Studied On The Chiral Stationary Phases Drived From Acylated And Alkyloxyformylated Chitosan Derivatives

Coating type chiral stationary phases(CSPs)based on polysaccharide derivatives show good chiral separation ability and high loading capacity,therefore being very essential for chiral high-performance liquid chromatography(HPLC).Some coating type CSPs derived from cellulose and amylose derivatives,such as cellulose tris(3,5-dimethylphenylcarbamate)(CDMPC)and amylase tris(3,5-dimethylphenylcarbamate)(ADMPC)have already been commercialized.But the applications of these CSPs are limited because the polysaccharide derivatives can dissolve in some organic solvents such as tetrahydrofuran,chloroform and ethyl acetate etc.,or highly swell in these solvents.Thus,these organic solvents can not be used as mobile phases,and the coating type CSPs based on these polysaccharide derivatives have a lower durability.However,if the polysaccharide derivatives bear good tolerability towards organic solvents,the corresponding CSPs will be obviously advantageous,because the solubility of chiral compounds and the degree of separation are closely related to organic solvents.If a chiral stationary phase can be used with more mobile phases consisting of organic solvents,its application will be widened.The goal of this thesis is to resolve this problem that restricts the application of polysaccharide derivatives CSPs.Hence,chitosan of ultrahigh deacetylation degree(DS)was derivatized with different groups for amino and hydroxyl groups,the resulting derivatives were then coated on 3-aminopropyl silica gel yielding new CSPs.The enantioseparation capability,tolerability to organic solvent and stability were evaluated,and the chiral recognition mechanism was preliminarily discussed.The works in this thesis are summarized as follows:(1)Chitosan with various molecular weights and ultrahigh DS was prepared from natural chitin in aqueous and alcoholic solutions of sodium hydroxide under different reaction conditions.The chitosan was derivatized twice by an excess of ethyl chloroformate,amyl chloroformate,isopropyl chloroformate and benzyl chloroformate yielding a series of chitosan alkoxyformamide derivatives that were employed as the intermediates for chiralselectors of new CSPs.These chitosan alkoxyformamide derivatives were characterized by1 H NMR,IR and elemental analysis confirming the amino groups on chitosan were selectively and almost completely alkoxyformylated.(2)The chitosan alkoxyformamides were dissolved in the solution of lithium chloride in N,N-dimethylacetamide(Li Cl/DMAc)and were further modified with an excess of4-methylphenyl isocyanate,affording four chitosan bis(4-methylphenylcarbamate)-(alkoxyformamide)derivatives.Four new CSPs were obtained by coating these derivatives on3-aminopropyl silica gel.The enantioseparation capability of the CSPs was investigated by HPLC.It was found that the enantioseparation capability of these CSPs was generally equivalent to that of the CSP of CDMPC.The chiral selectors with small size substituents on the C-2,such as ethyl and isopropyl showed better separation performance.The tolerability of the CSP with the best enantioseparation capability against organic solvents was studied.The results reveal that the CSP could be analyzed with a mobile phase of100% chloroform,or 100% ethyl acetate,therefore demonstrating good tolerance towards common organic solvents.(3)Chitosans of different molecular weights(Mv: 3.5×105,4.2×105 and 5.3×105)and ultrahigh of DS were modified with an excess of valeric anhydride affording completely N-acylated chitosan derivatives,which were then derivatized with the phenyl isocyanates with different substituents.These chitosan bis(arylcarbamate)-(valerylamide)derivatives.The derivatives were then coated on 3-aminopropyl silica gel to obtain new CSPs.Chromatographic separation results exhibited most CSPs possessed excellent enantioseparation capability.Under the same chromatographic conditions,the chiral recognition capabilities of these CSPs were generally equivalent to or even better than those of the CSPs of CDMPC and ADMPC.These chiral selectors were insoluble in most organic solvents,yet swelled or slightly dissolved in THF at different extent.The corresponding CSPs could be analyzed with a mobile phase of 100% ethyl acetate,or 70%THF.(4)Two CSPs were prepared from n-valerylated chitosans of two different molecularweights and 3,5-dichloropheny isocyanate.It was observed that the CSP with the chiral selector of lower molecular weight showed better enantioseparation ability.Compared the IR spectra of these two derivatives,the absorbance of carbonyl group at C-2 on low molecular weight derivative was found to be obviously stronger than the one on high molecular weight,suggesting that the chiral selector of lower molecular weight possessed a more regular suprastructure and further improving the resolution of chiral samples.Moreover,for the chiral selector of low molecular weight,the absorbance of carbonyl groups on C-3 and C-6 splitted,indicating that these carbonyl groups existed in two forms,i.e.free and bonded forms.The free form of C=O would result in more chiral recognitions,while the bonded form of C=O would benefit the improvement in resolution.These observations were consistent with the enantioseparation results.In contrast,the absorbance of amides corresponding to free and bonded forms in the chiral selectors of lower and higher molecular weights did not obviously change.Therefore,the prediction of enantioseparation based on the absorbance of the carbonyl group at C-2 position is more helpful than based on the absorbance of the amides at C-3 and C-6 positions.In summary,the CSPs derived from chitosan of ultrahigh DS not only exhibited high enantioseparation performance,but also could work in mobile phases containing most organic solvents.Practical range of mobile phases was greatly broadened by the CSPs prepared in this thesis,revealing prospective potential application foreground.In addition,it was found that molecular weight of chitosan significantly affected the enantioseparation performance of related CSPs.