bluuebunny/crossref_metadata_2025 huggingface dataset:
The solvolysis of tri-p-tolyl[2-13C]vinyl bromide in 70% HOAc – 30% H2O
Authors: Lee Choi Chuck , Ko Eric C. F. , Obafemi Craig A. , Quail J. Wilson
Published: May 2006

Tri-p-tolyl[2-13C]vinyl bromide (3-Br-2-13C) was solvolyzed in 70% HOAc – 30% H2O and the scrambling of the label from C-2 to C-1 in the reaction product was found to decrease with increasing concentrations of added NaOAc. For example, the 18.5% scrambling found for the solvolysis of 3-Br-2-13C in 70% HOAc in the absence of any NaOAc was decreased to 7.7% when the reaction was carried out in the presence of 4770 mM NaOAc. Common-ion rate depression as well as isotopic exchange between [82Br]Et4NBr and the organic substrate, 3-Br, were observed, indicating the formation of the dissociated tri-p-tolyivinyl cation. Apparently, the tri-p-tolylvinyl cation, similar to the trianisylvinyl cation, is of sufficient stability to allow it to discriminate between different nucleophiles. Thus the presence of increasing concentrations of the acetate ion would favor the product-forming capture of the tri-p-tolylvinyl cation at the expense of the 1,2-shift process, resulting in a decrease in the extent of scrambling. From the observed scrambling data, selectivity ratios such as KOAc/KSOH and KOAc/Kr(Tol) were calculated.
Enamines and iminium salts from amido-acids
Authors: McIntosh John M. , Pillon Lilianna Z. , Acquaah Samuel O. , Green James R. , White Graham S.
Published: May 2006

Distillation of ω-carboxyalkyl lactams from soda-lime leads to the formation of bicyclic enamines or imines with concomitant loss of carbon dioxide and water. Monoamides of dicarboxylic acids afford cyclic ketones. A mechanistic rationale is presented and it is concluded that the reaction is a variant of the classical Ruzicka cyclization of dicarboxylic acids. The method allows the regiospecific formation of iminium salts of bicyclic nitrogen heterocycles.
Preparation and stereochemistry of 8- and 9-hydroxy-2,5-ethano-3-benzazocines
Authors: Bélanger Patrice C. , Scheigetz John , Young Robert N.
Published: May 2006

Two isomers of a novel ring system, 1,2,3,4,5,6-hexahydro-2,5-ethano-3-benzazocine, were prepared by a Beckmann rearrangement of syn- and anti-2-hydroxy-11-oximino-6,9-methano-5,6,7,8,9,10-hexahydrobenzocyclooctene, 4a and 4b respectively. Because of the similarity in their ir, nmr, and ms, it was impossible to distinguish between the different isomers. To resolve this problem, selective benzylic oxidation and nmr shift reagent studies gave results that determined the structures.
Dissolution enthalpies of NaI in binary mixtures of water with aliphatic alcohols and methylcellosolve at 298.15 K. Enthalpic pair interaction coefficients of NaI–nonelectrolyte in water solution
Authors: Piekarski Henryk
Published: May 2006

The enthalpy of solution, ΔHsol, of NaI in methanol + water, ethanol + water, n-propanol + water, isopropanol + water, and methoxyethanol + water at all compositions and ΔHsol of NaI in s-butanol + water and tert-butanol + water from 0 to 5 mol% alcohol was measured.The NaI–nonelectrolyte pair interaction enthalpies were determined and correlated with the functions describing some properties of the nonelectrolyte. A functional group additivity concept, introduced by Savage and Wood for non-ionic solutes, was applied to the interactions between NaI and nonelectrolytes.
A kinetic and nuclear magnetic resonance study of methylated pyrimidine nucleosides
Authors: Allore B. D. , Queen A. , Blonski W. J. , Hruska F. E.
Published: May 2006

Alkylated pyrimidine nucleosides are of interest from the viewpoint of mutagenesis and carcinogenesis. 1H and 13C nmr data are presented for a series of 2′-deoxynucleosides methylated at the O2-, O4-, and N3-positions of the base, and discussed in terms of their physical properties. The pH dependence of the stability of the O2- and O4-methylated 2′-deoxyribosides as well as the corresponding ribosides was examined by 1H nmr and ultraviolet (uv) spectrophotometric methods.
15N and 199Hg magnetic resonance spectral investigation of inosine protonation and diamagnetic metal ion complexation
Authors: Buchanan G. W. , Bell M. J.
Published: May 2006

On the basis of complexation induced 15N chemical shift changes, the nucleoside inosine is shown to protonate preferentially at the N-7 site. This nitrogen is also favoured by Zn+2 and Hg+2 ions. 199Hg shifts support the notion that the binding of the first equivalent of Hg+2 is site selective.
A reversal in the order of H-6R and H-6S chemical shifts of some aldohexopyranose derivatives, associated with the acetylation of OH-4 and OH-6 groups. A distinction between 3- and 4-linked D-glucose residues in disaccharides
Authors: Rao Vanga S. , Perlin Arthur S.
Published: May 2006

On peracetylation of methyl α- or β-D-glucopyranoside, there is a reversal in the order of the chemical shifts of the 6,6′-methylene protons, i.e., whereas the H-6S signal appears downfield of H-6R in the spectra of the glucosides, their relative locations in the spectra of the tetraacetates are interchanged. Evidence is presented from comparative data for a wide range of regioselectively acetylated D-glucose derivatives, showing that the reversal in chemical shifts of the methylene protons occurs only when both O-4 and O-6 are acetylated. Consequently, in the disaccharide series, shift reversal is not observed with glucose residues that are bonded glycosidically through O-4, whereas the reversal occurs when the linkage is through O-3. A conformational model offered to account for these effects suggests that the 4- and 6-O-acetyl substituents are oriented by means of a weak, mutual, interaction in such a way that the magnetic anisotropy of the carbonyl group of the 6-O-acetyl substituent can induce selective deshielding of H-6R. The comparable influences of O-benzoyl and O-(4-nitro)benzoyl substituents on chemical shift are consistent with this proposal. D-Mannopyranosides exhibit characteristics analogous to those of their D-gluco epimers, whereas D-galactopyranosides give a different, more complex, chemical shift pattern.
Hydrogen bonded interactions of methanol and water with purines and pyrimidines
Authors: Spencer J. N. , Barton S. W. , Smith K. A. , Wolbach W. S. , Powell J. F. , Kirshenbaum M. R. , Firth D. W. , Harris E. M. , Judge T. A.
Published: May 2006

The solvation of benzimidazole, 4-azabenzimidazole, purine, adenine, uracil, thymine, and cytosine in water and methanol was determined by a combination of calorimetric and spectral analysis. Specific enthalpies have been assigned to each interaction site in the first solvation shell. Calculations, based on the enthalpies of hydrogen bond formation with water, and estimates for intermolecular hydrogen bond enthalpies, show that the self-association of purine and adenine in water through hydrogen bonded interactions is enthalpically possible.
A photoconductivity study of trapped electrons in γ-irradiated hydrocarbon glasses at 77 K
Authors: Okabe Naoto , Kimura Toyoaki , Fueki Kenji
Published: May 2006

Photoconductivity spectra of trapped electrons in γ-irradiated 3-methylpentane, 2-methylpentane, 3-methylheptane, and 3-methylpentane–methylcyclohexane (1:9 volume ratio) glasses at 77 K were measured. From the comparison of photoconductivity and optical absorption spectra in these systems we have obtained relative values of quantum efficiencies for a trapped electron in the ground state to be excited to the conduction band by photo-induced transition. For electrons stably trapped, we have measured the ranges of trap depths from the shallowest to the deepest in these systems.
Separation of U(VI) from aqueous solutions by precipitate flotation with 8-quinolinol and surfactants
Authors: Shakir Kamal , Benyamin Kamal , Aziz Mourad
Published: May 2006

Foam separation investigations over the 2.5–10.5 pH range show that U(VI) can be successfully separated from aqueous solutions by precipitation with oxine and subsequent flotation of the precipitate, either in absence or in presence of surfactant collectors. Collectors tested were: cetyl trimethyl ammonium bromide (CTAB) and sodium lauryl sulphate (NaLS). The extent of removal depends on many factors and removals as high as 98% could be achieved when uranyl oxinate precipitate was floated, under certain controlled conditions, in the presence of CTAB. Factors affecting the percentage removal were investigated and the best conditions were selected. The results obtained are discussed in terms of the hydrolytic behaviour of U(VI).
Résonance magnétique nucléaire 13C des dérivés allyliques de la colonne IV-A, de type (C6H5)3ECH2CH=CH—CH3. Effets électroniques du substituant du noyau aromatique
Authors: Matarasso-Tchiroukhine E.
Published: May 2006

The 13C nmr study of the series (C6H5)3ECH2CH=CH—CH31E shows, for the aromatic nucleus, a polarization consistent with a mesomeric electron withdrawing effect of the substituent in the case of E = Si, Ge, Sn; inversely, a mesomeric donating effect, in the case E = Pb, involving a higher electronegativity for Pb; and a donating π inductive effect, in the case E = C. The mean electronic excitation energy seems to dominate the chemical shifts of the aromatic carbons Cipso and Cortho, as well as the coupling 0J(13C—207Pb).
Etude par spectroscopie infrarouge de l'action de MgBr2 anhydre sur le N-méthylacétamide en solution dans CCl4
Authors: Blanchard Simone , Tistchenko Anne-Marie
Published: May 2006

The reaction between anhydrous MgBr2 and N-methylacetamide (NMA) in CCl4 solution has been studied by infrared spectrometry. The temporal variation of the spectra has been followed and the final spectrum is attributed to a compound of structure[Formula: see text]
The hydration parameter and the excess acidity functions in H2SO4–H2O and HClO4–H2O media
Authors: Traverso Pibtro Giuseppe
Published: May 2006

Excess acidities, X, for aqueous H2SO4 and HClO4 are derived under conditions of fixed [Formula: see text] and m* in both media, and of fixed m* only, and compared among themselves and with newly derived and previous scales obtained without constraint, or with fixed [Formula: see text] values. Analysis shows few or no real differences among these scales, which means that the equation [Formula: see text] applies without the necessity of involving a "second-order" m**X2 term, at least for these media.
Fall-off shape and incubation times in weak-collision thermal unimolecular reactions
Authors: Pritchard H. O.
Published: May 2006

A new formula for the weak-collision unimolecular rate constant is readily interpretable in terms of bottleneck concepts. This formula is used to explore the correlation between variations in relaxation structure and fall-off shape in weak-collision thermal unimolecular reactions; some information is also presented relating to collision efficiency factors βc, and to incubation times.It is conjectured that randomisation failure is an important feature which causes near-Lindemann behaviour to occur in practical weak-collision thermal reactions.
The intramolecular Diels–Alder reaction: recent advances and synthetic applications
Authors: Fallis Alex G.
Published: May 2006

Recent advances and examples of the intramolecular Diels–Alder reaction are summarized and applications to the total synthesis of natural products, both completed and in progress, noted. A detailed discussion of trienes leading to bicyclo[4.3.0]nonene skeletons is followed by examples of bicyclo[4.4.0]decenes and adducts arising from ortho-quinodimethane precursors. Cycloadditions affording bicyclo[n.4.0] systems and bridged-ring adducts from cyclic dienes conclude this survey which is followed by a discussion of unreactive trienes and a brief analysis of synthetic strategy to complete the review.
Intrazeolite photochemistry. I. Phosphorescence enhancement of aromatic ketones included in silicalite.
Authors: Casal H. L. , Scaiano J. C.
Published: May 2006

The triplet lifetime of β-phenylpropiophenone at room temperature is enhanced by ca. five orders of magnitude upon inclusion in the channels of silicalite, a hydrophobic zeolite. This behaviour is attributed to restrictions to mobility in this medium, and it is suggested that photochemical probes can provide a rather unique insight in the study of intrazeolite chemical processes.
Étude théorique de la structure du phosphate d'isaxonine
Authors: Monti Jean-Pierre , Sarrazin Marcel , Brouant Pierre
Published: May 2006

Protonations of isaxonine phosphate are studied by performing CNDO/2 and CNDO/S molecular orbital calculations. Results are compared with previously published data. Wiberg's bond indices and S character percentages calculated using electronic populations are shown to correctly predict variations of bond lengths and bond angles as well as [Formula: see text] coupling constants. A good prediction of proton chemical shift variations using a calculation of the screening constant was obtained.
Simulation of edge effects in electroanalytical experiments by orthogonal collocation. Part 4. Application to voltammetric experiments
Authors: Cassidy John F. , Pons Stanley , Hinman Scott , Speiser Bernd
Published: May 2006

Orthogonal collocation is used to simulate cyclic voltammograms that are influenced by non-linear diffusion due to edge effects at small disc electrodes.
Oxidation of trans-[RhCl(CO)(PPh2R)2] (R = Et, Pri, CH2SPh) with HgCl2 to form rhodium–mercury bonded complexes, and complexes of mercury(II) with phosphinoalkylthioethers or dithioethers
Authors: Sanger Alan R.
Published: May 2006

Mononuclear complexes of HgII of 1:1 stoicheiometry are formed with the chelating ligands Ph2P(CH2)nSPh (n = 1, 2) or (PhS)2(CH2)n (n = 1–4). In contrast to the behaviour of trans-[RhCl(CO)(PPh3)2], reactions of trans-[RhCl(CO)(PPh2R)2] (R = Et, Pri, or CH2SPh) with HgCl2 give the corresponding oxidative addition complexes [RhCl2(HgCl)(CO)(PPh2R)2].•For [RhCl2(HgCl)(CO)(PPh2CH2SPh)2] the pendant thioether groups coordinate to mercury. Reactions of HgCl2 with [MCl2(Ph2PCH2SPh)2] (M = Pd, Pt) give the corresponding heterodinuclear, Ph2PCH2SPh bridged complexes, [MHgCl4(μ-Ph2PCH2SPh)2]. The 199Hg nmr spectrum of [RhCl2(HgCl)(CO)(PEtPh2)2] has been obtained, and the first value for 1J(103Rh—199Hg) has been measured.
Capacités calorifiques, volumes, expansibilités et compressibilités des solutions aqueuses concentrées de LiOH, NaOH et KOH
Authors: Roux Alain H. , Perron Gérald , Desnoyers Jacques E.
Published: May 2006

The densities and heat capacities of various alkali hydroxides were measured in water with a vibrating tube densimeter and a flow microcalorimeter in the temperature range 4 to 55 °C. The concentration ranges investigated were respectively: 0.04 to 3.8 mol kg−1 for LiOH, 0.02 to 10.9 mol kg−1 for NaOH, and 0.02 to 15.5 mol kg−1 for KOH. The partial molar volumes [Formula: see text] and heat capacities [Formula: see text]were calculated from the least-squares fit of the apparent molar quantities [Formula: see text] as a function of molalities. From the temperature dependence of these functions the apparent molar expansibilities [Formula: see text] and [Formula: see text] were derived. The ultrasonic velocities were also measured for the dilute solutions of NaOH at 25 °C, and can be used to calculate the standard partial molar compressibilities and isochoric heat capacities. The present data were compared with literature values. The general trends as a function of concentration are similar to those of other 1:1 electrolytes even though the deviations from the Debye–Hückel theory are larger.
Primary kinetic isotope effect in the gas phase photochlorination of ethyl chloride-1-d1
Authors: Niedzielski Jan , Yano T. , Tschuikow-Roux E.
Published: May 2006

The abstraction of hydrogen/deuterium from CH3CHDCl by ground state chlorine atoms produced photolytically from Cl2 has been investigated at temperatures betwen 280 and 368 K. The relative rates for the internal competition[Formula: see text]are found to conform to an Arrhenius rate law:[Formula: see text]These data, taken together with the external competition results for the C2H5Cl/CH3CHDCl system, in conjunction with the competitive results using CH4 as a primary reference, have yielded the rate constants (cm3 s−1):[Formula: see text]The relatively weak primary kinetic isotope effect, kH/kD, decreases with increasing temperature from 1,855 at 280 K to 1.66 at 365 K. The results are compared with those obtained based on the BEBO method. While both the trend and the magnitude of the kinetic isotope effect are satisfactorily predicted, the activation energy is not.
Vapour pressure and thermodynamic data for the binary liquid mixture of pyridine and sulphur dioxide
Authors: Gardner C. L. , Day R. W.
Published: May 2006

Solubility and nmr data arc presented for the pyridine – sulphur dioxide system. The data have been analyzed to determine partial vapour pressures, activity coefficients, and thermodynamic functions. The results indicate that a 1: 1 complex of pyridine and SO2 is formed in these solutions.
Anomalous 5-endo-trig reversals: general reactions of 7-oxabicyclo[2.2.1]heptenes and heptanes
Authors: Keay B. A. , Rajapaksa D. , Rodrigo R.
Published: May 2006

Two general 5-endo-trig reversals of 7-oxabicyclo[2.2.1]heptanes and heptenes are described and discussed. The reverse-Michael reaction occurs with the aldehyde, ketone, ester, and nitrile derivatives while the reverse aldol reaction catalysed by acid is confined to the aldehydes and ketones. These properties of the title compounds are rationalized in terms of the geometric alignments of the bonding and antibonding orbitals of the bridging oxygen atom and its neighboring carbons. New isobenzofuran and cyclohexadiene syntheses form a part of the report.
The synthesis of 3,3-dimethyltricyclo[6.3.0.01,5]undecan-4-one via β-enolate rearrangement. A new approach to the pentalenene ring system
Authors: Jurlina Jeffrey L. , Patel Hemant A. , Stothers J. B.
Published: May 2006

The preparation of 3,3-dimethyltricyclo[6.3.0.01,5]undecan-4-one from 9,9-dimethyltricyclo[5.3.1.01,5]undecan-8-one by β-enolate rearrangement is described. The required [5.3.1.01,5]ketone was obtained in six steps from tricyclo[5.2.1.01,5]dec-8-ene. This sequence provides a new entry into the tricyclo[6.3.0.01,5]undecane ring system which is characteristic of a number of recently encountered naturally occurring terpenes. The 13C data for several of these tricyclic derivatives are presented.
Syntheses of some pyrimidine N-oxides
Authors: Jovanovic Misa V.
Published: May 2006

Various monosubstituted pyrimidines and methylpyrimidines were N-oxidized with a number of different peracids. In general, they are more susceptible to side reactions accompanying N-oxidation (i.e., decomposition, annular carbon oxidation, ring opening) than other -π-deficient diazine and triazine analogs. Unsymmetrical pyrimidines, which can potentially yield two isomeric products, were N-oxidized preferentially at the site para to strong electron-donating substituents. Weaker ring-activating groups, such as methyl, are mainly ortho directing and only aid in the N-oxidation of pyrimidine nuclei having ortho/para-directing substituents.
Preparation and characterization of some ruthenium(III) porphyrins, including the crystal structure of bromo(octaethylporphinato)(triphenylphosphine)ruthenium(III)
Authors: James Brian R. , Dolphin David , Leung T. W. , Einstein Frederick W. B. , Willis Anthony C.
Published: May 2006

Some ruthenium(III) complexes Ru(porp)(L)X and [Ru(porp)L2]X, (porp = dianion of octaethylporphyrin (OEP) or tetraphenylporphyrin (TPP); L = PPh3, P″Bu3; X = Br, Cl) have been prepared from the precursor complexes Ru(porp)L2 or Ru(porp)(CO)L using as oxidant the halogens, or air in the presence of the hydrogen halides. The X = PF6 salts can be made using Et3O+PF6− as oxidant. Ruthenium(II) porphyrin π-cation radical intermediates have sometimes been detected. The X-ray crystal structure of Ru(OEP)(PPh3)Br, the first reported for a ruthenium(III) porphyrin, reveals that metal is displaced 0.049 Å from the plane of the pyrrole N atoms towards the phosphine. The Ru(OEP)(py) Br and [Ru(OEP)(py)CH3CN]PF6 complexes have been synthesized from Ru(OEP)(CO)py. The ruthenium(III) complexes are low-spin as shown by magnetic and esr data. Optical and 1H nmr data, the latter showing large paramagnetic shifts, are also presented.
Effects of solvent structure on electron reactivity and radiolysis yields: 2-propanol/water mixed solvents
Authors: Cygler Joanna , Freeman Gordon R.
Published: May 2006

Reaction of solvated electrons with nitrobenzene, N, is nearly diffusion controlled in both pure solvents; kN ~ 1010 dm3/mol s. The value of kN is approximately proportional to the inverse viscosity η−1 in the pure solvents, and in the mixed solvents at different temperatures. However, on going from zero to 74 mol% water at the same temperature kN is independent of the 40% increase of η. Electron diffusion in the mixed solvents is not a simple function of fluidity.Reaction with the inefficient scavengers tryptophane (kS ~ 109 dm3/mol s) and phenol (kS ~ 107–108 dm3/mol s) correlates inversely with the electron optical absorption energy. The latter is related to the trap depth in the solvent; electrons in deeper traps have less tendency to react with molecules of low electron affinity.Addition of 3 mol% 2-PrOH to water at 296 K increases the value of Gεmax by 16%, although the value in pure 2-PrOH is three-fold smaller than that in pure water. The increase is attributed to an increase in the free ion yield, caused by an increase in the product of the electron thermalization range and the microscopic dielectric constant of the fluid between the ion and electron, averaged over the time that they exist as a correlated pair. Addition of a small amount of alcohol to water increases the orderliness of the water structure.
Nuclear magnetic resonance studies of the acid–base chemistry of amino acids and peptides. IV. Mixed disulfides of cysteine, penicillamine, and glutathione
Authors: Theriault Yvon , Cheesman Bruce V. , Arnold Alan P. , Rabenstein Dallas L.
Published: May 2006

The acid–base chemistry of the ammonium groups of penicillamine–glutathione mixed disulfide and cysteine–glutathione mixed disulfide has been characterized by 13C nmr and that for the ammonium groups of penicillamine–cysteine mixed disulfide by 1H nmr. The mixed disulfides were formed by thiol/disulfide exchange. Chemical shift titration data were obtained simultaneously for the mixed disulfides, the thiols, and the symmetrical disulfides in the mixture. Since the fractional titration of individual ammonium groups can be obtained directly from chemical shift data, it was not necessary to isolate the mixed disulfides. The chemical shift data indicate that the two ammonium groups of each of the mixed disulfides are titrated over the same pH range, with the acidity of the ammonium group of the penicillamine and cysteine parts of the penicillamine–glutathione and cysteine–glutathione mixed disulfides 4.4 and 3.9 times as acidic as those of the glutathione part. In the penicillamine–cysteine mixed disulfide, the penicillamine ammonium group is 1.7 times as acidic as that of the cysteine part. Macroscopic and microscopic acid dissociation constants are reported for each of the mixed disulfides. The advantages of nmr as a method for characterizing the acid–base chemistry of mixed disulfides are discussed.
Laser-induced chemical reactions in ethanol by multiphoton absorption
Authors: Andrade Leite Sergio Roberto de , Isolani Paulo Celso , Riveros José Manuel
Published: May 2006

The chemical reactions initiated by multiphoton absorption in gaseous ethanol were studied in the pressure range of 0.1 to 10 Torr under irradiation of a focused TEA CO2 laser. The predominant reaction is a unimolecular elimination: C2H5OH → C2H4 + H2O, which is particularly important at low pressures and lower laser intensities. This process has not been previously identified in pyrolysis studies. At higher pressures and higher laser intensities, H2, CO, andCH4 also become important reaction products which are claimed to originate from a higher energy channel. Several arguments are provided to show that the increasing yield of CO and CH4 at higher levels of excitation of the ethanol molecule are not consistent with a radical mechanism.
Nuclear magnetic resonance studies. XL. Tricyclooctanone rearrangement, [3.2.1.0] → [3.3.0.0], via γ-enolization and a 2Hmr study of H/D exchange in tricyclic ketones
Authors: Cheng A. K. , Ghosh Ashim K. , Stothers J. B.
Published: May 2006

The effect of strongly basic conditions (t-BuO−/t-BuOH/185 °C) on the exo and endo isomers of 7,7-dimethyltri-cyclo[3.2.1.02.4]octan-6-one has been examined. While the exo isomer is stable, the endo isomer readily rearranges to 4,4-dimethyltricyclo[3.3.0.02.8]octan-3-one (6) which subsequently undergoes reduction of the three-membered ring. The initial transformation is one of very few examples of γ-enolate rearrangement. Experiments in tert-buiyl alcohol-O-d1 were carried out to establish the several sites of deuterium incorporation in the initial ketones and in 6 using 2Hmr. These results reveal the stereochemistry of γ-enolate formation and cleavage as well as the stereoselectivities and relative rates of the exchange processes.
Electron spin resonance study of the hyperfine splitting constants of naturally abundant carbon-13 and nitrogen-15 in diphenylmethyl tert-butyl aminoxyl (nitroxide). Solvent and temperature effects
Authors: Janzen Edward G.
Published: May 2006

The β-13C and 15N hyperfine splitting constants in the phenyl radical adduct of phenyl tert-butyl nitrone (PBN), diphenylmethyl tert-butyl aminoxyl, have been measured as a function of solvent and temperature.
Total synthesis of d,l-coronafacic acid by an intermolecular Diels–Alder approach
Authors: Liu Hsing-Jang , Llinas-Brunet Montse
Published: May 2006

An efficient total synthesis of d,l-coronafacic acid (2) has been achieved from 4-cyclopentene-1,3-dione (4). The synthesis involves keto ester 7 as a key intermediate that is conveniently prepared by the Diels–Alder addition of enedione 4 to ethyl 4-ethyl-2,4-pentadienoate (5).
Conformational and steric effects on regioselectivity in the Baeyer–Villiger reaction
Authors: Dave Vinod , Stothers J. B. , Warnhoff E. W.
Published: May 2006

The Baeyer–Villiger oxidation of six β,β,γ-trisubstituted cyclopentanones and cyclohexanones leads to preferential migration of the α-carbon relative to the α′-carbon by a factor of 1.5–4:1. The origin of the preference is suggested to be steric and is ascribed to the greater relief of non-bonded interactions as the α—β bond lengthens in going to the transition state. Thus, in the carbonyl addition intermediate the 1,3-diaxial-like interaction between the hydroxyl and the closest γ-carbon is diminished to a greater extent when the α-carbon migrates relative to the α′-carbon.
Studies on γ-irradiated aqueous solutions of tripeptides composed of glycine and L-alanine by the spin-trap high-performance liquid chromatography – electron spin resonance method
Authors: Moriya Fumio , Iguchi Naohisa , Makino Keisuke , Rokushika Souji , Hatano Hiroyuki
Published: May 2006

The γ-irradiated aqueous solutions of four tripeptides were studied by the spin-trap hplc–esr method with 2-methyl-2-nitrosopropane. Four types of spin adducts were separated and identified as follows: (a) deamino adducts derived from short-lived radicals produced by reductive deaminations from the addition of hydrated electrons to the N-terminal peptide carbonyl groups, (b) C-terminal backbone adducts, (c) internal backbone adducts, and (d) side-chain adducts derived from short-lived radicals produced by hydrogen abstractions by hydroxyl radicals, mainly from so-called α-carbons in the C-terminal and internal main-chains and a carbon in an aliphatic side-chain, respectively. For polyglycine, internal backbone radicals produced by hydrogen abstraction were spin-trapped and detected.
Electrostatic effects on disproportionation equilibria: dihalogeno and methyl derivatives of benzene
Authors: Friedl Zdeněk
Published: May 2006

The disproportionation equilibria of benzene derivatives X—Ph—Y (X, Y = F, Cl, CH3) in the gas phase, were calculated from their enthalpies and entropies of formation at 298 K. Calculations of the electrostatic energy based on the point charges approximation method predict reasonably well the reaction enthalpies of all meta-benzene derivatives and predict nearly as well the other fluoro- and methyl-derivatives. The theory fails if strong resonance and/or steric interactions are present. The generally better ability of the electrostatic theory to treat disproportionation or conformational equilibria than the proton transfer reactions is also discussed.
Effet de substitution dans les positions 3, 5 et 7 en série spiropyrannique benzothiazolinique
Authors: Kister Jacky , Pommier Henri Paul , Metzger Jacques , Guglielmetti Robert
Published: May 2006

Benzothiazolinic spiropyrans substituted in the 3, 5, 6, 7, and 8 positions have been studied. The results obtained show the equivalent role of the 3 and 5 substituted positions during the synthesis of merocyanin type compounds. Non-bonded interactions play a direct role in ring closing to spiropyran compounds. The relative stabilities of open and closed ring compounds depend on stereoelectronic parameters.
Déplacements homolytiques intramoléculaires. 2. Décomposition du perpentène-4 oate de tert-butyle dans les éthers
Authors: Kharrat A. , Gardrat C. , Maillard B.
Published: May 2006

The thermolysis of tert-butyl perpent-4-enoate 1 in THF led to several compounds; the main product, 5-(2-tetrahydrofuryl)-4-pentanolide (yield 42%) comes from an induced decomposition of the perester occurring with the addition of 2-tetrahydrofuryl radical to the double bond. A similar reaction was performed at a lower temperature, using butyl perdicarbonate as initiator, but did not define the mechanism (concerted or two-step). The thermolysis of 1 in ethers (THP, oxepane, 1,4-dioxane, dipropyl ether) appears to be a useful synthetic tool: several new γ-lactones have been obtained in this way.
Synthesis, stereochemistry, and transformations of (E)-1,2-bis(benzenesulfonyl)ethylene cycloadducts to 2-oxa substituted 1,3-dienes
Authors: Lucchi Ottorino De , Lucchini Vittorio , Zamai Moreno , Modena Giorgio , Valle Giovanni
Published: May 2006

The cycloaddition of (E)-1,2-bis(benzenesulfonyl)ethylene (2) to 2-acetoxy- and 2-silyloxy-1,3-cyclodienes occurs in high yields and with high steroselectivity. Structure assignment has been based largely on 1H nmr nuclear Overhauser effect (nOe) experiments on the isolated products. The cycloadducts 4a and 4b derived from the 2-oxa substituted cyclopentadienes have been further elaborated to the parent norbornenone, to the respective unsaturated sulfones 5, and to the β-hydroxy substituted sulfone 6. The vinyl sulfones 5 have also been independently synthesized via cycloaddition of benzenesulfonyl acetylene. The sulfones obtained are potentially important starting molecules for the preparation of more complex targets.
A construction of the angularly hydroxylated cis 1,8-naphthalenedione system typical of the tetracyclines
Authors: Stork Gilbert , Yee Ying K.
Published: May 2006

This paper demonstrates the feasibility of constructing the angularly hydroxylated 1,8-decalindione system typical of the tetracycline group of antibiotics. The α-hydroxy-β-diketone structure is constructed by the action of fluoride ion on an enediol disilyl ether. The siloxyketone enolate thus generated is then intramolecularly trapped by an appositely placed ester.
Proton transfer reactions from 4-X-phenyl-4-nitrophenylcyanomethanes to tetramethylguanidine in acetonitrile solvent
Authors: Hojatti Mahmood , Leffek Kenneth T.
Published: May 2006

The reactions of 4-X-phenyl-4-nitrophenylcyanomethanes with tetramethylguanidine in acetonitrile solvent give carbanionic products absorbing in the range λmax 580–650 nm. The equilibrium constants are all large, in excess of 6000 dm3 mol−1. For a series of deprotonation reactions with X = NO2, Cl, Br, H, and OCH3, the Hammett ρ value is 2.03 ± 0.03. For the same compounds, the primary deuterium isotope effect, kH/kD, varies from 8.9 for X = NO2 to 14.0 for X = OCH3 at 25 °C.
A laser diffraction and nuclear magnetic resonance investigation of the cholesteric potassium N-dodecanoylalaninate mesophase system
Authors: Radley Keith , Tracey Alan S.
Published: May 2006

Nematic and twisted nematic mesophases have been prepared from various mixtures of potassium N-dodecanoyl-l-alaninate and its racemate. The induced pitch was determined as a function of optical purity as were nmr spectra from a variety of additives including several inorganic salts and d,l-alanine. Dipolar couplings from the CHCH3 moiety of the alaninate headgroup were studied, as were quadrupole splittings from the N—D of the amido group and the D2O of the water. It was proposed that with development of cholesteric behaviour on proceeding in a stepwise fashion from the d,l to l amphiphile, the individual disk-like micelles twisted into a chiral shape, then aggregated to form domain-like structures. The major axis of these aggregates is the axis along which the helix propagates and this local axis serves as the director axis for the aggregates. The individual aggregates reorient as a unit when placed in the magnetic field of an nmr spectrometer. As a consequence of this, the diamagnetic anisotropy of the mesophase prepared from the l amphiphile is positive in sign while that of the d,l analogue is negative. Despite this observed change in diamagnetic anisotropy, the alignment of the individual micelles is not different for the chiral and racemic materials.
Dominance of irregular structures in the formation of intramolecular antiparallel β sheets by homopolyamino acids
Authors: Mattice Wayne L. , Lee Eok , Scheraga Harold A.
Published: May 2006

A matrix formulation of the conformational partition function is used to assess the influence of irregular structures on the formation of intramolecular antiparallel β sheets. An antiparallel sheet is considered to be irregular if any pair of contiguous strands has an unequal number of residues. The regular structures in the model consist of antiparallel sheets in which every strand contains the same number of residues. Aside from a growth parameter t, the model contains two parameters that account for the influence of edge effects. Each tight turn contributes a factor δ, and each residue in the sheet that does not have a partner in a preceding strand contributes a factor τ. When τ < δ = 1, preferred sheets consist of an extremely large number of very short strands. Such sheets resemble those found in cross-β fibers. Irregular structures increase the cooperativity of the formation of cross-β fibers. They cause the fibers that are formed to be longer (have more strands) and thicker (have more residues per strand) than if all antiparallel sheets were regular. A much different result is obtained if end effects are modified so that the antiparallel sheets formed resemble those found in globular proteins. Formation of antiparallel sheets remains cooperative, but irregular sheets now markedly reduce the cooperativity of the transition. At high antiparallel-sheet content, irregular structures cause typical antiparallel sheets to be smaller. The behavior of the conformational partition function shows that irregular structures make the dominant contribution to both transitions. Therefore, formulations that restrict consideration to regular structures may provide a misleading picture of antiparallel-sheet formation.
Phase transfer catalysis in the tert-butyloxycarbonylation of alcohols, phenols, enols, and thiols with di-tert-butyl dicarbonate
Authors: Houlihan F. , Bouchard F. , Fréchet J. M. J. , Willson C. G.
Published: May 2006

It is shown that the tert-butyloxycarbonylation of phenols, alcohols, enols, and thiols can be accomplished by reaction of these functionalities with di-tert-butyl dicarbonate under phase transfer conditions. The reactions proceed in high yield and can also be used for the introduction of t-BOC groups onto functionalized polymer backbones such as novolac or poly(p-hydroxystyrene). In addition, a study is made of the selectivity towards tert-butyloxycarbonylation of various polyfunctional compounds.
Liquid crystal polymers. 17. Thermotropic properties of random copolyesters of monotropic and enantiotropic polymers
Authors: Zhou Q.-F. , Jin J.-I. , Lenz R. W.
Published: May 2006

A series of random copolyesters containing two different aromatic ester triads and a decamethylene flexible spacer was prepared and characterized for the effect of comonomer composition on glass, crystalline and liquid crystalline state properties. The homopolymer of one of the triads was enantiotropic, but the other was not. However, a copolymer containing 90 mol% of the latter units and 10 mol% of the former showed thermotropic behaviour, which suggested that the latter homopolymer was probably monotropic. The effects of comonomer content on glass, melting and isotropization temperatures of the copolymers were very different for the three transitions.
The influence of a base on the methylation of aminoalcohols
Authors: Kashima Choji , Harada Kazuo , Omote Yoshimori
Published: May 2006

A selective methylation towards N and O atoms on aminoalcohols was observed using metal hydride reagents as the base, and the predominant factors for the selectivity were investigated in detail.
Enthalpies of hydrogen bond formation of 1-octanol with aprotic organic solvents. A comparison of the solvation enthalpy, pure base, and non-hydrogen-bonding baseline methods
Authors: Stephenson W. Kirk , Fuchs Richard
Published: May 2006

Enthalpies of solution (ΔHs) of 1-octanol and five model compounds (di-n-butyl ether, n-heptyl methyl ether, 1-fluoro-octane, 1-chlorooctane, and n-octane) have been determined in 13 solvents (heptane, cyclohexane, CCl4, 1,1,1-trichloro-ethane, 1,2-dichloroethane, triethylamine, butyl ether, ethyl acetate, DMF, DMSO, benzene, toluene, mesitylene), and combined with heats of vaporization to give enthalpies of transfer from vapor to solvent (ΔH(v → S)). These values have been used to calculate the enthalpy of hydrogen bond formation (ΔHh) of 1-octanol with each solvent, using the pure base (PB), solvation enthalpy (SE), and non-hydrogen-bonding baseline (NHBB) methods. Evidence is presented suggesting that (a) the SE method is susceptible to mismatches of the 1-octanol vs. model polar and dispersion interactions, (b) the PB method is sensitive to polar interaction mismatches, whereas (c) the NHBB method compensates for both polar and dispersion interactions mismatches. The (apparent) ΔHh values determined by the SE and PB methods may be as much as several kcal/mol (nearly 50%) too large, because of the inclusion of other polar and dispersion interactions. The NHBB method is therefore preferred for determining enthalpies of H-bond formation from calorimetric data. However, apparent ΔHh values from the SE and PB methods can be incorporated into total solvatochromic equations using Taft–Kamiet π*, β, and ξ parameters, to provide enthalpies of H-bond formation in good agreement with ΔHh (NHBB).
An evaluation of high resolution nuclear magnetic resonance experiments and protocols in the structure elucidation of organic molecules. I. A relatively simple system, brucine
Authors: Bernstein Michael A. , Hall Laurance D.
Published: May 2006

Using a combination of one-dimensional (1D) and two-dimensional (2D) high resolution nmr methods, the 1H nmr spectrum of brucine was fully assigned. The 2D J-resolved and homonuclear chemical shift correlated (COSY) experiments provided assignments without full structural information; this was obtained from nuclear Overhauser effect (nOe) enhancement experiments (1D and 2D). With the proton spectrum fully assigned, proton-bearing carbons in the 13C nmr spectrum were easily assigned using the 2D heteronuclear chemical shift correlation map (CSCM) experiment.
On the structure of rubrifacine
Authors: Feistner Gottfried , Budzikiewicz Herbert
Published: May 2006

Evidence is presented that rubrifacine, the red pigment of Erwiniarubrifaciens, is 3-(3-carboxymethyl-4,5-dihydro-4,5-dioxo-2-pyrrolyl)-4,5,6-trihydroxypyridine-2-acetic acid, based on mass spectrometric and nmr data of rubrifacine, its hydrolysis and reduction products as well as their methyl derivatives.
Homoenolization of 1,3,3-trimethylbicyclo[2.2.1]heptan-2-one in perdeuterated tert-butyl alcohol. Evidence that the medium potassium tert-butoxide – tert-butyl alcohol is unstable at elevated temperatures
Authors: Werstiuk Nick Henry , Timmins George
Published: May 2006

Homoenolization of 1,3,3-trimethylbicyclo[2.2.1]heptan-2-one (1) (fenchone) in potassium tert-butoxide – tert-butyl alcohol-O-d and perdeuterated potassium tert-butoxide – tert-butyl alcohol at 220 °C established that there is significant deterioration of the medium and dilution of the deuterium pool. We show that the major pathway of deterioration of the medium and dilution of the pool involves formation of isobutene by elimination of water from the alcohol. Exchange data already accumulated in the literature are used to support our view that the elimination also occurs at 185 °C, the temperature commonly used to homoenolize ketones.
Reaction of hydroxide ion with 1,3,5-trinitrobenzene in cationic micelles: evidence of variable counterion binding to micellar head groups
Authors: Gan Leong-Huat
Published: May 2006

Cationic micelles of hexadecyltrimethylammonium bromide, chloride, and hydroxide (CTABr, CTACl, and CTAOH) greatly enhance the stability of anionic σ-complex formed by hydroxide ion and 1,3,5-trinitrobenzene (TNB). The counterion binding to the micellar head groups is assumed to vary but is governed by a distribution constant. Different distribution constants are used for different types of counterion. Using this treatment, the rate constant – surfactant concentration profiles for the reactions of TNB with hydroxide ion in CTAOH, CTABr, and CTACl can all be satisfactorily accounted for, yielding consistent results in various conditions. The ion distribution constants, KOH = 55 mol−1 dm3, KBr = 1800 mol−1 dm3, and KCl = 420 mol−1 dm3 and the rate constant KM = 3600 s−1 are obtained. In view of the success of this treatment, the assumption of a constant β value associated with the pseudophase model at best could only be viewed as an approximation, valid only for micellar systems with tightly bound counterions such as those in the micelles of CTABr.
The Diels–Alder reaction of 4-methoxy-7-hydroxyisobenzofuran with methyl vinyl ketone; a general method for identification of some regioisomeric α-naphthols
Authors: Keay B. A. , Rodrigo R.
Published: May 2006

Two regioisomeric α-naphthols 14 and 15 obtained by aromatization of the methyl vinyl ketone adducts of an unsymmetrical isobenzofuran 9 are differentiated from each other by the observation of significant upfield shifts of the peri proton resonances in the 1H nmr spectra of their acetates 16 and 17. Such upfield shifts of 0.3–0.6 ppm appear to be a general phenomenon and are probably due to the anisotropic effect of the acetate carbonyl group.
Indium(III) compounds with the 2-[(dimethylamino)methyl]phenyl ligand
Authors: Steevensz Richard S. , Tuck Dennis G. , Meinema Harry A. , Noltes Jan G.
Published: May 2006

The syntheses of the arylindium compounds L2InX (X = Cl, I), L2InR (R = CH3, C6H5), L3In and L2InSnPh3 (L = 2-[(dimethylamino)methyl]phenyl) are reported. The pathway of the redistribution reactions involving InCl3 and L3In are discussed. Mass and 1H nmr spectra are reported.
γ-Phosphono-γ-lactones. The use of allyl esters as easily removable phosphonate protecting groups
Authors: Kamber Matthias , Just George
Published: May 2006

During the synthesis of γ-lactones bearing a phosphonic acid group at the γ-position, difficulties were encountered generating the free phosphonic acids from corresponding esters. A protecting group used for carboxylic acids was adapted to phosphonic acids, making this transformation easy.
Total assignment of the 1H and 13C spectra of mikanokryptin and comparison of its solution and solid state conformations
Authors: Reynolds William F. , Enríquez Raul G. , Chavez Marco A. , Silba Ana L. , Martinez Miguel A.
Published: May 2006

The 1H and 13C spectra of mikanokryptin are unambiguously assigned using a variety of one-dimensional nmr experiments (selective homonuclear decoupling, use of inversion–recovery pulse sequences to reveal hidden peaks, and DEPT spectral editing) in conjunction with normal and "long range" two-dimensional heteronuclear correlated experiments. Detailed comparison of vicinal, allylic, and homoallylic 1H—1H coupling constants with dihedral angles determined from an earlier crystal structure determination indicates that solution and solid state configurations are similar.
Flow-tube studies of reactions of selected ions with cyanoacetylene
Authors: Raksit A. B. , Bohme D. K.
Published: May 2006

Rate constants and product distributions have been determined for ion/molecule reactions of cyanoacetylene (HC3N) with He+, C+(2P), CH3+, C2+, C2H+, C2H2+, CN+, C2N+, C2N2+, N2+, CO+, H3+, N2H+, HCO+, C2H3+, H3O+, CH3NO2H+, CH3CNH+, (CH3)2COH+, i-C3H7OH2, C3H+, and C4N+. The measurements were performed with the selected-ion flow-tube (SIFT) technique at 296 ± 2 K. The observed reactions exhibit a variety of pathways including charge transfer, proton transfer, condensation, and association. The condensation and association reactions are suited for molecular growth by ion/molecule reactions. Special consideration is given to the active role of cyanoacetylene in the possible molecular synthesis which occurs in dense interstellar gas clouds by ion/molecule reactions.
Cyclopentadienyl ruthenium complexes. Part III. Reactivity of some η5-cyclopentadienylbis(triphenylphosphine)ruthenium(II) complexes with nitrosyl tribromide and dinitrogen trioxide
Authors: Ashok R. F. N. , Gupta M. , Arulsamy K. S. , Agarwala U. C.
Published: May 2006

Mixed ruthenium(II) nitrosyls have been synthesized in yields larger than 60% by a general reaction of [Ru(η5-C5H5)(PPh3)L]+X− (L = 2,2′-bipyridine or 1,10-phenanthroline, X = Cl or Br) or [Ru(η5-C5,H5)(PPh3)(L)X] (L = PPh3, pyridine, 3-picoline, 4-picoline, [Formula: see text], or [Formula: see text]; X− = Cl−, Br−, I−, CN−, NCS−, H−, or SnCl3−) with NOBr3 and N2O3. In these complexes NO seems to bind with the metal ion as NO+. The reactions of N2O3 gave either nitrito or nitrosyl dinitrito complexes. The reactions of NOBr3 with trichlorostannate complexes did not yield nitrosyl complexes, instead nitrito complexes were isolated in which spectroscopic evidence (ir, 1H nmr) suggest π-interaction of one of the phenyl rings of the triphenylphosphine ligand to the ruthenium center. All products are characterised by elementary microanalyses, conductivity, magnetic moment measurements, electronic, ir, and 1H nmr spectral data.
Synthesis of D-norisomorphinans ((±)-[14α]-D-normorphinans)
Authors: Yardley John P. , Rees Richard W.
Published: May 2006

A synthesis of the D-norisomorphinan ring system is described. Conversion of the initial synthetic target, trans-1,3,4,9,10,10a-hexahydro-6-methoxy-9-oxo-4a(2H)-phenanthrenecarboxamide (10c) into the D-norisomorphinan (6b) proved possible only after removal of the carboxamide ambident functionality. The successful route proceeds via hypobromous acid addition to trans-1,3,4,10a-tetrahydro-6-methoxy-4a(2H)-phenanthrenemethanamine (21) followed by a facile intramolecular cyclization to the D-norisomorphinan (23).