX selectors, 12 new CSPs were successfully prepared starting from suitable functionalized small molecules including xanthone and benzophenone derivatives. The chiral selectors comprising one, two, three, or four chiral moieties were covalently bonded to a chromatographic support and further packed into LC stainless-steel columns (150 × 2.1 mm I.D.). The enantioselective performance of the new CSPs was evaluated by LC using different classes of chiral compounds. Specificity for enantioseparation of some CDXs was observed in the evaluation of the new CSPs. Besides, assessment of chiral recognition mechanisms was performed by computational studies using molecular docking approach, which are in accordance with the chromatographic parameters. X-Ray analysis was used to establish a chiral selector 3D structure.     

Interactions of chiral molecules with an (r)-n-(3,5-dinitrobenzoyl) phenylglycine HPLC stationary phase

Forty different chiral molecules were studied by liquid chromatography with a Pirkle-type, (R)-N-(3,5-dinitrobenzoyl) phenylglycine (DNBPG), chiral stationary phase column. The dramatic effect of a small molecular change on chiral recognition was demonstrated using DL-amino acid derivatives. The inductive effect on chiral recognition was also studied using trifluoro-, trichloro-, dichloro-, monochloroacetyl, and acetyl derivatives of four different chiral amines. The study of the enantiomer separation of 11 different crown ethers of 2,2′-binaphthyldiyl showed that the rigidity of the chiral center can be an additional parameter in chiral recognition for the DNBPG phase but not for a β-cyclodextrin bonded chiral phase. It is apparent from this study that steric effects, inductive effects, and molecular rigidity play important roles in chiral recognition with DNBPG chiral stationary phases.     

Synthesis of dendrimer‐type chiral stationary phases based on the selector of (1S,2R)‐(+)‐2‐Amino‐1,2‐diphenylethanol derivate and their enantioseparation evaluation by HPLC

In our recent work, a series of dendritic chiral stationary phases (CSPs) were synthesized, in which the chiral selector was L‐2‐(p‐toluenesulfonamido)‐3‐phenylpropionyl chloride (selector I), and the CSP derived from three‐generation dendrimer showed the best separation ability. To further investigate the influence of the structures of dendrimer and chiral selector on enantioseparation ability, in this work, another series CSPs ( CSPs 1‐4 ) were prepared by immobilizing (1S,2R)‐1,2‐diphenyl‐2‐(3‐phenylureido)ethyl 4‐isocyanatophenylcarbamate (selector II) on one‐ to four‐generation dendrimers that were prepared in previous work. CSPs 1 and 4 demonstrated the equivalent enantioseparation ability. CSPs 2 and 3 showed the best and poorest enantioseparation ability respectively. Basically, these two series of CSPs exhibited the equivalent enantioseparation ability although the chiral selectors were different. Considering the enantioseparation ability of the CSP derived from aminated silica gel and selector II is much better than that of the one derived from aminated silica gel and selector I, it is believed that the dendrimer conformation essentially impacts enantioseparation. Chirality, 2010. © 2009 Wiley‐Liss, Inc.