Chiral separation abilities of homogeneously synthesized cellulose 3,5-dimethylphenylcarbamates: Influences of degree of substitution and molecular weight

A series of cellulose 3,5-dimethylphenylcarbamates(CDMPCs) with different degrees of substitution(DS) and degrees of polymerization(DP) were homogeneously synthesized in 1-allyl-3-methylimidazolium chloride(Amim Cl). Then, the CDMPCs were coated on silica gel and used as chiral stationary phases(CSPs), and their chiral recognition abilities for seven racemates were evaluated by high performance liquid chromatography. The results showed that DS and DP of CDMPCs had a great influence on chiral recognition abilities of the CSPs. The CSPs with the DS ≈ 1 gives a low chiral recognition to most racemates. On the contrast, the CSPs with the DS ? 2 exhibited high chiral separation abilities. For example, six racemates could be separated on the CSP with CDMPC of DS ≈ 2(CSP-2). Especially, for the enantioseparation of 1-(2-naphthyl) ethanol and Tr?ger's base, CSP-2 gave the highest separation ability in all of CSPs. On the other hand, when the DP of cellulose was in a range from 39 to 220, the chiral separation abilities of CDMPCs increased as the DP increased. This work demonstrates that the structure of cellulose esters such as DS and DP has important effect on their chiral separation ability, and therefore provides a practical method to design and prepare desirable CSPs for different racemates.     

Effect of Single-walled Carbon Nanotubes on Cellulose Phenylcarbamate Chiral Stationary Phases

Single-walled carbon nanotubes(SWNTs)have a high adsorption ability and nanoscale interactions.Cellulose trisphenylcarbamates possess high enantioseparation ability in high-performance liquid chromatography(HPLC).Single-walled carbon nanotubes mixed with cellulose trisphenylcarbamate are coated on the silica gel as chiral stationary phases and higher enantioseparation factors are obtained.After a single-walled carbon nanotube is linked to the 6-position of cellulose 2,3-bisphenylcarbamate,its enantioseparation resolution increases compared to that of the cellulose trisphenylcarbamate.It is the first time that SWNTs have been applied to enantioseparation.The results indicate that the single-walled carbon nanotubes are good promoters of chiral recognition.This method can be used to improve the enantioseparation efficiency of the polysaccharide chiral stationary phases.     

Influence of Mobile Phase Composition on the Enantioseparation of Methoxyl Flavanones with Self-prepared CDMPC Column and Chiral Recognition Mechanism

Chiral stationary phases (CSPs) based on the cellulose derivatives have proven to be one type of the most useful CSPs in high-performance liquid chromatography because of their versatility, durability and loadability1, 2. Flavanone was often used as a standard chiral compound for evaluating cellulose derivative for chiral columns1, 3. Krause and Galensa4 reported the enantioseparation of flavanone and its seven derivatives on six kinds of commercial chiral column. 4-Methoxyl flavanone, 5-…     

COVALENTLY BONDING CHIRAL POLYURETHANE ON AMINATED SILICA GEL： A NEW STRATEGY TO PREPARE CHIRAL STATIONARY PHASE FOR HIGH PERFORMANCE LIQUID CHROMATOGRAPHY

Two polyurethanes of different molecular weights were prepared by the copolymerization of phenyl diisocyanate and diisopropyl tartrate. The polyurethanes having terminal isocyanate groups were reacted with 3-aminopropyl silica gel to afford two chiral stationary phases. The Mn of the two polyurethanes were 4057 g/mol and 6442 g/mol. The polyurethanes and the corresponding chiral stationary phases were characterized by FT-IR, 1H NMR and elemental analysis. The loading capacities of the polyurethanes on silica gel were 0.68 mmol units/g and 0.61 mmol units/g, respectively. The separation performance and the influence of additives, triethylamine and trichloroacetic acid, on the separation of chiral compounds were investigated by HPLC. The chiral stationary phase prepared from polyurethane with Mn of 4057 g/mol demonstrated better enantioseparation capability than that with Mn of 6442 g/mol. Additionally, it was found that the addition of triethylamine and trichloroacetic acid in the mobile phases significantly improved the enantioseparation for these two chiral stationary phases.     

Enantioseparation of Novel Chiral Tetrahedral Clusters on an Amylosoe Tris-（3,5-dimethylphenylcarbamate） Chiral Stationary Phase by Normal Phase HPLC

Amylose tris(3,5-dimethylphenylcarbamate) (ADMPC) coated on a kind of small particle silica gel was prepared. On this ADMPC chiral stationary phase (CSP), the direct enantiomeric separation of six novel chiral transition metal tetrahedral clusters has firstly been achieved using n-hexane as the mobile phase containing various alcohols as modifiers. The effect of mobile phase modifiers and the structural variation of the solutes on their retention factors (k‘) and resolutions (Rs) were investigated. The result suggests that not only the structure and concentration of alcohol in mobile phase, but also the structural differences in racemates can have a pronounced effect on enantiomeric separation. ADMPC-CSP is a suitable CSP for the optical resolution of chiral tetrahedral cluster by HPLC.     

Enantiomer Separation of α—Dimethyl Dicarboxylate Biphenyl and Related Biphenyl Compounds by Normal Phase HPLC on Polysaccharide Based Chiral Stationary Phases

Cellulose tris(4-methylphenylcarbamate),amylose tris(3,5-dimethylphenylcarbamate) and amylose tris (phenylcarbamate) were prepared by the method reported by Okamoto and were coated onto an aminopropylated mesoporous spherical silica gel.These final products were used as chiral stationary phases of high performance liquid chromatography for the eighteen structurally related biphenyl compounds.The resolution was made using normal-phase methodology with a mobile phase consisting of n-hexane-alcohol(ethanol,1-propanol,2-propanol or 1-butanol).The effects of various aliphatic alcohols in the mobile phase were studied.The structural features of the solutes that influence their k′ were discussed.A dominant effects of trifluoroacetic acid on chiral separation of acidic solutes was noted.     

Sensitive Method for Enantioseparation of Rivastigmine with Highly Sulfated Cyclodextrin as Chiral Selector by Capillary Electrophoresis

A sensitive method for enantioseparation of a basic drug rivastigmine and determination of its optical impurityby capillary electrophoresis with highly sulfated β-cyclodextrin(HS-β-CD)as the chiral selector is described.Ingeneral,enantioseparation of basic chiral compounds is carried out in acidic condition(pH 2.5)to prevent analytesfrom adsorption on the capillary wall.However,in the case of rivastigmine,the detection sensitivity was too limitedto determine the optical impurity of S-rivastigmine lower than 1% when buffer pH was 2.5.It was found that thedetection sensitivity was improved 1.6 times just by raising the buffer pH value from 2.5 to 5.8.The poor columnefficiency due to the adsorption of the analytes on the capillary wall was resolved by using dynamical coating of thecapillary wall with the linear polyacrylamide solution.The experirnental parameters such as the concentration ofHS-β-CD,buffer pH and buffer ionic strength were optimized,respectively.The method was validated in terms ofrepeatability,linearity,limit of detection(LOD)and limit of quantitation(LOQ).Using the present method,the op-tical purity of nonracemic rivastigmine with the enantiomeric excess(ee)value of 99.14% was determined.     

A chiral stationary phase with special hydrophobic framework for ligand-exchange chromatography

A novel chiral stationary phase (CSP1) for ligand-exchange chromatography (CLEC) was prepared by firstly using dimethyl- chlorosilane as an endcapping reagent for decreasing residual silanol group on the surface of silica gel,and then introducing L-Pro as a chiral selector and hydrophobic octyl group to the silica gel surface simultaneously.The enantioseparations of 14 DL-amino acids on CSP1 were achieved with the enantioselectivityαranging from 1.09 to 2.44 and the resolution Rs being between 0.8 and 6.3.The chromatographic performances of CSP1 with the bonded phase (CSP2) prepared using reference method were compared.The results showed that the column efficiency and resolution Rs of chiral stationary phase could be improved by using the above modifying method.     

改进的方法用于制备键合型纤维素－三（苯基氨基甲酸酯）手性固定相

The classical method for preparation of covalently boned cellulose derivative chiral stationary phases (CSP) with diisocyanate as spacer was improved. Diisocyanate was firstly allowed to react with 3-aminopropyltriethoxysilane, and the resulting product was then applied as the spacer reagent to immobilize cellulose derivatives onto silica gel. Influences of the amount and the length of the spacer on the optical resolution ability of the CSP were investigated. Comparing improved procedure to classical diisocyanate method, the cross-linking between the glucose units of the cellulose derivatives was avoided to the most extent. With the improved procedure, regio-nonselective ways could be adooted to prepare covalently bonded CSP, which showed an advantage for the rapid preparation.     

The Direct Resolution of the Enantiomers of Four Chiral Pesticides

Cellulose-tris(3,5-dimethylphenylcarbamate)(CDMPC) were synthesized as highperformance liquid chromatography (HPLC) chiral stationary phase(CSP). The successful resolution of four chiral pesticides on the CSP was realized and the influence of the temperature on the separation was studied.     

Partial Carbamoylation of Cellulose Microspheres:a New Method to Prepare Adsorbents for Liquid Chromatography简

Micron-sized cellulose microspheres were prepared through sol-gel method using NaOH/urea solution to dissolve cellulose, then cross-linked by 1,6-hexanylene diisocyanate （HDI）, toluene 2,4-diisocyanate （TDI） and 1,4-phenylene diisocyanate （PDI）, respectively. The reaction conditions for partial modification of the microspheres were studied. The degree of substitution （DS） in cellulose was controlled by adjusting the reaction conditions. HDI-crosslinked microspheres were partially modified with phenyl isocyanate to obtain chiral stationary phases （CSPs）. The CSPs of a lower degree of crosslinking （DC） showed better chiral recognition ability than those of a higher DC. Meanwhile the CSPs prepared by pre- modification exhibited better chiral recognition ability than those prepared by pre-crosslinking.     

Stereoselective interactions of chiral dipeptides on amylose based chiral stationary phases

Dipeptides are stereo-specifically involved in several biological functions that are challenging to separate enantiomerically. Elution order of enantiomers is an important issue in chiral chromatography. Amylose tris-(3,5-dimethylphenylcarbamate) chiral stationary phase(CSP) is the best and most-widely-used CSP in chiral separations, but experimental data of enantiomeric separation of dipeptides on this CSP is lacking. Simulation studies were conducted to determine the order of elution and the chiral recognition mechanism of didpetides on this CSP. Results indicated that the docking energy of SR-enantiomers were higher than SS-antipodes. The range of docking energies for SR-enantiomers was -7.44 to -5.92 kcal/mol with CSP, but -7.15 to -5.87 kcal/mol for SS-stereoisomers. Therefore it is predicted that SS-enantiomer will elute first, followed by SR-antipode. Furthermore, hydrogen bondings, van der Waal’s interactions and electrostatic interactions were observed among SR- and SSenantiomers and chiral grooves of CSP. The number of hydrogen bonds was one in each enantiomer binding except S-Ala-R-Tyr, which contained two hydrogen bonds. No hydrogen bond was found in S-Ala-R-Trp, S-Leu-S-Trp, and S-Leu-S-Tyr dipeptides bindings. The chiral recognition mechanisms dictate different strengths of stereoselective bindings of the enantiomers on CSP.     

Study of chiral recognition mechanism of O,O-diethyl (p-methylbenzenesulfonamindo)-aryl(alkyl)-methylphosphonates by HPLC with a series of CSPs

Five chiral stationary phases (CSPs) were used to separate the enantiomers of a series of O,O-diethyl (p-methyl-benzenesulfonamindo)- aryl(alkyl)-methylphosphonates. A chiral recognition mechanism was presented to explain the resolution of these compounds. Results show that CSP with strong π-acceptor 3,5-dinitrobenzoyl group and high steric hindrance has the best resolution ability in chiral separation of O,O-diethyi (p-methyl-benzenesulfonamindo)- aryl(alkyl)-methylphosphonates. When a CSP has just a strong π-acceptor 3,5-dinitrobenzoyl or high steric hindrance it does not have good chiral resolution ability. The chiral recognition is more difficult when the CSP has more than one asymmetric center.     

The Enantiomer Separations of Allethrone and Propargyllone Using Two Long Chain Acylated β-Cyclodextrin Derivatives as CGC Capillary Stationary Phases

Using two β-cyclodextrin derivatives (CDs) with long chain of acyl groups as chiral stationary phases (CSPs) of capillary gas chromatography (CGC), the enantiomers of racemic allethrone and propargyllone were well resolved after derived with acetyl chloride. The enantiomer excess values (e.e.%) of 1S-allethrone and 1S-propargyllone were also determined successfully using these CDs.     

Enantioseparation of Racemic Naproxen Esters on Cellulose Tris（4—methylbenzoate） Chiral Stationary Phase

Several kinds of racemic naproxen ester were successfully separated on CTMB chiral stationary phase with hexane-ethanol(98:2,vol./vol.) as the mobile phase. The influence of mobile phase composition and structure of racemic naproxen ester on chiral separation was studied and the chiral recognition mechanism of CTMB was discussed.     

Thin Layer Chromatographic Study of Bauxite and Quantitative Estimation of Co-Existing Al3+, Fe2+ and Ti4

 Thin layer chromatography in combination with spectrophotometry and titrimetry has been used to evaluate chromatographic characteristics of bauxite constituents. The retention behaviors of four major constituents (Al3+, Fe2+, Ti4+, Si4+) in bauxite mineral have been examined on plain and modified layers of silica gel G, silica gel H and cellulose with mobile phases containing aqueous sodium chloride, formic acid and hydrochloric acid. Ternary separation of Al-Fe-Ti was achieved on chromatographic plates made of silica gel H. The pH effect and presence of impurity elements in samples, nature of stationary phases on the ternary separation and detection limits of bauxite constituents were studied. Silicon in bauxite was detected on cellulose plates. Quantitative determinations of Al3+, Fe2+ and Ti4+ on silica gel H impregnated with sodium formate layers were achieved by titrimetry and spectrophotometry.     

Influence of Mobile Phase Composition on the Enantioseparation of Meythoxyl Flavanones with Self—prepared CDMPC Column and Chiral Recogniton Mechanism

The influence of different alcohol modifiers in mobile phase on the chiral separation of 4‘‘‘‘‘‘‘‘-methoxyl flavanone,5-methoxyl flavanone and 6-methoxyl flavanone on cellulose tris(3,5-dimethylphenylcarbamate)(CDMPC)column was studied and the chiral recognition mechanism was discussed.Using hexane-tert-butanol(1.31molL^-1) as the mobile phase,those three methoxyl flavanones were excellently separated on CDMPC chiral column.     

Chiral Borated Esters in Asymmetric Synthesis： 1. The First Asymmetric Reaction Catalyzed by Chiral Spiroborated Esters with an O3BN Framework

The first asymmetric reaction catalyzed by chiral spiroborated esters with an O3BN framework was reported. In the presence of 0.1 equivalent of (R,S)-1 or (S,S)-1, acetophenone was reduced by 0.6 equivalent of borane in THF at 0-5℃ for 2 h to give (R)-1-phenylethanol of up to 76% ee and 73% isolated yield. Influence of reaction conditions on the stereoselectivity of the reduction was investigated and a possible catalytic mechanism of the chiral spiroborated esters toward the reduction was also suggested.     

L—PROLINE—FUNCTIONALIZED POLYSTYRENE WITH 1—METHYLDECYLENE SPACER AS A CHIRAL STATIONARY PHASE FOR LIGAND—EXCHANGE CHROMATOGRAPHIC RESOLUTION OF AMINO ACID RECEMATES

The L-proline-functionalized polystyrene with 1-methyldecylene spacer was synthesized from 2% divinylbenzene-crosslinked polystyrene gel via 10-bromo-1-methyldecyl polystyrene intermediate.After complexed with copper(Ⅱ） ion,the polymer with L-proline ligand was used as the chiral stationary phase (CSP) for ligand exchange chromatography of amino acid racemates.The results showed that the CSP possessed powerful enantioselectivity and all racemates of the fifteen tried amino acids were completely separated.     

超声波技术制备功能化二氧化硅在苯酚和碳酸二甲酯甲基化反应中的应用

Porous silica modified with -（CH2）3NH2 （primary amine）, -（CH2）3NHCH2CH2NH2 （secondary/primary amine） and -（CH2）3N-cycl-（CH2）4 （tertiary amine） were synthesized by ultrasonic technique under mild conditions. The samples were characterized by BET, elemental analysis and TG, showing that the organosilane moieties were grafted onto the surface of porous silica by covalent bond. The structure of the organosilane moieties and ultrasonic treatment time were all significant for the quantities of grafted amino groups. The samples exhibited promising catalytic properties towards the methylation reaction of phenol with dimethyl carbonate （DMC）. The methylation reaction with the modified samples featured high selectivity at high conversion. The samples were subjected to utilization for a few recycles without obvious loss of activity to indicate that ultrasonic technique was effective for the preparation of organically modified porous silica catalysts.