Synthesis of water-soluble multidentate aminoalcohol β-cyclodextrin derivatives via epoxide opening

New highly soluble β-aminoalcohol β-cyclodextrin (β-CD) derivatives have been synthesized via nucleophilic epoxide opening reactions with mono-6-amino mono-6-deoxy-permethyl-β-CD and mono-6-amino mono-6-deoxy-β-CD. The binding properties of the β-CD were enhanced by linking aminoalcohol subunits which caused its solubility to improve markedly. The reaction conditions were optimised using microwave irradiation giving moderate-to-good yields with a series of epoxides. A regioselective epoxide opening reaction was observed in the reaction with styrene oxide while the stereoselectivity was strictly dependent on substrate structure.     

Preparation and evaluation of biselector bonded-type multifunctional chiral stationary phase based on dialdehyde cellulose and 6-monodeoxy-6-monoamino-β-cyclodextrine derivatives

A novel biselector bonded-type multifunctional chiral stationary phase (MCSP) was prepared by covalently crosslinking dialdehyde cellulose (DAC) with 6-monodeoxy-6-monoamino-β-cyclodextrine (CD) via Schiff base reaction. The biselector bonded-type MCSP had good chiral and achiral chromatographic performance in normal phase (NP) and reversed phase (RP) modes. Seven and eight enantiomers were successfully separated on the prepared biselector bonded-type MCSP in NP and RP modes, respectively. The biselector bonded-type MCSP showed enhanced chiral resolution ability compared with single selector chiral stationary phases due to the simultaneous introduction of DAC and 6-monodeoxy-6-monoamino-β-CD on the surface of silica gel. Aromatic compounds including polycyclic aromatic hydrocarbons, anilines, phenols, phenylates, and aromatic acids were choosed as analytes to investigate the achiral chromatographic performance of the biselector bonded-type MCSP in NP and RP modes. Chromatographic evaluation results showed that the above aromatic compounds were essentially capable of achieving baseline separation by hydrophobic interaction, π-π interaction, and π-π electron-donor-acceptor interaction. Moreover, the host-guest inclusion effect of 6-monodeoxy-6-monoamino-β-CD and the multiple interactions made the biselector bonded-type MCSP have good steric selectivity. The preparation method of the biselector bonded-type MCSP was simple and provided a new idea and strategy for the preparation of the subsequent novel biselector MCSP.     

Novel chiral stationary phases based on 3,5-dimethyl phenylcarbamoylated β-cyclodextrin combining cinchona alkaloid moiety

Novel chiral selectors based on 3,5-dimethyl phenylcarbamoylated β-cyclodextrin connecting quinine (QN) or quinidine (QD) moiety were synthesized and immobilized on silica gel. Their chromatographic performances were investigated by comparing to the 3,5-dimethyl phenylcarbamoylated β-cyclodextrin (β-CD) chiral stationary phase (CSP) and 9-O-(tert-butylcarbamoyl)-QN-based CSP (QN-AX). Fmoc-protected amino acids, chiral drug cloprostenol (which has been successfully employed in veterinary medicine), and neutral chiral analytes were evaluated on CSPs, and the results showed that the novel CSPs characterized as both enantioseparation capabilities of CD-based CSP and QN/QD-based CSPs have broader application range than β-CD-based CSP or QN/QD-based CSPs. It was found that QN/QD moieties play a dominant role in the overall enantioseparation process of Fmoc-amino acids accompanied by the synergistic effect of β-CD moiety, which lead to the different enantioseparation of β-CD-QN-based CSP and β-CD-QD-based CSP. Furthermore, new CSPs retain extraordinary enantioseparation of cyclodextrin-based CSP for some neutral analytes on normal phase and even exhibit better enantioseparation than the corresponding β-CD-based CSP for certain samples.     

High-performance liquid chromatographic study of the interactions between immobilized β-cyclodextrin polymers and hydrophobically end-capped polyethylene glycols

The formation of inclusion complexes between polyethylene glycols (PEGs) bearing hydrophobic ends (naphtyl and phenyladamantyl) and β-cyclodextrin polymers (polyβ-CD) immobilized onto silica particles was studied by high-performance liquid chromatography (HPLC). It was shown that hydrophobic interactions were involved in the retention mechanism of these compounds, since retention volumes decreased when organic solvents were added to the mobile phase while it was the contrary in the presence of salts. Moreover, the association could be reversed by adding a competitor (hydroxypropylβ-cyclodextrin) to the mobile phase. A theoretical model permitted the evaluation of affinity constants of 1:1 complexes formed between the modified PEGs and the immobilized polyβ-CD which depended on the type of hydrophobic groups grafted to the PEG.     

Improved functional properties of trypsin modified by monosubstituted amino-β-cyclodextrins

Bovine pancreatic trypsin was chemically modified by several β-cyclodextrin (β-CD) derivatives using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide as coupling agent. The modifying agents used were mono-6-amino-6-deoxy-β-cyclodextrin (CDNH₂), mono-6-ethylenediamino-6-deoxy-β-cyclodextrin (CDEN), mono-6-propylenediamino-6-deoxy-β-cyclodextrin (CDPN) and mono-6-butylenediamino-6-deoxy-β-cyclodextrin (CDBN). The enzyme–cyclodextrin conjugates contained about 2 mol of oligosaccharide per mol of trypsin. The catalytic and thermal stability properties of trypsin were improved by the attachment of cyclodextrin residues, and these effects were markedly noticeable for cyclodextrin (CD) derivatives having an even number of carbon atoms in the spacer arms. The thermostability of the enzyme was increased by about 2.4–14.5 °C after modification. The conjugates prepared were also more stable against thermal incubation at different temperatures ranging from 45 to 60 °C. In comparison with native trypsin, the enzyme–cyclodextrin complexes were markedly more resistant to autolytic degradation at pH 9.0. Attending to the results here reported, we suggest that conjugation of enzymes with β-CD derivatives might be an useful method for improving the stability and the catalytic properties of these biocatalysts.     

Preparation of novel β-cyclodextrin functionalized monolith and its application in chiral separation

A novel β-cyclodextrin (β-CD) functionalized organic polymer monolith was prepared by covalently bonding ethylenediamine-β-CD (EDA-β-CD) to poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate) (poly(GMA-co-EGDMA)) monolith via ring opening reaction of epoxy groups. SEM characterization was performed to confirm the homogeneity of the monolithic polymer. The resulting monolith was then characterized by DSC and XPS elemental analysis to study the thermal stability of the monolith, and to prove the successful immobilization of β-CD on the polymer substrate. The β-CD ligand density of 0.68 mmol g^?1 was obtained for the modified monolith, indicating the high reactivity and efficiency of the EDA-β-CD modifier. The ethylenediamine-β-CD functionalized monoliths were used for the chiral separation of ibuprofen racemic mixture and showed promising results.     

Enantioselective host-guest complexation of Ru(II) trisdiimine complexes using neutral and anionic derivatized cyclodextrins

Enantioselective host-guest complexation between five racemic Ru(II) trisdiimine complexes and eight derivatized cyclodextrins (CDs) has been examined by NMR techniques. The appearance of non-equivalent complexation-induced shifts of between the Δ and Λ-enantionomers of the Ru(II) trisdiimine complexes and derivatized CDs is readily observed by NMR. In particular, sulfobutyl ether-β-cyclodextrin sodium salt (SBE-β-CD), R-naphtylethyl carbamate β-cyclodextrin (RN-β-CD), and S-naphtylethyl carbamate β-cyclodextrin (SN-β-CD) showed good enantiodiscrimination for all five Ru complexes examined, which indicates that aromatic and anionic derivatizing groups are beneficial for chiral recognition. The complexation stoichiometry between SBE-β-CD and [Ru(phen)₃]²⁺ was found to be 1:1 and binding constants reveal that Λ-[Ru(phen)₃]²⁺ binds more strongly to SBE-β-CD than the Δ-enantiomer. Correlations between this NMR method and separative techniques based on CDs as chiral discriminating agents (i.e., selectors) are discussed in detail.     

Direct enantioseparation of mandelic acid by high-performance liquid chromatography using a phenyl column precoated with a small amount of cyclodextrin additive in a mobile phase

Direct enantioseparation of mandelic acid by high-performance liquid chromatography (HPLC) with a reversed phase column and a mobile phase containing a small amount of hydroxylpropyl-β-cyclodextrin (HP-β-CD) was studied as an efficient method for saving consumption of the CD additive. As a result, it was proposed that racemic mandelic acid can be analyzed with a phenyl column by using a mobile phase composed of 10 mM ammonium acetate buffer (pH 4.2) and 0.02% (w/v) HP-β-CD at a flow rate of 1.0 mL/min at 40°C after the passage of 10 mM ammonium acetate buffer (pH 4.2) containing 0.1% (w/v) HP-β-CD as a precoating mobile phase for 60 min. It is suggested that HP-β-CD is bound with a phenyl group on the surface of the stationary phase to allow a phenyl column to act as a transient chiral column, and injected mandelic acid can form the ternary complex with the adsorbed HP-β-CD. The longer retention time of D-mandelic acid than the L-isomer for HPLC can be explained from the higher stability of the HP-β-CD complex with D-mandelic acid, which was confirmed by CE experiment with HP-β-CD as a selector. The efficiency of a phenyl column compared with other stationary phases was also discussed.     

Formulation and Development of Extended-Release Micro Particulate Drug Delivery System of Solubilized Rifaximin

Rifaximin (RFX), a semi-synthetic antibiotic belonging to BCS class IV category, has been used in the treatment of traveler’s diarrhea. An attempt has been made to improve aqueous solubility of RFX in the presence of β-cyclodextrin (β-CD) and hydroxy propyl β-cyclodextrin (HP-β-CD) and control its release in the gut by enteric coating. The stoichiometric proportion of RFX and complexing agent’s β-CD and HP-β-CD were determined by phase solubility studies. RFX–β-CD and RFX–HP-β-CD were prepared in 1:2 ratio by solvent evaporation technique using rota-evaporator with yield of 78% and 84% respectively followed by their evaluation using different techniques such as saturation solubility, Fourier transform infrared, differential scanning calorimeter, powder X-ray diffractometer, in vitro antimicrobial activity. The saturation solubility of RFX had improved from 0.0736 mg/ml to 0.2354 mg/ml and 0.5681 mg/ml in presence of β-CD and HP-β-CD respectively resulting in an increased zone of inhibition in the later complex during antimicrobial studies. The RFX–HP-β-CD complex particles were coated with eudragit L 100 (EL 100) by spray drying technique. The 3² factorial design was applied to formulate the micro particles. All formulations exhibited pH dependant drug release. The % EE was 69% and the release of RFX was retarded by enteric coating in the optimized batch FB2. Therefore, it can be concluded that solubility of some BCS class IV drugs can be improved by β-CD complexation and release of such inclusion complexes can be retarded to increase the residence time of RFX in the gastrointestinal tract.     

Enzymatic synthesis of dimaltosyl-beta-cyclodextrin via a transglycosylation reaction using TreX, a Sulfolobus solfataricus P2 debranching enzyme

Di-O-α-maltosyl-β-cyclodextrin ((G2)₂-β-CD) was synthesized from 6-O-α-maltosyl-β-cyclodextrin (G2-β-CD) via a transglycosylation reaction catalyzed by TreX, a debranching enzyme from Sulfolobus solfataricus P2. TreX showed no activity toward glucosyl-β-CD, but a transfer product (1) was detected when the enzyme was incubated with maltosyl-β-CD, indicating specificity for a branched glucosyl chain bigger than DP2. Analysis of the structure of the transfer product (1) using MALDI-TOF/MS and isoamylase or glucoamylase treatment revealed it to be dimaltosyl-β-CD, suggesting that TreX transferred the maltosyl residue of a G2-β-CD to another molecule of G2-β-CD by forming an α-1,6-glucosidic linkage. When [¹⁴C]-maltose and maltosyl-β-CD were reacted with the enzyme, the radiogram showed no labeled dimaltosyl-β-CD; no condensation product between the two substrates was detected, indicating that the synthesis of dimaltosyl-β-CD occurred exclusively via transglycosylation of an α-1,6-glucosidic linkage. Based on the HPLC elution profile, the transfer product (1) was identified to be isomers of 6¹,6³- and 6¹,6⁴-dimaltosyl-β-CD. Inhibition studies with β-CD on the transglycosylation activity revealed that β-CD was a mixed-type inhibitor, with a K_i value of 55.6 μmol/mL. Thus, dimaltosyl-β-CD can be more efficiently synthesized by a transglycosylation reaction with TreX in the absence of β-CD. Our findings suggest that the high yield of (G2)₂-β-CD from G2-β-CD was based on both the transglycosylation action mode and elimination of the inhibitory effect of β-CD.     

Photoluminescent hyperbranched poly(amido amine) containing β-cyclodextrin as a nonviral gene delivery vector

Hyperbranched poly(amido amine)s (HPAAs) containing different amounts of β-cyclodextrin (β-CD) (HPAA-CDs) were synthesized in one-pot by Michael addition copolymerization of N,N'-methylene bisacrylamide, 1-(2-aminoethyl)piperazine, and mono-6-deoxy-6-ethylenediamino-β-CD. In comparison to pure HPAA, the fluorescence intensity of HPAA-CDs was enhanced significantly while the cytotoxicity became lower. Ascribed to plenty of amino groups and strong photoluminescence, HPAA-CDs could be used as nonviral gene delivery vectors, and the corresponding gene transfection was evaluated. The experimental results indicated that HPAA-CDs condensed the plasmid DNA very well. By utilizing the fluorescent properties of HPAA-CDs, the cellular uptake and gene transfection processes were tracked by flow cytometry and confocal laser scanning microscopy without any fluorescent labeling. The transfection efficiencies of HPAA-CDs were similar to that of pure HPAA. In addition, the inner cavities of β-CDs in HPAA-CDs could be used to encapsulate drugs through host--guest interaction. Therefore, the HPAA-CDs may have potential application in the combination of gene therapy and chemotherapy.     

Explanation of enantioseparation of amino acid derivatives in gas chromatography

The enantioseparation of amino acid derivatives by gas chromatography was investigated through molecular dynamics simulation. The chiral stationary phase was based on permethylated β-cyclodextrin (PM-β-CD). The model enantiomers were four amino acid derivatives. For the inclusion complexes of PM-β-CD with the enantiomers, we studied the binding energy. The competitive binding of l- or d-enantiomers to PM-β-CD was simulated. The interaction energy of the enantiomers with PM-β-CD and the appearing frequency of l- and d-enantiomers around a certain distance from the centre of mass of PM-β-CD were obtained. It was found that the appearing frequency is an important parameter to explain the enantioseparation of the amino acid derivatives in gas chromatography. The appearing frequency of the enantiomer together with the binding and interaction energy can be used to predict the elution orders of the enantiomers in gas chromatography.     

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.     

Synthesis of polymer-type chiral stationary phases and their enantioseparation evaluation by high-performance liquid chromatography

Two new chiral polymers of different molecular weights were synthesized by the copolymerization of (1R,2R)-(+)-1,2-diphenylethylenediamine, phenyl diisocyanate and terephthaloyl chloride. The polymers were immobilized on aminated silica gel to afford two chiral stationary phases. The polymers and the corresponding chiral stationary phases were characterized by Fourier transform-IR, elemental analysis, 1H and 13C NMR. The surface coverages of chiral structural units on the chiral stationary phases were estimated as 0.27 and 0.39 mmol/g, respectively. The enantioseparation ability of these chiral stationary phases was evaluated with a variety of chiral compounds by high-performance liquid chromatography. The effects of the organic additives, the composition of mobile phases, and the injection amount of sample on enantioseparation were investigated. A comparison of enantioseparation ability between these two chiral stationary phases was made. It was believed that the chain length of polymeric chiral selector significantly affected the enantioseparation ability of corresponding chiral stationary phase.     

The comparison in enantioseparation ability of the chiral stationary phases with single and mixed selector--the selectors derived from two D-tartrates

(2S,3S)-2,3-Bis(3,5-dimethylphenylcarbonyloxy)-3-(benzyloxycarbonyl)-propanoic acid and (2S,3S)-2,3-bis(1-naphthalenecarbonyloxy)-3-(benzyloxycarbonyl)-propanoic acid were synthesized from D-tartaric acid. These two compounds were chlorinated to afford two chiral selectors for chiral stationary phases (CSPs). The selectors were separately immobilized on aminated silica gel to give two single selector CSPs; and were simultaneously immobilized to obtain a mixed selector CSP. Comparing to the single selector CSPs, the mixed selector CSP bears the enhanced enantioseparation ability, suggesting that the two selectors in the mixed selector CSP are consistent for chiral recognition in most mobile phase conditions.     

Inducing enantioselective crystallization with and self-assembly of star-shaped hybrid polymers prepared via “grafting to” strategy

Helical polymers present some interesting and distinctive properties, and one of the most distinguished applications of them is the chiral recognition and resolution of enantiomers. In this work, star-shaped hybrid helical poly (phenyl isocyanide) (PPI) with polyhedral oligomeric silsesquioxanes (POSS) as the core was designed and synthesized by “grafting to” strategy. The homoarm star-shaped hybrid POSS-(PPI)₈ was first obtained by the click reaction between azide-modified POSS (POSS-(N₃)₈) and alkynyl-modified PPI (PPI-Alkynyl). The hybrid POSS-(PPI)₈ was with predominated left-handed helical conformation and exhibited excellent ability in the enantioselective crystallization of racemic compounds. In the meantime, heteroarm star-shaped hybrid (PEG)₄-POSS-(PPI)₄ was prepared by the click reaction of POSS-(N₃)₈ with PPI-Alkynyl and alkynyl-modified poly (ethylene glycol) (PEG-Alkynyl). The hybrid (PEG)₄-POSS-(PPI)₄ was amphiphilic, and it could self-assemble to form spherical micelles in aqueous solutions.     

Enantioselective influence of cyclodextrins on cleavage of chiralic esters

Assessing the reactivity of optical antipodes is of central importance in drug research. Using the model of 2-methoxy-2-phenylacetic acid-4-nitrophenylester (MPE), the rate of hydrolysis in the presence of β-cyclodextrin (CD), hydroxyethyl- and hydroxypropyl-β-CD, as well as methyl-β-CD is studied photometrically and by means of HPLC (Chiralcel-OD-R-column). Both β-CD and hydroxyalkylated-β-CD catalyze (?)-(R)-enantiomers to a larger extent than (+)-(S)-enantiomers, resulting in an enrichment of the latter. Methyl-β-CD stabilizes the ester trifold, thus abolishing chiral discrimination. © 1995 Wiley-Liss, Inc.     

龙芽草化学成分研究

从药用植物龙芽草(Agrimonia pilosa Ledeb)全草的95%乙醇提取物中分离得到4个化合物,通过理化性质及波谱学方法分别鉴定为豆甾-5-烯-3β,7β-二醇(1)、豆甾-5-烯-3β7,α-二醇(2)、豆甾-3β6,α-二醇(3)和β-谷甾醇(4)。化合物1～3均为首次从龙芽草中分离得到。     

β-Cyclodextrin-modified covalent organic framework as chiral stationary phase for the separation of amino acids and β-blockers by capillary electrochromatography

Covalent organic frameworks (COFs) as a novel stationary phase have attracted much attention in the field of chromatography owing to their permanent nanoscale porosity, higher surface area, and exceptional stabilities. Here, a novel isocyanate-β-cyclodextrin-modified COF (MDI-β-CD-modified COF) was synthesized using isocyanate-β-cyclodextrin as the chiral selector and imine-based TpPa-1 COF as the matrix by a bottom-up strategy. The reaction condition and the structure of MDI-β-CD-modified COF were optimized and characterized by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectra, nitrogen adsorption/desorption (Brunauer–Emmett–Teller [BET]), and thermogravimetric analysis (TGA). And then the coated open-tubular column (OT column) was prepared using MDI-β-CD-modified COF as chiral stationary phase (CSP) by in situ growth approach, which exhibited excellent stability and repeatability. For seven consecutive runs, the intraday and interday relative standard deviations (RSDs) were in range from 0.35% to 2.21% for the migration time of histidine. The column-to-column reproducibility ranged from 2.39% to 3.08%. Meanwhile, the separation of eight compounds including four amino acids and four β-blockers by capillary electrochromatography sufficiently verified the favorable chiral resolution properties of the MDI-β-CD-modified COF-coated OT column. This strategy of fabricating MDI-β-CD-modified COF-coated OT column expanded the application of imine-based COFs in chromatographic analytical fields.     

Novel chiral ionic liquids stationary phases for the enantiomer separation of chiral acid by high‐performance liquid chromatography

Novel chiral ionic liquid stationary phases based on chiral imidazolium were prepared. The ionic liquid chiral selector was synthesized by ring opening of cyclohexene oxide with imidazole or 5,6‐dimethylbenzimidazole, and then chemically modified by different substitute groups. Chiral stationary phases were prepared by bonding to the surface of silica sphere through thioene “click” reaction. Their enantioselective separations of chiral acids were evaluated by high‐performance liquid chromatography. The retention of acid sample was related to the counterion concentration and showed a typical ion exchange process. The chiral separation abilities of chiral stationary phases were greatly influenced by the substituent group on the chiral selector as well as the mobile phase, which indicated that, besides ion exchange, other interactions such as steric hindrance, π‐π interaction, and hydrogen bonding are important for the enantioselectivity. In this report, the influence of bulk solvent components, the effects of varying concentration, and the type of the counterion as well as the proportion of acid and basic additives were investigated in detail.