Extraction of technetium(vii) by calix[4]arene tetraketones and tetraesters from acidic and basic media

Tetrasubstituted on the lower rim calix[4]arenes, containing carbonyl and ester groups, and existing in a cone conformation are selective and efficient extracting agents for Tc^VII extraction from both acidic and basic solutions.     

Anion receptors based on ureido-substituted thiacalix[4]arenes and calix[4]arenes

The regioselective nitration of 25,27-dipropoxythiacalix[4]arene was carried out as a key step in the synthesis of thiacalix[4]arene derivative bearing two arylureido functions on the upper rim. The preorganisation of ureido units using the thiacalix[4]arene/calix[4]arene moieties as a molecular scaffold gave novel anion receptors. These compounds, albeit based on hydrogen bonding interactions, show good complexation ability even in highly HB-competitive solvent, such as DMSO. Direct comparison of otherwise identical structures 6a and 7a revealed remarkable dominance of the thiacalix[4]arene derivative over its classical analogue in anion binding.     

Fourfold [2]rotaxanes based on calix[4]arenes

Tetraurea calix[4]arenes with four loops form exclusively heterodimers with open-chain urea calix[4]arenes when they are dissolved in aprotic solvents. These assemblies can be considered as pseudorotaxanes. If open-chain tetraureas ending with maleic imide functions are used, their Diels-Alder reaction with 1,4,5,8-tetrapentoxyanthracene leads to tetra[2]rotaxanes which cannot be split into the single calixarene parts by hydrogen bond breaking solvents.     

Long synthetic nanotubes from calix[4]arenes

We report the synthesis and encapsulation properties of long (up to 5 nm) molecular nanotubes 1-4, which are based on calix[4]arenes and can be filled with multiple nitrosonium (NO(+)) ions upon reaction with NO(2)/N(2)O(4) gases. These are among the largest nanoscale molecular containers prepared to date and can entrap up to five guests. The structure and properties of tubular complexes 1(NO(+))(2)-4(NO(+))(5) were studied by UV/Vis, FTIR, and (1)H NMR spectroscopy in solution, and also by molecular modeling. Entrapment of NO(+) in 1(NO(+))(2)-4(NO(+))(5) is reversible, and addition of [18]crown-6 quickly recovers starting tubes 1-4. The FTIR and titration data revealed enhanced binding of NO(+) in longer tubes, which may be due to cooperativity. The described nanotubes may serve as materials for storing and converting NO(x) and also offer a promise to further develop supramolecular chemistry of molecular containers. These findings also open wider perspectives towards applications of synthetic nanotubes as alternatives to carbon nanotubes.     

Multiple catenanes based on tetraloop derivatives of calix[4]arenes

Four novel tetraarylurea calix[4]arenes (4a-d) have been synthesized, substituted by ω-alkenyloxy residues in 3,5-positions of the arylurea residues. The eight alkenyl groups were pairwise connected by olefin metathesis and subsequent hydrogenation. The ring-closure reaction was carried out with heterodimers exclusively formed by 4 with a tetratosylurea calix[4]arene 1, which serves as a template in this reaction step. The potential trans-cavity bridging is entirely suppressed in this way. Bis- and tetraloop calix[4]arenes cannot form dimers due to overlapping loops. However, they readily form heterodimers with open-chain tetraureas, as long as their urea residues can pass through the loops. Thus, five heterodimeric capsules 8a-e with bis[3]catenane structure were synthesized using again the olefin metathesis followed by hydrogenation. Two different strategies were compared for this reaction sequence, starting with heterodimers formed either by tetraloop derivatives 5 with tetraalkenyl tetraureas 6 (pathway A) or by bisloop derivatives 7 with octaalkenyl tetraureas 4 (pathway B). A distinct advantage of one of these pathways was not observed; the bis[3]catenanes were obtained with yields of 20-60%. Heterodimers formed by tetraloop derivatives 5b-d and octaalkenyl ureas 4b-d were converted analogous to three novel cyclic [8]catenanes 9a-c in 30-42% yield. The structure of the novel catenanes was unambiguously proved by ¹H NMR and ESI MS, and for 8a and 9a additionally by single crystal X-ray analysis.     

Regioselective ipso-nitration of calix[4]arenes

A novel protection/deprotection method leading to the regioselective ipso-substitution of calix[4]arenes is described. The introduction of nosyl (p-nitrobenzenesulfonyl) groups into the lower rim of partly alkylated tert-butylcalix[4]arenes leads subsequently to the exclusive ipso-nitration of the alkylated phenol rings, while the protecting groups can be easily removed in the next step. This method gives dialkoxy- or trialkoxy-substituted calix[4]arenes with nitro groups on the alkylated rings and tert-butyl groups on the remaining ones. The above substitution pattern is complementary to the isomers so far known in the chemistry of calix[4]arenes and could be used in the design of novel type of calixarene-based receptors.     

Water sorption on crystalline calix[4]arenes

The water sorption isotherms on seven samples of crystalline calix[4]arenes were obtained by the isopiestic method. The experimental isotherms were described by the Harkins-Jura equation for condensed films. The conclusion was made that water was adsorbed on the surface of calixarene crystals or through pores of a fairly large diameter with the formation of a condensed film. Original Russian Text ? O.V. Surov, M.I. Voronova, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 5, pp. 943–947.     

Palladium(II) extraction by sulfur-containing calix[4,6]arenes from hydrochloric acid solutions

The comparison of the extraction properties of calixarenes, thiacalixarenes, and calix[4,6]arene thioethers showed that methyl(thiamethyl)calix[4,6]arenes 3a and 4a have the highest extraction abilities. These extractants rapidly and completely extract palladium from hydrochloric acid solutions; regarding distribution factors achieved in the kinetic mode, they three to four orders of magnitude exceed their monodentate analogue, octylbenzyl sulfide (OBnS). Approaches are considered to enhance palladium extraction via generating mixed palladium species in low-acidity solutions and via intramolecular catalysis by the protonated oxygen atoms of alkoxy groups in the lower rim. For 1 M HCl, the kinetic order of diluent effects on palladium extraction was established. The substitution of sulfur atoms for bridging CH₂ groups was discovered to enhance palladium extraction by calix[4]arene thioether 3c.     

New preorganized calix[4]arenes. Part II. Double Schiff-based calix[4]arenes derived from 4,6,10,12,16,18,22,24,25,26,27,28-dodecamethyl-5,11,17,23-tetrahydroxycalix[4]arene

We describe the condensation of an appropriate tetraaldehyde derivative of mesitol calix[4]arene with 1,n-diaminoalkanes to afford double Schiff-base calix[4]arene receptors in the 1,3-alternate conformation.     

Specific extraction behavior of amide derivative of calix[4]arene for silver (I) and gold (III) ions from highly acidic chloride media

25,26,27,28-tetrakis(N,N-diethylaminocarbonylmethoxy)-5,11,17,23-tetrakis(1,1,3,3-tetramethylbutyl)calix[4]arene, a macrocyclic extraction reagent, and p-(1,1,3,3-tetra-methylbutyl)phenoxymethyl-N,N-diethylamide, an acyclic extraction reagent corresponding to the former one, were synthesized to investigate their extraction behavior for silver(I), gold(III), palladium(II), and platinum(IV) from highly acidic solution into chloroform. In the extraction of silver and gold from hydrochloric acid solution, a completely different extraction behavior was observed between these two types of the reagents. The extraction behavior was examined in detail for silver and was found to be dependent on whether silver ion was extracted as a cationic species or a anionic species complexed with chloride ion. This was supported by proton nuclear magnetic resonance study of the calix[4]arene derivative. As a result, the extraction of silver ion with calix[4]arene derivative was very peculiar which was attributable to the fitting between cyclic size of calix[4]arene and ionic radius of silver.     

Synthesis and structure of [2-oxo(thioxo)tetrahydropyrimidin-4-yl]calix[4]arenes

Cone-shaped di- and tetrapropoxycalix[4]arenes functionalized at the upper rim with one or two 2-oxo(thioxo)tetrahydropyrimidine residues were synthesized by the Biginelli reaction of formylcalixarenes with urea (thiourea) and methyl acetoacetate. The steric structure of the products was studied by NMR and X-ray diffraction. The Biginelli reaction with dipropoxyformylcalixarene was diastereoselective, and it quantitatively produced the corresponding meso form. Tetrapropoxydiformylcalixarenes under analogous conditions gave rise to equimolar mixtures of racemic and meso compounds. The macrocyclic skeleton of the synthesized pyrimidine-containing calixarenes in crystal and in solution has a flattened cone conformation. (5-Methoxycarbonyl-2-oxotetrahydropyrimidin-4-yl)calixarene molecules in crystal undergo self-organization to form infinite chains via repeated inclusion of the methoxy group into the cavity of the neighboring macrocycle. Selforganization of bis(5-methoxycarbonyl-2-oxotetrahydropyrimidin-4-yl)calixarene with formation of analogous chains involves intermolecular hydrogen bonding NH ... O=C.     

The inhibitory properties of acidic functionalised calix[4]arenes on human papillomavirus pentamer formation

ABSTRACT

Human Papillomavirus (HPV) is the leading cause of cervical cancer, with only some HPV types prevented with vaccines and no treatments for the viral infection itself. One way to target viral infection is by inhibiting the assembly of the L1 monomer into a pentamer, which forms the viral capsid. Four calix[4]arene compounds functionalised with D- and L-aspartic and glutamic acid and an iminodiacetic functionalised calix[4]arene were synthesised and tested for L1 pentamer formation inhibition. The amino acid functionalised calix[4]arene derivatives showed millimolar inhibition (IC₅₀ = 0.72 to 2.67 mM) of pentamer formation, with little difference between the stereoisomers. The iminodiacetic acid calix[4]arene derivative showed no inhibitory properties, despite sharing structural similarities with the four other calix[4]arenes. Confirmation of binding the negatively charged compounds to the positive residues of the L1 protein was achieved by trypsin digestion. This study is helpful in the development of cost-effective inhibitors to prevent HPV assembly.     

New artificial receptors from selectively functionalized calix[4]arenes

Abstract

The development of new synthetic methods for the monoalkylation of calix[4]arenes at the lower rim allows the synthesis of a new class of trihydroxamate siderophores. Three chelating hydroxamic acid units are introduced through a sequence of reactions which blocks the macrocycle in the cone conformation. The new ligands obtained form neutral 1:1 complexes (FeL) with iron (III), which are stable in EtOH/H₂O 9:1 at pH 2–7. Calix[4]arene bis-crown ethers are prepared by exploiting the selective 1,2-(proximal) functionalization of calix[4]arenes at the lower rim. These ligands are, however, less effective in complexing alkali metal cations compared with the 1,3-calix[4]arene crown-ethers which, in their partial cone structure, offer a better shielding for the complexed cations. Rigid upper rim-bridged calix[4]arenes potentially useful for the inclusion of neutral molecules are prepared by exploiting the selective 1,3-diformylation of calix[4]arene at the upper rim. Finally a new chloromethylation method for calix[4]arenes blocked in the cone conformation is described together with the synthesis of new cavitands.     

Synthesis of chiral calix[n]arenes. Part 2: Synthesis of new chiral calix[n]arenes based on (p-hydroxy-phenyl)-menthone

The synthesis of new chiral calix[n]arenes, related to Corey's phenyl-menthol, is described. Starting from enantiomerically pure (R)-(+)-pulegone, calix[n]arenes with different ring sizes could be obtained in reasonable yield.     

The synthesis of metallocene calix[4]arenes

The condensation of 1,1-bis(chlorocarbonyl)metallocenes andp-tert-butylcalix[4]arene in toluene leads to novel metallocene calix[4]arenes in which the metallocene subunit bridges the opposite hydroxy groups of the parent calixarene.Presented at the Fourth International Symposium on Inclusion Phenomena and the Third International Symposium on cyclodextrins, Lancaster, U.K., 20–25 July 1986.     

Dendritic octa-CMPO derivatives of calix[4]arenes

Calix[4]arenes substituted at the narrow or wide rim by eight carbamoylmethyl-phosphine oxide (CMPO) functions in a dendritic manner were synthesised and studied in extraction of Eu³⁺ and Am³⁺ from aqueous nitric acid into o-nitrophenylhexyl ether. ¹H NMR relaxivity titrations for a wide rim octa-CMPO reveal the clear formation of a solvent-free 1:2 ligand/metal complex, while the wide rim tetra-CMPO formed oligomeric complexes under similar conditions.     

Self-sorting dimerization of tetraurea calix[4]arenes

Calix[4]arenes substituted by four urea functions are self-complementary molecules that spontaneously combine in apolar solvents in the presence of an ammonium salt to form dimeric capsules held together by a belt of hydrogen bonds. In the presence of tetraethylammonium salts, the Et4N+ cation is included as a guest. The sorting between dimeric capsules formed in a mixture of calix[4]arenes directly depends on the steric crowding of the substituents grafted on the urea groups whether aromatic derivatives or aliphatic chains linking urea functions in mono-, di-, or tetraloop structures. Simple rules allow one to anticipate which capsules will be exclusively formed when calix[4]arenes are mixed in different proportions. The stabilization of the dimeric structures by hydrogen bonds is thwarted by the overlaps of aliphatic loops and/or by bulky groups that cannot pass through these loops. Despite the structural similarity of the calixarenes, the exclusive formation of dimers of well-defined compositions and clear titration breaks are observed by electrospray mass spectrometry. This technique yields reliable information on stoichiometries and composition despite measurements in the gas phase rather than in solution and it does not suffer from excessive peak overlaps in contrast with NMR.     

Conformational properties of cyanomethoxy calix[4]arenes

O-Alkylation of the dinitro calix[4]arene 2, easily available by selective ipso-nitration of the di-cyanomethyl ether 1, with allylbromide (DMF/Cs2CO3) gave tetraethers 3 and 4 with anti- and syn-orientations of the two allyl ether residues. The two possible stereoisomers of in the partial cone and 1,2-alternate conformation exist as an equilibrium mixture which could be quantitatively analysed by 1H NMR spectroscopy. The temperature dependence of this equilibrium leads to deltaH0 = -7.6 to -9.7 kJ mol(-1) in different solvents (tetrachloroethane, benzene, dimethylsulfoxide). Since 3(1,2-alt) could be obtained in pure form, its isomerisation to the equilibrium mixture with 3(paco) could be followed also kinetically. An activation energy of E(a) = 110.5 kJ mol(-1) was found for this reaction in DMSO-d6. The results were confirmed by similar studies with tetraethers 5 and 6 obtained by O-allylation of 1, although exact thermodynamic and kinetic studies were not possible in this case, since the NMR signals of the respective isomers were strongly overlapping. Single crystal X-ray structures were obtained for 2, 3(1,2-alt), 4(paco) and 5(1,2-alt).     

Porous molecular crystals of calix[4]arenes

X-ray diffraction, thermogravimetric, and adsorption measurements were applied to study the thermal decomposition of the complexes of 25,26,27,28-tetrahydroxycalix[4]arene and 4-tert-butylcalix[4]arene with the solvents resulting in the formation of porous phases. Diffusion of the "guest" (solvent, gas) through the molecular crystal of calixarene-host proceeds via the consecutive formation of a series of thermodynamically unstable porous phases of the "host." Owing to the structural reorganization of calixarene, the transport of the "guest" occurs through the "host" matrix followed by complexation and formation of the thermodynamically stable structure.