Click chemistry for facile immobilization of cyclodextrin derivatives onto silica as chiral stationary phases

Preparation of a bis-triazolyl bridged β-cyclodextrin stationary phase and its application for enantioseparation of chiral compounds by HPLC

A novel bis-triazolyl bridged β-cyclodextrin was first synthesized by the Click reaction between azido-β-cyclodextrin and 1,6-heptadiyne. Then it was bonded onto silica gel to obtain a bis-triazolyl bridged β-cyclodextrin-based chiral stationary phase (BCDP). After structure characterization, the HPLC performance of BCDP was systematically evaluated by using different types of compounds as probes. The results showed that BCDP could well separate 18 kinds of achiral aromatic compounds (homologues, positional isomers, etc.) and 35 kinds of chiral drugs or pesticides, such as triazoles (Rs = 1.33-3.15), flavanones (Rs = 1.49-2.62), dansyl amino acids (Rs = 0.96-1.99), and β-blocker drugs (Rs = 0.68-2.78). BCDP could separate a wider range of compounds (53 kinds); especially, some chiral substance pairs that were difficult to be resolved on the ordinary cyclodextrin CSPs, including triazoles containing two chiral carbons (triadimenol, bitertanol, metconazole, and triticonazole), strongly ionized amino acids (acidic Asp, alkalic Arg, and polar Thr) and β-blockers with bulky groups (carvedilol, propranolol, and pindolol). Obviously, the unique synergistic inclusion effect of bridged cyclodextrin with double cavities and the bis-triazole bridging group could provide multiple action sites, such as hydrogen bonding, π-π stacking and acid-base action sites, thus improving its chiral chromatographic performance.

Modular synthesis of 2-furyl carbinols from 3-benzyldimethylsilylfurfural platforms relying on oxygen-assisted C-Si bond functionalization

3-Silylated furfurals, readily prepared in three steps from biomass-derived furfural and 5-methylfurfural, are converted into 3-silylated 2-furyl carbinols upon condensation with organomagnesium or organolithium reagents. The hydroxy unit of the carbinol adducts can be exploited to promote C3(sp2)-Si bond functionalization through intramolecular activation. Two approaches were contemplated for this purpose. Activation by alkoxides of the C3-SiEt3 or C3-SiMe2 t-Bu bonds was ineffective. Conversely, treatment of the C3-benzyldimethylsilyl-appended derivatives with tetrabutylammonium fluoride led to cyclic siloxanes, which revealed to be competent donors for copper-catalyzed cross-coupling reactions, such as arylation reactions catalyzed by Pd2(dba)3/CuI, as well as allylation and methylation reactions catalyzed by CuI⋅PPh3. C3-Benzyldimethylsilyl-appended furfurals are thus introduced as versatile platforms, providing a modular access to 3-substituted 2-furyl carbinols from renewable feedstock.

Contribution of the "Click Chemistry" Toolbox for the Design, Synthesis, and Resulting Applications of Innovative and Efficient Separative Supports: Time for Assessment

The last two decades have seen the rapid expansion of click chemistry methodology in various domains closely related to organic chemistry. It has notably been widely developed in the area of surface chemistry, mainly because of the high-yielding character of reactions of the "click" type. Especially, this powerful chemical reaction toolbox has been adapted to the preparation of stationary phases from the corresponding chromatographic supports. A plethora of selectors can thus be immobilized on either organic, inorganic, or hybrid stationary phases that can be used in different chromatographic modes. This review first highlights the few different chemical ligation strategies of the "click" type that are up to now mainly devoted to the development of functionalized supports for separation sciences. Then, it gives in a second part an up-to-date survey of the different studies dedicated to the preparation of click chemistry-based chromatographic supports while highlighting the powerful and versatile character of the "click" ligation strategy for the design, synthesis, and developments of more and more complex systems that can find promising applications in the area of analytical sciences, in domains as varied as enantioselective separation, glycomics, proteomics, genomics, metabolomics, etc.

Water-Compatible Synthesis of 1,2,3-Triazoles under Ultrasonic Conditions by a Cu(I) Complex-Mediated Click Reaction

A new monophosphine Cu(I) complex bearing bis(pyrazolyl)methane (L 1 ) (CuIL 1 PPh 3 ) was synthesized and used as a catalyst for the three-component click reaction from an alkyl halide, sodium azide, and terminal alkyne to furnish 1,4-disubstituted 1,2,3-triazoles in up to 93% yield. The catalyst is highly stable, compatible with oxygen/water, and works with total efficiency under ultrasonic condition. The structure of the complex was studied and confirmed by X-ray crystallography, finding a riveting relationship with its catalytic activity. This sustainable triazoles synthesis is distinguished by its high atom economy, low catalyst loading (up to 0.5 mol %), broad substrate scope, short reaction times, operational simplicity, and an easy gram-scale supply of a functionalized product for subsequent synthetic applications.

Click chemistry at the microscale

This manuscript reviews recent developments in click chemistry in microscale systems.

HPLC Enantioseparation on Cyclodextrin-Based Chiral Stationary Phases

As one of the commonly used chiral separation materials, cyclodextrin-based chiral stationary phases (CD-CSP) have been developed rapidly in the past 30 years. A large number of CD-CSPs have been designed and applied for enantioseparation in high-performance liquid chromatography (HPLC). The development of novel CD-CSPs focuses on two aspects: the immobilization chemistry and the functionalization of the CD skeleton. Although such studies are not regarded as the prime research topic in analytical chemistry, there are still many recent works pushing this research forward tardily. In this chapter, the fabrication procedure of a triazole-bridged duplex CD-CSP and its application to HPLC enantioseparations is described.

Thiol–ene click chemistry for the design of diol porous monoliths with hydrophilic surface interaction ability: a capillary electrochromatography study

Thiol–ene click chemistry provides an efficient surface grafting strategy for designing diol monoliths meant for hydrophilic interaction capillary electrochromatography.

Toward multifunctional "clickable" diamond nanoparticles

Nanodiamonds (NDs) are among the most promising new carbon based materials for biomedical applications, and the simultaneous integration of various functions onto NDs is an urgent necessity. A multifunctional nanodiamond based formulation is proposed here. Our strategy relies on orthogonal surface modification using different dopamine anchors. NDs simultaneously functionalized with triethylene glycol (EG) and azide (-N3) functions were fabricated through a stoichiometrically controlled integration of the dopamine ligands onto the surface of hydroxylated NDs. The presence of EG functionalities rendered NDs soluble in water and biological media, while the -N3 group allowed postsynthetic modification of the NDs using "click" chemistry. As a proof of principle, alkynyl terminated di(amido amine) ligands were linked to these ND particles.

Cyclodextrin-functionalized chromatographic materials tailored for reversible adsorption

Novel dendronized silica substrates were synthesized. First- and second- generation polyaryl ether dendrons were appended to silica surfaces. Using Cu(I) mediated cycloaddition "click" chemistry, β-cyclodextrin was tethered to the dendronized surfaces and to a nondendronized surface for comparison purposes. This synthesis strategy affords a modular, versatile method for surface functionalization in which the density of functional groups can be readily varied by changing the generation of dendron used. The surfaces, which are capable of adsorbing target analytes, have been characterized and studied using X-ray photoelectron spectroscopy (XPS) and vibrational sum frequency spectroscopy (VSFS). Fluorescence spectroscopy was used to study the surfaces' ability to retain coumarin 152 (C152). These studies indicated that the β-cyclodextrin functionalized surfaces not only adsorbed C152 but also retained it through multiple aqueous washes. Furthermore, these observations were quantified and show that substrates functionalized with first-generation dendrons have a more than 6 times greater capacity to adsorb C152 than slides functionalized with monomeric β-cyclodextrin. The first-generation dendrons also have 2 times greater the capacity than the larger generation dendrons. This result is explained by describing a dendron that has an increased number of β-cyclodextrin monomers but, when covalently bound to silica, has a footprint too large to optimize the number of accessible monomers. Overall, both dendronized surfaces demonstrated an increased capacity to adsorb targeted analytes over the slides functionalized with monomeric β-cyclodextrin. The studies reported provide a methodology for characterizing and evaluating the properties of novel, highly functional surfaces.

Chiral differentiation of novel isoxazoline derivatives on “clicked” thioether and triazole bridged cyclodextrin chiral stationary phases

This work first demonstrates the complete chiral resolution of novel isoxazoline derivatives on smartly designed triazole- and thioether-bridged native cyclodextrin (CD) chiral stationary phases (CSPs).

Phenylboronic-acid-modified nanoparticles: potential antiviral therapeutics

Phenylboronic-acid-modified nanoparticles (NPs) are attracting considerable attention for biological and biomedical applications. We describe here a convenient and general protocol for attaching multiple copies of para-substituted phenylboronic acid moieties onto either iron-oxide-, silica- or diamond-derived NPs. The boronic acid functionalized NPs are all fabricated by first modifying the surface of each particle type with 4-azidobenzoic ester functions. These azide-terminated nanostructures were then reacted with 4-[1-oxo-4-pentyn-1-yl) amino]phenylboronic acid units via a Cu(I) catalyzed Huisgen cycloaddition to furnish, conveniently, the corresponding boronic-acid modified NPs (or "borono-lectins") targeted in this work. The potential of these novel "borono-lectins" as antiviral inhibitors was investigated against the Hepatitis C virus (HCV) exploiting a bioassay that measures the potential of drugs to interfere with the ability of cell-culture-derived JFH1 virus particles to infect healthy hepatocytes. As far as we are aware, this is the first report that describes NP-derived viral entry inhibitors and thus serves as a "proof-of-concept" study. The novel viral entry activity demonstrated, and the fact that the described boronic-acid-functionalized NPs all display much reduced cellular toxicities compared with alternate NPs, sets the stage for their further investigation. The data supports that NP-derived borono-lectins should be pursued as a potential therapeutic strategy for blocking viral entry of HCV.

Preparation and enantioseparation of a new click derived β-cyclodextrin chiral stationary phase

A new cyclodextrin-derived chiral stationary phase (denoted as CDA-CSP) was synthesized by immobilizing mono(6(A)-azido-6(A)-deoxy)-per(p-chlorophenyl carbamoylated) β-cyclodextrin derivative to alkynyl modified silica via click chemistry. This newly prepared CSP shows good enantioseparation performance for six chiral compounds (1-6), such as 4-phenyl-oxazolidine-2-thione, two kinds of aryl alcohols, substituted flavonoids and benzoin, in which baseline separation of Analytes 1-4 was achieved under the experimental conditions. The effects of column temperature, mobile phase pH and content of methanol on the enantioseparation characteristics of CDA-CSP were investigated in detail. Retention factor and resolution for Compound 3 gradually reduced with an increase of column temperature, and a good linear relationship was shown between napierian logarithm of selectivity factor and reciprocal of column temperature. In the pH range from 3.56 to 5.50, a change in pH hardly affected the resolution of Analyte 2. In addition, increasing methanol in the mobile phase resulted in rapid eluting of the analytes from the column in reversed-phase mode. The retention factors for Analytes 1 and 3 significantly decreased and their resolution showed different trends.

[Chromatographic evaluation of multi-mode retention behavior of thiol-ene click chemistry based beta-cyclodextrin stationary phase]

A native beta-cyclodextrin (beta-CD) stationary phase was prepared by covalent bonding of beta-CD on silica particles via thiol-ene click chemistry. The resulting beta-CD bonded silica (Click TE-CD) was characterized by elemental analysis, which proved the successful immobilization of beta-CD on the silica support with thiol-ene click chemistry. Click TE-CD was chromatographically evaluated with a set of flavone glycosides under hydrophilic interaction chromatography (HILIC) mode, reversed-phase chromatography (RPLC) mode and supercritical fluid chromatography (SFC) mode. The acetonitrile content dependent "U" retention curves indicated its HILIC/RPLC mixed-mode retention behavior. The difference of the separation selectivity between HILIC, RPLC and SFC was described as orthogonality by using geometric approach. The orthogonalities between HILIC/RPLC, HILIC/SFC, and RPLC/SFC reached 69. 8%, 50. 8% and 50. 8%, respectively. The separation of Chinese traditional medicine Lignum Dalbergia Odorifera extract under HILIC, RPLC and SFC modes indicated the potential of Click TE-CD stationary phase in the analysis of complex samples. The mixed-mode HPLC properties and excellent orthogonality demon strated its flexibility in HPLC method development and its great potential in two-dimensional liquid chromatography separation on one HPLC column by different separation modes.

HPLC enantioseparation on cyclodextrin-based chiral stationary phases

Cyclodextrin-bonded silica material is one of the most commonly used chiral stationary phases in liquid chromatography for pharmaceutical analysis and enantioseparations. The approaches for immobilization of cyclodextrins onto the silica surface influence both, the stability of the chiral stationary phase and the enantioselectivity. In this chapter, we describe an established example of the preparation of a cyclodextrin-based chiral stationary phase via "click chemistry" and their application for enantioseparations of racemic compounds by HPLC.

Cyclodextrin-mediated enantioseparations by capillary electrochromatography

Immobilized cyclodextrin derivatives are used as chiral selectors in various modes of capillary electrochromatography (CEC). The present chapter describes three techniques in detail utilizing CDs in CEC: (1) open-tubular capillary electrochromatography (o-CEC), (2) packed capillary electrochromatography (p-CEC), and (3) monolithic capillary electrochromatography (rod-CEC). Nanoparticle pseudostationary phase capillary electrochromatography (psp-CEC) is briefly discussed.

Recent advances of enantioseparations in capillary electrophoresis and capillary electrochromatography

A comprehensive survey of recent developments and applications of capillary electromigration techniques for enantioseparations from January 2006 to June 2010 is presented. The techniques include capillary electrophoresis, chip capillary electrophoresis and capillary electrochromatography. The separation principles and the chiral recognition mechanisms are discussed. Additionally, on-line preconcentrations in chiral capillary electrophoresis are also reviewed.

Application of click chemistry on preparation of separation materials for liquid chromatography

With the increasing requirement for analysis and separation of samples related to genomics, proteomics, metabolomics, pharmacology and agrochemistry, diverse stationary phases for liquid chromatography have been prepared by Cu(i)-catalyzed 1, 3-dipolar azide-alkyne cycloaddition reaction (CuAAC). It has been proved that CuAAC is a powerful tool for preparing covalently bonded stationary phases. In this tutorial review, we highlighted the preparation of separation materials by immobilization of functional groups on silica beads, polymer beads and agarose via CuAAC and their applications in liquid chromatography and related purposes, such as separation of polar compounds, enrichment of valuable bio-samples, orthogonal two-dimensional HPLC and chiral separation. Meanwhile, agarose-based separation materials for affinity chromatography are reviewed.

"Click" preparation of hindered cyclodextrin chiral stationary phases and their efficient resolution in high performance liquid chromatography

This communication reports the preparation of two new cyclodextrin (CD) chiral stationary phases (CSPs): heptakis(6-deoxy-6-azido)-β-CD and heptakis(6-deoxy-6-azido-phenylcarbamoylated)-β-CD CSPs that perform quite differently to our previously reported "click" immobilized CD-CSPs. These CSPs are sterically congested at the narrow mouth of the CD and exhibit chiral discrimination between over 40 pairs of enantiomers in high performance liquid chromatography. The free hydroxyl CSP afforded better separation of indoprofen, ketoprofen, Tröger's base, hydroxyl, carboxylic and dansyl amino acids than did the phenylcarbamoylated CSP, while the latter was better at resolving aryl alcohols, flavonoids, β-blockers and β-agonists. The current work shows that enantiodiscrimination achieved with different CSPs for different classes of analyte may be correlated with CD accessibility and peripheral functionality.

"Click" immobilized perphenylcarbamated and permethylated cyclodextrin stationary phases for chiral high-performance liquid chromatography application

Two cyclodextrin-based chiral stationary phases have been prepared by immobilization of functionalized mono-6-azido-beta-CD derivatives to alkynyl modified silica via "click" chemistry and applied to the HPLC enantioseparation of various chiral compounds. The perphenylcarbamated CD CSP (CCP-CSP) exhibited excellent chiral recognition of a wide range of analytes including racemic aryl alcohols, flavonoids, bendroflumethiazide, atropine and some beta-blockers. Methanol proved to be a better organic modifier than acetonitrile for most of the analytes with the exception of bendroflumethiazide. The "click" chemistry immobilized permethylated CD CSP (CCM-CSP) afforded poor chiral recognition for most analytes, but could resolve non-aromatic ionone derivatives which were not separated on CCP-CSP. These results suggest that resolution with cyclodextrin derived CSPs depend on a complex interplay of 'host'-'guest' inclusion, hydrogen bonding, pi-pi and hydrophobic interactions.