Structural characteristics of some soluble and insoluble β-cyclodextrin polymers

Several β-cyclodextrin polymers (βCDP) have been obtained by cross-linking β-cyclodextrin (βCD) with the reagent epichlorohydrin (EP). It is expected that these polymers are capable of retaining different organic molecules by adsorbing them on its network and also by forming inclusion complexes with βCDs. In this work, two soluble polymers containing 39% and 48% βCD and other insoluble ones with 65% and 74% βCD have been studied. The total amount of CD in the polymers could not be available for complexation. This parameter has been calculated by means of the decrease of colour intensity of phenolphthalein solutions when different amounts of βCDP were added. The insoluble polymer with 74% βCD appears to possess less CD available than that with 65% βCD, probably due to the higher cross-linking degree of the former. On the other hand, a higher availability of CD is found for the soluble polymer which contains 48% βCD. Moreover, the amount of glycerol monoether groups formed as a side effect during the cross-linking process has been determined and related to the epichlorohydrin content, structure and swelling properties of the polymers. It is concluded that, varying the synthesis conditions, it is possible to induce structural modifications in the hydrogel networks which can improve their practical applications.     

Regioselective monobromination of substituted phenols in the presence of β-cyclodextrin

Cyclodextrin acts as a restricting nanovessel to enhance regioselectivity in bromination of substituted phenols such as 3-nitrophenol, 2-chlorophenol, 3-chlorophenol, and 4-chlorophenol. In contrast to solution bromination, cyclodextrin facilitates regioselective monobromination and formation of polybrominated products are substantially reduced. Selectivities in brominations are also observed in water and in the solid state. The observed results are rationalized on the basis of specific modes of inclusion of substituted phenols inside the cyclodextrin cavity and find strong support from energy minimization studies and ¹H-¹H NOESY.     

The resolution of some chiral compounds in reversed-phase high-performance liquid chromatography by means of β-cyclodextrin inclusion complexes

Summary -Cyclodextrin is applied as the chiral component of the mobile phase in a systematic study of the resolution —into enantiomers — of mandelic acid and its derivatives by reversed-phase liquid chromatography. It is found that the stereoselectivity arising from inclusion in -cyclodextrin molecules is significant (=1.02÷1.05) only for the compounds showing at the asymmetric carbon atom the presence of first, an intact carboxylic group and second, a functional group capable of hydrogen bonding with the hydroxyl groups of -cyclodextrin. Results are discussed in the light of the three-point attachment model of stereoselectivity as well as of the structure of the inclusion complexes.Presented at the 14th International Symposium on Chromatography London, September, 1982     

A new asymmetric synthetic route to substituted piperidines

An asymmetric synthesis of substituted piperidines has been described. β-Cyclodextrin- or oxazaborolidine-catalyzed asymmetric reduction of α-azido aryl ketones to the corresponding alcohols has been employed as the key step along with ring closing metathesis and selective dihydroxylation.     

Electrodialytic transport properties of anion-exchange membranes in the presence of α-cyclodextrin

Electrodialysis of mixed salt solutions, sodium chloride and sodium sulfate, and sodium chloride and sodium nitrate, was carried out in the presence of α-cyclodextrin using commercial anion-exchange membranes. It was confirmed by several methods that the compound existed in the membrane matrix when the membrane had been immersed in its aqueous solution, though the molecular weight of α-cyclodextrin is relatively high. In electrodialysis, sulfate ions, large and strongly hydrated anions, easily permeated through the membranes and nitrate ions, less hydrated anions, permeated with difficulty through the membranes in the presence of α-cyclodextrin. Because α-cyclodextrin is a hydrophilic compound, which has many ether and alcoholic groups, the hydrophilicity of the anion-exchange membranes is thought to increase. Thus, sulfate ions easily permeate and nitrate ions permeate with difficulty. This proves that the hydrophilicity of the anion-exchange membranes controls permselectivity between anions through the membranes. Received: 8 August 2000 Accepted: 24 October 2000     

Sub-1-micron mesoporous silica particles functionalized with cyclodextrin derivative for rapid enantioseparations on ultra-high pressure liquid chromatography

Mesoporous silica particles of relatively uniform sub-1-micron size (0.6-0.9 μm) were successfully prepared by a modified synthesis strategy and applied in chiral separation in an ultra-high pressure liquid chromatography system. These particles were prepared via a ternary surfactant system (Pluronic P123, F127 and hexadecyltrimethyl-ammonium bromide) and subsequently derivatized with perphenylcarbamoylated-β-cyclodextrin moieties. The mesoporous silica particles, despite their submicron size, enabled low back-pressure operation on an ultra-high pressure liquid chromatography system at a maximum flow rate of 2 ml/min. In addition, the particles possessed high surface area (480 m(2)/g) and thus afforded high cyclodextrin derivative loading (32 μmol/g), demonstrating rapid enantioseparation and good resolution of 6 basic and neutral racemates.     

Influence of substituted groups on the binding ability of benzoyl-modified β-cyclodextrins

A series of substituted benzoyl modified β-cyclodextrins, including mono-6-O-(p-methylbenzoyl)-β-CD (1), mono-6-O-(m-methylbenzoyl)-β-CD (2), mono-6-O-(o-methylbenzoyl)-β-CD (3), mono-6-O-(p-methoxylbenzoyl)-β-CD (4), mono-6-O-(m-methoxylbenzoyl)-β-CD (5), mono-6-O-(o-methoxylbenzoyl)-β-CD (6), mono-6-O-(m, p-dimethoxylbenzoyl)]-β-CD (7), mono-6-O-(o,m-dimethoxylbenzoyl)-β-CD (8), and mono-(6-O-benzoyl)-β-CD (9) were synthesized and their inclusion properties were studied by using fluorescence spectroscopy. The binding constants (K_a) of the modified β-CD derivatives with 2-p-toluidinylnaphthalene-6-sulfonate (TNS) were determined on the basis of the fluorescence spectroscopy. The effect of types and location of substituted groups of the benzene ring of the modified β-cyclodextrins on the binding property was discussed. Results indicated that the substituents had significant influences on the binding abilities of modified β-cyclodextrins.     

Chiral discrimination of ibuprofen isomers in β-cyclodextrin inclusion complexes: experimental (NMR) and theoretical (MD, MM/GBSA) studies

In this paper, we analyze the energetic and conformational preferences involved in the chiral discrimination of ibuprofen (Ibu) isomers by beta-cyclodextrin (β-CD) when forming inclusion complexes in water. This study was performed by means of atomistic molecular mechanics simulations upon four different penetration modes of the guest, and a structural 2D NMR experiment. The trajectories of these simulations were treated with the MM/GBSA method in order to obtain the relative weights of the different free energy components. The resulting values of the free energy of binding and other geometrical features indicate that this chiral selectivity is influenced by a preferred penetration mode involving the S-(+)-Ibu isomer. The calculated ΔΔG of binding is in good agreement with published experiments.     

Thermal release behavior of Fe, CpFe and (Cp)2Fe ions from the molecular aggregate of β-cyclodextrin with ferrocene

The second-sphere interactions between ferrocene (Fc, guest) analogs and cyclodextrins (CD, host) have attracted sufficient attention in the past decades. However, there are some unanswered questions related with the interactions. For example, how such interactions are reflected by the thermal decomposition process? The present report describes our efforts to elucidate the correlation between the destruction of a second-sphere complex through the release of its members and the separate destruction of its members by a direct sample introduction device with programmed temperature in a gas chromatography coupled to time-of-flight mass spectrometry. Different kinds of mass spectra are analyzed to elicit accurate details about sample decomposition processes. It is found that on the one hand, the presence of Fc effectively delays the decomposition temperature of the complexed β-CD though a great part of Fc has been released from the complex. On the other hand, the special molecular structure of β-CD has served to prevent the sublimation of Fc to a certain degree. Furthermore, the thermal decomposition mode of the β-CD molecules complexed by Fc is drastically changed compared with that of free β-CD. Also, this study provides a paradigm that ferrocene decomposes in the β-CD cavity through the release of a CpFe⁺ moiety, and the remaining Cp ring remains contained in the host.     

Synthesis and characterization of a persistent paramagnetic rotaxane based on α-cyclodextrin and α,ω-alkyl disulfides

A new synthetic strategy for the preparation of persistent paramagnetic cyclodextrin-based rotaxanes is described. The method consists in the formation of inclusion complexes between α-cyclodextrin (α-CD) and α,ω-dithiols containing an octamethylene chain covalently trapped by bulky stoppers composed of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical fragments. Interaction of α-CD (the bead) and 1,8-octanedithiol (the thread) occurs in aqueous alkaline media and encapsulation is obtained by nucleophilic substitution at both termini of the linear component with a bulky paramagnetic iodide [2,2,6,6-tetramethyl-4-(2-iodoacetamide)piperidine-N-oxyl]. Structure determination of the new [2]rotaxane by ¹H NMR is reported and the spectroscopic data are discussed.