bluuebunny/crossref_metadata_2025 huggingface dataset:
Micellar effects upon reactions of the 2,2′,4,4′,4″-pentamethoxytrityl cation with nucleophiles
Authors: Bunton Clifford A. , Cuenca Angela
Published: May 2006

Cationic micelles of cetyltrimethylammonium chloride and bromide (CTACl and CTABr) speed attack of water upon the 2,2′,4,4′,4″-pentamethoxytrityl cation by a factor of ca. 5. The first-order rate constant in water is 5.51 s−1 at 25.0 °C. Anionic micelles of sodium dodecyl sulfate (SDS) have little effect on this reaction, but they strongly inhibit attack of OH−. In water, second-order rate constants for attack of OH−, CN−, and N3− are, respectively, 235, 177, and 2.8 × 105 M−1 s−1. Rate constants of reaction in CTACl go through maxima with increasing [surfactant] and analysis of the data shows that second-order rate constants at the micellar surface are similar to those in water.
Metal catalysis in oxidation by peroxides. 24. Extraction of aqueous peroxomolybdenum species into organic media and their reactivity
Authors: Bortolini Olga , Bragante Letanzio , Furia Fulvio Di , Modena Giorgio
Published: May 2006

The effect of the lipophilicity and coordinating ability of neutral ligands, belonging to the classes of phosphoric amides and pyridine oxides, on the extraction of the neutral oxidants molybdenum(VI)–peroxo complexes from an aqueous acidic phase to an organic one, has been studied. The nature of the Mo(VI)–peroxo species present both in aqueous and organic phase has also been investigated. Sizable amounts of mineral acid, added to adjust pH, may be extracted into the organic phase. The various factors affecting this process such as the nature of the acid added, the organic solvent and the neutral ligand, as well as the effect of this feature on the partition of the oxidant between the two phases are briefly discussed.
Motion about the Csp2—S bond in thioanisole and some derivatives by the J method
Authors: Schaefer Ted , Baleja James D.
Published: May 2006

Conformations about the Csp2—S bond in thioanisole and eight of its derivatives in solution are investigated by means of long-range spin–spin coupling constants over six bonds between the sidechain 13C nucleus and the para ring proton or 19F nucleus. According to geometry optimized STO 3G MO calculations the internal barrier to rotation is predominantly twofold in the gas phase in thioanisole and is 6.2 kJ/mol. In benzene solution the coupling constant yields 5.5(4) kJ/mol. Para fluorine and methyl substituents reduce the magnitude of the internal barrier, but meta methyl or chlorine substituents cause significant increases. In the presence of two ortho fluorine substituents the conformation of lowest energy has the C—S bond in a plane perpendicular to the aromatic plane, but die barrier may now contain a fourfold component. Addition of further fluorine substituents in the meta or para positions causes characteristic changes in conformational preferences of the thiomethyl group.
Fluorination of protected mannose derivatives using diethylaminosulfur trifluoride
Authors: Street Ian P. , Withers Stephen G.
Published: May 2006

3,4,6-Tri-O-benzyl-D-mannose and methyl 3,4,6-tri-O-benzyl-α-D-mannopyranoside were subjected to fluorination by diethylaminosulfur trifluoride and each produced two products. The former suffered fluorination at the anomeric centre to give the α-fluoride and, to a lesser extent, additionally at C-2 to give the 2-fluoro-2-deoxy-β-D-glucopyranosyl fluoride. The methyl mannoside underwent a methyl ether migration reaction to produce a 1:1 mixture of the α- and β-2-O-methyl glucosyl fluorides. A mechanism for this rearrangement is proposed and 1H and 19F nmr data are presented for all products.
Preparation of exo-6-benzyl-exo-2-(m-hydroxyphenyl)-1-dimethylaminomethylbicyclo[2.2.2.]octane. A non-peptide mimic of enkephalins
Authors: Bélanger Patrice C. , Dufresne Claude
Published: May 2006

A model for the active conformation of methionine enkephalin was derived from computer modeling. From this model, a target was designed and synthesized using bicyclo[2.2.2]octane as a structural template. Thus, a key intermediate, exo-6-benzoyl-1-carboethoxybicyclo[2.2.2]-2-octene was prepared via a Diels–Alder reaction using ethyl dihydrobenzoate and phenylvinylketone. It was subsequently modified to exo-6-benzyl-1-dimethylaminobicyclo[2.2.2]-2-octene. This intermediate was hydroborated and oxidized to the ketone on which the second aromatic group was introduced using a Grignard reaction, eventually giving rise to the desired target, exo-6-benzyl-exo-2-(m-hydroxyphenyl)-1-dimethylaminobicyclo[2.2.2]octane. Biological testing demonstrated weak activity with this compound.
Vitamin D relatives. Part I. B-thiophene-des-A-cholestanes. Solvolytic reactions of some derivatives of 2,2-disubstituted cyclohexane-1,4-diol and 4-hydroxycyclohexan-1-one
Authors: Morzycki Jacek W.
Published: May 2006

Some derivatives of des-A-cholestane with a thiophene ring B have been prepared from triketone 2. The reactions of tosylates of C-10 alcohols (7b, 9b, and 8c) leading to A-ring ethers (10 and 11) and olefin 5, respectively, have been studied. The solvolysis of tosylate 4c and the Bamford–Stevens reactions of p-tosylhydrazones 3c and 13b are also discussed.
Squaraine chemistry. On the anomalous mass spectra of bis(4-dimethylaminophenyl)squaraine and its derivatives
Authors: Law Kock-Yee , Bailey F. Court , Bluett Lynn J.
Published: May 2006

The electron impact (EI) mass specra of bis(4-dimethylaminophenyl)squaraine 1 and its derivatives 2–14 have been studied. Ions of mass number higher than the molecular ion, which are shown to be precursors of fragment ions of lower mass numbers, are observed. From the structure–property relationship, these ions are assigned to [Formula: see text] and [Formula: see text] where CH2R is the alkyl group in the N,N-dialkylanilino moiety of squaraine. Evidence is provided that the formation of [Formula: see text] and [Formula: see text] is the result of alkyl transfer and H transfer reactions within a squaraine aggregate rather than intermolecular vapor phase reactions in the mass spectrometer. This molecular aggregate is later shown to be a trimer by analysis of metastable ion data and chemical ionization mass spectrometry. The fragmentation sequence of this trimeric species is elucidated with the assistance of metastable ions. Results show that the trimer may break down to monomelic and dimeric species upon electron impact; alternatively, the four-membered ring of the central squaraine in the trimer may cleave to generate two species of approximately equal mass number. These two species usually dominate the mass spectrum and further fragment into [Formula: see text] and [Formula: see text] Detailed fragmentation schemes for [Formula: see text] and [Formula: see text] are proposed and discussed.
Reactions of cis-[Pt(sulfoxide)2Cl2] with silver salts and synthesis of hydroxo-bridged platinum(II) complexes with sulfoxides
Authors: Rochon Fernande D. , Kong Pi-Chang , Girard Louise
Published: May 2006

Platinum(II) compounds of the type cis-[Pt(L)2X2] where L = tetramethylenesulfoxide (TMSO), ethylmethylsulfoxide (EMSO), di-n-propylsulfoxide (DPSO), benzylmethylsulfoxide (BMSO), and dibenzylsulfoxide (DBSO) and X = Cl− and I− have been synthesized. The reactions of these compounds with a silver salt were studied in aqueous solution. Monomers of the type cis-[Pt(L)2(ClO4)2] (L = TMSO, EMSO, and DBSO), cis-[Pt(EMSO)2(SO4)] and hydroxy-bridged oligomers were isolated. Dimers, cis-[Pt(L)2(OH)]2(NO3)2 (L = TMSO, DPSO), and cis-[Pt(EMSO)2(OH)]2SO4 and possibly a trimer (cis-[Pt(EMSO)2(OH)]3)2(SO4)3 were characterized. The compounds were studied by infrared spectroscopy and by 1H nmr.
Formation of blue diquinolinyl methine dyes by irradiation of N-acylated primaquine in chloroform solution with ultraviolet light
Authors: Gessner Wieslaw P. , Venugopalan Bindumadhavan , Brossi Arnold , Jurgens Alex R. , Hufford Charles D.
Published: May 2006

Irradiation of N-acylated primaquines 1b–d in chloroform solution with ultraviolet light afforded blue diquinolinyl methine dyes 3b–d. The structures of compounds 3b–d are supported by spectral data, synthesis of 3c from 1c with orthoformate, and chemical reduction of 3d affording known dimer 5d.
Light-promoted catalysis of nickel hydride complexes in the isomerization and hydrogenation of cis,cis-1,5-cyclooctadiene: mechanistic studies
Authors: Chow Yuan L. , Li Huali
Published: May 2006

Xanthone-sensitized photoreduction of Ni(acac)2 in benzene under hydrogen, in the presence of cis,cis-1,5-cyclooctadiene (1,5-COD), causes isomerization and hydrogenation of the diene according to the consecutive transformation 1,5-COD → 1,4-COD → 1,3-COD → cyclooctene → cyclooctane. Evidence was provided that (i) a nickel hydride complex was generated, (ii) the sensitized excitation of this complex caused addition to the double bond, (iii) subsequent elimination caused isomerization, and (iv) triplet excited xanthone sensitized the transformations.
A highly convergent and enantiospecific synthesis of acyclic subunits of ionomycin from a single chiral progenitor
Authors: Hanessian Stephen , Murray P. J.
Published: May 2006

The C2–C10 and C11–C22 subunits of ionomycin were synthesized from a chiron derived from L-glutamic acid or D-ribonolactone. A strategy is presented in which C-methyl and hydroxyl groups are introduced on appropriate carbon frameworks in a completely predictable fashion based on the replicating lactone method, thus chemically matching any substitution pattern arising from the propionate biosynthetic pathway.
Electrosorption of vinyltriphenyl and propargyltriphenylphosphonium cations from methanolic solutions at the dropping and hanging mercury drop electrodes
Authors: Christodoulou Anastasia , Anastopoulos Anastos , Jannakoudakis Demetrios
Published: May 2006

The interfacial behaviour of vinyltriphenyl and propargyltriphenylphosphonium cations at the mercury electrode – methanolic solution interface is studied by phase selective ac polarography using dropping Hg electrodes of short (≤5 s) and long [Formula: see text] drop times and the hanging Hg drop electrode.The various adsorption parameters are determined on the basis of non-ionic adsorption isotherms involving capacitance data independent as well dependent on time.
Basicity of the thionamide group of α-aminothionamides. Attenuation of electrostatic effects by solvation
Authors: Paventi Martino , Heirtzler Fenton , Edward John T.
Published: May 2006

The first dissociation constants pK1of four diprotonated α-aminothionamides, determined spectrophotometrically, are between −8.1 to −8.8; the second dissociation constants pK2, of three of these compounds, determined titrimetrically, are between 7.3 and 7.7. Both pK1and pK2 values are close to those calculated from current electrostatic theory for equilibria in solutions of zero ionic strength, in spite of the fact that data for pK1values were obtained in solutions of high ionic strength. Results indicate a much larger effect of the charged ammonium group on the basicity of the thionamide group than on the basicity of the amide group. A possible explanation based on the greater hydration of the protonated amide group is advanced.
Study of the CEE mechanism by voltammetry and chronoamperometry
Authors: Lasia Andrzej
Published: May 2006

The electroreduction of bivalent metal cations may proceed by a CEE mechanism with a heterogenous chemical reaction on the electrode surface. The applications of the convolutive linear sweep voltammetry and chronoamperometry to study that mechanism are presented. The behaviour of the electrochemical reactions was simulated using an implicit finite difference technique for different values of kinetic parameters. The simulated curves were analysed and an agreement between the introduced and obtained data was found.
Total syntheses of the sesquiterpenoids (±)-axamide-1, (±)-axisonitrile-1, and the corresponding C-10 epimers
Authors: Piers Edward , Yeung Bik Wah Anissa
Published: May 2006

Total syntheses of (±)-axamide-1 (1), (±)-axisonitrile-1 (2), and the corresponding C-10 epimers 16 and 17 are described. The key steps of the synthetic sequence involve methylenecyclohexane annulation of 2-methyl-2-cyclopenten-1-one and TiCl4-catalyzed conjugate addition of 3-methyl-1,1-bis(trimethylsiloxy)-1-butene to the bicyclic enone 8.
The formylacetate synthon. Studies related to the synthesis of the penam and oxapenam ring systems
Authors: Wolfe Saul , Sterzycki Roman Z.
Published: May 2006

Methyl penicillanate has been synthesized by condensation of D-penicillamine with three different variations of the synthon RO2CCH2CHO, followed by esterification of the resulting 4-carboxythiazolidine-2-acetic acid ester, removal of R, and Mukaiyama ring closure. The conditions for the synthesis of an analogous 4-carboxyoxazolidine-2-acetic acid ester, containing an N-benzoyl protecting group, have been worked out. The condensation of threonine with diethinylketone does not lead directly to a vinylogous oxapenam.
(Trialkylstannyl)copper(I) reagents: preparation and reaction with α,β-unsaturated carbonyl systems. Preparation of β-trialkylstannyl α,β-unsaturated ketones
Authors: Piers Edward , Morton Howard E. , Chong J. Michael
Published: May 2006

Reaction of Me3SnLi with 1 equivalent of CuBr•Me2S, PhSCu, Me2(MeO)C—C≡C—Cu, or CuCN in tetrahydrofuran (THF) affords the (trimethylstannyl)copper(I) reagents Me3SnCu•Me2S (1) and [Me3SnCuY]Li (Y = PhS (2), Me2(MeO)C—C≡C (4), CN (5)), respectively. Similarly, treatment of a THF solution of Me3SnLi with 0.5 equivalent of CuBr•Me2S provides (Me3Sn)2CuLi (3), while reaction of n-Bu3SnLi with PhSCu (1 equivalent) gives [n-Bu3SnCuSPh]Li (6). Reagents 2 and 6 readily transfer (in a conjugate sense) the R3Sn group to conjugated enones (e.g. 7–9) and structurally simple enoates (e.g. 10), but do not react with β,β-disubstituted α,β-unsaturated esters (e.g. 11–13). Reaction of reagent 2 with the enone 26 under "normal" conditions produces, in addition to the expected ketone 28, a significant amount of the double conjugate addition product 30. A similar result is obtained with ethyl propenoate (27). However, reaction of 26 and 27 with reagent 2 in the presence of acetic acid provides cleanly and efficiently the "simple" conjugate addition products 28 and 29, respectively. Reagent 1 does not react, or reacts very sluggishly, with α,β-unsaturated carbonyl compounds. β-Trialkylstannyl α,β-unsaturated ketones (e.g. 37–46) can be produced smoothly and efficiently by reaction of the corresponding β-iodo enones (e.g. 32–36) with reagents 2 (or 1) and 6.
Synthesis of tricyclic azetidinones by intramolecular free radical cyclization
Authors: Just George , Sacripante Guerino
Published: May 2006

The synthesis of strained tricyclic azetidinones was achieved by free radical cyclization of N-substituted azetidinones involving a tributyltin hydride/AIBN mediated chain reaction. The 5-exo cyclization led to benzo carbapenems 10, 12, and 13, which were relatively unstable in solution. The 6-exo mode, however, afforded stable benzo carbacephems 20, 22, 24, and 25.
Preparation of α-aminothioamides from aldehydes
Authors: Paventi Martino , Edward John T.
Published: May 2006

The conversion of aldehydes into α-aminonitriles and thence into imidazolidin-4-thiones has been studied. Hydrolysis of the appropriate imidazolidin-4-thiones gave thioamides of nine naturally occurring α-amino acids. The possible pre-biotic significance of these compounds is discussed.
Uranium(IV) trifluoromethanesulfonate and some ternary uranium(IV) trifluoromethanesulfonates
Authors: Gill Madanmohan S. , Sethi Anil K. , Verma Rajendra D.
Published: May 2006

U(SO3CF3)4 and MIIUIV(SO3CF3)6, M = Mg, Mn, Fe, Zn, have been prepared by the reaction of their respective trifluoroacetates with HSO3CF3 acid in CF3COOH medium at room temperature. Infrared data indicate bidentate bridging —SO3CF3 groups in these compounds. Magnetic moments show the presence of uranium(IV) species. Diffuse reflectance spectra suggest six coordinate uranium(IV) although the presence of possible eight or higher coordinate uranium(IV) cannot be ruled out. X-ray diffraction studies reveal them to be definite entities.
Kinetics of acid-catalyzed hydration in aqueous solution of 1-methoxy- and 1-methylthio-2-phenylethyne and some related acetylenes
Authors: Banait N. , Hojatti M. , Findlay P. , Kresge A. J.
Published: May 2006

The rates of conversion of C6H5C≡COCH3 to C6H5CH2CO2CH3 were measured in dilute HClO4/H2O, DCIO4/D2O, and H3PO4–H2PO2−/H2O buffer solutions, and the rates of conversion of C6H5C≡CSCH3 to C6H5CH2COSCH3, C6H5C≡CH to C6H5COCH3, 2,4,6-(CH3)3C6H2C≡CH to 2,4,6-(CH3)3C6H2COCH3, and p-CH3OC6H4C≡CCH3 to p-CH3OC6H4COCH2CH3 were measured in concentrated HClO4/H2O solutions, all at 25 °C. The reaction of C6H5C≡COCH3 showed general acid catalysis and gave the isotope effect [Formula: see text], which indicates that it proceeds through rate-determining proton transfer from catalyst to substrate. The hydronium ion catalytic coefficient for this reaction is [Formula: see text], and those for the other four, in the order given above, are [Formula: see text], and 8.5 × 10−6 M−1 s−1. Relative reactivities based on these rate constants are discussed.
Conformational preference and internal rotation about the C1—Cα bond in phenylacetaldehyde and some benzyl alkyl ketones from 1H nuclear magnetic resonance and abinitio molecular orbital calculations
Authors: Penner Glenn H.
Published: May 2006

Analysis of the 1H nuclear magnetic resonance spectra of the benzyl moieties in phenylacetaldehyde, benzyl methyl ketone, benzyl ethyl ketone, benzyl isopropyl ketone, and 3,5-dichlorobenzyl tert-butyl ketone yields the long-range couplings between ring and α protons. These stereospecific couplings change very little upon replacement of the aldehydic hydrogen by various alkyl groups. The couplings for all the molecules studied fall within the ranges 4J(CH2, Ho) = −0.566 ± 0.008 Hz, 5J(CH2, Hm) = 0.278 ± 0.002 Hz, and 6J(CH2, Hp) = −0.409 ± 0.010 Hz, suggesting that in the ketones the alkyl group prefers to be trans to the phenyl ring and does not interfere with rotation about the C1—Cα bond. The long-range couplings are consistent with a potential function V(θ) = 8.4 ± 1.2 sin2 θ for two-fold rotation about the C1—Cα bond; θ is the angle between the carbonyl and benzene ring plane. Abinitio molecular orbital calculations on phenylacetaldehyde at the STO-3G level with the C=O bond cis to the phenyl group yield a potential of V(θ) = (8.65 ± 0.73) sin2 θ + (1.27 ± 0.80) sin2 2θ, rather close to the experimental potential but with a small fourfold component. The spin–spin coupling constant between the aldehydic and α protons displays a solvent dependence consistent with previously reported values. The insensitivity of 4J(CH2, Ho), 5J(CH2, Hm), and 6J(CH2, Hp) to solvent suggests that [Formula: see text] is very weakly dependent on the rotation of the aldehyde group.
Determination of polyacrylamides by maximum suppression
Authors: Wentzell Byron R. , Smith-Palmer Truis , Donini John C.
Published: May 2006

The use of polarographic maximum suppression methods for determining polyacrylamides and polyacrylic acid-based polymers was investigated. Using the oxygen, Co2+, and Hg2+ maxima these polymers were determined at the low ppm level. The effectiveness of polymers as maximum suppressors was dependent on the charge of the polymer and that of the electrode at the potential at which the polarographic wave occurred. The maximum suppression of various binary mixtures of polyacrylamides indicated that in some mixtures suppression was additive while in other mixtures, particularly those containing both anionic and cationic polymers, the suppression was greatly reduced. Aqueous extracts of chosen coals contained various interfering substances which could be removed most efficiently using Sephadex G-75, although this resin was only compatible with anionic and neutral polymers.
Structural studies of organoboron compounds. XXV. Synthesis and structure of (maltolato)diphenylboron
Authors: Orvig Chris , Rettig Steven J. , Trotter James
Published: May 2006

The reaction of maltol (3-hydroxy-2-methyl-4-pyrone) with diphenylborinic acid affords a high yield of the title compound. Crystals of (maltolato)diphenylboron are monoclinic, a = 14.3481(6), b = 8.3994(3), c = 12.6657(5) Å, β = 100.892(4)°, Z = 4, space group P21/n. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.041 and Rw = 0.052 for 2052 reflections with I ≥ 3σ(1). The molecule contains a five-membered C2O2B ring having a flattened B-envelope conformation, the B atom being displaced 0.081(2) Å from the C2O2 plane. Structural and spectroscopic data are consistent with weak binding of the maltolate oxygen atoms to boron. Bond distances (corrected for libration) are O—B = 1.533(2) and 1.600(3), B—C = 1.600(3) and 1.605(3) Å.
Isomeric 6H,8H-naphthyridino-1,3-benzothiazin-8-ones
Authors: Fodor Lajos , MacLean David B.
Published: May 2006

The synthesis of the ring system present in two of the four possible isomeric 6H,8H-naphthyridino-1,3-benzothiazines is reported here for the first time. 2,3-Dimethoxy-6H,8H-[2,7]naphthyridino[2,3-c][1,3]benzothiazin-8-one (5) and several related compounds have been prepared by both chemical and photochemical methods. 2,3-Dimethoxy-6H,8H-[1,6]naphthyridino[6,7-c][1,3]benzothiazin-8-one (9) has been isolated in 15% yield along with 5 in 38% yield on irradiation of 6,7-dimethoxy-3-nicotinoyl-4-methylidene-2H-1,3-benzothiazine (8).
Nouveaux modèles du NADH en série oxazolyl-3 dihydro-1,4 pyridine: synthèse, réactivité, rôle de la complexation dans la réactivité
Authors: Binay P. , Dupas G. , Bourguignon J. , Queguiner G.
Published: May 2006

A new class of NADH models is described: they contain the 3-oxazolyl-1,4-dihydropyridine structure. They have been synthesized by regioselective addition at the 4 position of organometallic derivatives (one of them bearing a chiral group) on 3-(4,4-dimethyl-2-oxazolyl)pyridine. The 1,4-dihydropyridine structure is obtained after quaternarization and reduction by sodium dithionite. Quaternarization through Zincke's reaction leads to models having a chiral group at the pyridine nitrogen. This chirality is necessary to induce asymmetry at the 4 carbon in the corresponding 1,4-dihydropyridine. This asymmetry plays an important role in the low enantioselectivity of reduction of a prochiral substrate such as methyl benzoylformate. The chemical reactivity of the models has been studied by performing the reduction of p-nitrobenzaldehyde in the presence of magnesium perchlorate. All models gave quantitative yields. However, reactions are slower than those performed, in the same conditions, with N-benzyl-1,4-dihydronicotinamide (BNAH). A 13C nmr study shows that this behaviour is probably a consequence of the high complexation of magnesium with the oxazoline moiety.
Protonation of a conjugated Schiff base by weak to strong acids: a 13C and 15N nuclear magnetic resonance study
Authors: Cossette Daniel , Vocelle Daniel
Published: May 2006

The interactions (mostly protonation) between several carboxylic and mineral acids and trans,trans-2,4-heptadienylidene tert-butylamine (HBA) were determined by 13C and 15N nuclear magnetic resonance spectroscopies. Results indicate that protonation is only partial with weak acids in both polar and nonpolar solvents. No interaction between polar groups located on the acids and the polyenic chain of HBA seems to occur. The chemical shifts of all carbon atoms and also of the Schiff base nitrogen are linearly correlated with the acidity strength of the carboxylic acids.
Versatile thin layer spectroelectrochemical cell employing specular reflectance
Authors: Hinman A. Scott , Pavelich Brad J.
Published: May 2006

A versatile thin layer spectroelectrochemical cell employing specular reflection of the incident light beam from the electrode surface is described. Its application to in-situ uv–vis and FTIR characterization of the products of electrochemical reactions and to thin layer voltammetry and coulometry as well as conventional cyclic voltammetry is demonstrated for the oxidation of tetraphenylporphinatozinc in dichloroethane/tetrabutylammonium perchlorate solution. The advantages and disadvantages of this type of cell as compared to more conventional sandwich type optically transparent thin layer electrodes are discussed.
Determination of total tin in National Research Council of Canada marine reference materials
Authors: Sturgeon R. E. , McLaren J. W. , Willie S. N. , Beauchemin D. , Berman S. S.
Published: May 2006

An intralaboratory determination of total Sn in three National Research Council of Canada reference materials, i.e., estuarine sediments BCSS-1 and MESS-1 and lobster hepatopancreas TORT-1 was undertaken. Samples were solubilized by acid mineralization and analyzed by stable isotope dilution inductively coupled plasma mass spectrometry, direct solution injection graphite furnace atomic absorption spectrometry (GFAAS), and hydride generation-GFAAS. The materials were homogeneous at the 500 mg subsample size. Good agreement was obtained amongst the results from the three instrumental methods of analysis. Certified values of total Sn in these materials are 1.85 ± 0.20 μg/g in BCSS-1, 3.98 ± 0.44 μg/g in MESS-1, and 139 ± 11 ng/g in TORT-1. The above uncertainties represent 95% tolerance limits for an individual (500 mg) subsample.
The X-ray photoelectron spectroscopy of ancient murals in the tombs at Beni Hasan, Egypt
Authors: Wilson-Yang K. M. , Burns George
Published: May 2006

The once colourful mural paintings at the Beni Hasan tombs (ca. 2100 B.C.), an important Egyptian archaeological site, are covered with an obscuring grey deposit. Qualitative observations made over the past 150 years were assembled and correlated with each other by us. Cumulatively, they indicate that recently the development of this layer has accelerated. To study this surface degradation, X-ray photoelectron spectra of fragments from the painted walls have been collected. Empirical XPS sensitivity factors have been employed to treat the data semiquantitatively. Three types of surfaces were identified. The first surface consists of CaCO3 and SiO2 as major constituents, and was the only surface identified previously. Our XPS data indicate that it also contains Al2O3, NaCl, MgO, sulphates, and phosphates as minor and trace components. The second surface consists predominantly of aluminium silicates. The third surface consists mostly of calcium carbonate. A preliminary mechanism for opaque layer formation is proposed herein. It suggests that the principal physicochemical processes are adsorption of water in the presence of carbon dioxide, followed by dissolution of the substrate walls upon which the murals were painted. This mechanism is consistent with our finding that mural deterioration is recent; it also indicates that deterioration is progressive, continuous, and cumulative. Further experiments are designed to verify the proposed mechanism. Two mechanisms proposed earlier are found to be inconsistent with at least some of our findings. Relatively recent efforts in removing obtrusive surface layers with dilute HCl were found to be performed carefully; there was no evidence of excess chlorides on cleaned surfaces. However, this finding does not provide a guarantee that murals were undamaged by HCl below the surface. It does suggest that progressive damage due to the formation of hygroscopic CaCl2 on the surface has been avoided.
The voltammetric determination of trace U(VI) in seawater —adsorptive preconcentration of the 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol complex
Authors: Elwerfalli Jamal , Page John A. , VanLoon Gary W.
Published: May 2006

Trace U(VI) in seawater has been determined by voltammetry after preconcentration by adsorption of the U(VI)/Br—PADAP complex (Br—PADAP = 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol).The reagent and the metal complex are adsorbed at a Hg drop electrode polarized at −0.40 V vs. a Ag/AgCl reference from a seawater electrolyte containing 3 × 10−7 M added Br—PADAP and buffered at pH 7.8 by the addition of triethanolamine/HClO4. The adsorption is at a mass transfer controlled rate. Voltammetry of the adsorbed electroactive material gave two peaks, one at −0.56 V for reduction of adsorbed Br—PADAP and one at −0.65 V for reduction of adsorbed U(VI)—PADAP complex. The height of the second peak was a sensitive measure of the concentration of U(VI) in the electrolyte. Analysis is carried out with preconcentration restricted to a low fractional electrode coverage with calibration by the method of standard additions. The method is subject to interference from organic surfactants but not from common metal ions. Interferences were minimized by the use of short adsorption times in the preconcentration step.For a 60 s adsorption time, a scan rate of 0.050 V s−1 and peak current measurement, the sensitivity was 8.2 nA/(μg L−1 U(VI)). Values of 3.44 and 3.21 μg L−1 U (VI) were determined for NASS-1 and CASS seawater reference standards.
Ba(OH)2 as the catalyst in organic reactions. Part XIV. Mechanism of Claisen–Schmidt condensation in solid–liquid conditions
Authors: Aguilera A. , Alcantara A. R. , Marinas J. M. , Sinisterra J. V.
Published: May 2006

The mechanism of the Claisen–Schmidt condensation, catalyzed by a series of activated barium hydroxide catalysts, is discussed. Two kinds of ketones are studied: R—CO—CH3 (R = Me, Et, Pri, But) and acetophenone (R = Ph). The influence of the chemical and textural properties of the catalysts, as well as the nature of the solvent, are studied. The process is an interfacial solid–liquid reaction between the adsorbed carbanion and the benzaldehyde. The different structures of the adsorbed carbanion produced from Ph—CO—CH3 and R—CO—CH3 are analyzed by infrared spectroscopy, and could explain the different reactivities observed in the process. The kinetic runs are discussed.
Notes on the directed aldol synthesis and reduction of some compounds related to cycloheximide
Authors: Piatak David M. , Tang Pui-Fun Louisa
Published: May 2006

Several N-phenylcycloheximide analogues were prepared by TiCl4-promoted aldol condensation between trimethylsilyl enol ethers of cyclic ketones and (N-phenylglutarimide)-β-acetaldehyde. Favored stereoisomers were isolated in several instances and identified from nuclear magnetic resonance data. Only for the analogue from 4-tert-butylcyclohexanone was it possible to obtain both erythro and threo isomers. Catalytic reduction of the β-hydroxyketone system overPtO2 gave dihydrocycloheximide analogues possessing an axial ring hydroxyl group. Reduction in glacial HOAc was noted as promoting fission of the glutarimide ring with formation of lactone amides and, in several instances, concomitant reduction of the phenyl ring to a cyclohexyl group.
High-yield regioselective synthesis of 9-glycosyl guanine nucleosides and analogues via coupling with 2-N-acetyl-6-O-diphenylcarbamoylguanine
Authors: Zou Ruiming , Robins Morris J.
Published: May 2006

Treatment of 2-N,9-diacetylguanine with diphenylcarbamoyl chloride followed by heating with aqueous ethanol gave 2-N-acetyl-6-O-diphenylcarbamoylguanine (2-acetamido-6-diphenylcarbamoyloxypurine). Bis-trimethylsilylation of this product followed by coupling with glycosyl acetates (trimethylsilyl triflate catalysis) or α-haloethers in anhydroustoluene gave 9-substituted guanine compounds in high yields with no 7-isomers detected.
Azides. Part VIII. Thermolysis of ethyl azidoformate in acetic and propionic anhydrides: facilitation of intramolecular cyclization of the resultant carbethoxynitrene to 2-oxazolidinone
Authors: Ayyangar N. R. , Brahme K. C. , Srinivasan K. V.
Published: May 2006

The thermolysis of ethyl azidoformate in acetic and propionic anhydrides as solvents yielded the corresponding N-acyl-2-oxazolidinones (2a,b) in relatively significant yields. The other major products isolated were N-carboxyethyl-N,O-diacyl hydroxylamine (1a,b) and N,N,O-triacetyl hydroxylamine (3), The intramolecular cyclization is facilitated by the solvent.
Hydration reactivity of ketenes generated by flash photolysis
Authors: Allen Annette D. , Kresge A. Jerry , Schepp Norman P. , Tidwell Thomas T.
Published: May 2006

The ketenes n-BuCH=C=O, PhCH=C=O, and Ph2C=C=O have been generated by flash photolysis and their reactivities toward water, including acid and base catalysis, have been measured in wholly aqueous media by ultraviolet spectrophotometry. The phenyl group electronically enhances the reactivity of ketenes relative to n-butyl toward water and hydroxide, but retards reaction with protic acids. The presence of bulky groups, including phenyls, on each side in the ketene plane sterically retards nucleophilic attack. The reactions are interpreted as involving in-plane attack by nucleophilic water or hydroxide in the neutral and base-induced reactions, and electrophilic protonation perpendicular to the ketene plane for the acid-catalyzed reaction.
Arylphosphonic acids. II. General acid and general base catalysis of acetone enolization
Authors: Shelly Kevin P. , Nagarajan K. , Stewart Ross
Published: May 2006

We have measured the rate constants for the enolization of acetone catalyzed by 29 arylphosphonate dianions (ArPO32−) and by 20 arylphosphonic acids (ArPO3H2). An excellent Brønsted correlation is found for the former reaction, with most ortho substituted compounds falling on the line drawn for the meta and para compounds (β = 0.72). The largest deviation is found for o-iodo, whose small positive deviation is ascribed to a polarizability effect in the transition state. The arylphosphonic acids give a fairly good Brønsted plot (α = 0.37) but here the ortho substituents tend to react slightly faster than would be expected on the basis of their equilibrium acid strengths. Catalysis by the monoanion ArPO3H− is difficult to detect; such ions appear to be acting as general acids, not general bases, and do not appear to act as bifunctional catalysts; it is shown that protonating acetone is 3.4 × 104 times as effective as deprotonating ArPO3H−. The Brønsted coefficients (β) for the rate-controlling steps for the enolization of acetone by the principal routes are shown to be inversely related to the magnitude of the rate constants.
General solution to axially symmetric quadrupole interaction in nuclear magnetic resonance of solids
Authors: Sanctuary B.C. , Campolieti G.
Published: May 2006

Spins with magnitude I greater than [Formula: see text] have nuclear electric quadrupole interactions that perturb the Zeeman splitting. To calculate the spin dynamics of multipulse sequences, it is necessary to know how the system evolves between pulses under the quadrupole. Prior work gave specific solution for [Formula: see text] in tabulated form. In this paper, a single expression is obtained, which is valid for all.I.
Particular solution for any consecutive second-order reaction
Authors: Paventi Martino
Published: May 2006

Arguments are presented against the accepted notion that multiple rate constants may be obtained from observed singular or pooled kinetic runs. For any competitive consecutive second-order reaction, the particular solution satisfying the total differential equation, derived from (n + 1) simultaneous differential equations, is S = (aA0 − S0){(aA0/S0)[exp (aA0 − S0)kt] − 1}−1, where S is the concentration of reactant common for all the steps, A is the concentration of the substrate with a reactive sites, A0 and S0 are the concentration of reactants at zero time, k is the observed rate constant, and t is the time. This equation is shown to reproduce the experimental reported data and yields k = min(k(1), …, k(n)), where k(i) is the rate constant assigned to step i. It is also shown that the initial conditions need not be known. For experiments with the initial condition (aA0 = S0), with only species A and S are present at zero time, the expression [Formula: see text] may be useful for an approximate evaluation of k when values of S do not approach zero after an infinite time (i.e., Sx = α ≠ 0, where α is the asymptote for the data).
Orbital control in the 1,3-dipolar cycloaddition of benzonitrile oxide to benzylideneanilines
Authors: Alcaide Benito , Mardomingo Carmen L. , Plumet Joaquín , Cativiela Carlos , Mayoral José A.
Published: May 2006

Twelve para-substituted benzylideneanilines selected according to the optimality criteria of experimental matrices were prepared in order to study the influence of the substituents on the cycloaddition rate with benzonitrile oxide. The results obtained indicate that the cycloaddition is a concerted process with a high degree of synchrony.
La dissolution anodique du cuivre en présence d'ions F− dans des solutions aqueuses acides
Authors: Potvin E. , Drogowska M. , Ménard H. , Brossard L.
Published: May 2006

The influence of F− ions on the anodic dissolution of copper in aqueous acidic solutions (pH 1.6–6), mainly at pH 5 with 0.1 M KF, has been studied. The nature of the changes that occur on the electrode during the oxidation has been examined by cyclic voltamperometry as well as by different techniques of surface analysis (SEM, XPS, AES). To determine the mechanism of the electrochemical oxidation of copper at pH 5, studies with rotating disk electrodes have been carried out under conditions giving rise to a general dissolution of copper. At pH 1.6–6, the copper electrode dissolves uniformly, i.e., without preferential zones on the surface, when it is submitted to a potentiodynamic polarization (10 mV s−1) at potentials above 0 VECS. At slightly acidic pH (pH 5, KF0.1 M), with a sweep of potential up to +0.5 V, the oxidation of copper begins with a generalized corrosion of the electrode and it is followed by the formation of a film of Cu2O that is not passivating; as the oxidation proceeds, the current drops and the electrode becomes passivated. One model for the anodic dissolution is presented for the results obtained at pH 5. When a uniform corrosion develops on the rotating electrode, the copper is oxidized to Cu2+ ions. When ω → 0, the rate of dissolution of the electrode is limited by the diffusion of the Cu2+ ions from the electrode to the solution, whereas when ω → ∞ the process of charge transfer is the limiting factor to the kinetics of the dissolution. [Journal translation]
Secondary α hydrogen–deuterium kinetic isotope effects in the syn-elimination reaction of 2-phenylethyldimethylamine oxide
Authors: Smith Peter James , Westaway Kenneth Charles
Published: May 2006

The secondary α-deuterium kinetic isotope effect has been measured for the thermal reaction of 2-phenylethyldimethylamine oxide in 90 mol% DMSO–H2O at 60.0 °C. A large secondary α-deuterium isotope effect of 1.158 per α-D was found, which indicates significant [Formula: see text] bond rupture and very little double bond formation at the transition state for this concerted syn-elimination process. The observed large normal value for (kH/kD)α is discussed in terms of the use of secondary α-D isotope effects for the determination of stereochemistry for a concerted elimination process. Various isotope effects found for the syn elimination of the above amine oxide and those for the anti elimination of 2-arylethyltrimethylammonium salts with ethoxide are considered with respect to the relative transition state structures for syn- and anti-elimination reactions.
Determination of the intercarbonyl dihedral angle of 1,2-diketones by 17O nmr
Authors: Cerfontain Hans , Kruk Cor , Rexwinkel Roel , Stunnenberg Frank
Published: May 2006

This study describes a method using 17O nmr to determine the intercarbonyl dihedral angle in a homologous series of seven 1,2-diketones. It is shown that the 17O nmr shifts of the 1,2-diketones are linearly dependent on both the electron density at the oxygen atoms and the n→π* excitation energy of the 1,2-diketo chromophore. Reduction of the overlap in the π system of the 1,2-diketo moiety leads to a gradual upfield shift of the 17O nmr signal, due to the increase in both the π-electron density at the oxygen atoms [Formula: see text] and the mean transition energy ΔE.
Radical ions in photochemistry. 18. The photosensitized (electron transfer) tautomerization of alkenes; the 1,1-diphenyl alkene system
Authors: Arnold Donald R. , Mines Shelley A.
Published: May 2006

The photosensitized (electron transfer) irradiation of several conjugated 1,1-diphenyl alkenes, in acetonitrile with 1,4-dicyanobenzene or 1-cyanonapthalene as electron accepting sensitizer and 2,6-lutidine as base, leads essentially quantitatively to tautomerization to the less stable unconjugated isomer(s). The proposed mechanism for this reaction involves formation of the alkene radical cation and sensitizer radical anion followed by deprotonation of the radical cation, reduction of the resulting radical to the ambident anion by back electron transfer from the radical anion, and reprotonation. There are several steps in this mechanism that could control the ratio of isomers. Evidence is provided that, at least in some cases, it is the relative rate of deprotonation from the isomeric radical cations that is the determining factor. This rate is influenced by the conformation of the radical cation; the carbon–hydrogen bond involved in the deprotonation step must overlap with the singly occupied molecular orbital.
Preparation of (4R,5S)-5-methyl-2,2-dimethyl-1,3-dioxolane-4-ylmethyl)-phosphonium iodide. A synthetic approach to olguine: further model studies
Authors: Valverde S. , Herradón B. , Rabanal R. M. , Martín-Lomas M.
Published: May 2006

not available
Preparation, reactions, and spectroscopic investigation of the 1,1,3,3-tetramethylimidazolidinium and 1,1,4,4-tetramethylpiperazonium dications
Authors: Creighton J. Alan , Taylor Michael J.
Published: May 2006

The title dications are prepared in a single step by the reaction of N,N,N′,N′-tetramethyl-1,2-diaminoethane (tmen) with the halogenoalkanes CH2X2 (X = Cl, Br, or I)orCH2XCH2X (X = Cl or Br), respectively. These bulky dications are useful for the precipitation of complex ions from solution, and show selectivity for doubly-charged anions. Tmen also reacts with CBr4 to yield a dibromo-imidazolidinium heterocyclic dication, whereas reaction with CHBr2CHBr2 produces the dihydrobromide of tmen. Vibrational and nmr spectra are reported, and the protons of the CH2 group between the two quaternary nitrogen atoms of the tetramethylimidazolidinium dication are shown from the nmr data to possess acidic character.
Complexes of 18-crown-6 macrocyclic ethers containing both an alkali metal phenoxide salt and phenol. Crown:salt:phenol ratios of 1:1:1 and 1:1:2
Authors: Rodrigue André , Bovenkamp John W. , Murchie Michael P. , Buchanan Gerald W. , Fortier Suzanne
Published: May 2006

Sodium and potassium phenoxide, in the presence of one equivalent of dicyclohexano-18-crown-6 (cis–anti–cis isomer) and dibenzo-18-crown-6, form complexes that have a ratio of 1:1:1 (crown:salt:phenol) in ethereal solvents containing excess phenol. On the other hand, complexes having 1:1:2 ratios (crown:salt:phenol) are obtained under the same conditions when the macrocycle is 18-crown-6 or dicyclohexano-18-crown-6 (cis–syn–cis isomer). When only one equivalent of phenol is present, then 1:1:1 complexes are obtained with 18-crown-6 and dicyclohexano- 18-crown-6 (cis–syn–cis isomer). No complexes containing only the metal salts were isolated from these solutions. The infrared spectral data obtained from the complexes show that the phenol hydroxyl hydrogens in the complexes form either strong or very strong hydrogen bonds. The nuclear magnetic resonance chemical shifts of the phenol protons in the complexes have been obtained from deuterated dimethoxyethane solutions. Comparisons are made with the chemical shifts of the phenol hydroxyl protons obtained from the corresponding solutions containing only the equivalent salt/phenol solutions.
Forbeside C, a saponin from Asteriasforbesi. Complete structure by nuclear magnetic resonance methods
Authors: Findlay John A. , Jaseja Mahesh , Brisson Jean-Robert
Published: May 2006

The structure of forbeside C, a saponin from the starfish Asteriasforbesi, has been deduced totally by nuclear magnetic resonance methods applied to the undegraded molecule. Using principally 2D-COSY, HCORR, RCT, and 1D-nOe experiments, forbeside C is shown to be the hydrate of the monosodium salt of 6α-O-{β-D-fucopyranosyl(1 → 2)-O-β-D-fucopyranosyl(1 → 4)-O-[β-D-quinovopyranosyl(1 → 2)]-O-β-D-quinovopyranosyl(1 → 3)-O-6-deoxy-β-D-xylo-hexos-4-ulopyranosyl}-20S-hydroxy-3β-sulfo-oxy-5α-cholest-9(11)-en-23-one, the structure reported previously for ovarian asterosaponin 1.
Enzymes in organic synthesis. 40. Evaluation of the enantioselectivity of the pig liver esterase catalyzed hydrolyses of racemic piperidine carboxylic acid esters
Authors: Toone Eric J. , Jones J. Bryan
Published: May 2006

Pig liver esterase (PLE) catalyzed hydrolysis of racemic piperidine esters proceeds enantioselectively to give product acids and recovered esters in 0–47% enantiomeric excess.
Reactions of dimethylsilylene with pinenes
Authors: Wang Dong , Chan T. H.
Published: May 2006

Photolysis of β-pinene and dodecamethylhexasilane (2) followed by methanol quenching gave the allylsilane 5 and the methoxysilane 6. The intermediacy of the silirane 7 was proposed. Photolysis of α-pinene and 2 gave 6 only. The formation of another silirane 15 as intermediate was verified by deuterium labelling experiment. The potential of silylene chemistry in organic synthesis was examined.
Dielectric polarization and molecular interaction. Part I. Acetoacetanilide and benzoylacetanilide–triethylamine–benzene systems
Authors: Naoum Magdi M. , Saad Gamal R. , Abdel-Moteleb Mohamed M. , Shinouda Hakim G. , Rizk Hanna A.
Published: May 2006

The apparent molecular polarization at infinite dilution of acetoacetanilide, and benzoylacetanilide in five constant-ratio mixtures of triethylamine in benzene leads to the inference that the equilibrium constants of the association process between the anilide and the amine in benzene are 357 and 600, and that the dipole moments of the complexes formed are 4.43 and 5.25 D, respectively.
Synthetic oligosaccharides related to Group B streptococcal polysaccharides. The rhamnotriose moiety of the common antigen
Authors: Pozsgay Vince , Brisson Jean-Robert , Jennings Harold J.
Published: May 2006

The methyl glycoside (1) of the α-L-Rhap-(1 → 2)-α-L-Rhap-(1 → 2)-α-L-Rhap-(1 → portion of the group-specific, common antigen polysaccharide of Group B Streptococci has been synthesized. The key compound of the synthesis was methyl 3,4-di-O-benzyl-α-L-rhamnopyranoside (4). Controlled acetolysis of 4 gave 1,2-di-O-acetyl-3,4-di-O-benzyl-α-L-rhamnopyranose (10), which upon reaction with 4 under catalysis by trimethylsilyl trifluoromethanesulfonate (TMS-Tf) yielded the α-(1 → 2)-linked dirhamnoside. Subsequent rhamnosylation followed by deprotection gave 1. TMS-Tf was also used in rhamnosylation reactions with tetra-O-acetyl-α-L-rhamnopyranose. Acetylation of acceptor alcohols was found to be a major side reaction in the TMS-Tf assisted glycosylations. One-dimensional nuclear Overhauser enhancement differential spectroscopy indicated that, in solution, 1 takes up a bent instead of an extended conformation.
Biologically active metabolites from Agelas mauritiana
Authors: Fathi-Afshar R. , Allen T. M.
Published: May 2006

Two novel bicyclic diterpenoides, agelasimine-A (9), and agelasimine-B (10), have been isolated from the orange sponge Agelas mauritiana. Also, a new bromine-containing alkaloid, 5-debromomidpacamide (12), along with midpacamide (13) and methyl N-methyl-4,5-dibromopyrrole-2-carboxylate (11), has been isolated. The structures were determined by interpretation of their spectral data. Agelasimine-A and -B exhibit a wide range of interesting biological activities such as cytotoxicity, inhibition of adenosine transfer into rabbit erythrocytes, Ca2+-channel antagonistic action, and α1 adrenergic blockade.