NMR of a series of novel hydroxyflavothiones

Alkylated hydroxyflavothiones, namely flavothione, 5‐hydroxyflavothione, 5,7‐dihydroxyflavothione (chrysinthione), 7‐dodecyloxy‐5‐hydroxyflavothione, 7‐butyloxy‐5‐hydroxyflavothione, 2′,3,4′,7‐tetramethoxy‐5‐hydroxyflavothione, 3,3′,4′,7‐tetramethoxy‐5‐hydroxyflavothione, 7‐butyloxy‐4′,5‐dihydroxyflavothione and 7‐butyloxy‐4′,5‐hydroxyflavanonethione have been synthesized from the corresponding hydroxyflavones in two steps, alkylation of the non‐hydrogen‐bonded hydroxyl groups by bromoalkanes or dimethyl sulfate followed by conversion of the carbonyl group to a thione using Lawesson's Reagent under microwave irradiation and solvent‐free conditions. Part of the alkylated flavanone, 7‐butyloxy‐4′,5‐dihydroxyflavanone, was oxidized during the treatment with Lawesson's reagent to yield a second product 7‐butyloxy‐4′,5‐dihydroxyflavothione in addition to the target product butyloxy‐4′,5‐hydroxyflavanonethione. Deuterium isotope effects on 13C chemical shifts have been measured in hydroxyflavones, isoflavones, flavanones and the thio analogs. Formal four‐bond deuterium isotope effects on 13C chemical shifts, nΔC?S(OD) are very sensitive to variations in structures and substitution patterns. Density functional theory (DFT) calculations are carried out to obtain geometries. Correlations relating distances around the hydrogen bond system to the deuterium isotope effects on 13C chemical shifts are discussed. 13C chemical shifts are calculated by DFT methods. Effects of thiocarbonyl anisotropies are suggested. Copyright © 2009 John Wiley & Sons, Ltd.     

1H NMR,IR and UV/VIS Spectroscopic Studies of Some Schiff Bases Derived from 2‐Aminobenzothiazole and 2‐Amino‐3‐Hydroxypyridine

By condensing 2‐aminobenzothiazole with 2‐hydroxy‐1‐naphthaldehyde, 2‐hydroxybenzaldehyde, 4‐methoxybenzaldehyde, 4‐hydroxybenzal‐dehyde, benzaldehyde and 4‐dimethylaminobenzaldehyde, and five Schiff bases Ia‐Ie are prepared. Also, two Schiff bases IIa and IIb are prepared by condensation of 2‐amino‐3‐hydroxypyridine with 2‐hydroxy‐1‐naphthaldehyde and 2‐hydroxybenzaldehyde. The ¹H NMR, IR and UV/Vis spectra of these seven Schiff bases are investigated. The signals of the ¹H NMR spectra as well as the important bands in the IR spectra are considered and discussed in relation to molecular structure. The UV/Vis absorption bands in ethanol are assigned to the corresponding electronic transitions and the electronic absorption spectra of Schiff bases Ib and IIb are studied in organic solvents of different polarities. The UV/Vis absorption spectra of 2‐amino‐3‐hydroxypyridine Schiff bases IIa and IIb are investigated in buffer solutions of different pH values containing 5% (v/v) methanol, and the results are utilized for the determination of pK_a and ΔG^* of the ionization of the phenolic OH‐groups. The fluorescence spectra of IIa and IIb are studied in organic solvents of different polarities. The obtained spectral results are confirmed by some molecular calculations using the atom super position and electron delocalization molecular orbital theory for the Schiff base IIb.     

β‐Sheet to Helical‐Sheet Evolution Induced by Topochemical Polymerization: Cross‐α‐Amyloid‐like Packing in a Pseudoprotein with Gly‐Phe‐Gly Repeats

Protein‐mimics are of great interest for their structure, stability, and properties. We are interested in the synthesis of protein‐mimics containing triazole linkages as peptide‐bond surrogate by topochemical azide‐alkyne cycloaddition (TAAC) polymerization of azide‐ and alkyne‐modified peptides. The rationally designed dipeptide N₃‐CH₂CO‐Phe‐NHCH₂CCH ( 1 ) crystallized in a parallel β‐sheet arrangement and are head‐to‐tail aligned in a direction perpendicular to the β‐sheet‐direction. Upon heating, crystals of 1 underwent single‐crystal‐to‐single‐crystal polymerization forming a triazole‐linked pseudoprotein with Gly‐Phe‐Gly repeats. During TAAC polymerization, the pseudoprotein evolved as helical chains. These helical chains are laterally assembled by backbone hydrogen bonding in a direction perpendicular to the helical axis to form helical sheets. This interesting helical‐sheet orientation in the crystal resembles the cross‐α‐amyloids, where α‐helices are arranged laterally as sheets.     

Chiral and water‐soluble poly(2‐oxazoline)s

We describe the synthesis and characterization of the first water‐soluble and chiral poly(2,4‐disubstituted‐2‐oxazoline)s. While poly(2,4‐dimethyl‐2‐oxazoline)s are water soluble up to 100 °C, aqueous solutions of poly(2‐ethyl‐4‐methly‐2‐oxazoline) exhibit a lower critical solution temperature. This is discussed in context with its constitutional isomers poly(2‐oxazoline)s and poly(2‐oxazine)s. Circular dichroism spectroscopy revealed strong Cotton effects, which are also responsive to temperature in aqueous solution. It is therefore hypothesized that structures, comparable to polyproline helices, are formed in aqueous solution. In contrast to polyproline, poly(2,4‐disubstituted‐2‐oxazoline)s are highly water soluble and therefore represent very interesting pseudo‐polypeptides that may be useful to develop responsive biomimetic biomaterials. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Hydrolytic Heterocyclization of the 1,1,2,3,3‐Pentacyanopropene Salts

Salts of 5‐amino‐4‐cyano‐3‐dicyanomethylene‐2‐pyrrolinone are formed by action of dilute aqueous solutions of alkalis on pentacyanopropenide‐anion salts. These cyclic salts are transformed to 2,6‐diamino‐3,5‐dicyanoisonicotinates by subsequent action of alkalis.     

Supramolecular porphyrin assemblies through amidinium-carboxylate salt bridges and fast intra-ensemble excited energy transfer

Well‐defined supramolecular assemblies of Zn and free‐base porphyrins are constructed through the formation of amidinium–carboxylate salt bridges. A one‐to‐one donor–acceptor pair and a four‐to‐one antenna‐type assembly are investigated. The steady‐state and time‐resolved fluorescence measurements unequivocally showed that efficient singlet–singlet excited energy transfer from the Zn–porphyrin complex to the free‐base porphyrin takes place in these assemblies. Indeed, the observed energy‐transfer rates in both types of assemblies are much faster than those the Förster mechanism would suggest, implying the involvement of an intermolecular through‐bond mechanism.     

Synthetic approaches to 4,8‐dimethyl‐5′‐(N‐pyridiniummethyl)‐ 4′,5′‐dihydropsoralens and 4,8‐dimethyl‐5′‐ (N‐aminomethyl)‐ 4′,5′‐dihydropsoralens

New synthetic approaches to 4,8‐dimethyl‐5′‐(N‐pyridiniummethyl)‐4′,5′‐dihydropsoralens and 4,8‐dimemyl‐5′‐(N‐aminomethyl)‐4′,5′‐dihydropsoralens are described. The 5′‐halomethyl‐4′,5′‐dihydro‐psoralen precursors are formed by electrophilic ring closures of 4,8‐dimethyl‐6‐allyl‐7‐hydroxycoumarin. The ring‐closure reactions may also be applied to the synthesis of 5′‐halomethyl‐4‐methyl‐4′,5′‐dihydroangelicins. The compounds are potential therapeutic agents for improved psoralen ultraviolet A radiation treatment.     

Intramolecular Participation of Amino Groups in the Cleavage and Isomerization of Ribonucleoside 3′‐Phosphodiesters: The Role in Stabilization of the Phosphorane Intermediate

A dinucleoside‐3′,5′‐phosphodiester model, 5′‐amino‐4′‐aminomethyl‐5′‐deoxyuridylyl‐3′,5′‐thymidine, incorporating two aminomethyl functions in the 4′‐position of the 3′‐linked nucleoside has been prepared and its hydrolytic reactions studied over a wide pH range. The amino functions were found to accelerate the cleavage and isomerization of the phosphodiester linkage in both protonated and neutral form. When present in protonated form, the cleavage of the 3′,5′‐phosphodiester linkage and its isomerization to a 2′,5′‐linkage are pH‐independent and 50–80 times as fast as the corresponding reactions of uridylyl‐3′,5′‐uridine (3′,5′‐UpU). The cleavage of the resulting 2′,5′‐isomer is also accelerated, albeit less than with the 3′,5′‐isomer, whereas isomerization back to the 3′,5′‐diester is not enhanced. When the amino groups are deprotonated, the cleavage reactions of both isomers are again pH‐independent and up to 1000‐fold faster than the pH‐independent cleavage of UpU. Interestingly, the 2′‐ to 3′‐isomerization is now much faster than its reverse reaction. The mechanisms of these reactions are discussed. The rate accelerations are largely accounted for by electrostatic and hydrogen‐bonding interactions of the protonated amino groups with the phosphorane intermediate.     

Theoretical studies on geometry,solvent effect,and photochromic mechanism of two bis‐heterocyclic compounds containing pyrazolone ring

Two bis‐heterocyclic compounds containing pyrazolone ring, 1‐phenyl‐3‐methyl‐4‐(6‐hydro‐4‐amino‐5‐sulfo‐2,3‐pyrazine)‐pyrazole‐5‐one and 1‐phenyl‐3‐methyl‐4‐(6‐hydro‐4‐methylamino‐5‐sulfo‐2,3‐pyrazine)‐pyrazole‐5‐one, are investigated to gain a deeper insight into their geometries and photochromic mechanism by applying density functional theory. The solvent effects are simulated using the polarizable continuum model of the self‐consistent reaction field theory. Bader's atom‐in‐molecule theory is used to investigate the nature of hydrogen bonds. The data of energy, dipole moments, and the condensed Fukui functions have been calculated to assess the stability and reactivity of the title compounds. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

The Quest of Excited‐State Intramolecular Proton Transfer via Eight‐Membered Ring π‐Conjugated Hydrogen Bonding System

Searching for eight‐membered ring π‐conjugated hydrogen bonding (8‐MR H‐bonding) systems with excited‐state intramolecular proton transfer (ESIPT) property is seminal and synthetically challenging. In this work, a series of π‐conjugated molecules ( 8‐HB‐1 , 8‐HB‐L1 and 8‐HB‐2 ) potentially possessing 8‐MR H‐bonding are strategically designed, synthesized and characterized. The configurations of these three potential molecules are checked by their X‐ray structures, among which 8‐HB‐L1 (a structurally locked 8‐HB‐1 core chromophore) is proved to be an 8‐MR H‐bonding system, whereas 8‐HB‐1 and 8‐HB‐2 are too sterically hindered to form the 8‐MR intramolecular H‐bond. The ESIPT property of 8‐HB‐L1 is confirmed by the dual fluorescence consisting of normal and proton‐transfer tautomer emissions. The insight into the ESIPT process of 8‐HB‐L1 is provided by femtosecond fluorescence upconversion measurements together with computational simulation. The results demonstrate for the first time a successful synthetic route to attain the 8‐MR H‐bonding molecule 8‐HB‐L1 with ESIPT property.     

The characteristic of photoluminescence of tris‐(7‐substituted‐8‐hydroxyquinoline) aluminum complexes and polymeric complexes

The 7‐allyl‐ and 7‐(2‐methylvinyl)‐functionalized derivatives of 8‐hydroquinoline are synthesized by Claisen rearrangement and double bond rearrangement respectively. Then 7‐allyl‐8‐hydroquinoline (C) and 7‐(2‐methylvinyl)‐8‐hydroquinoline (D) are reacted with aluminum chloride to afford the corresponding tris‐(7‐allyl‐8‐hydroxyquinoline) aluminum complex (F) and tris‐(7‐(2‐methylvinyl)‐8‐hydroxyquinoline) aluminum complex (G). The photoluminescence of complex (F) or (G), compared with that of tris‐(8‐hydroxyquinoline) aluminum complex (E), all showed a red shift in emission wavelengths in different solvents, such as chloroform, hexane and ethanol. For two substituents containing an external double bond, the 2‐methylvinyl group gives a larger red shift in the emission wavelength than the allyl group. The X‐ray crystal structure indicates that 7‐(2‐methylvinyl)‐8‐hydroxyquinoline (D) is a trans‐isomer. The styrene and 7‐allyl‐8‐hydroxyquinoline copolymer, and the styrene and 7‐(2‐methylvinyl)‐8‐hydroxyquinoline copolymer are also reported. Further reactions of the copolymer are then performed by adding aluminum(III) chloride and ligands 8‐hydroxyquinoline. The spectroscopic characteristics of these aluminum(III) polymeric complexes are discussed. Copyright © 2003 John Wiley & Sons, Ltd.     

Enthalpy and Entropy Barriers Explain the Effects of Topology on the Kinetics of Zeolite‐Catalyzed Reactions

The methylation of ethene, propene, and trans‐2‐butene on zeolites H‐ZSM‐58 (DDR), H‐ZSM‐22 (TON), and H‐ZSM‐5 (MFI) is studied to elucidate the particular influence of topology on the kinetics of zeolite‐catalyzed reactions. H‐ZSM‐58 and H‐ZSM‐22 are found to display overall lower methylation rates compared to H‐ZSM‐5 and also different trends in methylation rates with increasing alkene size. These variations may be rationalized based on a decomposition of the free‐energy barriers into enthalpic and entropic contributions, which reveals that the lower methylation rates on H‐ZSM‐58 and H‐ZSM‐22 have virtually opposite reasons. On H‐ZSM‐58, the lower methylation rates are caused by higher enthalpy barriers, owing to inefficient stabilization of the reaction intermediates in the large cage‐like pores. On the other hand, on H‐ZSM‐22, the methylation rates mostly suffer from higher entropy barriers, because excessive entropy losses are incurred inside the narrow‐channel structure. These results show that the kinetics of crucial elementary steps hinge on the balance between proper stabilization of the reaction intermediates inside the zeolite pores and the resulting entropy losses. These fundamental insights into their inner workings are indispensable for ultimately selecting or designing better zeolite catalysts.     

Selenoacetalyzation of 4‐Formylpyrazoles in the Presence of Trimethylchlorosilane

The reaction of 3,5‐dimethyl‐4‐formylpyrazoles, bearing various substituents at N‐1 atom, with propane‐1,3‐diselenol and 2‐hydroxypropane‐1,3‐diselenol in the presence of ТМSCl proceeds without heating to chemoselectively give hitherto unknown 2‐(pyrazol‐4‐yl)‐1,3‐diselenane hydrochlorides in high yields. The latter are easily transformed to the corresponding free bases—2‐(pyrazol‐4‐yl)‐1,3‐diselenanes. The ¹⁵N chemical shifts of the pyrazole ring in 2‐(pyrazol‐4‐yl)‐1,3‐diselenanes obtained by 2D HMBC‐gp (¹⁵N‐¹H) technique are indicative of the N‐2 atom protonation in hydrochlorides.     

An Efficient Synthesis of 3‐Substituted Isoquinoline and Pyridine Derivatives by Gold Catalyzed Intramolecular Cyclization from o‐Alkynyloximes

A one‐pot reaction was developed efficiently by AuCl₃ catalyzed intramolecular cyclization of aromatic o‐alkynyloximes and 2‐alkynylcycloalkene‐1‐carbaldoximes leading to the formation of isoquinoline and pyridine derivatives with high yields. This methodology has been applied for aromatic as well as aliphatic systems. Aromatic o‐alkynyloximes are benzene and naphthalene, whereas electron‐donating groups are 4‐methoxybenzene, 4‐methylbenzene, and 4‐methoxy‐5‐methylbenzene. There are electron‐withdrawing groups such as chloro and nitrobenzene o‐alkynyl oximes, and the same methodology has been successfully applied to pyridine and piperonal, which is also extended to aliphatic rings such as five‐member, six‐member, seven‐member, and eight‐member 2‐alkynylcycloalkene‐1‐carbaldoximes.     

Tunable Crystallinity and Charge Transfer in Two‐Dimensional G‐Quadruplex Organic Frameworks

DNA G‐quadruplex structures were recently discovered to provide reliable scaffolding for two‐dimensional organic frameworks due to the strong hydrogen‐bonding ability of guanine. Herein, 2,7‐diaryl pyrene building blocks with high HOMO energies and large optical gaps are incorporated into G‐quadruplex organic frameworks. The adjustable substitution on the aryl groups provides an opportunity to elucidate the framework formation mechanism; molecular non‐planarity is found to be beneficial for restricting interlayer slippage, and the framework crystallinity is highest when intermolecular interaction and non‐planarity strike a fine balance. When guanine‐functionalized pyrenes are co‐crystallized with naphthalene diimide, charge‐transfer (CT) complexes are obtained. The photophysical properties of the pyrene‐only and CT frameworks are characterized by UV/Vis and steady‐state and time‐resolved photoluminescence spectroscopies, and by EPR spectroscopy for the CT complex frameworks.     

Nitrite Ionic Liquids (IL‐ONO and [bmim]NO2) as Effective Nitrosonium Sources for the Synthesis of α‐Oximinoketones under Mild Heterogeneous Conditions

Ketones and β‐diketones were nitrosated and converted to their corresponding α‐oximinoketones using task‐specific ionic liquids, 1‐(4‐nitritobutyl)‐3‐methylimidazolium chloride, IL‐ONO, and 1‐butyl‐3‐methylimidazolium nitrite at room temperature. The results from two ionic liquids are comparable and showed that these IL's are effective nitrosonium sources for the preparation of oximinoketones. The protocol is rapid, the yields are excellent, and the method is simple.     

Polyion Detection via All‐solid‐contact Paper‐based Polyion‐sensitive Polymeric Membrane Electrodes

The first all‐solid‐contact paper‐based single‐use polyion‐sensitive ion‐selective electrodes (ISEs) are described. These polyion‐sensitive ISEs are fabricated using cellulose filter paper coated with a carbon ink conductive layer. A polyanion sensing membrane is cast on a section of the coated paper and the sensor is insulated, resulting in a disposable, single‐use device. Various polyanions are shown to yield large negative potentiometric responses when using these disposable devices for direct polyanion detection. These new sensors are further demonstrated to be useful in indirect polycation detection when polycations (i. e., polyquaterniums (PQs)) are titrated with polyanionic dextran sulfate (DS). Titrations monitored using these paper‐based, all‐solid‐contact devices yield endpoints proportional to the given PQ concentration present in the test sample.     

Stability of 7,8‐Dihydropterins in Air‐Equilibrated Aqueous Solutions

6‐Substituted 7,8‐dihydropterins (=2‐amino‐7,8‐dihydropteridin‐4(1H)‐ones) are heterocyclic compounds that occur in a wide range of living systems and participate in relevant biological functions. In air‐equilibrated aqueous solutions, these compounds react with dissolved O₂ (autooxidation). The rates of these reactions as well as the products formed strongly depend on the chemical structure of the substituents. 7,8‐Dihydro‐6‐methylpterin and 7,8‐dihydro‐6,7‐dimethylpterin that bear electron‐donor groups as substituents are the most reactive derivatives and undergo oxidation of the pterin moiety to yield the corresponding oxidized derivatives (6‐methylpterin and 6,7‐dimethylpterin, resp.). The oxidations of 7,8‐dihydrobiopterin, 7,8‐dihydroneopterin, and 7,8‐dihydrofolic acid are slower, and they yield 7,8‐dihydroxanthopterin as the main product. 7,8‐Dihydroxanthopterin, 6‐formyl‐7,8‐dihydropterin, and sepiapterin are rather stable, and their consumption in air‐equilibrated solutions is negligible for several days. The pseudo‐first‐order rate constants of the reactions between these compounds and O₂ at 25° and 40° are reported. The biological implications of the results obtained are also discussed.     

Ab initio and DFT study of the electronic structures and spectroscopic properties of pyrene ligands and their cyclometalated complexes

Two ligands 1‐diphenylphosphinopyrene (1‐PyP) ( L ₁ ), 1,6‐bis(diphenylphosphino)‐pyrene (1,6‐PyP) ( L ₂ ) and their cyclometalated complexes [Pt(dppm)(1‐PyP‐H)]⁺ ( 1 ), [Pt₂(dppm)₂(1,6‐PyP‐H₂)]²⁺ (dppm = bis(diphenylphosphino)methane ( 2 ), and [Pd(dppe)(1‐PyP‐H)⁺ (dppe = bis(diphenylphosphino)ethane) ( 3 ) are investigated theoretically to explore their electronic structures and spectroscopic properties. The ground‐ and excited‐state structures are optimized by the density functional theory (DFT) and single‐excitation configuration interaction method, respectively. At the time‐dependent DFT (TDDFT) and B3LYP level, the absorption and emission spectra in solution are obtained. As revealed from the calculations, the lowest‐energy absorptions of 1 and 3 are attributed to the mixing ligand‐to‐metal charge transfer (CT)/intraligand (IL)/ligand‐to‐ligand CT transitions, while that of 2 is attributed to the IL transition. The lowest‐energy phosphorescent emissions of the cyclometalated complexes are attributed to coming from the ³ILCT transitions. With the increase of the spin‐orbit coupling effect, the phosphorescence intensities and the emissions wavelength are correspondingly increased. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Self‐Assembly in Helical Columnar Mesophases and Luminescence of Chiral 1H‐Pyrazoles

Chiral polycatenar 1H‐pyrazoles self‐assemble to form columnar mesophases that are stable at room temperature. X‐ray diffraction and CD studies in the mesophase indicate a supramolecular helical organization consisting of stacked H‐bonded dimers. The liquid‐crystalline compounds reported are 3,5‐bis(dialkoxyphenyl)‐1H‐pyrazoles that incorporate two or four dihydrocitronellyl chiral tails. It can be observed that the grafting of these branched chiral substituents onto the 3,5‐diphenyl‐1H‐pyrazole core has a beneficial role in inducing mesomorphism, because isomeric linear‐chain compounds are not liquid crystalline; this is not the usual scheme of behavior. Furthermore, the molecular chirality is transferred to the columnar mesophase, because preferential helical arrangements are observed. Films of the compounds are luminescent at room temperature and constitute an example of the self‐organization of nondiscoid units into columnar liquid‐crystalline assemblies in which the functional molecular unit transfers its properties to a hierarchically built superstructure.