Total synthesis of myxol and deoxymyxol stereoisomers and their application to determining the absolute configurations of the natural products

The total synthesis of myxol stereoisomers 1a,b and deoxymyxol (plectaniaxanthin) stereoisomers 2a,b was accomplished by Wittig reaction of (S)- and (R)-C₁₀-phosphonium salts 8a,b bearing a silyl-protected 1,2-dihydroxy-ψ end group with C₃₀-apocarotenals 6 and 7. The phosphonium salts 8a,b were derived from aldehydes 11a,b possessing a cyclopentylidene ketal moiety, prepared via Sharpless asymmetric epoxidation of allylic alcohol 12 followed by regioselective cleavage of the oxirane ring. We established an analytical HPLC method using a chiral column to separate stereoisomers 1a,b and 2a,b and thus determined the absolute configurations of the natural products. The HPLC analyses established that both myxol and deoxymyxol isolated from bacteria have the 2′S-configuration.     

Molecularly imprinted polymer based potentiometric sensor for the determination of hydroxyzine in tablets and biological fluids

Molecular imprinting is a useful technique for the preparation of functional materials with molecular recognition properties. In this work, a biomimetic potentiometric sensor, based on a non-covalent imprinted polymer, was fabricated for the recognition and determination of hydroxyzine in tablets and biological fluids. The molecularly imprinted polymer (MIP) was synthesized by precipitation polymerization, using hydroxyzine dihydrochloride as a template molecule, methacrylic acid (MAA) as a functional monomer and ethylene glycol dimethacrylat (EGDMA) as a cross-linking agent. The sensor showed a high selectivity and a sensitive response to the template in aqueous system. The MIP-modified electrode exhibited a Nernstian response (29.4 ± 1.0 mV decade⁻¹) in a wide concentration range of 1.0 × 10⁻⁶ to 1.0 × 10⁻¹ M with a lower detection limit of 7.0 × 10⁻⁷ M. The electrode demonstrated a response time of ∼15 s, a high performance and a satisfactory long-term stability (more than 5 months). The method has the requisite accuracy, sensitivity and precision to assay hydroxyzine in tablets and biological fluids.     

Synthesis and Characterization of Pyrrolthiosemicarbazone Complexes of Palladium(II). Crystal Structures of [{Pd[C4H4NC(H)=NNC(S)NHMe](Cl)}2{μ‐Ph2P(CH2)3PPh2}] and [Pd{C4H4NC(H)=NNC(S)NHMe}{Ph2P(CH2)2PPh2‐P,P}](Cl)

Reaction of the thiosemicarbazone ligands C₄H₄NC(H)=NN(H)C(S)NHR (R = Me, a ; Et, b ) with Li₂[PdCl₄] gave the dinuclear complexes [Pd{C₄H₄NC(H)=NNC(S)NHR}(μ‐Cl)]₂ (R = Me, 1a ; Et, 1b ) with a central Pd₂Cl₂ core and with deprotonation of the thiosemicarbazones at the hydrazinic nitrogen atom. Treatment of 1a and 1b with triphenylphosphine gave the mononuclear compounds [Pd{C₄H₄C(H)=NNC(S)NHR}(Cl)(PPh₃)] (R = Me, 2a ; Et, 2b ), whereas reaction of 1a and 1b with tertiary diphosphines gave mono‐ and dinuclear compounds, as appropriate, with the corresponding diphosphine acting as a monodentate ( 6b ), chelating ( 3a ) and bridging ligand ( 4a, 5a , 4b, 5b ). Treatment of 1a and 1b with (Ph₂PCH₂CH₂PPh₂)W(CO)₅ gave the new heterobimetallic complexes 7a and 7b . The crystal structures of complexes 3a and 4a are described.     

Enhanced resolution comprehensive two-dimensional gas chromatography applied to the analysis of roasted coffee volatiles

The present research is based on the full exploitation of the separation power of a 0.05 mm internal diameter (ID) capillary, as a comprehensive two-dimensional (2D) GC (GC × GC) secondary column, with the objective of attaining very high-resolution second dimension separations. The aim was achieved by using a split-flow system developed in previous research [P.Q. Tranchida, A. Casilli, P. Dugo, G. Dugo, L. Mondello, Anal. Chem. 79 (2007) 2266], and a dual-oven GC × GC instrument. The column combination employed consisted of a polar 30 m × 0.25 mm ID column connected, by means of a T union, to a detector-linked high-resolution 1.1 m × 0.05 mm ID apolar analytical column and to a 0.33 m × 0.05 mm ID retention gap; the latter was connected to a manually operated split valve. As previously demonstrated, the use of a split valve enables the regulation of gas flows through both analytical columns, generating the most appropriate gas linear velocities. Comprehensive 2D GC experiments were carried out on Arabica roasted coffee volatiles (previously extracted by means of solid-phase microextraction) with the split-valve closed (equal to what can be defined as conventional GC × GC) and with the split-valve opened at various degrees. The reasons why it is absolutely not effective to use a 0.05 mm ID column as second dimension in a conventional GC × GC instrument will be discussed and demonstrated. On the contrary, the use of a 0.05 mm ID column as second dimension, under ideal conditions in a split-flow, twin-oven system, will also be illustrated and discussed.     

A sensitive mediator-free tyrosinase biosensor based on an inorganic-organic hybrid titania sol-gel matrix

A novel inorganic-organic hybrid titania sol-gel nanocomposite film was prepared to fabricate a sensitive tyrosinase biosensor for the amperometric detection of trace phenolic compounds without additional electron mediators. Acetylacetone worked as a complexing ligand to chelate with Ti atom in the synthesis process, and the pH of the titania solution could be adjusted to the value which was optimum for retaining tyrosinase activity and such a membrane was stably attached on to the surface of a glassy carbon electrode (GCE). This titania matrix could supply a good environment for enzyme loading, which resulted in a high sensitivity of 15.78 μA μM⁻¹ cm⁻² for monitoring phenols with a detection limit of 1×10⁻⁸ M at a signal-to-noise ratio of 3. The TiO₂ sol-gel derived biosensor exhibited a fast response less than 10 s and a good stability for more than 2 months.     

Calculated equivalence volume in complexometric titrations

Summary Using a digital pH-meter, a motorized burette and a programmable pocket calculator, complexometric titrations were executed with a high degree of precision, a high degree of accuracy and in a time comparable to the one needed for performing a titration with a visual indicator. If a suitable sensing electrode is at hand, concentrations as low as 0.001 M may be analyzed without difficulties.

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Berechnetes Äquivalenzvolumen bei komplexometrischen Titrationen

Zusammenfassung Mit Hilfe eines Digital-pH-Meßge-räts, einer Motorbürette und einem programmierbaren Taschenrechner konnten komplexometrische Titrationen mit hoher Präzision und Genauigkeit in einer der visuellen Indikation vergleichbaren Zeit durchgeführt werden. Mit einer geeigneten Meßelektrode können Konzentration bis herab zu 0,001 M ohne Schwierigkeiten erfaßt werden.

     

“Spontaneous” Ambient Temperature Dehydrocoupling of Aromatic Amine–Boranes

The dehydrocoupling/dehydrogenation behavior of primary arylamine–borane adducts ArNH₂ ? BH₃ ( 3 a – c ; Ar= a : Ph, b : p‐MeOC₆H₄, c : p‐CF₃C₆H₄) has been studied in detail both in solution at ambient temperature as well as in the solid state at ambient or elevated temperatures. The presence of a metal catalyst was found to be unnecessary for the release of H₂. From reactions of 3 a , b in concentrated solutions in THF at 22 °C over 24 h cyclotriborazanes (ArNH‐BH₂)₃ ( 7 a , b ) were isolated as THF adducts, 7 a , b? THF, or solvent‐free 7 a , which could not be obtained via heating of 3 a – c in the melt. The μ‐(anilino)diborane [H₂B(μ‐PhNH)(μ‐H)BH₂] ( 4 a ) was observed in the reaction of 3 a with BH₃?THF and was characterized in situ. The reaction of 3 a with PhNH₂ ( 2 a ) was found to provide a new, convenient method for the preparation of dianilinoborane (PhNH)₂BH ( 5 a ), which has potential generality. This observation, together with further studies of reactions of 4 a , 5 a , and 7 a , b , provided insight into the mechanism of the catalyst‐free ambient temperature dehydrocoupling of 3 a – c in solution. For example, the reaction of 4 a with 5 a yields 6 a and 7 a . It was found that borazines (ArN‐BH)₃ ( 6 a – c ) are not simply formed via dehydrogenation of cyclotriborazanes 7 a – c in solution. The transformation of 7 a to 6 a is slowly induced by 5 a and proceeds via regeneration of 3 a . The adducts 3 a – c also underwent rapid dehydrocoupling in the solid state at elevated temperatures and even very slowly at ambient temperature. From aniline–borane derivative 3 c , the linear iminoborane oligomer (p‐CF₃C₆H₄)N[BH‐NH(p‐CF₃C₆H₄)]₂ ( 11 ) was obtained. The single‐crystal X‐ray structures of 3 a – c , 5 a , 7 a , 7 b? THF, and 11 are discussed.     

Transition of polymers from rubbery elastic state to fluid state

On increasing the temperature of a polymer, the transition of the polymer from a rubbery elastic state to a fluid state could occur. The transition temperature is termed the fluid temperature of the polymer, T _f, which has a direct relationship with the polymer molecular weight. As one of polymer parameters, T _f is as important as the glass transition temperature of a polymer, T _g. Moreover, special attention to T _f should be paid for polymer processing. In research on the transition of a polymer from a rubbery elastic state to a fluid state, the concept of T _f would be more reasonable and more effective than the concept of T _l,l because it is neglected in the concept of T _l,l in that the molecular weight of a polymer may affect the transition of the polymer. In this paper the discussion on the fluid temperature involves the characters of polymers, such as the deformation—temperature curve, the temperature range of the rubbery state and the shear viscosity of polymer melt. From the viewpoint of the cohesional state of polymers, the transition of a polymer from a rubbery elastic state to a fluid state responds to destruction and construction of the cohesional entanglement network in the polymer. The relaxing network of polymer melt would be worthy to be considered as an object of study. __________ Translated from Huaxue Tongbao (Chemistry), 2008,71(3) (in Chinese)     

Conformational Changes and Redox Properties of Bimetallic Single Helicates of Hexapyrrole-α,ω-dicarbaldehydes

The hexapyrrole-α,ω-dicarbaldehydes 1 a and 1 b were metallated with Cu^II, Ni^II, and Pd^II to give bimetallic complexes where a pair of 3 N+O four-coordinate metal planes are helically distorted and the central 2,2′-bipyrrole subunit adopts a cis or trans conformation. X-ray crystallographic analysis of the bisCu complex revealed a closed form with a cis-2,2′-bipyrrole subunit and an open form with a trans-2,2′-bipyrrole subunit. The bisPd complexes took a closed form both in the solid state and in solution. They are regarded as single helicates of two turns and the energy barrier for the interchange between an M helix and a P helix was remarkably influenced by the bulky 3,3′-substituent of the central 2,2′-bipyrrole subunit. Although the bisNi complexes adopt a closed form in the solid state, they exist as a homohelical open C₂-symmetric form or a heterohelical open C_i-symmetric form in solution. A theoretical study suggested that the closed form of 1 a Pd was stabilized by the Pd–Pd interaction. Compound 1 a Pd was reversibly oxidized by one electron at 0.14 V versus ferrocene/ferrocenium (Fc/Fc⁺) and this oxidized species showed Vis/NIR absorption bands at λ=767 and 1408 nm.     

DNA Computing of Bipartite Graphs for Maximum Matching

Let M be a matching in a graph G. It is defined that an M-augmenting path must obtain one element of M. In this paper, it is obtained that a matching M in a graph G is a maximum matching if and only if G contains no M-augmenting path and M is a maximal matching in G. It supplies a theoretical basis to DNA computing. A detailed discussion is given of DNA algorithms for the solutions of the maximal matching problem and maximum matching one in a bipartite graph.     

Matrix diversion methods for improved analysis of perchlorate by suppressed ion chromatography and conductivity detection

Two inline matrix diversion methods were developed for the sensitive analysis of perchlorate in a matrix comprising up to 1000 mg l⁻¹ of chloride, sulfate and bicarbonate ions using suppressed ion chromatography and conductivity detection. The first method used a cryptand C1 concentrator column, which exhibited a high selectivity for perchlorate ion over the other matrix anions. After retaining the sample anions in a concentrator column derivatized with a crytpand phase, a rinse step was implemented with a weak base to divert the matrix ions to waste while selectively retaining perchlorate in the concentrator column for subsequent analysis. The analysis was done using a 2 mm IonPac^® AS16 or 2 mm IonPac^® AS20 separator column. The second method was a two-dimensional matrix diversion method with a focus on improving the detection sensitivity. The first dimension was used to achieve some resolution of the matrix ions from perchlorate. The perchlorate ion was then diverted into a concentrator column for subsequent analysis in the second dimension. By pursuing analysis using a 4 mm IonPac^® AS16 or IonPac^® AS20 column in the first dimension and subsequently pursuing analysis using a 2 mm IonPac^® AS16 or IonPac^® AS20 column format, excellent sensitivities were achieved when the first and second dimensions were operated at the same linear flow velocity (cm min⁻¹). While sensitive detection of perchlorate in the low μg l⁻¹ regime was achieved by the above methods in the presence of matrix ions, superior recovery for perchlorate was demonstrated under a variety of matrix concentrations by the second method.     

Supramolecular polymer networks based on calix[5]arene chained poly(p-phenyleneethynylene) and C60 fulleropyrrolidine

Abstract

A poly(p-phenyleneethynylene) polymer (PCF[5]), bearing two π-rich cone-like calix[5]arene cavities (assembling cores) attached to a rigid p-phenyleneethynylene spacer, was synthesised by a Pd-catalysed cross-coupling reaction. UV–vis absorption and fluorescence spectroscopies combined with dynamic light scattering measurements provide evidence for the self-assembly of PCF[5] (homopolytopic host molecule) with a complementary C₆₀ fulleropyrrolidine (C₆₀-Pyr) guest in solution, in the construction of a supramolecular polymer network. Atomic force microscopy analysis of PCF[5]/C₆₀-Pyr highlights the formation of a bicontinuous network consisting of a uniform distribution of prominent structures, within a polymeric background forming a biphasic structure.     

A Ruthenium(II)–Copper(II) Dyad for the Photocatalytic Oxygenation of Organic Substrates Mediated by Dioxygen Activation

Dioxygen activation by copper complexes is a valuable method to achieve oxidation reactions for sustainable chemistry. The development of a catalytic system requires regeneration of the Cu^I active redox state from Cu^II. This is usually achieved using extra reducers that can compete with the Cu^II(O₂) oxidizing species, causing a loss of efficiency. An alternative would consist of using a photosensitizer to control the reduction process. Association of a Ru^II photosensitizing subunit with a Cu^II pre‐catalytic moiety assembled within a unique entity is shown to fulfill these requirements. In presence of a sacrificial electron donor and light, electron transfer occurs from the Ru^II center to Cu^II. In presence of dioxygen, this dyad proved to be efficient for sulfide, phosphine, and alkene catalytic oxygenation. Mechanistic investigations gave evidence about a predominant ³O₂ activation pathway by the Cu^I moiety.     

Sandwich-like mesoporous graphene@magnetite@carbon nanosheets for high-rate lithium ion batteries

Sandwich-like mesoporous GS@Fe₃O₄@C nanosheets with a 2D nanoarchitecture have been successfully synthesized by one-step solvothermal treatment. Such type of 2D nanoarchitecture is made up of a number of Fe₃O₄ nanoparticles uniformly grown on a graphene sheet and an even amorphous carbon layer covering on their surface. The Li-cycling properties of GS@Fe₃O₄@C nanosheets have been evaluated by galvanostatic discharge-charge cycling and impedance spectroscopy. Results indicate that the GS@Fe₃O₄@C nanosheets with about 5 wt % of graphene content provides a very high discharge capacity of 913.2 mAh g⁻¹ at a current densities of 200 mA g⁻¹ after 100 cycles and reveals a stable discharge capacity of 483.2 mAh g⁻¹ at a rate of 1600 mA g⁻¹.     

SYNTHESIS OF SOME BICYCLIC THIAZOLO AND THIAZEPINOPYRIMIDINE DERIVATIVES

Thiazolo 3 a–c and oxothialo 5 a–b pyrimidine compounds were synthesized by a simple one-pot condensation reaction of the pyrimidine derivative i 1a and 1,2-dibromoethane i 2 or 2-bromopropanoic acid 4 . In a similar way the thiazepinopyrimidine compounds 7 (a–b) were synthesized by reaction of 1b and 1,4-dichlorobutane 6 in dimethylformamide under reflux condition. The yields of products following recrystallization were of the order of 60–80%.     

Dissolved oxygen amperometric sensor based on layer-by-layer assembly using host-guest supramolecular interactions

The development of a simple, efficient and sensitive sensor for dissolved oxygen is proposed using the host-guest binding of a supramolecular complex at a host surface by combining a self-assembled monolayer (SAM) of mono-(6-deoxy-6-mercapto)-β-cyclodextrin (βCDSH), iron (III) tetra-(N-methyl-4-pyridyl)-porphyrin (FeTMPyP) and cyclodextrin-functionalized gold nanoparticles (CDAuNP). The supramolecular modified electrode showed excellent catalytic activity for oxygen reduction. The reduction potential of oxygen was shifted about 200 mV toward less negative values with this modified electrode, presenting a peak current much higher than those observed on a bare gold electrode. Cyclic voltammetry and rotating disk electrode (RDE) experiments indicated that the oxygen reduction reaction involves probably 4-electrons with a rate constant (k_obs) of 7 × 10⁴ mol⁻¹ L s⁻¹. A linear response range from 0.2 up to 6.5 mg L⁻¹, with a sensitivity of 5.5 μA L mg⁻¹ (or 77.5 μA cm⁻² L mg⁻¹) and a detection limit of 0.02 mg L⁻¹ was obtained with this sensor. The repeatability of the proposed sensor, evaluated in terms of relative standard deviation was 3.0% for 10 measurements of a solution of 6.5 mg L⁻¹ oxygen.     

Development of a method for estimating an accurate equivalence point in nickel titration of cyanide ions

In a nickel titration of cyanide ions using murexide as indicator, an accurate equivalence point was determined by a non-linear least-squares curve-fitting for a titration curve. This method was developed to establish a standard solution for cyanide ions. In a curve-fitting procedure, a theoretical titration curve was calculated, assuming that nickel ion formed only a 1:4 Ni²⁺:CN⁻ complex with cyanide ions and formed only a 1:1 complex with murexide. Results of the curve-fitting were reasonable at any pH and any indicator concentration studied. The combined standard uncertainty for a concentration of a 1000 mg kg⁻¹ cyanide solution by this method was 0.079%.     

Synthesis and conformational behavior of pseudopeptides containing δ-azaproline. A cis conformational preference for Xaa1–δ-azaPro bond

δ-Azaproline, a new bis-nitrogen proline surrogate has been used in order to control the conformation of an AA–ΨPro bond. Conformational analysis of Xaa₁–δ-azaPro–Xaa₃ performed by NMR, IR experiments, and molecular modeling revealed a preference for a trans conformation of the Xaa₁–δ-azaPro bond when δ-azaPro is protected by a Boc group. The removal of the Boc protection leads to the establishment of a C₁₀ pseudocycle via a hydrogen bond network favoring the cis conformation of the Xaa₁–δ-azaPro bond.     

Synthesis and reactivity of thiophene palladium and thiophene dipalladium complexes with unsaturated molecules

Reactions of 2,5‐dibromothiophene, 1 , with [Pd₂(dba)₃]^?dba [Pd(dba)₂; dba = dibenzylideneacetone] in the presence of N‐donor ligands such as 2,2′‐bipyridine (bpy) and 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine (dtbbpy) give arylpalladium complexes of cis‐[2‐(5‐BrC₄H₂S)PdBrL₂], 2a, b [L₂ = bpy ( 2a ), L₂ = dtbbpy ( 2b )], and cis‐cis‐L₂PdBr[2,5‐(C₄H₂S‐)PdBr(L₂)], 3a, b [L₂ = bpy ( 3a ), L₂ = dtbbpy ( 3b )]. Treatment of cis complexes 2a, b and 3a, b with CO causes the insertion of CO into the Pd? C bond to give the aroyl derivatives of palladium complexes of cis‐[2‐(5‐BrC₄H₂S)COPdBrL₂], 4a, b [L₂ = bpy ( 4a ), L₂ = dtbbpy ( 4b )], and cis‐cis‐[(L₂)(CO)BrPdC₄H₂S‐PdBr(CO)(L₂)], 5a, b [L₂ = bpy ( 5a ) and L₂ = dtbbpy ( 5b )], respectively. Treating complexes 2a, b with 1 mole equivalent of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) gave iminoacyl complexes cis‐[2‐(5‐BrC₄H₂S)C?NXyPdBrL₂], 6a, b [L₂ = bpy ( 6a ), L₂ = dtbbpy ( 6b )], and a 3‐fold excess of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) gave triiminoacyl complexes [2‐(5‐BrC₄H₂S)(C?NXy)₃ PdBr], 7 . Cyclization reactions of 6a, b with 3 mole equivalents of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) or cyclization reaction of 7 with 1 mole equivalent of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) both gave tetraiminoacyl complexes of [2‐(5‐BrC₄H₂S)(C?NXy)₄PdBr], 8 , which was also obtained by the reaction of 1 or 2a, b with a 4‐fold excess of isocyanide XyNC with or without add Pd(dba)₂. Similarly, complexes 3a and b were also reacted with 2 mole equivalents of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) to give iminoacyl complexes cis‐cis‐[(L₂)(CNXy)BrPdC₄H₂S‐PdBr(CNXy)(L₂)], 10a, b [L₂ = bpy ( 10a ), L₂ = dtbbpy ( 10b )] and an 8‐fold excess of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) afforded tetraiminoacyl complexes of [2,5‐(C₄H₂S)(C?NXy)₈Pd₂Br₂], 11 . Complexes 2a, b and 3a, b reacted with TlOTf [(TfO = CF₃SO₃)] in CH₂Cl₂ to give 9a, b and 12a, b , respectively, in a moderate yield. Copyright © 2007 John Wiley & Sons, Ltd.     

Highly Crystalline Mesoporous C60 with Ordered Pores: A Class of Nanomaterials for Energy Applications

Highly ordered mesoporous C₆₀ with a well‐ordered porous structure and a high crystallinity is prepared through the nanohard templating method using a saturated solution of C₆₀ in 1‐chloronaphthalene (51 mg mL^?1) as a C₆₀ precursor and SBA‐15 as a hard template. The high solubility of C₆₀ in 1‐chloronaphthalene helps not only to encapsulate a huge amount of the C₆₀ into the mesopores of the template but also supports the oligomerization of C₆₀ and the formation of crystalline walls made of C₆₀. The obtained mesoporous C₆₀ exhibits a rod‐shaped morphology, a high specific surface area (680 m² g^?1), tuneable pores, and a highly crystalline wall structure. This exciting ordered mesoporous C₆₀ offers high supercapacitive performance and a high selectivity to H₂O₂ production and methanol tolerance for ORR. This simple strategy could be adopted to make a series of mesoporous fullerenes with different structures and carbon atoms as a new class of energy materials.