Foldamer-based pyridine-fullerene tweezer receptors for enhanced binding of zinc porphyrin

This paper reports the design and synthesis of a new series of hydrogen bonding-mediated foldamer-derived tweezer receptors that are used for efficient complexation of zinc porphyrin guest. One end of the rigidified aromatic amide backbone is incorporated with one fullerene unit, while another end is connected to one pyridine or imidazole unit. The ¹H NMR, UV-vis, and fluorescent investigations in chloroform revealed that, due to the intramolecular hydrogen bonding-driven preorganized folded conformation, the fullerene and pyridine units of the receptors are located with suitable spatial separation and consequently able to co-complex zinc porphyrin with remarkably increased stability. In contrast, the imidazole-incorporated receptor displays a weakened binding affinity possibly due to structural mismatching and large steric hindrance. The association constants of the complexes of the new receptors with zinc porphyrin have been determined.     

Hydrogen bonding-driven elastic bis(zinc)porphyrin receptors for neutral and cationic electron-deficient guests with a sandwich-styled complexing pattern

This Letter reports the design and synthesis of a new type of hydrogen bonding-mediated foldamer-derived tweezer receptors that are incorporated with two peripheral (zinc)porphyrin units. Due to the existence of four intramolecular hydrogen bonds, the (zinc)porphyrin units are forced to approach and stack with each other. ¹H NMR and fluorescent studies revealed that the new receptors could form 1:1 complexes with planar electron-deficient molecules such as naphthalene and benzene diimides and paraquat through a unique sandwich-styled binding pattern. The association constants of the new complexes have been evaluated by the ¹H NMR or fluorescent titration methods.     

Strapped porphyrin rosettes based on the melamine-cyanuric acid motif. Self-assembly and supramolecular recognition

This paper describes studies on the synthesis, self-assembly behavior, and complexing properties of several strapped porphyrin-incorporated melamine-cyanuric or melamine-barbiturate-based rosette supramolecules in chloroform-d. Strapped porpyrin cyanuric acid H₂1 and its Zn (II) complex Zn1 were designed and synthesized. Both H₂1 and Zn1 could combine melamine derivatives 11 or 12 to afford porphyrin rosettes, which are more stable than the model rosette initially reported by Whitesides due to the larger size of the porphyrin unit. The new porphyrin rosettes could efficiently complex tripyridyl derivative 13 through intermolecular, cooperative coordination between Zn (II) and pyridine. Two new pyridine-bearing barbiturates 18a and 18b were also synthesized. Mixing the identical amount of 18a or 18b with 11 or 12 in chloroform-d led to the formation of new isomeric rosettes as a result of different orientation of the pyridine unit of 18a or 18b in the rosettes. ¹H NMR study also revealed that porpyrin-bearing rosette Zn1₃·11₃ could complex pyridine-bearing rosette 11₃·18a₃, leading to the formation of new two-layer-typed supramolecular architectures.     

Chiral eighteen-component three-dimensional supramolecular entities stabilized by the hydrogen bonding and coordination interactions

A new class of chiral eighteen-component three-dimensional supramolecular entities has been assembled in toluene and chloroform from twelve zinc porphyrin-appended 2-(ethylamino)- pyrimido[4,5-b][1,8]naphthyridin-4(3H)-one monomers and six chiral bipyridyl compounds. The heterocyclic segments form two C₆-symmetric cyclic hexamers, which are stabilized by a well-established DDA-AAD hydrogen bonding motif, while the six chiral bispyridine ligands are coordinated to the corresponding zinc porphyrin units to give the two-layered architectures. The structures have been characterized by the ¹H NMR, UV-vis and circular dichroism experiments, which also reveals that, when the concentration of the monomers is high enough, the chiral supramolecular entity can be formed exclusively.     

Synthesis of triazole analogues of the nanaomycin antibiotics using ‘click chemistry’

A series of triazole analogues of the nanaomycin family of antibiotics have been prepared using a ‘click’ dipolar cycloaddition of a naphthalene azide to various alkynes, followed by oxidation to the desired pyranonaphthoquinones.     

Chiral supramolecular metal-organic architectures from dinuclear copper complexes

Two new chiral dinuclear Cu(II) complexes [Cu₂(μ-Cl)₂(HL¹)₂] · C₂H₅OH (1) and [Cu₂(μ-Cl)₂(HL²)₂] · CH₃OH (2), have been synthesized and structurally characterized, where the chiral ligands H₂L¹ and H₂L² are derived from the chiral amino alcohols (S)-(−)-2-amino-3-phenyl-1-propanol and (S)-(+)-2-phenylglycinol. Single-crystal X-ray crystallographic analyses revealed that in these complexes, the dominant hydrogen bonding property of metal bound chloride anion directs the self assembly of complex molecules through C–H···Cl hydrogen bonding interactions leading to the formation of intriguing hydrogen bonded metallo-supramolecular architectures in their respective crystal lattices. The supramolecular systems described here belong to the rare class of metal-organic architectures that are formed as a result of metal directed hydrogen bonding interactions among chiral complex molecules. Complexes 1 and 2 are further characterized by IR, ESR, UV–Vis and CD spectroscopy.     

The first one-pot ‘alkane-like’ reactions of carbonyl-containing adamantanes

A novel ‘alkane-like’ methodology for the direct and very simple one-pot functionalization of amides and an ester of 1-adamantanecarboxylic acid provides access to new and synthetically challenging 1,3-dicarbonyl-containing adamantanoid compounds with the same or different functional groups.     

‘Click’ chemistry as a tool for the facile synthesis of fullerene glycoconjugate derivatives

A bis-malonate C₆₀ derivative bearing terminal alkyne groups prepared by the Bingel reaction has been used as a building block under copper-catalyzed azide-alkyne cycloaddition conditions to produce a series of new fullerene glycoconjugate derivatives.     

Synthesis of azide-alkyne fragments for ‘click’ chemical applications; formation of oligomers from orthogonally protected trialkylsilyl-propargyl azides and propargyl alcohols

A series of orthogonally protected 1,4-disubstituted-1,2,3-triazoles were prepared from the corresponding alkynols and trialkylsilyl-propargyl azides via 1,3-dipolar cycloaddition. These cycloadducts were selectively deprotected and extended in a stepwise fashion via further ‘click’ reactions to form oligomeric peptidomimetic compounds. This methodology gives access to triazole-based peptidomimetics in a controlled fashion and lays the foundation for a fragment-based approach to drug discovery.     

Reaction of unsymmetrical trifluoromethyl-containing 1,3-dicarbonyl compounds with ‘push-pull’ enamines

The reaction of ‘push-pull’ enamines with trifluoromethyl-containing 1,3-dicarbonyl compounds was investigated. It was found that the reaction is sensitive both to the structure of the enamines and to reaction conditions. As a result, a set of various trifluoromethyldialkylanilines was obtained.     

Molecular recognition studies with a simple dipyrrinone

We present our investigations of 2-ethyl-3-methyl-(10H)-dipyrrin-1-one, its self-association, and anion binding properties. This receptor is easily accessible in a facile single step synthesis with a straightforward workup. An examination of the concentration dependence of the dipyrrinone NH chemical shifts in CDCl₃ and (CDCl₂)₂ over the temperature range from −20 °C to 100 °C determined the self-association constant to be 3850 M⁻¹. Molecular recognition studies have shown that it has a preference for guests with an OH moiety, such as hydrogen sulfate (HSO₄⁻) and carboxylic acids (RCO₂H).     

Zr-promoted ‘pair’-selective and regioselective synthesis of penta-substituted benzene derivatives

Tetra-substituted zirconacyclopentadiene derivatives, obtainable via in situ generation of zirconacyclopropenes and their cyclic carbozirconation with alkynes, can be treated with alkynyllithiums to induce 1,2-migration accompanied by aromatization and protonolysis, leading to the formation of penta-substituted benzene derivatives, in which all five substituted may be different.     

Synthesis of ‘difficult’ peptides free of aspartimide and related products, using peptoid methodology

We developed an efficient, cost effective strategy for Fmoc-based solid phase synthesis of ‘difficult’ peptides and/or peptides containing Asp/Asn-Gly sequences, free of aspartimide and related products, using a peptoid methodology for the preparation of N-substituted glycines.     

Fluorophore-linked zinc(II)dipicolylamine coordination complexes as sensors for phosphatidylserine-containing membranes

A series of Zn²⁺-2,2′-dipicolylamine (Zn²⁺-DPA) coordination complexes with an attached NBD fluorophore are synthesized and evaluated as fluorescent sensors. The sensors do not respond to vesicles composed of zwitterionic phosphatidylcholine, but the NBD fluorescence emission is enhanced in the presence of anionic vesicles. A sensor with two Zn²⁺-DPA units and a hydrophilic tris(ethyleneoxy) linker produced a larger emission enhancement than an analogue with a butyl linker, and titration with 1:1 POPC:POPS vesicles lead to an apparent phospholipid association constant of 5.3×10⁴ M⁻¹. The sensor can detect the presence of vesicles containing as little as 5% phosphatidylserine. The sensing effect apparently requires a membrane surface because the sensors do not respond to a phosphatidylserine derivative that is monodispersed in aqueous solution.     

Selective recognition of sodium cyanide and potassium cyanide by diaza-crown ether-capped Zn-porphyrin receptors in polar solvents

Two new ditopic porphyrin receptors Zn1, incorporating a diaza-15-crown-5 unit, and Zn2, incorporating a diaza-18-crown-6 unit, have been prepared and characterized. UV-vis study in polar methanol has revealed that Zn1 is able to selectively recognize sodium cyanide over potassium cyanide (the ratio of their binding constant is ca. 56), whereas Zn2 exhibits a higher binding affinity for potassium cyanide over sodium cyanide (the ratio of their binding constant is ca. 12). In contrast, both receptors display substantially weaker binding affinity for sodium thiocyanate and potassium thiocyanate presumably due to a monotopic binding fashion.     

Recognition of a dicarboxylic acid with dipicolyl urea in solution and in solid phases: intramolecular hydrogen bond inhibiting both pyridine nitrogens from binding carboxyl groups

Recognition of a dicarboxylic acid in solution as well as in the solid phase by the pyridyl urea based pseudoditopic receptor 1 has been studied. The X-ray structures of both the receptor and its complex with 1,4-phenylenediacetic acid are also presented. Intramolecular hydrogen bonding inhibits both the pyridine ring nitrogens from forming hydrogen bonds with the carboxyl group and force the receptor to behave in a monotopic manner, using the syn urea amide moiety to bind carboxyl group of a dicarboxylic acid to form a 2:1 complex. Binding of receptor 1 with a monocarboxylic acid is also compared.     

Synthesis,spectral, magnetic,electrochemical and kinetic studies of copper(II), nickel(II) and zinc(II) acetate complexes derived from phenol based ‘end-off’ ligands: Effect of p-substituents

A new class of symmetric, end-off, N-methyl piperazine armed binucleating ligands 2,6-bis(4-methyl piperazin-1-yl-methyl)-4-acetyl phenol (HL¹) and 2,6-bis[(4-methyl piperazin-1-yl-methyl)]-(4-methylcarboxy) phenol (HL²) were synthesized by the Mannich reaction. Their mononuclear and binuclear Cu(II), Ni(II) and Zn(II) complexes have been synthesized. These complexes were characterized by elemental analysis, infra-red and electronic spectral analysis. In the electronic spectra, the lower electron withdrawing nature of the C(O)CH₃p-substituent (HL¹) compared with the C(O)OCH₃p-substituent (HL²) of the phenolic ring causes a red shift in the LMCT-charge transfer band. The mononuclear Cu(II) complexes 1 and 7 have a magnetic moment value close to the spin only value with four hyperfine EPR signals. The binuclear Cu(II) complexes 4 and 10 illustrate an antiferromagnetic interaction (μ_eff 1.56 and 1.55 BM) at 298 K with a broad EPR signal. A variable temperature magnetic moment study of the binuclear copper(II) complexes shows that the extent of antiferromagnetic coupling increases in the order: CHO [K. Shanmuga Bharathi, A. Kalilur Rahiman, K. Rajesh, S. Sreedaran, P.G. Aravindan, D. Velmurugan, V. Narayanan, Polyhedron 25 (2006) 2859] < C(O)CH₃ < C(O)OCH₃ (−2J values 134 [Shanmuga Bharathi et al., mentioned above], 149 and 158 cm⁻¹, respectively). The mononuclear Ni(II) complexes 2 and 8 are square planar and diamagnetic. The six coordinated binuclear Ni(II) complexes 5 and 11 show a magnetic moment value of 2.96 and 2.95 BM, respectively. Electrochemical studies of the complexes reveal that all the mononuclear complexes show a single irreversible one-electron transfer reduction wave and the binuclear complexes show two irreversible one-electron transfer reduction waves in the cathodic region. There is an anodic shift in the reduction of the metal centres when the electron withdrawing nature of the p-substituent of the phenolic ring increases. The catecholase activity of the mono and binuclear copper(II) complexes, using pyrocatechol as a model substrate, and the hydrolysis of 4-nitrophenyl phosphate using the mono and binuclear copper(II), nickel(II) and zinc(II) complexes as catalysts showed that the binuclear complexes have higher rate constant values than those of the corresponding mononuclear complexes. A comparison of the spectral, electrochemical and magnetic behaviour of the complexes derived from the ligands is discussed on the basis of the substituent at the para position of the phenolic ring.     

Comparative analysis of volatile compounds of ‘fino’ sherry wine by rotatory and continuous liquid-liquid extraction and solid-phase microextraction in conjunction with gas chromatography-mass spectrometry

Different headspace solid-phase microextraction (HS-SPME) methods have been selected and applied to the analysis of volatile compounds in ‘fino’ sherry wine by gas chromatography-mass spectrometry. A method based on rotary and continuous liquid-liquid extraction (LLE) for analysis of these same compounds has been optimised. The best conditions to extract this type of compounds using SPME and LLE were determined and both methods were validated. Both methodologies show adequate detection and quantitation limits, and linear ranges for correctly analysing these compounds. The recoveries obtained were close to 100%, with good repeatability values. The analytical and procedural advantages and disadvantages of these two methods have been compared. In general, SPME presented higher sensitivities. Both analytical methods were used to analyse five samples of ‘fino’ sherry wine supplied by different producers. No significant differences were found between the techniques at a significance level of 5%. The regression coefficients (r²) for analysis using LLE and SPME exceeded 0.94 for all compounds. The LLE procedure is a method with high repeatability and has the possibility of simultaneous extraction of several samples (up to 12), however the SPME technique is a solvent-free method presenting major advantages, such as small sample volume and higher sensitivity and simplicity.     

Mechanistic studies of DCC/HOBt-mediated reaction of 3-phenylpropionic acid with benzyl alcohol and studies on the reactivities of ‘active ester’ and the related derivatives with nucleophiles

Despite of the extensive study for peptide synthesis, DCC-mediated esterification is left still unclear. Therefore, DCC- and DCC/HOBt-mediated reactions of 3-phenylpropionic acid (1) with benzyl alcohol were carried out under several mechanistic considerations. Further, in order to determine the reactivities of the so-called ‘active esters’ compounds changing the substituents bearing carbonyl and related derivatives group for the purpose of the development of new class of non-symmetry cross-linkers, we have studied the reaction of model compounds, N-(3-phenylpropionyloxy)benzotriazole (6), N-(3-phenylpropionyloxy)phthalimide (7), 3-phenylpropionyloxybenzothiazole (8), and N-(3-phenylpropionyl)benzotriazole (9) with various nucleophiles under similar conditions were carried out for the comparison. It was revealed to exhibit the order of 6>>8>9>7.