Upper Rim-Bridged Calix[4]arenes via Cyclization of meta Alkynyl Intermediates with Diphenyl Diselenide.

A Sonogashira coupling of meta-iodocalix[4]arene with various terminal acetylenes confirmed that the meta position of calixarene is well addressable, and that both thermal and microwave protocols led to good yields of alkynylcalixarenes. Alkynes thus obtained were subjected to the ferric chloride and diphenyl diselenide-promoted electrophilic closure. It turns out that the calix[4]arenes give completely different bridging products than those described for the non-macrocyclic starting compounds. This can be demonstrated not only by the isolation of products with a six-membered ring (6-exo-dig), but mainly by the smooth formation of the 5-endo-dig cyclization, which has never been observed in the aliphatic series. An attempt at electrocyclization led to a high yield of the 1,2-diketone (oxidation of the starting alkyne), again in contrast to the reaction described for the acyclic derivatives. The structures of the unexpected products were unequivocally established by X-ray analysis and clearly demonstrate how the preorganized macrocyclic skeleton favors a completely different regioselectivity of cyclization reactions compared to common aliphatic compounds.

Chemoselective Electrochemical Cleavage of Sulfonimides as a Direct Way to Sulfonamides.

A new method for selective cleavage of sulfonimides into sulfonamides in high yields using a simple electrochemical approach is shown. As revealed by the electrochemical study, the aromatic sulfonimides can be selectively cleaved by electrolysis of the starting compound at a given potential (only -0.9 V vs SCE for the nosyl group). The high chemoselectivity was confirmed by preparative electrolysis, and the results were supported with DFT calculations of a set of substances bearing different sulfonimide functions. Moreover, various experimental setups together with other attempts to simplify the procedure were tested. Finally, the removal of the p-nosyl group from the corresponding sulfonimides proceeds smoothly regardless of the number of nosyl groups and the overall shape of the complex molecule. Thus, the method is interesting for use in the field of multifunctional molecules such as calix[n]arenes.

An Insight into Anion Extraction by Amphiphiles: Hydrophobic Microenvironments as a Requirement for the Extractant Selectivity.

Coupling of electron-deficient urea units with aliphatic chains gives rise to amphiphilic compounds that bind to phosphate and benzoate anions in the hydrogen bonding competitive solvent (DMSO) with KAss = 6 580 M-1 and KAss = 4 100 M-1, respectively. The anchoring of these receptor moieties to the dendritic support does not result in a loss of anion binding and enables new applications. Due to the formation of a microenvironment in the dendrimer, the high selectivity of the prepared compound toward benzoate is maintained even in the presence of aqueous media during extraction experiments. In the presence of binding sites at 5 mM concentration, the amount of benzoate corresponding to the full binding site occupancy is transferred into the chloroform phase from its 10 mM aqueous solution. A thorough investigation of the extraction behavior of the dendrimer reported here, supported by a series of molecular dynamics simulations, provides new insight into the fundamental principles of extraction of inorganic anions by amphiphiles.

An Electrochemical Sensor for Detection of Neuroblastoma Markers: Complexation Studies as a Tool for the Selection of a Suitable Receptor for Electrode Coating.

Homovanillate (HVA) and vanilmandelate (VMA) are recognized markers of diseases, including neuroblastoma. However, their detection in urine represents a challenging task due to the complexity of the matrix. Here, a design, synthesis and thorough investigation of polymerizable urea-based receptors interacting with HVA and VMA are reported. The selection of receptor with the best anion recognition properties for electrode coating is based on 1 H-NMR and UV-Vis complexation studies. The sensor is prepared by electropolymerization with progress monitoring by cyclic voltammetry. The deposited layer is characterized by IR and scanning electron microscopy. The obtained sensor shows an electrochemical impedance spectroscopy response to VMA with linear range 9.9×10-6 to 1.2×10-3  M and LOD of 3.4×10-6  M. The sensor selectivity was demonstrated by the determination of VMA level in the presence of 16 µM HVA and in artificial urine with and without phosphates, with standard deviations of 0.11, 0.17 and 0.09, respectively.

Controlled Anchoring of (Phenylureido)sulfonamide-Based Receptor Moieties: An Impact of Binding Site Multiplication on Complexation Properties.

The repetition of urea-based binding units within the receptor structure does not only lead to monomer properties multiplication. As confirmed by spectroscopic studies, UV-Vis and 1H-NMR in classical or competitive titration mode, the attachment to a carrier allocates the active moieties to mutual positions predetermining the function of the whole receptor molecule. Bivalent receptors form self-aggregates. Dendritic receptors with low dihydrogen phosphate loadings offer a cooperative complexation mode associated with a positive dendritic effect. In higher dihydrogen phosphate concentrations, the dendritic branches act independently and the binding mode changes to 1:1 anion: complexation site. Despite the anchoring, the dendritic receptors retain the superior efficiency and selectivity of a monomer, paving the way to recyclable receptors, desirable for economic and ecological reasons.

Breaking thiacalix[4]arene into pieces - a novel synthetic approach to higher calixarenes bearing mixed (-S-, -CH2-) bridges.

A novel approach to calix[5-7]arenes possessing mixed (S and CH2) bridges within the skeleton is based on the reaction of thiacalix[4]arene monosulfoxide with BuLi leading to a linear phenolic tetramer in essentially quantitative yield. This key intermediate is then cyclized with suitable building blocks to give macrocyclic calixarene analogues. Compared to the traditional stepwise construction of similar systems, this procedure based on thiacalixarene cleavage represents a scalable, robust, and straightforward synthesis and enables the preparation of larger calixarenes on a gram scale. As shown by 1H NMR and UV-vis titration experiments, the mixed-bridge calix[7]arene is able to recognize fullerenes C60 and C70, thus showing possible applications of such systems. The structures of the mixed bridge systems were confirmed by single crystal X-ray analysis, and the behavior of novel macrocyclic skeletons in solution was studied using dynamic NMR techniques.

Sulphonamidic Groups as Electron-Withdrawing Units in Ureido-Based Anion Receptors: Enhanced Anion Complexation versus Deprotonation.

A sulphonamidic moiety was utilized as an electron-withdrawing group for enhancement of anion complexation features of urea-based receptors. A series of receptors varying in acidity of sulphonamidic and urea NH groups was synthesized and thoroughly tested. The individual complexation properties reflect deprotonation/complexation equilibrium in a given molecule as a function of the substitution. The receptors containing electron-donating groups in conjugation to the sulphonamidic moiety showed higher association constants towards H2 PO4- and carboxylate anions, while those containing electron-withdrawing groups inclined to deprotonation of sulphonamidic NH. The deprotonation issue can be avoided by alkylation at the early step of receptor synthesis or it can be utilized for insertion of suitable groups that enable its anchoring on various substrates to form more elaborated receptor structures.

Thermodynamics of Halide Binding to a Neutral Bambusuril in Water and Organic Solvents.

Driving forces of anion binding in water in contrast to nonpolar environments are of high interest because of their relevance to biology and medicine. Here we report a neutral bambusuril macrocycle (1), soluble in both water and nonpolar solvents due to decoration with 12 polyethylene glycol-based substituents. The new bambusuril has the highest affinity for I- in pure water ever reported for a synthetic macrocycle relying on hydrogen bonding interactions rather than metal coordination or Coulombic forces. Isothermal titration calorimetry (ITC) experiments in nine different solvents, ranging from polar water to nonpolar carbon tetrachloride, provided insight into the forces responsible for halide binding by bambusurils. The different importance of anion solvation and solvent expulsion from the cavity of the macrocycle in various solvents is illustrated by the fact that halide binding in water and chloroform is exclusively driven by favorable enthalpy with an entropic penalty, while in alcohols and nonpolar solvents, both favorable enthalpy and entropy contribute to anion encapsulation. DFT calculations and correlation of thermodynamic data with the solvent Swain acity parameter further underscore the importance of solvent effects on anion binding by bambusurils.