Biomimetic synthesis of a preclavulone a model

A synthetic route to the preclavulone-A (3) analogs 11 and 14 via allene oxide and 2-oxidopentadienyl cation intermediates is described which is based on the proposed biosynthesis of 3.     

Thermal properties of a disc-like compound benzene-hexa-n-pentanoate: a precursor of a discotic mesogen

The heat capacity of a disk-like compound, benzene-hexa-n-pentanoate (BH5), a precursor of a discotic mesogen, has been measured by adiabatic calorimetry between 13 and 390 K. BH5 showed four different crystalline phases, but no liquid crystalline phase. Molar entropy and transition entropies were determined and compared with those of other homologues (BH6, BH7, and BH8). An odd-even effect with respect to the number of carbon atoms in the alkyl chain was observed for the cumulative entropies of the phase transitions occurring in the solid state. This effect is discussed by comparing with data for the n-alkanes. The molar entropies of the crystalline state, especially below 250 K, exhibited a peculiar 'pairing effect' between BH(2m- 1) and BH(2m), where m is an integer.     

Partial incorporation of a cyclopentadienyl ligand into a molybdaborane to form a molybdacarbaborane

Reaction of the molybdaborane arachno-2-[Mo(η-C₅H₅)(η⁵:η¹-C₅H₄)B₄H₇] (I) with NEt₃ in toluene at 120 °C for 7 days gives a 90% yield of the molybdacarbaborane nido-1-[Mo(η-C₅H₅)(η³:η²-C₃H₃)C₂B₃H₅] (II). Two of the carbon atoms in the substituted cyclopentadienyl ring in I are incorporated into the metallacarbaborane cluster II. The carbaborane {C₂B₃H₅} fragment in II is attached to an allylic {C₃H₃} group and can be thought of as a new non-planar {C₅B₃H₈} ligand providing seven electrons to the molybdenum atom. Reaction of I with KH in thf at 20 °C gives the anion via deprotonation of a B-H-B bridging proton.     

Formation of a dinaphthanthracene by a stilbene-like photocyclization

Photocyclization of 1,3-bis[2-(2-naphthyl)vinyl]benzene leads regioselectively to dinaphth[1,2-$\text{a}$:2′,1′-$\text{j}$]anthracene without the need for a blocking bromine substituent at C-2 in the starting material as assumed previously by other workers.     

Impedance of a passive iron electrode in a solution containing a reducing agent

Frequency dependences of impedance of a passive iron electrode in 0.5 M Na₂SO₄ solutions with and without 0.01 M K₄[Fe(CN)₆] are obtained by a pulsed method and the Fourier transform. At 1–1000 Hz, the results in 0.5 M Na₂SO₄ satisfactorily agree with our previous results obtained by a lock-in method. With the [Fe(CN)₆]^4- ions present, the impedance decreases faster at lower frequencies. The impedance of the oxide film/solution interface relaxes longer the film impedance. These conclusions are similar to those obtained earlier.Translated from Elektrokhimiya, Vol. 41, No. 1, 2005, pp. 97–101.Original Russian Text Copyright © 2005 by Klyuev, Rotenberg, Batrakov.To the Centennial of B.N. Kabanov.     

Dynamic molecular recognition in the generation of a new crystal-engineering motif: a unique case study of a dicarboxylic acid with a ditopic receptor favouring a polymeric over a dimeric hydrogen-bonded supramolecular complex

A new crystal-engineering motif has been developed where a ditopic receptor 1 shows a novel syn-syn hydrogen-bonded polymeric supramolecular complex (Fig. 4b) (instead of a 1:1 dimeric syn-syn or polymeric syn-anti complex) giving rise to a hydrogen-bonded stair-like polymeric ribbon structure between the binding groups of the receptor pyridine amide and the carboxyl groups of the guest substrate.     

Mobility of a tackifying resin in a pressure-sensitive adhesive

A detailed study of the mobility of a tackifying resin in a pressure-sensitive adhesive (PSA) has been done for the first time. The objective of this work is to relate changes in adhesive performance with tackifier loading to tackifier mobility. Tackifiers are low-molecular weight resins that improve the overall performance of PSAs. They increase the adhesive tack or the ability to form a bond of measurable strength after brief contact under slight applied pressure. In this study the diffusion of n-butyl ester of abietic acid (n-BEAA) in either polyisoprene (PI) (M_w = 195,000 M_w/M_n ∼ 1.05) or poly(ethylene-propylene) (PEP) (M_w = 40,000 M_w/M_n ∼ 2.30) was measured by Pulsed Gradient Spin Echo-Nuclear Magnetic Resonance (PGSE-NMR) as a function of both tackifier concentration and temperature. The concentration dependence of the tackifier's diffusion coefficient was weak for both systems. The weak variation in mobility with composition for the PI/n-BEAA system was consistent with that system's weak variation in tack with composition. On the other hand, blends of PEP/n-BEAA showed only modest variation in mobility, even though these adhesive systems showed appreciable enhancement of tack at intermediate compositions. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 373–381, 1998     

Miscibility of a polyarylate with a liquid crystalline copolyester

The miscibility has been investigated for binary blends of a polyarylate (PAr) with a liquid crystalline copolyester of p-hydroxybenzoate and ethylene terephthalate units in a 6/4 molar ratio (PET/PHB). The binary blends were prepared by solution precipitation. The transitions of the PET/PHB have been measured with a rheometrics dynamic spectrometer. The phases in blends have been studied with a differential scanning calorimeter, by ther-mogravimetry and with a polarizing optical microscope. The blends exhibit two glass transitions (T_gs) over the composition range 10–90 wt %. The amorphous PET phase from the PET-PHB is found to be partially miscible with PAr, which leads to a decrease of the PAr T_g. The amount of this partially miscible portion of PET has been estimated by the Couch-man equation. On heat treatment of the blends at 250 to 300°C, transesterficiation takes place, as judged by the shift of the higher of the two T_gs. © 1993 John Wiley & Sons, Inc.     

Synthesis of a tetraazacyclotetradecadiene and a pentaazacyclotetradecatriene

Useful procedures for preparing a novel tetraazacyclotetradecadiene and a pentaazacyclotetradecatriene are reported. J. Heterocyclic Chem., 14, 1097 (1977)     

The Topochemical Pseudomorphosis of a Chloride into a Bismuthide

The heterogeneous reaction of crystals of the novel intermetallic subhalide Bi₁₂Rh₃Cl₂ with a solution of n‐butyllithium at 70 °C led to the complete topochemical exchange of chloride ions for bismuth atoms, that is, the transformation into the isostructural metastable intermetallic superconductor Bi₁₄Rh₃. The crystals underwent the reductive pseudomorphosis almost unchanged except some fissures perpendicular to the a‐axis. Detailed inspections of the transformed crystals by electron microscopy indicated no volume defects that would indicate internal chemical reactions. Thus, extensive mass transport must have occurred through the seemingly dense crystal structure. An efficient transport mechanism, based on an unusual breathing mode of the three‐dimensional network formed by edge‐sharing [RhBi₈] cubes and antiprisms, is proposed. The replacement of ionic interaction in the chloride by metallic bonding in the binary intermetallic compound closes the pseudo gap in the density of states at the Fermi level. As a result, the rod‐packing of conducting, yet electrically isolated strands of [RhBi₈] cubes in Bi₁₂Rh₃Cl₂ turns into the three‐dimensional metal Bi₁₄Rh₃.     

Isotope labelling in ring a of gibberellin a 20

Full assignments of the ¹H-nmr chemical shifts of the ring A protons in gibberellin A₂₀ methyl ester 13-acetate have been made on the basis of ¹H-, ²H- and ¹³C-nmr data of various deuteriated derivatives. These assignments have been used to prove that catalytic deuteriogenation of GA₅-16, 17- epoxide-13-acetate is a syn-addition from the less hindered β-face accompanied by allylic exchange at C-1 giving isotopic labels at the 1β-, 2β- and 3β- positions. The position and stereochemistry of isotopic labelling was confirmed by comparison with an authentic sample of [1β,2β,3β-²H₃] GA₂₀ methyl ester 13-acetate prepared by methods which introduce deuterium stereoselectively at C-1, C-2 and C-3. The preparation of [2α-²H]GA₂₀ and [3α-²H]GA₂₀ is described.     

Hydroacridines XXV [1]. First Synthesis of (4aα,8aα,9aβ,10aβ)-Tetradecahydroacridine and New Syntheses of (4aα,8aα,9aα,10aβ)- and (4aα,8aβ,9aα,10aβ)-Tetradecahydroacridine

Summary. On heating in dry DMSO, in the presence of potassium t-butoxide, the N-nitrosamine of (4aα,8aβ,9aβ,10aα)-tetradecahydroacridine is completely converted into the N-nitrosamine of (4aα,8aα,9aα,10aβ)-tetradecahydroacridine. Under similar conditions, the N-nitrosamine of (4aα,8aα,9aβ,10aα)-tetradecahydroacridine yields a ternary equilibrium mixture containing itself (19%), and the N-nitrosamines of (4aα,8aβ,9aα,10aβ)-tetradecahydroacridine (46%) and the so far unknown (4aα,8aα,9aβ,10aβ)-tetradecahydroacridine (35%). The resulting N-nitrosamines can be smoothly denitrosated to the corresponding secondary amines.     

Configurational degeneracy of a set of dipoles in a quasi-two-dimensional system

The purpose of this paper is to provide an exact evaluation of the configurational degeneracy when an arbitrary number (k) of dipoles are placed in a quasi-two-dimensional space (Q2D). This Q2D is made up of three contiguous diagonals 3 × N. Our Q2D space gives to the central sites of the lattice their full coordination number of nearest neighboring compartments. We are going to determine the exact configurational degeneracy W(k, N) when an arbitrary number k of the above mentioned particles are placed in this 3 × NQ2D space. We found that W(k, N) is exactly described by

Hydrolytic cleavage of a DNA-model phosphodiester: a new inorganic–organic hybrid constructed from a Zn-cluster with a polyoxometalate

An inorganic–organic hybrid constructed from a Zn-cluster with a polyoxometalate {[Zn₃Na₂(μ-OH)₂(bpdo)₆(H₂O)₁₆][PW₁₂O₄₀]₂}·(bpdo)₃·C₂H₅OH·2H₂O (bpdo?=?4,4′-bis(pyridine-N-oxide)) (1) has been synthesized by hydrothermal reaction and characterized by elemental analyses, IR spectra, and single crystal X-ray diffraction. The structural analysis indicates that 1 is an S-like complex constructed by [Zn₃Na₂(μ-OH)₂(bpdo)₆(H₂O)₁₆]^6? with two PW₁₂O₄₀ ^3? with water occupying several coordination sites and have the potential to act as labile ligands, allowing for substrate and nucleophile binding. Kinetic experiments for hydrolytic cleavage of the DNA-model phosphodiester bis(p-nitropheny1)phosphate (BNPP) were followed spectrophotometrically for absorbance increase at 400?nm in 4-(2-hydroxyethyl)piperazine-1–propane sulfonic acid (EPPS) buffer solution due to the formation of p-nitrophenoxide with 1 at pH 4.0 and 50?°C. UV spectroscopy indicates cleavage of the phosphodiester bond proceeds with pseudo-first-order rate constant 6.7(±0.2)?×?10^?7?s^?1, giving an inorganic phosphate and p-nitrophenol as the final products of hydrolysis. The results demonstrate that 1 exhibits good catalytic activity and reusability for hydrolytic cleavage of BNPP.     

Rearrangement of a pyranone to a benzothiazole

The conversion of 2-chloromethyl-5-methoxy-4H-pyran-4-one to 2-amino-5-hydroxy-6-methoxybenzothiazole hydrochloride under mild reaction conditions has been observed. The synthesis of the identical benzothiazole by an alternate unequivocal method is also reported.     

On a diffusion model of a catalytic reaction

A model to describe a catalytic reaction with simultaneous diffusion is analyzed. The possibility of formation of periodic structures in space is shown.

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Self-assembly of a nucleotide-calixarene hybrid in a triangular supramolecule

The self-assembly of a thymine nucleotide-calixarene hybrid (1) in CDCl₃ as a solvent was investigated. FT-IR, ESI-MS, ¹H and DOSY-NMR spectra evidenced that compound 1 (ammonium or sodium salt) self-assembles in a triangular trimeric supramolecule by thymine-thymine hydrogen bonding. The saline form is crucial for the arrangement in the cyclic trimer as the protonation of the nucleotide phosphate groups leads the assembly toward a dimeric species.     

Recoordination of a metal ion in the cavity of a crown compound: a theoretical study

The absorption spectra of styrylbenzothiazolium dye derivatives were calculated by the time-dependent density functional (TD DFT) method. The dyes of interest were (p-dimethylamino)styrylbenzothiazolium dye and its protonated form as well as aza-15(18)-crown-5(6)-containing dyes and their complexes with alkali (K⁺ and Na⁺) and alkaline-earth (Ca²⁺, Sr²⁺, and Ba²⁺) cations. Several low-lying conformers of the azacrown-containing dyes were considered. The electronic and geometric structures of the excited states responsible for the appearance of the long-wave (π-π*) absorption bands are studied. Complexation causes a hypsochromic shift of the long-wave absorption band correlating with the pyramidality of the crown ether nitrogen in the complex. The interaction of the cation with 3–4 solvent molecules or a counterion (ClO₄ ^?) considerably reduces this shift, especially in the conformers without the metal-nitrogen bond. In some cases, the long-wave absorption band is close to the absorption band of the free dye. Similar results were obtained using the polarizable continuum model of solvation. Excited-state structures of the free model dye and the free azacrown-containing dyes exhibit a tendency to bond alternation. Conversely, the cationic complexes of the crown-containing dyes and the protonated model dye exhibit a tendency to bond equalization in the excited state. The changes in the excited-state geometries of the free dyes and their complexes account for the complexation-induced fluorescence enhancement observed in the experiments.