Regioselective double intramolecular bridging of p-tert-butylcalix[7]arene

The first examples of doubly bridged calix[7]arenes 2a-h have been obtained by base-promoted O-alkylation of p-tert-butylcalix[7]arene 1 or 1,4-tetramethylene-bridged calix[7]arene 3a with a variety of bis-electrophiles including BrCH₂Cl, oligoethylene glycol ditosylates, and 1,2-bis(bromomethyl)benzene. In the presence of Cs₂CO₃ as the base, in acetone, the syn-1,4:2,3-bis-bridged regioisomer was obtained in yields up to 76%. Assignment of bridging pattern was based on chemical shift of OH groups in conjunction with chemical correlations with known compounds. Stereochemical and conformational features were investigated with the aid of 2D and Dynamic NMR studies and MM3 calculations.     

Synthesis and dichromate anion extraction ability of p-tert-butylcalix[4]arene diamide derivatives with different binding sites

The article describes the synthesis and evaluation of the dichromate anion (Cr₂O₇²⁻/HCr₂O₇⁻) extraction properties of p-tert-butylcalix[4]arene diamide derivatives (5-7) containing different binding sites. Among these compounds, 6 and 7 have been synthesized via aminolysis in a toluene-methanol solvent system with 3-aminomethylpyridine and 3,6-dioxa-1,8-diamino octane, respectively. On the other hand, compound 5 has been synthesized via an acid chloride method due to its inefficiency under aminolysis. The extraction properties of these diamides toward dichromate anions are studied by liquid-liquid extraction. The results show that p-tert-butylcalix[4]arene diamide derivative 7 exhibited a much higher affinity toward dichromate anions than that of 6 due to its special structure, while 5 was an ineffective ligand for these anions.     

Dioxamethylene intramolecular bridging of p-tert-butylcalix[8]arene

The first examples of dioxamethylene bridged calix[8]arenes 2-6 have been obtained by Cs₂CO₃-promoted direct O-alkylation of p-tert-butylcalix[8]arene with BrCH₂Cl. Assignment of the 1,2-, 1,2:3,4-, 1,2:3,4:6,7-, 1,4:2,3:5,6:7,8-, and 1,2:3,4:5,6:7,8-bridging pattern of 2-6, respectively, was mainly based on chemical shift of OH groups and chemical correlations. Dynamic ¹H NMR studies and MM3 calculations indicated that in these compounds the dioxocine subunit adopts a boat-chair conformation.     

New water-soluble iminecalix[4]arene with a deep hydrophobic cavity

We report the synthesis of a new water-soluble iminecalix[4]arene host 4c with a deep hydrophobic cavity. The negatively charged four carboxylate functions on the top of the cavity play a major role in the recognition of charged molecular species. The ¹H NMR titration experiments revealed that host 4c binds with cationic (10-12) and neutral guests (6-9) in water with high binding constants in the order of 10⁴-10⁵ M⁻¹. Cationic guest 9 showed highest binding constant of 2.81 × 10⁵ M⁻¹ with host 4c amongst all tested guests. Selectivity over anionic guests (13-17) is established by the presence of negative charges at the top of the deep hydrophobic cavity, as guests 15 and 17 were not recognized by host 4c. Neutral pyridine derivatives with hydrophobic chains at para positions showed high binding constants of 6.02 × 10⁴-2.23 × 10⁵ M⁻¹. The data obtained for the recognition of the guests by host 4c revealed that the ionic as well as the hydrophobic-hydrophobic interactions are crucial in the molecular recognition in aqueous medium.     

Selective recognition of the ditopic trimethylammonium cations by water-soluble aminocalix[4]arene

In this Letter, the role of the depth of the cavity in the recognition processes of the guests by the hosts has been investigated. The hosts 2, 3 interact with both the cationic function and the aromatic moiety in the guests 5, 6 but with a slight preference for the cationic functions. The host 4 selectively recognizes the trimethylammonium functions of the guests 5 and 6. However, the host 1 selectively recognizes the aromatic moiety of the ditopic trimethylammonium guests 5 and 6. The recognition and orientation of the guest in the cavity of the host are directly dependent on the depth of the hydrophobic cavity of the host.     

Induced-fit recognition by p-carboxylatocalix[4]arene hosts

Calix[4]arene derivatives bearing anionic carboxylato groups at the upper rim (p-carboxylatocalix[4]arenes) are able to ‘grab’ paraquat dicationic guest through an induced-fit mechanism, which originates from the conformational mobility of calixarene skeleton.     

An expedient route to p-tert-butylthiacalix[4]arene 1,3-diethers via Mitsunobu reactions

Regioselective distal dialkylation of p-tert-butylthiacalix[4]arene with alcohols was performed under the Mitsunobu protocol using the DEAD/TPP system. The method provides a versatile tool for obtaining 1,3-diethers with different functional groups in the alkyl chains.     

Regioselective ipso formylation of p-tert-butylcalix[4]arene

A convenient procedure for direct introduction of one formyl group into p-tert-butylcalix[4]arenes through ipso substitution is described.     

A new fluorescent double-cavity calix[4]arene: synthesis and complexation studies toward nitroanilines

A novel bicyclic calix[4]arene (4) molecular receptor has been synthesized, and its sensory abilities toward noxious aromatic nitroamines evaluated in apolar media. Molecular host–guest interactions examined through fluorescence and ¹H NMR spectroscopies showed the formation of 1:1 type endo-inclusion complexes with all the nitroaniline guests. The potential of 4 as a chemical sensor is established.     

Functionalized thiacalix- and calix[4]arene-based Ag ionophores: synthesis and comparative NMR study

Thiacalix[4]arene ionophores comprised of cyclic or linear O,S,N ligating and/or π-coordinate groups on the lower rim were synthesized and their Ag⁺ binding was studied by ¹H NMR methods in comparison with the respective known and novel calix[4]arene counterparts. Calix[4](O,S,N)crowns were found stronger binders than the π-coordinate molecules and thiacalixarene ionophores were generally superior to calixarenes. This study helped to develop silver ion-selective electrodes working in the subnanomolar region.     

Calix[4]arene based molecular probe for sensing copper ions

A novel calix[4]arene based molecular probe for metal ions has been designed, synthesized and evaluated. Studies on its binding with different metal ions reveal a noticeable naked eye color change, bathochromic shift in absorption spectrum and remarkable enhancement in fluorescence emission in the presence of Cu²⁺ only. The role of calix[4]arene scaffold for selective recognition of Cu²⁺ has been demonstrated by repeat evaluation and analysis of an appropriate reference molecule. A rational explanation for fluorescence enhancement in 3 on interaction with copper has been suggested.     

Synthesis of pillar[7]arene

The first synthesis of pillar[7]arene is reported with two methods.Method A:the FeCl₃-catalyzed condensation reaction of 1,4- dimethoxybenzene（1） with paraformaldehyde in CHCl₃ gave dimethoxypillar[7]arene（3）.Method B:the p-toluenesulfonic acid catalyzed condensation reaction of 2,5-bis（benzyloxymethyl）-1,4-dimethoxybenzene（2） in CH₂Cl₂ gave compound 3.Demethylation of 3 with BBr₃ gave pillar[7]arene（4）.The pillar[7]arene might be a perspective macrocyclic host in host-guest chemistry.     

Regioselective intramolecular bridging of p-tert-butylcalix[7]arene

The first examples of singly bridged calix[7]arenes 2–4 have been obtained by base-promoted direct O-alkylation of p-tert-butylcalix[7]arene with a variety of bis-electrophiles including BrCH₂Cl, oligoethylene glycol ditosylates, and 1,4-bis(bromomethyl)benzene. 1,2-Bridging was favored with ‘short bite’ spanning elements, while the 1,4-isomer predominated with the others (yields up to 72% in the presence of Cs₂CO₃). Assignment of bridging pattern was mainly based on chemical shift of OH groups, in some cases confirmed by 2D NMR experiments. A hampered conformational mobility, depending on the position and nature of the bridge, was observed for compounds 2–4.     

Preparation and characterization of p-tert-butylcalix[8]arene bonded capillaries for open-tubular capillary electrochromatography

p-tert-Butylcalix[8]arene bonded capillaries for open-tubular capillary electrochromatography were prepared with γ-glycidoxypropyltrimethoxysilane as a bridge. The bonded capillary displayed low and steady electroosmotic flow (EOF) values over the pH range from 4 to 9. Detection limits for direct spectrophotometric detection at 277 nm for benzenediols (at a signal to noise ratio of 2) were 0.96 mg l⁻¹ for the unbonded capillary and 1.48 mg l⁻¹ for the bonded capillary, showing that the bonded layer did not show significant absorbance and hence decreased sensitivity. The bonded capillaries showed good separation selectivity for o-, m- and p-benzenediols, α- and β-naphthols, and α- and β-naphthylamines. This selectivity was attributed to significant interactions between the analytes and the bonded p-tert-butylcalix[8]arene, which contributed to the electrochromatographic separation mechanism. The bonded capillaries gave high stability and reproducibility.     

Efficient synthesis of water-soluble calix[4]arenes via thiol-ene “click” chemistry

Several water-soluble calix[4]arenes were synthesized via radical addition reaction between thiols and alkenes under UV lamp irradiation (λ= 365 nm) in good yields. The structures of these compounds synthesized herein were fully confirmed by 1H NMR, ESI-MS and elemental analysis.     

Synthesis of calix[4]arene(amido)monocrowns and their photoresponsive derivatives

A series of new calix[4]arene(amido)mono-crown compounds have been synthesized through aminolysis of calix[4]arene esters and intramolecular cyclization of the intermediates. The title compounds were converted into their nitro and azo substituted derivatives to provide novel photoresponsive molecular receptors for transition metal ions. Single crystal X-ray analysis of calix[4]arene(ethyleneamido)mono-crown (2a) revealed that the compound is present in a cone conformation with an amido loop that caps the lower rim of calix[4]arene cavity to result in stacking along axis a and axis c to provide supramolecular aggregates in the solid state. Evaluation of synthesized macrocycles in the solution phase for recognition of transition metal cations (Cr³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Cd²⁺, Pb²⁺, Hg⁺, Hg²⁺, Pd²⁺, and Pt²⁺) by UV-visible spectroscopy revealed that p-tert-butyl-calix[4]arene mono-(amidocrown) 1c selectively shows a blue shift at 38 nm on interaction with Hg⁺ ions.     

Aminolysis of p-tert-butyltetrathiacalix[4]arene tetraethylacetates in cone, partial cone and 1,3-alternate conformation: synthesis of amide based receptors for oxyanions

Cone, partial cone and 1,3-alternate conformers of tetrathiacalix[4]arene tetraethylacetate were synthesized and subjected to aminolysis with alkyl amines [CH₃(CH₂)_nNH₂; n=2, 3, 5] to yield mono-, di-, tri- or tetrasubstituted p-tert-butyltetrathiacalix[4]arene amides which were characterized by detailed analysis of their NMR spectral and single crystal X-ray crystallography. It has been observed that while the 1,3-alternate and cone conformers of the tetrathiacalix[4]arene tetraethylacetate gave corresponding tetrathiacalix[4]arene tetraamides under different experimental conditions, the corresponding partial cone conformer undergoes a cascade of regioselective reactions with the same amines. Variable temperature ¹H NMR experiments allowed the determination of relative stability of different conformers within the temperature range of 298-333 K. The synthesized derivatives were evaluated as molecular extractants for cations and anions and were determined to facilitate extraction of oxyanions (CrO₄²⁻ and Cr₂O₇²⁻) from aqueous to the organic phase. The studies have a significance in the design of tetrathiacalix[n]arene based molecular receptors for innovative applications.     

An approach to the synthesis of chemically modified bisazocalix[4]arenes and their extraction properties

Bisazocalix[4]arenes [N,N′-bis(5-azo-25,26,27-tribenzoyloxy-28-hydroxycalix[4]arene)benzene (1), N,N′-bis(5-azo-25,26,27-tribenzoyloxy-28-hydroxycalix[4]arene)biphenyl (2) and N,N′-bis(5-azo-25,26,27-tribenzoyloxy-28-hydroxycalix[4]arene)-2,2′-dinitro biphenyl (3)] have been synthesized from 25,26,27-tribenzoyloxy-28-hydroxycalix[4]arene by diazocoupling with the corresponding aromatic diamines (p-phenylenediamine, 4,4′-diamino biphenyl and 4,4′-diamino-2,2′-dinitrobiphenyl). Extraction studies of bisazocalix[4]arenes 1, 2, and 3 show no difference in their extraction behavior and selectivity, whereas azocalix[4]arenes are a poor extractant for heavy metal cations. The absorption spectra of the prepared bisazocalix[4]arenes are discussed, both the effect of varying pH and solvent upon the absorption ability of bisazocalix[4]arenes.     

Water-soluble pentasulfonatocalix[5]arene: selective recognition of ditopic trimethylammonium cations by a triple non-covalent interaction

The inclusion of tetramethylammonium and ditopic trimethylammonium cations by the water-soluble pentasulfonatocalix[5]arene 1 has been studied at neutral pH by ¹H NMR and compared with the homologous tetrasulfonatocalix[4]arene 2. Unlike host 2, host 1 selectively binds the ditopic trimethylammonium ions by three different non-covalent interactions. Remarkably the flexible host 1 exhibits both more efficiency and selectivity in the complexation of ditopic methylammonium ions with respect to similar more preorganised calix[4]arene receptors.     

p-tert-Butylcalix[8]arene catalysed synthesis of 3,5-dinitrothiophene scaffolds: antiproliferative effect of some representative compounds on selective anticancer cell lines

A new efficient protocol for the synthesis of 3,5-dinitrothiophene scaffolds was developed by using simple p-tert-butylcalix[8]arene in aqueous medium. Biological activities of some representative compounds were also studied to inhibit the cell growth on selective anticancer cell lines.