bluuebunny/crossref_metadata_2025 huggingface dataset:
The metathesis of N-silylamines and benzeneselenenyl chloride. An efficient selenenamide synthesis
Authors: Back Thomas G. , Kerr Russell G.
Published: May 2006

The metathesis reactions of N-(trimethylsilyl)dialkylamines with benzeneselenenyl chloride afford high yields of N,N-dialkylselenenamides 1a–1f. Similarly, N-(trimethylsilyl)acetamide produces the unstable N-(phenylseleno)amide 2insitu while N-(trimethylsilyl)imidazole and N-(trimethylsilyl)benzimidazole undergo selenenylation at the 4- and 2-position, respectively, to provide the selenides 3 and 4. N-(Trimethylsily)phthalimide failed to react under these conditions.
Motional properties of spin labels in proteins: Effects of hydration
Authors: Ruggiero J. , Sanches R. , Tabak M. , Nascimento O. R.
Published: May 2006

Conventional and saturation transfer (electron spin resonance) techniques are used to study the motional properties of several spin labels introduced in three proteins: lysozyme, sperm whale myoglobin and human hemoglobin. The mobilities of a maleimide spin label which binds covalently to the proteins, as well as of two small probes TEMPO and PD-TEMPOL were monitored in the temperature range from –−10 to −150 °C for samples in the dry and solution states. The three proteins show a similar temperature dependence as indicated by the parameters 2Azz and ΔH. A small linear increase in 2Azz with decrease in temperature is observed for the dry samples. For the proteins in solution, on the other hand, the 2Azztemperature dependence shows a change of behaviour around −60 °C that is related to the freezing of the water molecules in the hydration shell. The changes observed for the parameter ΔH are such that at temperatures below −60 °C ΔH is greater for the solution sample, while at temperatures above −60 °C ΔH is greater for the dry sample. Saturation transfer measurements show that the motion of the spin label is very restricted in all systems (τc > 10−5s) in the temperature range studied, so that the residual librational motion of the label is sensitive to the hydration, being responsible for the observed changes of the esr parameters with temperature.
Iminium ion-mediated cyclizations of 4-aryl-1,4-dihydropyridines. Alternate cyclization pathways
Authors: Hartman George D. , Halczenko Wasyl , Cochran David W.
Published: May 2006

Acid-catalyzed cyclization of dimethyl 2,6-dimethyl-4-[(2-ethenyl-5-methoxy)phenyl]-1,4-dihydropyridine-3,5-dicarboxy-late affords novel products via competing intramolecular processes. The present mechanistic pathways differ from previous iminium ion-mediated cycloadditions, to afford conformationally constrained dihydropyridine analogs for study as calcium antagonists. These results illustrate the ability of the dihydropyridine nucleus to function as either a nucleophile or an electrophile, depending upon substituents in the aryl ring.
Des 6H-thiazines-1,3 aux céphèmes par cyclocondensation
Authors: Pradere J. P. , Roze J. C. , Quiniou H. , Danion-Bougot R. , Danion D. , Toupet L.
Published: May 2006

The synthesis of new thiazines is described. Their structural determination and their spectroscopic parameters are examined. The [2 + 2] cyclocondensation of these thiazines with ketenes is studied along with the reversibility of the reaction as a function of the nature of the substituents on position 4 and (or) 5 of 6H-1,3-thiazines.
Dithiaphosphagermetannes-1,3,2,4 et dithiaphosphadigermolannes-1,4,5,2,3
Authors: Barrau J. , Amine M. El , Rima G. , Satgé J.
Published: May 2006

The synthesis of the new germylated and phosphorylated heterocycles 2-anisyl-4,4-dimethyl-2-thio-1,3,2,4-dithiaphos-phagermetane, [Formula: see text] (1), and 5-anisyl-2,2,3,3-tetramethyl-5-thio-1,4,5,2,3-dithiaphosphadigermolane, [Formula: see text] (2), are described. The decomposition of 1 with formation of (Me2GeS)3 and (AnPS2)n is observed at room temperature. At 200 °C 2 undergoes thermal fragmentation leading to (Me2GeS)3 and (AnPS)n. These results are explained by two mechanisms of decomposition involving formation of transient species [Me2Ge = S] and [AnPS2] or [AnPS], respectively. Exchange reactions between 2 and various metal (MIVB) chlorides are described. With Me2SiCl2 the reaction leads to silathione [Me2Si = S], probably via transient dithiaphosphasiletane.
A rate and equilibrium study of the addition of acetone enolate ion to the 2-methyl-5-nitroisoquinolinium cation in aqueous solution
Authors: Bunting John W. , Tam James W.
Published: May 2006

Rate and equilibrium constants for the reaction between acetone and the 2-methyl-5-nitroisoquinolinium cation to give 1-acetonyl-1,2-dihydro-2-methyl-5-nitroisoquinoline (2) have been evaluated over the pH range 10.0–11.3 in aqueous solutions at 25 °C. This reaction is shown to occur under much milder conditions (temperature, pH) and in much shorter reaction times than previously used for the synthesis of this adduct. Analogous data for the reaction of hexadeuteroacetone with this heterocyclic cation are also presented. The formation of 2 is shown to be first order in each of acetone, isoquinolinium cation, and hydroxide ion, and is not catalyzed by carbonate buffer species. These data are consistent with the rate-determining attack of the acetone enolate anion upon the isoquinolinium cation. The microscopic reverse of this reaction is the uncatalyzed decomposition of 2, which is consistent with the observed pH independence of the first-order rate constant for this process. Quantitative comparisons of rates and equilibria for the addition of hydroxide ion and acetone enolate ion to this isoquinolinium cation and to substituted benzaldehydes are now available.
Mechanisms of elimination reactions. 40. Attempted study of stereochemistry of elimination from 2-(p-nitrophenyl)ethyltrimethylammonium ion. Base-promoted cis–trans isomerization of p-nitrostyrene-β-d
Authors: Dohner Brent R. , Saunders Jr. William H.
Published: May 2006

Stereospecifically deuterated ArCHDCHDNMe3+ I − and ArCHDCHDNMe2O have been prepared, where Ar=C6H5 and p-NO2C6H4. When Ar=C6H5, the elimination reaction of the quaternary salt with ethoxide in ethanol goes with >98% anti stereochemistry, and the Cope elimination of the amine oxide with >98% syn stereochemistry. When Ar=p-No2C6H4, however, both reactions lead to apparent 50:50 anti/syn product. Subjection of (E)-p-nitrostyrene-β-d to the conditions of both the ethoxide-promoted and Cope eliminations results in complete cis–trans equilibration. No loss of deuterium from p-nitrosryrene-α-d occurs under either set of conditions, excluding isomerization via an α-arylvinyl carbanion. The most likely mechanism for isomerization is reversible addition of ethoxide under E2 conditions and ArCHDCHDNMe2O under Cope conditions to the β-carbon of p-nitrostyrene. The cis–trans isomerization of the p-nitrosryrene is sufficiently rapid to preclude determination of the stereochemistry of base-promoted eliminations leading to it.
Mechanism of azo coupling reactions. Part 34. Reactivity of five-membered ring heteroaromatic diazonium ions
Authors: Diener Heinz , Zollinger Heinrich
Published: May 2006

The azo coupling reactions of six five-membered ring heteroaromatic diazonium ions with 2-naphthol-3,6-disulfonic acid are investigated kinetically at various pH values. The dependence of the measured rate constants on the acidity of the aqueous reaction system is evaluated. It can be shown that the 2-naphtholate-3,6-disulfonate trianion reacts 4 × 108–8 × 108 times faster than the 2-naphthol-3,6-disulfonate dianion. The rate constants of the six diazonium ions vary by more than four orders of magnitude. The logarithms of the rate constants of all comparable diazonium ions correlate linearly with 1H nmr chemical shifts of the respective unsubstituted heteroaromatic parent compounds. An analogous correlation was found for azo couplings with substituted benzenediazonium ions. Diazotization of heteroaromatic amines does not go to completion, rather to an equilibrium. It is shown therefore that in acidic coupling systems the azo compound is only the kinetically controlled product. The thermodynamic products are 1-nitroso-2-naphthol-3,6-disulfonic acid and the heteroaromatic amine.
Chlorine isotope effects in the solvolysis of substituted 1-phenylethyl chlorides
Authors: McLennan Duncan J. , Stein Allan R. , Dobson Brian
Published: May 2006

Kinetic chlorine isotope effects attending the solvolysis of several ring-substituted 1-phenylethyl chlorides in alcohol–water solvent mixtures are reported. The k35/k37 values are insensitive to the identity of ring substituents and to solvent composition. Results are interpreted in terms of an SN1 heterolytic process incorporating a significant amount of internal return. Theoretical calculations suggest that the incipient chloride ion in the transition state may be strongly hydrogen-bonded.
Rates and equilibrium constants for the covalent hydration of 5-bromo-2(1H)-pyrimidinone in aqueous solution
Authors: Tee Oswald S. , Pika Jana , Kornblatt M. Judith , Trani Michael
Published: May 2006

The kinetics of bromination of the title compound (1) have been measured in aqueous solutions of pH 0–6. The change in the order of reaction which occurs around pH 2.5 is explained by 1 reacting via its covalent hydrate, 3. Furthermore, there is sufficient 3 present at equilibrium that the kinetics of its equilibration with 1 were also measured. From these two studies the extent of covalent hydration of 1 is estimated to be 5%.Kinetic studies of the bromination of the dimethyl cation 5 and of its equilibration with the pseudobase 6 were also carried out for the purposes of comparison.The present results for 1, 3, 5, and 6 are compared to earlier results for 2-pyrimidinone and analogous derivatives.
Pyrrolyl complexes of the early transition metals. 3. Preparation and crystal structure of (η5-C5H5)2Zr(η1-NC4H2Me2)2 and Zr(η1-NC4H2Me2)4
Authors: Bynum R. Vann , Zhang H.-M. , Hunter William E. , Atwood Jerry L.
Published: May 2006

The title compounds were synthesized by the reactions of NaNC4H2Me2 with the appropriate complex metal chloride in THF. The crystal structures have been determined from X-ray data measured by counter methods. (η5-C5H5)2Zr(η1-NC4H2Me2)2 crystallizes in the orthorhombic space group Pbca with cell dimensions a = 15.797(6), b = 14.327(5), c = 16.417(6) Å, and ρcalcd = 1.46 g cm−3 for Z = 8. Full-matrix least-squares refinement led to a final R factor of 0.041 based on 536 observed reflections. Zr(η1-NC4H2Me2)4 belongs to the monoclinic space group P21/n with a = 14.065(5), b = 10.717(4), c = 15.733(6) Å, β = 90.61(4)°, and ρcalcd = 1.31 g cm−3 for Z = 4. A final R value of 0.029 resulted from the refinement on the basis of 2649 observed reflections. The dicyclopentadienyl derivative exhibits two features of importance. The Zr—N—centroid angles are 159 and 168°, and the Zr—N bond lengths are 2.24(2) Å. In the homoleptic complex the corresponding values are 164–169° and 2.069(3)–2.090(3) Å. In both cases the bonding parameters are indicative of a substantial amount of π overlap between the zirconium atom and the pyrrolyl nitrogen atom.
2H nuclear magnetic resonance studies of motions in tetramethylammonium salts: the question of methyl reorientation
Authors: Ratcliffe Christopher I. , Ripmeester John A.
Published: May 2006

The 2H nmr lineshapes of a number of tetramethylammonium salts have been studied as a function of temperature. It is shown from this and previous evidence that in certain cases the lowest temperature motions should be assigned to whole ion [Formula: see text] or pseudo-isotropic motion, rather than to methyl group motions as has been assumed in previous 1H nmr studies. Steric potential calculations have been used to investigate a previously suggested model of concerted partial reorientations of methyl groups.
Further analysis of surface bond lengths measured for chemisorption on metal surfaces
Authors: Mitchell K. A. R. , Schlatter S. A. , Sodhi R. N. S.
Published: May 2006

This paper compares bond lengths deduced from the methods of surface crystallography with predictions from the Pauling–Schomaker–Stevenson approach and from a new alternative approach suggested by recent work of Brown and Altermatt. Examples considered are specifically for X—M surface bond lengths where atoms X from groups 16 or 17 are adsorbed on well-defined surfaces of a metal M. The alternative approach introduced here is parametrised with reference to structural data from solid compounds of formula MX. The two predictive approaches considered, when used together, appear to be quite adequate for guiding choices of trial model structures to be included in surface crystallographic analyses with low-energy electron diffraction (LEED); also they seem reasonable for checking the general reliability (or otherwise) of surface bond length data. Two further features introduced by this work are (i) evidence that the Cl—Ag distance reported by LEED for Cl adsorbed on the Ag(100) surface is broadly consistent with the structure of solid AgCl; (ii) evidence for S adsorbed on the Fe(110) surface that these analyses can guide investigations of lateral relaxations of surface metal atoms. As more reliable structural data become available, extensions of these analyses should help to identify the finer details in X—M bond lengths which result from the special coordination arrangements occurring at surfaces.
Reactions of alicyclic ketones in carbon tetrachloride. II. Kinetics of the chlorination of 2-chlorocyclopentanone and 2-chlorocyclohexanone, catalyzed by hydrogen chloride
Authors: Stamhuis Eize J. , Maatman Henk , Joosten Geert E. H.
Published: May 2006

The kinetics of the direct chlorination of 2-chlorocyclopentanone (2-mccp) and 2-chlorocyclohexanone (2-mcch) in carbon tetrachloride, catalyzed by hydrogen chloride, were studied. Reaction products are all the possible 2,2-, 2,5-, and 2,6-dichloro compounds. The ratios depend on the concentrations of the monochloro compound and hydrogen chloride. Surprisingly, even at conversions of the monochloro compound as low as 2%, 2,2,5-trichlorocyclopentanone and 2,2,6-trichlorocyclohexanone, respectively, are also formed. The chlorination reaction of both monochloro ketones shows zero order in chlorine. The order in hydrogen chloride is 1.3. The order in 2-mccp and 2-mcch varies somewhat with the concentration of the ketone and was found to be roughly 1.7. The variation in reaction order is explained by a partial self-association of the ketones. The ketones act as substrates as well as basic catalysts in the rate-determining α- or α′-carbon deprotonation. General base catalysis is clearly demonstrated by a strong increase in the rate of chlorination of 2-mccp upon addition of cyclopentanone (cp) to the reaction mixture, which agrees with the mechanism as presented in a previous paper. Kinetic equations derived from the reaction models for the "separate" and "mixed" ketone chlorinations accurately describe the observed rates of the chlorination of 2-mccp and 2-mcch in the concentration range of 0.04–1.0 M.
Study of photocatalytic reaction of methanol with water over Rh-, and Pd-loaded TiO2 catalysts. The role of added alkali metal cations
Authors: Naito Shuichi
Published: May 2006

The product selectivity of the photocatalytic reaction of methanol with water is changed drastically by the addition of alkali metal cations to Rh- and Pd-loaded TiO2 catalysts. Over alkali metal cation free catalysts, the main products are 1:1 ratio of H2 and dimethoxymethane, which is replaced with H2, methyl formate, and CO2 over alkali metal cation added catalysts. The role of added alkali metal cations is the stabilization of the reaction intermediate as adsorbed formate instead of adsorbed formaldehyde, which causes the selectivity change from dimethoxymethane to methyl formate.
Chiral building blocks for amphotericin B
Authors: Liang David , Pauls Henry W. , Fraser-Reid Bert , Georges Michael , Mubarak Azeez M. , Jarosz Slawomir
Published: May 2006

A general plan is outlined for disconnection of the polyene macrolides, which is aimed at achieving their syntheses, as well as facilitating the determination of their absolute configurations. The plan is exemplified by amphotericin B, I, the only family member of known absolute configuration. The major chiral component is a 20-carbon chain, III, which upon further retrosynthetic disconnection leads to three subunits, two of which correspond to the dideoxy hexoses IV and V. Although these are formally mirror images, they are best represented as the 3,4-dideoxyhexopyranoside, 1, and the 3,5-dideoxyfuranose, 2. Syntheses of 1 and 2 from readily available starting materials are described.
Rotations of linear triatomic anions in aqueous solution by nuclear magnetic resonance spectroscopy
Authors: Nicholas A. Martin de P. , Wasylishen Roderick E.
Published: May 2006

The temperature dependence of nitrogen-14 nuclear magnetic resonance (nmr) spin-lattice relaxation times (T1) for dilute aqueous solutions of the linear triatomic anions, SeCN−, SCN−, OCN−, and N3−, is investigated in the temperature range 3–80 °C. These nmr relaxation times (T1) yield rotational correlation times, τ2R, for the reorientation of the C∞ symmetry axis in these anions. The observed nonlinear dependence of log T1(14N) on temperature suggests that the rotations of the anions are coupled to the transport properties of water. For the thiocyanate anion the nmr correlation times are compared with those obtained previously from Raman spectroscopic data and Rayleigh scattering results. 17O T2 and I4N T1measurements for the NCO− anion are used along with abinitio calculations to estimate the 17O nuclear quadrupolar coupling constant in this ion.
Oxydoréduction photochimique et thermique entre le chrome(VI) et un acide α aminé (glycine, alanine, hydroxy proline et méthionine)
Authors: Bolte M. , Robert B. , Lemaire J.
Published: May 2006

Photochemical and thermal oxidation that occurs between chromium(VI) and various amino acids leads quantitatively to chromium(III). Spectroscopic studies rule out the formation of a chromium(VI) – amino acid complex in the ground state. Thermal oxidoreduction involves HCrO4−, the protonated amino acid [Formula: see text] and H+ ions. Chromium(V) appears to be an intermediate species in the reaction. HCrO4− is the only absorbing compound and the photochemical chromium(VI) reduction proceeds through a reaction between [HCrO4−]* and the amino acid. Chromium(V) is the primary product of the charge transfer reaction. Reduction quantum yields are proportional to the amino acid concentration and pH independent in the HCrO4− existence range.
Ion–ion–solvent interactions in aqueous ionic cosolvent systems. I. Transfer thermodynamics of hydrogen chloride in aqueous sodium nitrate solutions from emf measurements
Authors: Rudra Sibaprasad , Talukdar Himansu , Chakravarti Bijoy P. , Kundu Kiron K.
Published: May 2006

Standard potentials (E0) of the Ag–AgCl electrode have been determined in 1, 2, and 4 m NaNO3 + water mixtures at five equidistant temperatures ranging from 15–35 °C from the emf measurements of the cell: Pt, H2 (g, 1 atm)/HCl (m) NaNO3 +water/AgCl–Ag. These values have been used to evaluate the transfer energetics [Formula: see text] accompanying the transfer of 1 mole of HCl from the standard state in water to the standard state in each of the NaNO3 + water mixtures. Transfer free energies [Formula: see text] of HCl and that of the individual ions obtained from a separate study, and those obtained after correcting the "cavity effect" and Born-type electrostatic effect, as estimated tentatively by the scaled-particle theory (SPT) and simple Born equation respectively, have been discussed in the light of ion–ion–solvent interactions. The observed [Formula: see text]–composition profile as well as that obtained after correcting for the "cavity effect" were examined in the light of semiquantitative theory proposed by Kundu etal. earlier and are found to substantiate this theory.
Aqueous solution interaction of the methylmercuric cation with the dinucleotides CpG and dCpdG as studied by carbon-13 nuclear magnetic resonance spectroscopy
Authors: Buchanan G. W. , Bell M. J.
Published: May 2006

13C nuclear magnetic resonance chemical shifts and 13C–31P coupling constants are reported for the self-complementary dinucleotides CpG and dCpdG in aqueous solution. The influence of methylmercuration at pH 6.0 on these spectral parameters has been examined. Results are interpreted in terms of preferential methylmercuration at the N-7 site of the guanine base of each dinucleotide with concomitant base destacking.
Analysis of intramolecular hydrogen bonding in terms of the topological properties of the charge density. The protonated fluoroacetones
Authors: Choi Sai Cheng , Boyd Russell J.
Published: May 2006

A new method for the analysis of intramolecular hydrogen bonding is proposed and applied to the equilibrium structures of the protonated fluoroacetones. The method, based on a theory of molecular structure due to Bader, uses the topological properties of the charge density to elucidate the types of interactions within a molecule of interest. The calculations show a strong basis set dependence. In particular, the STO-3G optimized geometries exhibit intramolecular hydrogen bonding between a fluorine substituent and the carbonyl proton, whereas some calculations with geometries optimized at the 4-31G and 6-31G* levels indicate a direct bonding interaction between the fluorine and oxygen atoms. Two types of catastrophe point, namely conflict and bifurcation, are possible in protonated monofluoroacetone. Only the latter was located in the present calculations.
The pressure dependence of rates of homolytic fission of metal–ligand bonds in aqueous solution
Authors: Ishihara Koji , Swaddle Thomas Wilson
Published: May 2006

The volume of activation for the exclusively homolytic decomposition of protonated 4-pyridylmethylchromium(III) ion in aqueous HClO4 at 63.4 °C is +19 cm3 mol−1, with negligible dependence on pressure up to 350 MPa at least. The origins of the strongly positive volumes of activation that characterize homolysis of complex cations in aqueous solution are examined.
ipso Nitration. XXVII. The crystal structure and stereochemistry of 3-bromo-6-methyl-6-nitrocyclohexa-2,4-dienyl acetate, 5-bromo-2-methyl-6-nitrocyclohexa-2,4-dienyl acetate, and 3-bromo-6-methyl-6-nitrocyclohexa-2,4-dienyl chloride
Authors: Bushnell Gordon W. , Fischer Alfred , Henderson George N. , Mahasay Sumit Ray
Published: May 2006

The adduct obtained on nitration of 4-bromotoluene in acetic anhydride is (Z)-3-bromo-6-methyl-6-nitrocyclohexa-2,4-dienyl acetate. Its stereoselective rearrangement product, obtained on thermolysis in the presence of p-cresol, is (Z)-5-bromo-2-methyl-6-nitrocyclohexa-2,4-dienyl acetate. Reaction with hydrogen chloride in ether is also stereospecific and gives (Z)-3-bromo-6-methyl-6-nitrocyclohexa-2,4-dienyl chloride. The crystal structures of these compounds are reported.
The synthesis, X-ray structure, and substitution lability of chloro(2,3,7,8,12,13,17,18-octaethylporphinato)(triphenylphosphine)rhodium(III)
Authors: Thackray David C. , Ariel Sara , Leung Tak W. , Menon Kusum , James Brian R. , Trotter James
Published: May 2006

The rhodium(III) octaethylporphyrin complex Rh(OEP)(PPh3)Cl (1) has been synthesized via Rh(III) or Rh(I) precursors, and fully characterized both by spectroscopy and single crystal data. The crystals, available as a bis(chloroform) solvate are triclinic, P1, a = 13.478(5), b = 14.300(5), c = 15.346(4) Å, α = 102.33(2), β = 102.89(2), γ = 90.56(3)°, Z = 2, Dx = 1.384 g cm−3. The structure was determined from Mo diffractometer data and refined by least-squares methods to R = 0.095, Rw = 0.068 for 5189 reflections. The octahedrally coordinated rhodium atom is displaced by 0.077 Å from the mean plane of the four N atoms, towards the triphenylphosphine group. The average Rh – ring nitrogen distance is 2.024 Å, Rh—P is 2.306(3) Å and Rh—Cl, 2.442(2) Å. Solution equilibria studies on 1 describe formation of Rh(OEP)L2+ (L = PPh3, PnBu3) via thermal reactions (including thermodynamic data for the PPh3 system), and formation of Rh(OEP)Cl(L′) species (L′ = CO, THF, MeCN) via photochemical processes.
Studies in anthracyclinone synthesis. The oxidative decyanation route to aklavinone and 4,11-dideoxy-α-rhodomycinone
Authors: Rao J. Appa , Ravichandran K. , O'Malley G. J. , Cava M. P.
Published: May 2006

The formal synthesis of aklavinone and the first synthesis of 4,11-dideoxy-α-rhodomycinone are described. Oxidative decyanation of the tetracyclic nitrile to the corresponding ketone using a strong base (LDA) constitutes an important strategy in the synthesis of both these compounds.
Schmidt reaction on camphor. Part I. Structure of the products
Authors: Hunter Norman R. , Khan M. Zafar , Marat Kirk , El-Kabbani Ossama A. L. , Delbaere Louis T. J.
Published: May 2006

A reinvestigation of the major product from the Schmidt reaction on camphor has led to the assignment of the structure as 1,9,9-trimethyl-3-oxo-2,8-diazabicyclo[3.3.1]nonane (5). The structure has been established by high-field proton and carbon-13 nuclear magnetic resonance, as well as degradation to the imine ester 4. Confirmation of the structure of 5 was achieved by X-ray crystallography. Compound 5, C10H18N2O, crystallized in space group P21/a with a = 14.934(1) Å, b = 10.973(1) Å, c = 12.341(1) Å, β = 92.0°, and Z = 8. The relatively rare cis amide linkage is present in the molecule. In addition to the amino amide 5, the Schmidt reaction on camphor leads to small amounts of the tetrazole 8 and the imine nitrile 9.
Fluorescence properties of polyene antibiotics in phospholipid bilayer membranes
Authors: Petersen N. O. , Gratton R. , Pisters E. M.
Published: May 2006

The fluorescence characteristics of the polyene antibiotic Nystatin have been studied by measurements of quantum yields, lifetimes, and anisotropies in a model bilayer membrane. These measurements have been performed as a function of temperature and fluorophore-to-phospholipid molar ratio. Comparisons with data available for the parinaric acids demonstrate that the photophysics of the two polyene chromophores is similar. The quantum yield decreases at increasing Nystatin densities while the lifetimes are constant. These observations combined with quantitative comparisons with calculations of the density dependence of dynamic quenching in two-dimensional systems show that Nystatin fluorescence is quenched in bilayer membranes by a static process. This is likely due to the formation of complexes within the bilayer. The quenching is equally efficient in both the gel and liquid crystalline phases of the bilayer which suggests that the formation of the complexes occurs at all temperatures. At high Nystatin densities, there is also a quenching of the polarization, but the mechanism whereby this occurs is not clear. The limiting anisotropy at low concentrations is between 0.33 and 0.35 at all temperatures, suggesting that the Nystatin monomer is highly restricted in its reorientational motion in both gel and liquid crystalline phases.
Pronounced solvent effect on the absorption spectra of the photochemically produced 2,4-dinitrobenzyl carbanion
Authors: McClelland Robert A. , Steenken Steen
Published: May 2006

The photochromism of 2,4-dinitrotoluene in aqueous solution has been attributed to formation of the 2,4-dinitrobenzyl carbanion (J. Chem. Phys. 39, 1218 (1963)). The spectrum of the colored transient is, however, significantly different from that of ground state carbanion when formed in aprotic solvents through thermal decarboxylation of potassium 2,4-dinitro-phenylacetate (J. Org. Chem. 49, 413 (1984)). In this paper absorption spectra obtained upon 248 nm laser photolysis are reported, for solutions in water, acetonitrile, and their mixtures. The spectrum in acetonitrile does closely resemble that of the carbanion formed by decarboxylation. It is suggested that the same ion is formed in water, and a solvent effect is responsible for the significantly different spectrum. Spectra in the mixed solvents suggest that there is an equilibrium between differently solvated forms of the anion, the extremes being an unspecifically solvated form with λmax 400 and 640 nm as in acetonitrile and a form in water that is hydrogen bonded through the nitro group with λmax 350 and 500 nm. Experiments with conductivity detection provide corroborative evidence for carbanion formation. In acetonitrile a non-conducting transient is produced as a precursor to the anion. This is suggested to be the conjugate acid of the aci-nitro anion, formed by intramolecular hydrogen abstraction in the excited nitrotoluene.
The temperature dependence of the Raman spectrum and gauche interactions of tri-N-butylamine: a conformational study
Authors: Costa A. M. Amorim da , Carvalho L. A. E. Batista de , Teixeira-Dias J. J. C. , Barbosa Ester F. G. , Lampreia Isabel M. S.
Published: May 2006

Raman spectra of tri-n-butylamine show pairs of bands whose temperature-dependent intensities clearly suggest their assignment to different conformers in simultaneous equilibria. These spectroscopic data are interpreted and correlated with structural information obtained from statistical analysis of gauche skeletal arrangements in tri-n-butylamine. The average numbers of gauche interactions in various tri-n-alkylamines are used to evaluate partial molal volumes which show excellent agreement with experimental data, thus imparting statistical meaning and usefulness to the concept of "average conformation".As the temperature decreases, the band at 904 cm−1 in pure liquid tri-n-butylamine increases in intensity at the expense of the band at 880 cm−1. Both of these bands are ascribed to CH2 and CH3 rocking vibrations. In solution, the conformer represented by the band at 904 cm−1 is favoured by polar solvents, whereas the conformer represented by the 860 cm−1 band is favoured by non-polar solvents.The relative intensity changes with increasing temperature of the bands at 1456 and 1440 cm−1 is explained by an increase in the number of CH2 groups adjoining two gauche bonds and a decrease in the number of CH2 groups adjoining a trans and gauche bond.Temperature dependence of the relative intensities of bands in the CH stretching region suggest, by comparison with mono- and di-n-butylamine, that the increase in the number of alkyl chains around the nitrogen atom makes molecular packing less restrictive and unique, giving rise to mesophases, as the overall shapes of the molecules become more spherical.
Spectral studies on the interaction of iodine with thiazoles
Authors: Abdel-Hamid Refat , El-Samahy Ahmed A. , El-Taher Abdel-Hafiz , El-Sagher Hussein
Published: May 2006

The electron donor–acceptor molecular complexes between thiazole and some of its derivatives and iodine in several solvents have been investigated spectrophotometrically at different temperatures. Spectral characteristics, equilibrium constants, K, extinction coefficients, ε, and thermodynamic functions of formation, ΔH0, ΔG0, and ΔS0 have been determined. It was found that the data fit satisfactorily the 1:1 stoichiometric equilibrium: D + I2 = DI2 "outer complex". The EDA complexes obtained are of "n–σ*" type in which the nitrogen atom of thiazole is the donor center. Moreover, the effect of solvents on the K values is discussed in terms of the solute–solute and solute–solvent competing equilibria.
Structures and stabilities of C2H2F3+ cations: an abinitio molecular orbital study
Authors: Charpentier Micheline , Fossey Jacques , Tidwell Thomas T. , Wolfe Saul
Published: May 2006

Eleven stationary points on the singlet C2H2F3+ potential energy surface have been calculated using the 3-21G basis set, and characterized as minima (four structures) or first-order saddle points (seven structures) by vibrational analysis. To check the reliability of this basis set, three of the structures have also been optimized at the 6-31G* level; although the geometries change somewhat, the relative energies and nature (maxima, minima) of the structures remain the same. For CF3CH2+ the minimum energy structure has one C—F bond coplanar with the vacant p-atomic orbital at the cationic centre. The structure is 16.4 kcal/mol less stable than the lowest energy conformation of FCH2CF2+, and the barrier for the 1,2 fluorine migration which connects the two structures is low. The cation F2CHCHF+ has a conformation that is a minimum on the potential energy surface that is 16.9 kcal/mol higher in energy than FCH2CF2+; the two structures are separated by a barrier for 1,2 hydrogen migration of 23.5 kcal/mol. The electronic effects in the various structures have been studied using a quantitative PMO analysis of the interactions between the two carbon fragments of the ions. For CF3CH2+ the net effect of the fluorine is highly destabilizing; the principal stabilizing interactions between CF3+ and CH2 consist of π donation from CF3+ to CH2 and homoconjugation of a fluorine lone pair with the cationic centre. No net stabilization attributable to fluorine bridging could be found.
The potassium permanganate oxidation of steroidal homoannular dienes
Authors: Bończa-Tomaszewski Zbigniew
Published: May 2006

The oxidation of 3β-acetoxycholesta-5,7-diene (1) with potassium permanganate–sodium periodate reagent gave epoxy-diol 2 with almost quantitative yield. Similar oxidation of cholesta-2,4-diene (3) afforded, as well as epoxy-diols 5 and 6, products of cleavage of the double bonds. These results show that formation of epoxy-diols predominates in the case of hindered steroidal dienes (e.g., diene 1), whereas oxidation of unhindered steroidal dienes (e.g., diene 2) gives, in addition to epoxy-diols, products of cleavage of the double bonds.
Synthesis of functionalized bicyclo[3.2.1]octa-2,6-dienes by thermal rearrangement of substituted 6-exo-(1-alkenyl)bicyclo[3.1.0]hex-2-ene systems
Authors: Piers Edward , Jung Grace L. , Ruediger Edward H.
Published: May 2006

Thermolysis of each of the enol silyl ethers 31–35 affords, cleanly and efficiently, the bicyclo[3.2.1]octadienes 36–40, respectively. Similarly, thermal rearrangement of the enol silyl ether 50 provides the diene 51. Hydrolysis of 36, 37, and 39, and decarbomethoxylation of the resultant keto esters 41, 42, and 44, gives the ketones 46–48, respectively. The ketone 46 is also obtained by acid hydrolysis of 51. Conversion of 6-methyl-1-hepten-4-yn-3-ol (56) into the enol ether 63 is described. Thermolysis of 63 gives 64, which, upon acid hydrolysis, affords 65. Thermolysis of the enones 68 and 69 produces the bicyclic dienones 70 and 71, respectively. On the other hand, thermolysis of 73 and 74, derived from the enone 69, provides the C-8 exo substituted bicyclo[3.2.1]octa-2,6-dienes 75 and 76, which are transformed smoothly, by acid hydrolysis, into the ketones 77 and 78, respectively.
High performance liquid chromatography as an analytical tool for the determination of sulfate in coconut and caffeine in tea
Authors: Ramakrishna Rasiah S. , Dias M. , Palamakumbura S. , Jeganathan M.
Published: May 2006

The need for a rapid method for detecting sulphate sulphur in coconut and caffeine in manufactured tea initiated a study to compare high performance liquid chromatography with classical methods. We compare the methods in their applicability in rapid and selective analysis of coconut water, plant tissues, and soil, as well as of caffeine in tea samples from different regions. We present a statistical correlation between the methods and discuss the significance of our results.
Ternary charge transfer complexes. V. Complexes with thiazyl salts
Authors: Brownstein Sydney , Louie Brenda
Published: May 2006

Charge transfer complexes NS+MFn−•arene, where M = B, n = 4; M = P, As, or Sb, n = 6; and arene is hexamethylbenzene, mesitylene, napthalene, or 9,10-dimethylanthracene, have been prepared and characterized. The reaction of NS + MF6− (M = As or Sb) with highly oriented pyrolytic graphite results in graphite intercalated only with the metal fluoride anions.
Synthesis and chemiluminescence of new derivatives of isoluminol
Authors: Bélanger Alain , Brassard Paul , Laquerre Sylvie , Mérand Yves
Published: May 2006

In an attempt to improve the sensitivity of luminescent immunoassays, we have prepared some new isoluminol, 7-(N,N-dialkylamino)-5-methyl-2,3-dihydrophthalazine-1,4-diones by means of a novel procedure involving the cycloaddition of dienamines to maleic acid derivatives. These compounds are characterized by the presence of a methyl group at C-5 and give quantum yields three to five times greater than those of the most efficient isoluminols in use at present.
Two dimensional fluorescent sensors: the different dependences of the fluorescence band position and the fluorescence quantum yield of 1,5-diphenyl-3-vinyl-Δ2-pyrazoline upon solvent dipolarity and hydrogen bond acidity
Authors: Premakumara de Costa Merinnage Dayal , Prasanna de Silva Amilra , Pathirana Suram Therese
Published: May 2006

1,5-Diphenyl-3-vinyl-Δ2-pyrazoline (1) has a fluorescence quantum yield [Formula: see text] which is strongly controlled by the hydrogen bond acidity of the solvent (α) and to a minor extent by the dipolarity of the solvent (π*) according to the equation:[Formula: see text]The wavenumber of the fluorescence band maximum [Formula: see text] is controlled nearly equally by π* and a according to the equation:[Formula: see text]Thus the fluorescence spectrum of 1 delivers two distinct and independent parameters of its microenvironment while its electronic absorption spectrum is essentially solvent invariant.
The α(C—C)/β(C—H) ratio of the primary processes in the 184.9 nm photolysis of gaseous cis- and trans-2-butene
Authors: Deslauriers Hélène , Makulski Wlodzimiercz , Collin Guy J.
Published: May 2006

A complete study of the 184.9 nm photolysis of cis- and trans-2-butene has been done in the gas phase. The main primary processes (>90%) are the α(C—C) and β(C—H) bond ruptures. Both processes have similar quantum yield values in cis-2-butene: [Formula: see text]. Conversely, the α(C—C) bond rupture is more important in trans-2-butene: [Formula: see text]. These values are compared with those measured in propene (0.57) and isobutene (0.95). No simple molecular property seems sufficient to explain such an effect.
Synthesis and Diels–Alder reactions of 1-((E)-2-tert-butyldimethylsiloxy)ethylidene-4a-carbomethoxy-1,2,3,4,4a,5,6,7-octahydronaphthalene and related substances. Preparation of functionalized decahydro-1H-phenalenes
Authors: Piers Edward , Friesen Richard W.
Published: May 2006

The preparation of the structurally novel dienes 7 and 8 is described. The results of a study of the Diels–Alder reactivity of these substances and the "parent" diene 9 are summarized and the potential use of the resultant Diels–Alder adducts in organic synthesis is demonstrated.
The conformational preference of methoxyl and acetoxyl groups in 3-oxycyclohexenes, 2-oxymethylenecyclohexanes, and 1-oxytetralins and the generalized anomeric effect
Authors: Ouédraogo Adama , Viet Minh Tan Phan , Saunders John K. , Lessard Jean
Published: May 2006

The 13C and 1H nuclear magnetic resonance data of 3-oxycyclohexenes, 2-oxymethylenecyclohexanes, and 1-oxytetralins show (i) that the pseudoaxial preference is larger for the acetoxyl group than for the methoxyl group in 3-oxycyclohexenes, (ii) that the methoxyl and the acetoxyl groups have the same pseudoaxial preference in 1-oxytetralins, and (iii) that in 2-oxymethylenecyclohexanes, the acetoxyl group prefers the equatorial orientation whereas the methoxyl group prefers to be axial. These results are interpreted in terms of the orbital picture (π–σ* stabilization of the axial conformer) of the generalized anomeric effect to which is opposed a conformational effect termed the "unsaturation effect" in 2-acetoxymethylenecyclo-hexane and probably also, but to a much smaller extent, in 1-acetoxytetralin. The results also show that cis and transtert-butyl analogues of 3-oxycyclohexenes do not represent the true limit conformers whereas cis and trans 4-tert-butyl derivatives of 2-oxymethylenecyclohexanes do not differ drastically from the limit conformers.
Cyclic ether synthesis: sulfenyletherification with benzenesulfenyl chloride/N, N-diisopropylethylamine and sulfenate ester cycloadditions
Authors: Tuladhar Sudersan M. , Fallis Alex G.
Published: May 2006

A general method for the formation of the cyclic ethers 9, 10, 13, 14, 16, and 28 and the lactone 30 is described. The procedure employs benzenesulfenyl chloride prepared insitu in acetonitrile and N,N-diisopropylethylamine to generate a thiiranium (episulfonium) ion intermediate from which the cyclic products arise by internal nucleophilic displacement. In the special case of the norbornene alcohols 1 and 2 the oxetanes 5 and 6 are formed by intramolecular sulfenate ester cycloaddition.
The benzylic anomeric effect. Internal rotational potentials of ArCH2X compounds (X = F, Cl, SH, SCH3, S(O)CH3, SO2CH3)
Authors: Penner Glenn H. , Schaefer Ted , Sebastian Rudy , Wolfe Saul
Published: May 2006

The apparent twofold barriers to rotation about the exocyclic C—C bonds in ArCH2X (X = SH, SMe, S(O)Me, SO2Me) compounds have been determined in CS2 and in benzene-d6, from the magnitudes of long-range proton–proton coupling constants. The work extends earlier studies of the systems X = F, Cl. Except for X = F, all of the title compounds have a low energy conformation in which the C—X bond is perpendicular to the plane of the benzene ring. The introduction of two meta-chlorine substituents leads to a 2 kJ/mol relative destabilization of this structure for X = F, Cl, SH. The trend in the rotational barriers associated with the sulfur substituents is SH, [Formula: see text], SO2Me. Abinitio calculations, with geometry optimization at the STO-3G level for X = F, Cl, SH, reproduce the experimental results, and also the magnitudes of the rotational barriers. Quantitative PMO analyses of these abinitio wavefunctions have been employed to distinguish between "steric" and "other" interpretations, and reveal that the preferred conformation of X = F is determined by a first order perturbational effect, manifested in an electrostatic attraction between the heteroatom and a syn-periplanar ortho-hydrogen. For X = Cl, the PMO analysis suggests that a [Formula: see text] interaction, analogous to the [Formula: see text] interaction postulated as the origin of the anomeric effect in heterocyclohexanes, plays a dominant role. For X = SH, both [Formula: see text] and [Formula: see text] interactions are found; the latter is not possible in the anomeric effect itself. It is concluded that a benzylic anomeric effect exists, and that its magnitude, as a function of X, is S(O)Me, SO2Me > Cl > SH, SMe > F.
Nuclear magnetic resonance studies of ionomers. 5. Spectra of poly (methyl methacrylate-co-4-vinyl pyridine), poly (methyl methacrylate-co-2-methyl-5-vinyl pyridine), and their quaternized derivatives
Authors: Natansohn Almeria , Eisenberg Adi
Published: May 2006

Proton and carbon nuclear magnetic resonance spectra in solution of poly (methyl methacrylate-co-4-vinyl pyridine), poly(methyl methacrylate-co-2-methyl-5-vinyl pyridine), and of their derivatives obtained by quaternization with methyl iodide are presented. The vinyl pyridine structural units content of the samples is ca. 10%. In different solvents, the spectra provide information about the sequence distribution and configuration of the copolymers. A "coisotactic" alternating addition, defined as the probability of finding a methoxy group and a pyridine ring neighboring each other on the same side of the main chain, can be estimated as about 0.5 in both cases. This information arises from the splitting of the aromatic and of the methoxy signals (both in proton and carbon spectra). As expected, quaternization does not affect the copolymer configuration. Heteronuclear-correlated two-dimensional spectra can help in assigning the spectra.
Synthesis and characterization of picryl cellulose
Authors: Strauss Michael J. , Torres Reuben , Phelan John , Craft Andy , Pitner Bruce , Nason Deane , Carignan Yvon , Dust Julian M. , Buncel Erwin
Published: May 2006

2,4,6-Trinitrophenyl cellulose (picryl cellulose) was synthesized by SNAr displacement of chloride from picryl chloride by sodium cellulosate. The cellulosate was prepared insitu from microcrystalline cellulose and sodium methoxide. Depending upon the procedure used, two products with different degrees of picrylation resulted; one contained one picryl ring per ca. 6.5 glucosyl units (PC-6), while the other had ca. one picryl ring per 12 (PC-12). These picryl ethers were characterized by several independent methods: 400-MHz 1H nuclear magnetic resonance spectroscopy of the DMSO-soluble material, temperature-dependent mass spectrometry (ion pyrograms), and differential scanning calorimetry (DSC). The nuclear magnetic resonance spectrum displays peaks in the low field region (8–9.5 ppm) assigned to the picryl rings; these resonances are well separated from those for possible alternative compounds such as unreacted picryl chloride, picric acid, or 2,4,6-trinitroanisole. It is suggested that the various picryl resonances arise primarily from different microenvironments and (or) conformational preferences of the polymer chain rather than from different substitution sites. Substitution at a primary C-6 position of the glucosyl moiety is favoured. DSC shows that while cellulose undergoes an endothermic decomposition between 320 and 350 °C, picryl cellulose exhibits an exothermic decomposition at ca. 230 °C. This exotherm is accompanied by the liberation of picric acid, as confirmed by nuclear magnetic resonance; mass spectral results indicate concurrent chain cleavage to yield smaller polysaccharides. A mechanism for initiation of pyrolytic decomposition is proposed, in which loss of picric acid is accompanied by rupture of a glycosidic bond with concomitant formation of glucosan and enolic end groups on the remaining fragments.
Nouvelle synthèse de chloro-1 méthyl-5 isoquinoléines fusionnées à divers systèmes aromatiques par leur liaison [g]
Authors: Bisagni Emile , Rautureau Marilys , Croisy-Delcey Martine , Huel Christiane
Published: May 2006

Lithiation of 4-acetyl 2-chloro pyridine ethylene glycol ketal by exchange with lithium diisopropylamine (LDA) led to 4-acetyl 2-chloro 3-lithio pyridine ethylene glycol ketal. This lithio derivative was reacted with aromatic aldehydes giving the expected alcohols, which were reduced by triethylsilane and trifluoroacetic acid at room temperature. The resulting 4-acetyl 3-arylmethyl 2-chloro pyridine ethylene glycol ketals were hydrolysed and cyclized in acidic medium, leading to polycondensed heterocyclic systems that have a 1-chloro 5-methyl isoquinoline nucleus fused by its [g] bond to the starting aromatic aldehyde.
(±)-4-Oxo-9-deoxy-9-azaprostaglandin I2 derivatives. Very stable prostacyclin analogs
Authors: Guzmán Angel , Martinez Elfida , Velarde Esperanza , Maddox Michael L. , Muchowski Joseph M.
Published: May 2006

The synthesis of 4-oxo-9-deoxy-9-azaprostaglandin I2 (23a) and two ω-chain analogues thereof (23c and 23e) is described. The most salient features of the synthetic process used were (a) introduction of the nitrogen functionality at C-9 by [3,3] sigmatropic rearrangement of the trichloroacetamidate 5b to the trichloroacetamide 6, (b) transformation of 6 into the bicyclic lactam 7 with sodium borohydride, (c) stereospecific introduction of the 11α-hydroxyl group via the bromohydrin 8a, and (d) attachment of the α chain by extrusion of sulfur from the thioimidates 20 by the Eschenmoser sulfide contraction process.