bluuebunny/crossref_metadata_2025 huggingface dataset:
Unambiguous structural and nuclear magnetic resonance spectral characterization of two triterpenoids of Maprouneaguianensis by two-dimensional nuclear magnetic resonance spectroscopy
Authors: McLean Stewart , Perpick-Dumont Marion , Reynolds William F. , Jacobs Helen , Lachmansing Sagar Singh
Published: May 2006

It is shown that 1H–13C shift-correlated two-dimensional spectra obtained for polarization transfer via two-bond and three-bond 13C–1H coupling can, in conjunction with related experiments, be used to assign unambiguously the molecular skeletons of two of the less common triterpenes, moretenone and 3-acetylaleuritolic acid. It is concluded that this is a technique of considerable generality for assigning structures of triterpenes and is more reliable than alternative approaches such as mass spectral fragmentation patterns. It has the additional benefit of simultaneously providing reliable 13C and 1H spectral assignments for these compounds.
The crystal structures of the 1:1:1 complexes of dicyclohexano-18-crown-6 (isomer B) with potassium phenoxide and phenol and dicyclohexano-18-crown-6 (isomer A) with sodium phenoxide and phenol
Authors: Fraser Marie E. , Fortier Suzanne , Markiewicz Mary K. , Rodrigue André , Bovenkamp John W.
Published: May 2006

The crystal structures of the 1:1:1 complexes of dicyclohexano-18-crown-6 (isomer B) with potassium phenoxide and phenol, and of dicyclohexano-18-crown-6 (isomer A) with sodium phenoxide and phenol have been determined. The potassium phenoxide complex crystallizes in space group Pnca with a = 14.150(3), b = 23.794(6), c = 9.491(1) Å, and Z = 4. Thesodium phenoxide complex crystallizes in space group Pbca with a = 21.201(4), b = 24.406(6), c = 12.492(3) Å, and Z = 8. Both structures were solved by direct methods and refined by full matrix least-squares calculations to residuals, R, of 0.059 for the potassium phenoxide complex, and 0.069 for the sodium phenoxide complex. In the potassium phenoxide complex, the cation sits on a centre of symmetry, in the centre of the cavity formed by the crown ether in its regular conformation. The potassium is coordinated to all six crown oxygens as well as to two phenol/phenoxides, one on each side of the crown. Groups of phenol/phenoxide–potassium–phenol/phenoxide are linked by the hydrogen atoms to form a chain: K+… O … H … O … K+. The distance between the oxygens of the phenol/phenoxides is short, 2.48(1) Å. The hydrogen was initially located halfway between the two oxygens, on a 2-fold axis special position. Inspection of the difference Fourier map and refinement on the hydrogen position revealed, however, the presence of statistical or dynamical disordering. In the sodium phenoxide complex, the hexaether adopts an irregular conformation. The distances between the sodium and the crown oxygens range from 2.45 to 2.89 Å, while a distance of 2.363 Å is found with the phenoxide oxygen. Strong hydrogen bonding is found between the oxygens of the phenol and phenoxide. The O ….O distance is 2.471(4) Å.
Reactions of some N-acyl-1-alkylamines with poly phosphoric ester PPE: nuclear magnetic resonance and stereochemistry of reaction products
Authors: Alesso Elba N. , Tombari Dora G. , Iglesias Graciela Y. Moltrasio , Aguirre José M.
Published: May 2006

Phenylindanes were obtained from the reaction of N-acyl-1-arylalkylamines with polyphosphoric ester (PPE). cis-1-Methyl-3-(3,4-dimethoxyphenyl)-5,6-dimethoxyindane was synthesized to determine which stereoisomer was produced by N-formyl and N-acetyl-1-(3,4-dimethoxyphenyl)ethylamine with PPE. Nuclear magnetic resonance specra (1H and 13C) of several mono-, 1,3-di-, and 1,2,3-tri-substituted indanes were fully analyzed to provide information on steric interactions and conformation of the cyclopentene ring. Possible cyclodimerization pathways are proposed.
Convenient synthesis and cycloaddition reactions of 2-phenylseleno-1,3-butadiene and 2-trialkylstannyl-1,3-butadienes
Authors: Bates Gordon S. , Fryzuk Michael D. , Stone Charles
Published: May 2006

The facile preparation of 2-trialkylstannyl-1,3-butadienes and 2-phenylseleno-1,3-butadiene by reaction of 2-(1,3-butadienyl)magnesium chloride with trialkylstannyl chlorides and phenylselenium chloride, respectively, is reported. The Diels–Alder reactivity of these dienes with a variety of activated dienophiles is also described. Finally, a novel transmetallation of tin, in vinyl stannanes, to selenium by use of phenylselenium chloride is outlined.
Complexation of aqueous ammonia by cyclodextrins
Authors: Hirsch Warren , DiMartini Carol , Fried Vojtech , Ling William
Published: May 2006

The complexation of aqueous ammonia with α, β, and γ cyclodextrins has been monitored by pH measurements at (23 ± 1) °C. A mathematical model has been developed yielding reproducible equilibrium association constants for the formation of these complexes. The constants are quite large, suggesting the possibility of hydrogen bond formation.
Structure and absolute configuration of [1-(N,N-dimethylammonium)ethyl]ferrocene tartrate dihydrate
Authors: Luo Yao Guang , Barton Richard J. , Robertson Beverly E.
Published: May 2006

Crystals of [1-(N,N-dimethylammonium)ethyl]ferrocene tartrate dihydrate are triclinic, space group P1, a = 7.235(1), b = 11.372(2), c = 12.724(3) Å, α = 90.78(1), β = 102.75(1), γ = 91.76(2)°, V = 1020.3 Å3 at 20 ± 2 °C with Z = 2. The structure was solved by direct methods. The correct enantiomer was chosen based on full-matrix least-square refinements of each enantiomer. For the correct enantiomer, R = 0.031, Rw = 0.035 for 11052 reflections with I ≥ 2σ(I). Although the two crystallographically independent units of the complex in the structure are chemically equivalent, they are structurally different. The absolute configuration of the amino-substituted carbon atom is (S), and that of the tartrate anion is (R,R). The mean ferrocenyl Fe—C bond lengths are 2.037(2) Å and 2.027(3) Å for each molecule, respectively. The NMe2 group is located outside the planes of the cyclopentadienyl rings. The tartrate ions are connected through water molecules by hydrogen bonds.
Tandem Wessely oxidation and intramolecular Diels–Alder reactions. III. Synthesis of isotwistanes
Authors: Yates Peter , Macas Tadas S.
Published: May 2006

Wessely oxidation of o-(3-alkenyl) phenols (5) with lead tetraacetate gives 6-acetoxy-6-(3-alkenyl)-2,4-cyclohexadien-1-ones (6), which undergo intramolecular Diels–Alder reactions to give isotwistene (hexahydro-1,5-methanoindene) derivatives (7). These, on hydrolysis followed by oxidation with periodic acid, give hexahydro-1-oxo-1H-indene-5-carboxylic acids (41, 46). Compounds 5 were prepared either by Grignard coupling of o-methoxybenzyl chloride with allyl halides followed by de-O-methylation with sodium thioethoxide or via reduction of dihydrocoumarins (18–20) to 2-chromanols (21–23) with diisobutylaluminum hydride, followed by Wittig reaction of these with ethyl (triphenylphosphoranylidene)acetate.
The electrochemical oxidation of silver and tetraethylammonium salts of formamides and imides. N,N-Coupling of formanilidyl radicals
Authors: Huot Jean-Yves , Serve Denis , Desjardins Sylvie , Lessard Jean
Published: May 2006

The electrochemical oxidation of [amide-Ag-amide]Ag, [amide-Ag-amide]Et4N, and [amide]Et4N salts of imides and formanilides, in acetonitrile containing tetraethylammonium tetrafluoroborate, at platinum and vitreous carbon electrodes, is a one-electron and irreversible (αn < 1) process leading to an amidyl radicals that preferentially abstracts hydrogen from the medium to give the parent amide. N,N-Coupling (formation of hydrazine derivatives) was observed in the oxidation of the amide-Ag-amide anions of formanilide and p-methoxyformanilide. No coupling was observed in the oxidation of the amide-Ag-amide anion of p-cyanoformanilide and of imides, and in the oxidation of any of the tetraethylammonium salts (amidyl anions) studied. The formanilidyl radical was trapped by N-tert-butyl-α-phenylnitrone. Attempts to trap the succinimidyl radical by addition to olefins were unsuccessful.
Balance between van der Waals and coordination bond energies—Unified Model of coordination bond length in lanthanide complexes
Authors: Feng Xi-Zhang , Xu Ying-Ting , Sun Peng-Nian
Published: May 2006

This paper presents the variation of coordination bond lengths in lanthanide complexes as a balance between van der Waals and coordination bond energies. Each coordination atom moves in a unified potential well that is a combination of the Morse potential of its coordination bond and the van der Waals potentials of its neighboring coordination atoms. This is termed the Unified Model of coordination bond length.
Mixed alkali effect in sodium thiosulfate pentahydrate melt
Authors: Islam Shakira S. , Ismail Kochi
Published: May 2006

Density and electrical conductance measurements of 0.35[XNaNO3 + (1 − X)KNO3] + 0.65Na2S2O3•5H2O melt were made as functions of temperature and X. Molar volume, V, is found to be additive. The percent deviation of Vext (extrapolated V of the pure solute from the plot of V vs. total added alkali ion fraction) from Vcal (calculated V of the pure solute from its high temperature density data) increases monotonically as the amount of NaNO3 in the hydrate melt increases, thereby manifesting a "structure-forming" tendency of NaNO3. The non-Arrhenius temperature dependence of molar conductance, Λ is analyzed in terms of the Vogel–Tammann–Fulcher (VTF) equation. Mixed alkali effect (MAE) has been observed on Λ and T0 (ideal glass transition temperature). A competitive polarization model has been used to explain the MAE on Λ.
Spectroscopic studies of some Pd(II), Pt(II), Ag(I), and Au(III) complexes of 4,6-diamino-2-thiopyrimidine and 4,6-diamino-2-methylthiopyrimidine. Structure and binding site determination
Authors: Gutiérrez M. D. , López R. , Romero M. A. , Salas J. M.
Published: May 2006

Interaction of Pd(II), Pt(II), Ag(I), and Au(III) with 4,6-diamino-2-thiopyrimidine (DATP) and 4,6-diamino-2-methyl-thiopyrimidine (DAMTP) in aqueous medium resulted in the formation of new complexes which have been characterized by elemental analysis, ir, 1H-nmr spectroscopic techniques, magnetic susceptibility measurements in the solid state, and conductivity measurements in DMF solution. The ir and 1H-nmr spectra were consistent with coordination involving the non-protonated ring nitrogen atom, the exocyclic sulphur atom, or N,S-chelation.
Archaeometry of medieval Islamic glazed ceramics from North Yemen
Authors: Hallett Jessica R. , Thompson Michael , Keall Edward J. , Mason Robert B.
Published: May 2006

Microscopic examination and electron microprobe analysis were employed to determine the materials and techniques used to decorate ten glazed types of medieval Islamic ceramics from North Yemen. Eight types were underglaze-painted, one was slip-painted, and one was monochrome glazed. The glazes were of two compositions, soda–silica and lead oxide – silica with annealing temperatures of approximately 835 and 640 °C, respectively. The colourants used in the glazes and paints were cobalt (blue), iron (green), copper (green and blue), and antimony (yellow). Where clay slips or slip-paints were present, alumina enrichment of the glaze had occurred during application or firing, and pigment-paints on top of slip grounds masked enrichment. The Mellor ratio for the lead glazes ranges from 0.68 to 0.74 and is well above the acceptable safety limit of 0.5.
Nuclear magnetic resonance and infrared study of lithium coordination by phosphonates and phosphine oxides R2P(O)CH2COY and their carbanions. Comparison between ketone, ester, and amide COY groups
Authors: Strzalko Tekla , Seyden-Penne Jacqueline , Froment Françoise , Corset Jacques , Simonnin Marie-Paule
Published: May 2006

Lithiated carbanionic chelates α to P(O) groups are formed from four title compounds. Their infrared and 13C and 31P nuclear magnetic resonance spectra show that chelation induces peculiar structural perturbations that depend upon the nature of Y. The corresponding neutral precursors 1–4 are able to form bidentate coordinates with the Li cation, which can be observed in THF when Y = OMe or NMe2 but in benzene only when Y = Me. They are probably intermediates in the lithiated base deprotonation of neutral compounds 1–4.
Molecular orbital calculations and 1H nuclear magnetic resonance spectra as indicators of internal rotational potentials in some methyl derivatives of styrene
Authors: Schaefer Ted , Sebastian Rudy , Penner Glenn H.
Published: May 2006

The 1H nuclear magnetic resonance spectra of the α-methyl, cis and trans-β-methyl, 4-methyl, and β,β-dimethyl styrènes are analyzed to yield long-range proton–proton coupling constants. With the assumptin that the internal rotational potential for styrene in the gas phase is unaltered in solution, a consistent treatment of over 40 of the long-range coupling constants is given in terms of the known coupling mechanisms. Expectation values of sin2 θ, where θ is the angle of twist about the exocyclic carbon–carbon bond, are presented for these molecules. These are compared with theoretical potentials at the 6-31 G level of molecular orbital theory. The present data indicate rather larger average twist angles than those in the literature. The extrema (at θ = θ° and 90°) in the angle dependent long-range coupling constants appear to be rather smaller in magnitude than are theoretical values obtained from valence bond and molecular orbital approaches.
Electron paramagnetic resonance study of the reactions of the spin trap 3,5-dibromo-4-nitrosobenzenesulfonate
Authors: Smith Peter , Robertson Jill Suzanne
Published: May 2006

Using rapid-mixing, continuous-flow TiCl3-based techniques and also by means of static-sample studies involving the thermal decomposition of symmetric aliphatic azo compounds, we have characterized by electron paramagnetic resonance 22 spin adducts of 3,5-dibromo-4-nitrosobenzenesulphonate, 1, in aqueous solution at 25 °C. These spin adducts, all nitroxides, exhibit a moderately wide range of a-nitrogen and β-CH proton splitting constants, which we discuss in terms of steric and electronic effects. In connection with these studies, blank experiments showed that aqueous solutions of 1 gave no radicals when exposed to light and heat. In addition, we have studied by electron paramagnetic resonance at 25 °C static samples of aqueous solutions of 1 both by itself and in the presence of each of several acrylic and methacrylic monomers and in both the presence and absence of light. These solutions yielded radicals, namely, nitroxides, only when containing methacrylic monomers, the presence of light having no effect. These observations support the "ene" addition/oxidation mechanism of nitroxide formation.
Conductivity and ionic association of tetraalkylammonium halides in tert-butanol–water mixture at 25 °C
Authors: Ansari Aijaz Ahmad , Islam M. R.
Published: May 2006

The molar conductivities of Pr4NBr, Bu4NBr, Pr4NI, and BU4NI have been measured in tert-butanol–water (t-BuOH–H2O) mixtures (61.30 ≥ D ≥ 16.50) over the maximum concentration range (2 × D3 × 10−7 mol dm−3) along with the densities and viscosities of the solvent mixtures at 25 °C. The conductance data have been analysed by using the Fuoss-1978 (F78) conductance equation and the results compared with the values obtained from the Justice (J), Pitt's (P), and Fuoss–Onsager–Skinner (FOS) equations. Molar conductivities at infinite dilution (Λ0), the thermodynamic ion association constants (KA) and the distance or ion-size parameters (R0 or a0) are evaluated from these data. The F78 equation fitted the data better and yielded KA and R values which are in accord with the ion-association theories. The interpretation of these parameters is discussed to provide some insight on the magnitude of the ionic association and the ion–solvent interaction.
Reactions of 4-tert-butyl-5-methylene-3,3-dimethyl-Δ1-1,2,4-triazoline with dimethyl acetylenedicarboxylate.
Authors: Schwan Adrian L. , Warkentin. John
Published: May 2006

not available
Synthesis of a lactosamine-type trisaccharide: 6-(bromoacetamido)-hexyl 2-O-(4-O-β-D-galactopyranosyl-2-acetamido-2-deoxy-β-D-glucopyranosyl)-α-D-mannopyranoside, designed to affinity label the leukoagglutinin sugar binding site
Authors: Ammann Hélène , Dupuis Gilles
Published: May 2006

We describe the synthesis of a D-lactosamine-type trisaccharide, 6-(bromoacetamido)hexyl 2-O-(4-O-β-D-galactopyranosyl-2-acetamido-2-(deoxy-β-D-glucopyranosyl)-α-D-mannopyranoside (18), designed for affinity labeling the Phaseolus vulgaris leukoagglutinin sugar binding site. The trisaccharide derivative was obtained by condensation of 4-O-(2, 3, 4, 6-tetra-O-acetyl-β-D-galactopyranosyl)-3,6-di-O-acetyl-2-deoxy-2-phthalimido-β-D-glucopyranosyl trichloroacetimidate (8) with 6-(N-benzyloxycarbonylamino)-hexyl 3,4,6-tri-O-benzyl-α-D-mannopyranoside (13). Removal of the protecting groups afforded 6-aminohexyl 2-O-(4-O-β-D-galactopyranosyl-2-acetamido-2-deoxy-β-D-glucopyranosyl)-α-D-mannopyranoside (17), which was converted into the corresponding bromoacetamido derivative by reaction with N-hydroxysuccinimido bromoacetate.
An EPR/ENDOR study of aminoxyls (nitroxides) capable of intramolecular bonding: hydroxyalkyl radical spin adducts of nitrones
Authors: Haire D. Larry , Kotake Yashige , Janzen Edward G.
Published: May 2006

A comparison of the effect of solvent and 13C labeling (of the radical addend) on the EPR (electron paramagnetic resonance) spectra of hydroxyalkyl vs. alkyl radical adducts of α-phenyl-N-tert-butyl nitrone (PBN) and 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) has been investigated. The solution ENDOR spectra in toluene and in ethanol are the first examples studied of aminoxyls with hydroxyl substituents in close proximity to the free radical centre. Diastereomeric mixtures of hydroxyalkyl radical adducts are clearly resolved by ENDOR spectroscopy. Conformations of these radicals have been assigned based on the 1H and 13C hyperfine splittings (HFS's) and by differential optimum ENDOR temperatures. Definitive assignments of carbon-centered radical adducts of DMPO and PBN are shown to be feasible by monitoring the β-13C HFS's of the radical addend. Long-range γ-H HFS's from the added radical group can be observed when the deuterated spin trap PBN-d14 is used. The production of 13C labeled hydroxyalkyl adducts of nitrones (e.g., DMPO–13CH2OH, from the reaction of hydroxyl radicals with added 13CH3OH) is shown to be useful as an improved EPR spectroscopic method for the verification of the presence of hydroxyl radicals.
1H nuclear magnetic resonance, differential scanning calorimetry, and electrical conductivity studies on the phase transitions of pentylammonium chloride and the cationic self-diffusion in its rotator phase
Authors: Iwai Shigehiko , Ikeda Ryuichi , Nakamura Daiyu
Published: May 2006

Measurements of 1H nmr, differential thermal analysis, and differential scanning calorimetry have revealed that solid pentylammonium chloride, cooled rapidly from room temperature, forms a mixture of the stable and metastable low-temperature phases below the phase transition temperature, Ttr (238 K). The sample annealed just below Ttr for 3 h was shown to consist of a single stable phase down to about 100 K, indicating the existence of a time-consuming phase transition taking place successively just below the normal transition. In the rotator phase obtainable above Ttr (257 K), 1H nmr studies proved the presence of axial rotation of the whole cation about the long axis, similar to n-paraffins in their rotator phase. Above room temperature, the onset of the translational self-diffusion of the cations within 2D planes in the layer structure of the rotator phase was detected by measuring the temperature dependence of electrical conductivity as well as 1H T1 and T1p. 2D self-diffusion constants and diffusional correlation times evaluated from the electrical conductivity observed between room and the melting temperatures indicate that the 2D cationic diffusion near the melting point is as fast as 3D self-diffusion in usual plastic crystals. Cationic motions in the two different rotator phases α and α′ are compared.
Proton spin–lattice relaxation in a solid protein Streptomyces subtilisin inhibitor. Enhancement of segmental motions by adsorbed water
Authors: Akasaka K. , Takegoshi K. , Terao T. , Ganapathy S.
Published: May 2006

Proton spin–lattice relaxation times (T1) were measured on a protein Streptomyces subtilisin inhibitor, in the solid state, in the presence and the absence of adsorbed 2H2O in the temperature range between −170 and 100 °C at 30 and 60 MHz. In both samples, a common relaxation process is attributed to the methyl group rotations of amino acid side chains which show minimum values of T1 of 0.11 s at −105 °C at 30 MHz and 0.23 s at −80 °C at 60 MHz. In the presence of adsorbed 2H2O, another, effective relaxation process was found above 0°C, showing a minimum value of T1 of 0.12 s at about 80 °C at 60 MHz. The latter relaxation process was attributed to internal motions of exposed peptide segments, a large part of which is located in the crucial enzyme-binding region of the inhibitor.
Symmetries of configurations of charges on a sphere
Authors: Frickel R. H. , Bronk B. V.
Published: May 2006

New equilibrium figures are obtained for finite numbers of point charges confined to a sphere and interacting via a Coulomb potential. The symmetries of the configurations are classified for all cases up to a charge number of forty. Features of the geometry of the configurations are discussed and shown on equal area projections. The substantial number of configurations now available permit study of frequency of occurrence of various symmetries and persistence of certain geometries over a range of charge numbers, as well as detailed comparison of similarities and differences of the present global minimum energy figures with corresponding close packed structures.
Electron microprobe studies of chemical reactions in ancient painted murals: The Beni Hasan Tombs, Egypt
Authors: Wilson-Yang K. M. , Burns George
Published: May 2006

The Egyptian tombs at Beni Hasan (ca. 2100 B.C.) have developed an ubiquitous, progressive grey surface layer which disfigures their painted surfaces. Recently, we published a series of X-ray photoelectron spectra of samples from these tombs and proposed a mechanism for surface layer formation. It involves CO2-saturated condensed water which gradually erodes the mural substrate. However, no direct evidence for this erosion was available. To test the proposed mechanism, light microscopy was used to study cross-sections of the murals. Areas of repeated solution and precipitation, in the form of dendritic structures were discovered in the subsurface regions, especially in yellow pigmented samples. In red pigmented samples, this degradation is not seen to the same extent. A series of electron microprobe analyses of the surface and subsurface regions of small painted fragments also were performed. Porous, heterogenous subsurface regions with pockets of phosphates have been found, predominantly in the yellow samples. These findings support the mechanism proposed earlier. Pore size measurements, estimated by photographic measurements, indicate that subsurface alteration took place before the bulk of the surface deposit layer had formed and that the existing surface deposit protects, as well as obscures, the painted walls. The surface layer development described here has some aspects common to other natural systems, i.e. the interaction of CO2 in water with carbonate-bearing rocks and deposits. However, at Beni Hasan there is effectively no source of groundwater and the painted surfaces are sheltered from direct exposure to the environment. Therefore the Beni Hasan deterioration has unique mechanistic features.
Influence of cyclodextrins on the photo-Fries rearrangement of acetanilide
Authors: Nassetta Mirtha , De Rossi Rita H. , Cosa Juan J.
Published: May 2006

The effect of α-, β-, and -γ-cyclodextrins on the quantum yield for the formation of o- and p-aminoacetophenone from the photolysis of acetanilide at 254 nm was determined. There is an increase in the ratio of o/p products when cyclodextrins are added and the total quantum yield is about the same as in water in the presence of γ-CD, higher with β-CD, and lower with α-CD. The association constants between acetanilide and CD were determined and the values are 22.7 M−1 and 30.7 M−1 for α- and β-CD respectively. The quantum yields for the formation of o- and p-aminoacetophenone were measured. The rearrangement occurs in water and in the included acetanilide. The quantum yields for the formation of o-aminoacetophenone and p-aminoacetophenone from the included substrate could be calculated for α- and β-cyclodextrin. Comparing these values with those obtained in water, it is concluded that there is some steric inhibition for rearrangement at the para position when α-cyclodextrin is the host, whereas with β-cyclodextrin the less polar microsolvent effect is responsible for the increase in the total quantum yield relative to water.
Confirmation of the polyketide origin of kotanin by incorporation of [1,2-13C2]acetate
Authors: Stothers J. B. , Stoessl Albert
Published: May 2006

Incorporation experiments with [1,2-13C2]acetate establish that the biosynthesis of kotanin, a dimeric 5-methylcoumarin, proceeds through a polyketide pathway and, therefore, represents an exception to Turner's rule regarding the relative lengths of the uncyclized residues of polyketide chains. The previous 13C signal assignments are also revised.
Modelling the binding step in dopamine receptor–antagonist interactions
Authors: Boudon Alain , Szymoniak Jan , Chrétien Jacques R. , Dubois Jacques-Emile
Published: May 2006

Modelling of the binding step in dopamine receptor–antagonist interactions was undertaken using sixteen antagonists belonging to the following five chemical series: phenothiazines, thioxanthenes, butyrophenones, benzamides, and benzisoxazoles. The Lower Unoccupied Molecular Orbitals (LUMOs) of the antagonists used for these interactions were compared using a similarity τij index, which enabled us to define the characteristic orbital forms of the active molecules. The result of the intersection of these representative orbital forms was the form common to the antagonists' LUMOs. This form corresponds to the orbital form of the indole's Higher Occupied Molecular Orbital (HOMO), and thus suggests that the aromatic binding site of the dopamine receptor is part of a tryptophan type structure.
Fe(CO)3 and Rh(acac) complexes of ergosteryl and 7-dehydrocholesteryl acetates: A high field nuclear magnetic resonance study
Authors: Perkier Richard E. , Mcglinchey Michael J.
Published: May 2006

The 500 MHz 1H and 125 MHz 13C spectra of ergosteryl and 7-dehydrocholesteryl acetate were recorded in C6D6 and CDCl3 and assigned using two-dimensional NMR techniques. It is shown that incorporation of an Fe(CO)3 or a Rh(acac) moiety onto the ring B diene system affects the chemical shifts of neighbouring protons and carbons. These changes are discussed in terms of the anisotropic properties of the organometallic fragments.
Electron spin resonance study of the structure of some σ* disulfur radical cations
Authors: Bonazzola L. , Michaut J.P. , Roncin J.
Published: May 2006

The radical cations generated by exposure to 60Co γ rays, at 77 K. of dilute solutions of 1,4-dithiane, 1,5-dithiacyclooctane, and thiirane in freon matrices, have been investigated by ESR spectroscopy. The relation between the giso value and the S—S σ bond length is studied quantitatively.
Configurationally dependent hydroxyl group effects on 13C nuclear magnetic resonance chemical shifts of olefinic carbons in cyclic six-membered allylic alcohols
Authors: Kaufman Teodoro S.
Published: May 2006

The differences in chemical shifts of olefinic carbons, Δδ(sp2), of pseudoequatorial and pseudoaxial six-membered allylic alcohols were correlated with the Δδ(sp2) values of their parent olefins. The results obtained reflect configurationally dependent substituent effects, the magnitude of which could be used for the stereochemical assignment of the hydroxyl group in these compounds.
Electron paramagnetic resonance study of the reactions of the spin trap 3, 5-dibromo-4-nitrosobenzenesulfonate
Authors: Smith Peter , Robertson Jill Suzanne
Published: May 2006

not available
Bifunctional reagents in organic synthesis. Total syntheses of the sesquiterpenoids (±)-pentalenene and (±)-9-epi-pentalenene
Authors: Piers Edward , Karunaratne Veranja
Published: May 2006

Transformation of the readily available keto acetal 14 into the enone 23 was accomplished via a seven-step sequence of reactions. Reaction of 23 with the bifunctional Grignard reagent 5 in the presence of copper(I) bromide – dimethyl sulfide, followed by intramolecular alkylation of the resultant chloro ketone 24, gave the tetracyclic ketone 25. Treatment of 25 with hydrogen in the presence of platinum afforded a mixture of the ketones 27 and 28 (42:58, respectively), which was transformed into a separable mixture of (±)-pentalenene (1) (32%) and (±)-9-epi-pentalenene (30) (33%). Keywords: ±-pentalenene synthesis, ±-9-epi-pentalenene synthesis, sesquiterpenoid synthesis, methylenecyclopentane annulation, bifunctional reagents, triquinane synthesis.
Nucleophilic photosubstitutions of o-methoxynitrobenzenes
Authors: Bunce Nigel J. , Stephenson Karen Labonte
Published: May 2006

We report a new synthesis of 3-nitroveratrole, based on the directed lithiation of veratrole, and photochemical substitutions of both 3-nitroveratrole and o-nitroanisole with several nucleophiles. Both aromatic substrates undergo photocyanation meta to the nitro group. With hydroxide ion, 3-nitroveratrole reacts meta to the nitro group, but 2-nitroanisole undergoes replacement of either substituent, the proportion of reaction at each site depending upon the OH− concentration. 3-Nitroveratrole undergoes an inefficient reaction with butylamine at each methoxy group; this reaction is apparently second order in amine. The reaction between o-nitroanisole and diethylamine results only in photohydrolysis. Keywords: photosubstitution, nucleophilic, 3-nitroveratrole, o-nitroanisole.
Single-ion transfer energetics of some ions using tetraphenylarsonium tetraphenylborate reference electrolyte assumption in aqueous mixtures of urea and glycerol
Authors: Talukdar Himansu , Rudra Sibaprasad , Kundu Kiron K.
Published: May 2006

Single-ion transfer free energies [Formula: see text] and entropies [Formula: see text] of some ions from water to aqueous mixtures of urea and glycerol have been determined using the widely used tetraphenylarsonium tetraphenylborate reference electrolyte assumption from solubility and emf measurements of some appropriate electrolytes at five different temperatures (15 to 35 °C). Analysis of [Formula: see text] and [Formula: see text] values of the ions as well as their respective "chemical" effect, [Formula: see text] and [Formula: see text] as obtained after correcting for their cavity and Born-type electrostatic effects, estimated by the scaled particle theory (SPT) and simple Born equation, respectively, show a complex dependence upon solvent composition. Attempts have been made to explain the observed mirror-image entropie behaviour of simple cations and anions in the light of Kundu etal.'s four-step transfer process and to compare the results with those obtained in other aquo-ionic and nonionic systems. Keywords: single ion, transfer energetics, TATB assumption, aqueous glycerol, aqueous urea.
Soluble polymer supported synthesis of a monosubstituted tetraaryl porphyrin
Authors: Takahashi Y. , Guillet J. E. , Winnik M. A.
Published: May 2006

A new synthesis of monosubstituted tetraarylporphines is described based upon the soluble polymer supported synthesis strategy. Low molecular weight poly(ethylene glycol) is chloromethylated at both ends. These ends are then used to build up the polymer-bound ether of 5-(4′-hydroxymethyl)-10,15,20-tritolylporphine, which, after purification of the polymer, can be cleaved in high yield with TiCl4. While the method offers useful conveniences, such as the ease of separation and purification of the desired material, the overall yield is low: 100 mg from 40 g of polymer. Oligomerization of the polymer during chloromethylation increases the polymer molecular weight and thereby decreases the number of end groups per gram of polymer. Keywords: porphyrin, tetraaryl, monosubstituted poly(ethylene glycol) oligomerization, polymer-supported synthesis.
Excess partial molar enthalpies of 2-butoxyethanol and water in 2-butoxyethanol–water mixtures
Authors: Siu William , Koga Yoshikata
Published: May 2006

Excess partial molar enthalpies of 2-butoxyethanol (BE) and water, [Formula: see text](I = BE or H2O), were measured from 25 to 35 °C in the entire concentration range. The results indicated that there are three concentration regions bounded at about xB = 0.02 and xB = 0.5, xB being the mole fraction of BE. In each region, the concentration and temperature dependence of [Formula: see text] (I = BE or H2O), is distinctively different from those in the other regions, and appears to support the following views: In the water-rich region, xB < 0.02, BE molecules cause an enhancement in the structure of water, and this effect spans a long range via a structurally enhanced network of water. The solute–solute (BE–BE) interaction is repulsive and of a long range character in terms of enthalpy. As xB increases, the repulsive solute–solute interaction becomes stronger sharply to the threshold value, xB ≈ 0.017, whereupon the mode of such mixing no longer becomes possible. In the intermediate range, 0.02 < χB < 0.5, a new scheme whereby BE molecules tend to associate is predominant. Two clathrate-like aggregates of the types h[BE(H2O)k] with k = 55 and 4 may exist together with BE clusters (BE)n In the third region, 0.5 < xB, BE molecules are exactly in the same environment as in pure liquid, while water molecules are almost in the same environment as in pure water. The solution consists of BE and water clusters. Keywords: excess partial molar enthalpy, 2-butoxyethanol–water.
Thermal diffusion of aqueous alcohols
Authors: Mukherjee Ashis Kumar , Sanyal S. K.
Published: May 2006

The thermal diffusion of aqueous solutions of methanol, ethanol, propan-1-ol, butan-1-ol, ethan-1,2-diol, andpropan-1,2-diol contained in the pores of a sintered glass disc (of porosity G4) has been studied. The Soret coefficient (σ) and the heat of transport [Formula: see text] values are reported in the temperature range of 25–40 °C. The heat capacities of transport are ascertained at 30 °C from the temperature coefficient of heat of transport values. The results are explained on the basis of changes in local entropy in the solvent (water), arising out of solute–solvent interactions. Correlations of the observed experimental values with certain relevant thermodynamic parameters, taken from the literature, have also been sought, with encouraging results. Keywords: thermal diffusion, heat of transport, entropy of hydration.
The biosynthesis of ephedrine
Authors: Grue-Sørensen Gunnar , Spenser Ian D.
Published: May 2006

It is shown by 13C nuclear magnetic resonance spectroscopy that the labelled C2 fragment of [2,3-13C2]pyruvic acid is transferred intact into the C-methyl group and the adjacent carbon atom of the Ephedra alkaloids, norephedrine, ephedrine, norpseudoephedrine, and pseudoephedrine, in growing plants of Ephedragerardiana. This finding serves to identify pyruvate as the elusive precursor of the aliphatic C2 terminus of the skeleton of the alkaloids. In earlier experiments with C-labelled substrates, label from [3-14C]pyruvic acid was incorporated mainly, but not exclusively, into the C-methyl group of ephedrine, and label from [2-14C]pyruvate was incorporated similarly into the carbon atom adjacent to the C-methyl group. A C6–C1 unit related to benzaldehyde or benzoic acid has long been known to generate the benzylic fragment of the carbon skeleton of the Ephedra alkaloids. It is likely that the carbon skeleton of ephedrine is generated from pyruvate and either benzaldehyde or benzoic acid, by a reaction analogous to the formation of acetoin or diacetyl from pyruvate and acetaldehyde or acetic acid, respectively. Keywords: biosynthesis of ephedrine, Ephedra alkaloids, 13C NMR spectra, ephedrine, biosynthesis of pyruvic acid, incorporation into ephedrine13C NMR spectra.
Characterization of thermal modulation of electrical conductivity: a small volume absorbance measurement technique
Authors: McLaren Robert , Dovichi Norman J.
Published: May 2006

Laser-induced thermal modulation of electrical conductivity is a new photothermal method for the measurement of minute absorbance in sub-nanoliter liquid samples. In this paper, a model is presented for the technique. This model is based upon the interaction of a Gaussian laser beam with a cylindrical, homogeneous electrolytic resistor. Particularly simple results are produced if the laser beam spot size is much smaller than the diameter of the electrodes. The theory is verified with respect to the electrode spacing and to the laser power, spot size, and chopping frequency; the highest sensitivity occurs with low chopping frequency, high laser power, and small electrode spacing. The signal is independent of laser spot size, as long as the beam does not illuminate the electrode surface. The precision of the measurement is dominated by shot noise in the current flow through the electrolytic resistor. One point absorbance detection limits of 5.5 × 10−6 measured across the 76-μm diameter electrode wires are obtained in aqueous solution with a 5- mW helium–neon laser. With this instrument, the difference in the absorbance of water and deuterated water is easily determined at 632.8 nm. Keywords: thermal modulation of electrical conductivity.
Nitrogen-15 shielding in trans-azobenzene
Authors: Wasylishen Roderick E. , Power William P. , Penner Glenn H. , Curtis Ronald D.
Published: May 2006

The principal values of the nitrogen nuclear magnetic resonance shielding tensor for trans-azobenzene have been measured and compared with theoretical values recently reported for trans-diazene. The shielding anisotropy for the two crystallographically nonequivalent molecules in trans-azobenzene is 925 and 880 ppm respectively, about 40% smaller than the value calculated for trans-diazene. Interestingly, the isotropic shielding constants for the two nonequivalent molecules are the same with in experimental error. The most shielded component of the 15N shielding tensor, σ33, is approximately perpendicular to the molecular plane, as it is in other planar π-systems; σ11, the least shielded component, is directed in the same general direction as the N—Cipso bond. The theoretical calculations overestimate the paramagnetic contribution to σ11. The magnitude and relative orientation of the nitrogen shielding tensor in trans-azobenzene are also compared with the corresponding phosphorus values in a diphosphene. Keywords: 15N chemical shielding anisotropy, solid state NMR, trans-azobenzene.
Structural studies of oligonucleotides with an O4-isopropylated thymine. d(i4TpT), d(Tpi4T), and d(Tpi4TpT)
Authors: Buchko Garry W. , Hruska Frank E. , Sadana Krishan L.
Published: May 2006

Syntheses are described for d(i4TpT) (1), d(Tpi4T) (2), and d(Tpi4TpT) (3), the O4-isopropylated analogues of d(TpT) (4) and d(TpTpT) (5). The detection of the molecular [M – H]− ions and some [2M – H]− aggregates shows that Fast Atom Bombardment mass spectrometry operating in the negative ion mode is useful for structural analysis of these DNA oligomers. In contrast with the monomer, i4dT, the two O4-isopropyl methyl groups of 1 and 2 are magnetically nonequivalent and give rise to separate 1H and 13C NMR signals. Their chemical shift difference (Δ) decreases gradually with increasing temperature, and can be interpreted as reflecting intramolecular base stacking, in line with circular dichroic data. This chemical shift difference is not seen in the 1H NMR spectra of 3, and is attenuated in the 13C spectra of this trimer. Coupling constant data indicate a shift towards the 3′-endo conformation of the sugar of the i4Tp moiety of 1, a trend observed for the corresponding O4-ethylated moiety of d(e4TpT). 13C–31P and 1H–31P coupling constants indicate that the structural features monitored by these couplings are not significantly altered by O4-isopropylation of the thymine base. Keywords: oligonucleotides, O-alkylation, stereochemistry, NMR, mass spectrometry.
Xestolide and secoxestenone, degraded triterpenoids from the sponge Xestospongiavanilla
Authors: Northcote Peter T. , Andersen Raymond J.
Published: May 2006

Xestolide (5) and secoxestenone (6), two metabolites with new terpenoid carbon skeletons, have been isolated from the northeastern Pacific sponge Xestospongiavanilla. Proposed structures for 5 and 6 are based on spectroscopic analysis and chemical interconversions. The similarity in structure between xestolide (5), secoxestenone (6), xestenone (2), and the aglycone of secoxestovanin A (4) has prompted the biogenetic proposal that metabolites 2, 5, and 6 are degraded triterpenoids. Keywords: sponge, terpenoids, Xestospongia.
Synthesis of a new unsaturated 16-membered heterocycle with alternating CC and NS building blocks
Authors: Sundermeyer Jörg , Roesky Herbert W. , Noltemeyer Mathias
Published: May 2006

The reaction between C6H4(SCl)23 and (PhC=NSiMe3)24 leads to the 16-membered heterocycle (SC6H4SN=CPhCPh=N)25. The X-ray structure analysis of 5 shows a nonplanar macrocyclic system with alternating CC and NS units. Keywords: 16-membered heterocycle, X-ray structure, sulphur–nitrogen.
One-electron transfer processes in the reaction of elemental gallium, indium, and tin with tetrahalogeno-ortho-benzoquinones
Authors: Annan Theodore A. , Tuck Dennis G.
Published: May 2006

Tin reacts with tetrahalogeno-o-benzoquinones X4C6O2 (X = Cl, Br) in refluxing toluene to give the tin(IV) catecholato species Sn(O2C6X4)2, and neutral and anionic derivatives have also been obtained. A mixture of Sn + X4C6O2-o + I2 + 1,10-phenanthroline (phen) yields SnI2(O2C6X4)phen. With gallium or indium (= M), the corresponding experiments give derivatives of MII (e.g., [M(O2C6X4)phen]2), or MIII (e.g., MI(O2C6X4)phen). Intermediate paramagnetic species were detected in the case of [Formula: see text], and a reaction mechanism involving one-electron transfer processes is proposed on the basis of these results. Solvated crystals of the compound InCl(O2C6Cl4)(C6H7N)3 were obtained from CH2Cl2 solution, and the structure of this molecule has been determined by X-ray crystallographic methods. Keywords: gallium, indium, tin, benzoquinones, oxidation.
Interfacial behaviour of isopropanol at the Hg electrode from solutions of H2O-isopropanol-HCl
Authors: Mazhar Amina A.
Published: May 2006

The adsorption of isopropanol on mercury in aqueous solutions of HCl at constant chemical potential of the acid has been determined from capacity and electrocapillary measurements. The maximum adsorption occurs at a charge of −2.2 μC cm−2. The Frumkin isotherm is obeyed, the a factor in the isotherm changes from a positive value for mixtures rich in water to a negative value for mixtures rich in alcohol. The standard Gibbs energy of adsorption depends quadratically on the potential. The orientation of the alcohol molecule in the interface is discussed. Keywords: interface, medium effects, adsorption isotherm, aliphatic alcohol, Gibbs energy of adsorption.
The voltammetric determination of cobalt(II) in seawater – adsorptive preconcentration of the dimethylglyoxime complex
Authors: Westenbrink W. W. , Page J. A. , VanLoon G. W.
Published: May 2006

Conditions for the determination of trace Co in seawater are described. The method involves formation of the dimethylglyoxime complex and adsorptive accumulation onto a hanging mercury drop electrode at an applied potential of −0.70 or −0.96 V vs. SCE. The adsorbed complex is then reduced by scanning the electrode potential to −1.20 V; the reduction peak potential (Ep) is −1.12 V. Nickel behaves in a similar manner with Ep of −0.99 V but the Co complex appears to be preferentially adsorbed, making analysis for Co possible even in the presence of a large excess of Ni. Adsorption of organic matter also occurs; this does not affect analytical sensitivity but limits the maximum deposition time allowed in the analysis.Determination of Co in six seawater samples yielded results ranging from 4 to 114 ng L−1. Good agreement was obtained with the one reference sample (NASS-1) for which there was a certificate value. Keywords: cobalt, dimethylglyoxime, stripping voltammetry, adsorption, seawater.
Bacteriorhodopsin analogue from anthryl chromophores
Authors: Singh Anil K. , Roy Mita
Published: May 2006

Preparation and properties of the bacteriorhodopsin (bR) analogue having the 3,7-dimethyl-9-(9-anthryl)-2E,4E,6E,8E-nonatetraenal (12) chromophore is described. Synthesis of the chromophore has been achieved by successive introduction of C5 units to 9-anthraldehyde (3) via the Horner reaction. The all-trans chromophore has been characterized by its ultraviolet–visible and 1H nuclear magnetic resonance spectra. Incubation of 12 with bacterioopsin suspension (prepared by photobleaching of bR isolated from Halobacteriumhalobium) at ambient temperature in the dark gave the new bR analogue 15, which showed an absorption band at 545 nm, and an opsin shift of 5575 cm−1. The new pigment is stable to hydroxylamine in the dark. It showed light–dark adaptation with the light-adapted form absorbing at a slightly red-shifted value of 550 nm. All-trans-retinal did not replace the anthryl chromophore in competitive bindings. Photolysis of the bR analogue 15, followed by difference spectrophotometric analysis, indicated formation of a photoproduct with an absorption band near 400 nm. The results are discussed in terms of the stereoelectronic requirements of the bR reaction centre. Keywords: bacteriorhodopsin (bR), retinal analogue, reconstitution, opsin shift (OS), external point charge model (EPC).
Conductimetric titrations of symmetrical aliphatic dicarboxylic acids in solvents pyridine and pyridine–benzene mixtures
Authors: Gündüz Turgut , Kiliç Esma , Tastekin Mustafa , Ozkan Güleren
Published: May 2006

Nine symmetrical aliphatic dicarboxylic acids, namely oxalic, malonic, succinic, glutaric, adipic, pimelic, suberic, azelaic, and sebacic acids, were titrated conductimetrically with tetrabutylammonium hydroxide in pyridine and pyridine–benzene mixtures ((2 + 1), (1 + 1), (1 + 2), (1 + 3), and (1 + 4)). Titration curves of the acids exhibited two distinct and stoichiometric end-points in media of dielectric constants 13.5, 10.0, 8.2, 6.3, 5.3, and 4.7, respectively. The closer investigations of the titration curves revealed that three factors mainly influence the shapes of the titration curves: dielectric constant of the medium, number of the methylene groups in the acid, and formation of hydrogen bonds between several species. As dielectric constant of the medium decreases, conductivity of the medium also decreases at any point in the titration curve. Increase in the number of methylene groups from one to five decreases conductivity of the medium linearly at the first end-points of the acids, from then the conductivity becomes practically constant in acids having five to eight methylene groups in pyridine and pyridine–benzene mixtures of 2 + 1 and 1 + 1 ratios. Keywords: conductimetric titration in pyridine, conductimetric titration in pyridine–benzene mixtures, symmetrical aliphatic dicarboxilic acids.