bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of a trimeric chloroguaiacol quinone in the preparation of tetrachloroguaiacol and crystal structure of its tetramethoxy derivative
Authors: Burlinson Nick , Rettig Steven J. , Trotter James , McKague Bruce
Published: May 2006

A by-product formed in the preparation of tetrachloroguaiacol is shown to be a trimeric quinone. Derivatization of the quinone to a crystalline tetramethoxy derivative allowed X-ray determination of its crystal structure as 3,6-dichloro-4,5-dimethoxy-1,2-bis(2′-methoxy-3′,4′,5′,6′-tetrachlorophenoxy)benzene, 4. Rarely observed long range JCH coupling between ring carbons and methoxyl hydrogens as well as conformational isomerism were observed in 1H and 13C nmr studies of 4. Crystals of 4 are triclinic, a = 12.562 (1), b = 12.708(1), c = 9.223(1) Å, α = 96.93(1), β = 97.478(7), γ = 101.285(8)°, Z = 2, space group[Formula: see text]. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.056 and Rw = 0.056 for 2946 reflections with I ≥ 1.5σ(I). The molecule contains a central six-membered dichloro-tetraoxo substituted aromatic ring, linked via oxygen bridges to two identical tetrachloro-dioxo substituted rings; intramolecular steric overcrowding causes significant deviations from a symmetrical conformation. Bond lengths, angles, and intermolecular distances are generally close to expected values.
Bridged polycyclic compounds. 88. Multiple intermediates in solvolysis of certain bridged bicyclic and tricyclic compounds
Authors: Cristol Stanley J. , Schloemer George C. , Braun Dieter , Mayo Gwendolyn O.
Published: May 2006

Acetolyses of 7-chloromethyl- and 7-bromomethyldibenzobicyclo[2.2.2]octatrienes, 8-chloro- and 8-bromo-7-methylenedibenzobicyclo[2.2.2]octa-2,5-dienes, 4-bromo-8-methylenedibenzobicyclo[3.2.1]octa-2,6-diene, and 2-chlorodibenzotricyclo[3.2.2.02,4]nonadiene were carried out. The results observed were consistent with the involvement of two carbocations interconverting by a Wagner–Meerwein rearrangement, and with the absence of the 1-dibenzosemibullvalenylcarbinyl cation, which is the cyclopropylcarbinyl isomer of one of the cations produced. The preparations of the reactants and products are described.
Importance of repulsion of lone electron pairs in the enhanced reactivity of 1,8-naphthyridine and the large α-effect of hydrazine in the aminolyses of p-toluenesulfonyl chloride
Authors: Oae Shigeru , Kadoma Yoshihito
Published: May 2006

The rates of aminolyses of p-toluenesulfonyl chloride with primary and tertiary amines have been determined both in acetonitrile and in ethanol. The Brönsted plots of log krel again pKa′ values of amines (except hydrazine and 1,8-naphthyridine in acetonitrile) gave a good correlation when the aminolyses were carried out in acetonitrile. In ethanol, however, although Brönsted plots with all tertiary amines show a good correlation, less basic hydrazine shows a higher reactivity than n-butylamine. The abnormal rate enhancement found with hydrazine is undoubtedly due to the α-effect, while that with 1,8-naphthyridine in acetonitrile is considered to be due to the repulsion of two lone electron pairs on the two nitrogen atoms in 1,8-naphthyridine
Kinetics of the equilibration of 3-hydroxyphthalide and o-formylbenzoic acid. Hemiacetal breakdown with a carboxylic acid leaving group
Authors: McClelland Robert A. , Sørensen Poule E.
Published: May 2006

A temperature-jump relaxation study is reported for the equilibration: 3-hydroxyphthalide (SH) [Formula: see text]o-formylbenzoate (R−) [Formula: see text]o-formylbenzoic acid (RH). A kinetic analysis is carried out in which SH and R− interconvert with catalysis in the ring opening direction by water and by added general bases. Excellent Brønsted plots based upon a series of oxyacid buffer catalysts are obtained. These have slopes β for the base-catalyzed ring opening of 0.81 and α for the reverse acid-catalyzed ring closing of 0.19. A mechanism where S−, the conjugate base of SH, is a discrete intermediate can be ruled out on the basis of the Brønsted values and the magnitudes of the rate constants. The lifetime of S− is estimated to lie in the range 10−11–10−15 s. Two mechanisms can be proposed. A fully concerted mechanism "enforced" by lifetimes less than 10−13 s involves direct interconversion of SH and R− with no intermediate. A preassociated mechanism "enforced" by lifetimes in the 10−11–10−12 s range requires, in the ring closing direction, that an acid catalyst be hydrogen bonded to the carbonyl in R−.
Kinetics of ionisation of carbon acids in non-aqueous media: detritiation of [2-3H1]diethyl malonate by heterocyclic bases
Authors: Davey John P. , Jones John R. , Buncel Erwin
Published: May 2006

To investigate proton transfer processes in non-aqueous media, two procedures have been developed of general applicability for the measurement of detritiation rates of carbon acids. One method is a variation of an existing solvent extraction procedure but with inclusion of a trace of trifluoroacetic acid, while the other involves the simple modification of a gas chromatograph so as to function as a radio-gas chromatograph. They have been established by studying the detritiation of [2-3H1]diethyl malonate in six different solvents (dimethylformamide, dimethyl sulfoxide, sulfolane, hexamethylphosphortriamide, tetrahydrofuran, ethanol) as catalyzed by various heterocyclic bases (substituted pyridines, imidazoles, benzimidazoles, pyrrole, pyrazole, purine, adenosine). The results are discussed in terms of solvation effects and catalyst structure. An approximate Brønsted correlation is found to exist between [Formula: see text] for detritiation determined in DMSO and the pKa of the conjugate base measured in water at 25 °C.
Controlled oxidations of benzo[a]pyrene
Authors: Lee-Ruff E. , Kazarians-Moghaddam H. , Katz M.
Published: May 2006

The four diones derived from benzo[a]pyrene oxidation have been characterized by high-field nuclear magnetic resonance techniques including 2-D COSY and selective nuclear Overhauser enhancement. All the proton chemical shifts for these four quinones have been uneqivocally assigned. The direct photoxidation of benzo[a]pyrene gives a product distribution very similar to the TPP photosensitized oxygenation, suggesting singlet oxygen is involved in the former. A major product, which was characterized as the 6-seco derivative 6 and not previously reported, was detected in the singlet oxygen reaction. The presence of this product suggests a possible mechanism for quinone formation in the singlet oxygen reaction. One-electron oxidations of benzo[a]pyrene were carried out using tris(p-bromophenyl)aminium hexachloroantimonate and quenching of the radical cation with superoxide or water. The product distribution in this case was quite different from that obtained in the direct photooxidation.
Structure et conformation du complexe du N-acétyl-aspartate avec un cation lanthanide. Étude par résonance magnétique nucléaire du 1H et du 13C
Authors: Baron Daniel , Lumbroso-Bader Nicole
Published: May 2006

Lanthanide induced shifts by Lu3+, Yb3+, Tm3+, Er3+, Ho3+, and Dy3+ in NAcAsp (0.14 M in D2O, pH 5) are observed for ten magnetic sites (1H and 13C). The averaged binding constant for 1:1 complexes is 72 M−1 (for chloride solutions of ca. 0.05 M). Determination of the pseudo-contact geometrical factors (under axial symmetry approximation) requires taking into account a contact term and discarding the Tm3+ results. Data from the Asp residue are in agreement with 6 structures of this residue such that chelation occurs through the two carboxylates. Oβ−… Cα′ length seems to be the main factor while COOα− orientation is a minor one. The entire set of results is consistent with only three structures where the nitrogen atom is far from the carboxylates, and the Cα—N rotamers have extended conformations. Three oxygen atoms (two from the COOβ− group) appear to be involved in the chelation. However, the data do not exclude another minor conformational species.
High-temperature thermodynamic properties of several 1:1 electrolytes
Authors: Saluja Preet P. S. , Pitzer Kenneth S. , Phutela Ramesh C.
Published: May 2006

Comprehensive equations for the thermodynamic properties of aqueous NaI, CsF, CsCl, CsI, NaIO3, KIO3, CsIO3, KClO4, and HCl are generated by combining the heat capacity and density measurements reported by the first author with literature data for the enthalpy and Gibbs energy, or activity, as a function of molality at 298 K. The composition dependence is represented by the equations of Pitzer, which combine a theoretical form and Debye–Hückel terms with empirically evaluated parameters for short-range ion interactions. Temperature dependencies are represented by simple empirical equations, which should be reliable to 413 K or a little higher.
Use of camphor in pseudoguaianolide synthesis
Authors: Hutchinson John H. , Money Thomas , Piper Susan E.
Published: May 2006

Stereoselective alkylation of a bicyclic lactone (9) derived from (+)-camphor (5) is the basis of a synthetic route to bicyclic enone (3) which could be used as a key intermediate in helenanolide synthesis.
Improved syntheses of hydroxy acid precursors of macrolide pheromones of cucujid grain beetles
Authors: Oehlschlager A.C. , Czyzewska E. , Aksela R. , Pierce Jr. H. D.
Published: May 2006

Syntheses of 11- and 12-hydroxy-(Z)-3-dodecenic, 11- and 12-hydroxy-(Z,Z)-3,6-dodecadienic, and 13-hydroxy-(Z,Z)-5,8-tetradecadienic acids are reported. These unsaturated hydroxy acids are precursors of male-produced macrolide aggregation pheromones of grain beetles of the genera Cryptolestes and Oryzaephilus. The sequence of reactions for synthesis of the hydroxy-(Z)-3 acids was carboxylation of the dilithium salt of 10- or 11-hydroxy undecyne, deconjugation of the resulting carboxylic acids with NaNH2 in THF/liquid ammonia to a mixture of 2,3-allenic and 3-alkynoic acids, and hydrogenation of the mixture with P-2 nickel catalyst. This latter reaction gave only the desired hydroxy-(Z)-3-alkenoic acids. The convergent point in the syntheses of the diunsaturated hydroxy-(Z,Z) acids was the coupling of the dimagnesium salt of 3-butyn-1-ol or 5-hexyn-1-ol to the appropriate functionalized propargylic bromide. The resulting 1,4-diacetylenic alcohols were oxidized with Jones reagent to the corresponding carboxylic acids, which were hydrogenated over P-2 nickel catalyst and deprotected.
Alternariol: evidence for biosynthesis via norlichexanthone
Authors: Stinson E. E. , Wise W. B. , Moreau R. A. , Jurewicz A. J. , Pfeffer P. E.
Published: May 2006

Alternaria molds produce numerous mycotoxins including many α-dibenzopyrones such as alternariol (AOH) and related polyketides. AOH, presumed to be the initial α-dibenzopyrone produced, has for 20 years been considered to be biosynthesized from a single polyketide chain in a single step reaction. The present study presents evidence that the reaction may proceed through an intermediate, norlichexanthone (NLX). Bond cleavage and rearrangement of NLX to form AOH may be similar to aflatoxin B1 formation from sterigmatocystin. The 2-D INADEQUATE experiment was used to assign the 13C spectrum of AOH and to distinguish between possible mechanisms by which AOH may be synthesized from 1-13C and 2-13C acetates via NLX.
Concerning the Si—Li bonding in phenylsilyllithiums as studied by variable temperature Li nuclear magnetic resonance
Authors: Buncel Erwin , Venkatachalam T. Krishnan , Edlund Ulf
Published: May 2006

A variable temperature 7Li nmr study of equimolar mixtures of two structurally related phenylsilyllithiums supports a bimolecular exchange mechanism between monomeric structures as being responsible for the collapse of the 7Li signals of the two species. The different exchange rates indicated in MTHF and THF parallel the observations from a Si—Li coupling study. The experimental data provide support for a mainly ionic Si—Li bond, although some orbital interaction is indicated.
Contraction de cycle dans la substitution d'un triflate osidique
Authors: Grouiller Annie
Published: May 2006

Benzoate displacement of the 2-triflyl derivative of methyl 3-azido or 3-O-benzoyl-4-deoxy-α-DL-thero-pentopyranoside (1 or 4) occurs with a pyranose to furanose contraction reaction at C-2. This contraction is not observed with an azido displacement.
Syntheses of model oligosaccharides of biological significance. 7.Synthesis of a fucosylated N,N′-diacetylchitobioside linked to bovine serum albumin and immunochemical characterization of rabbit antisera to this structure
Authors: Lee Ho-Huat , Schwartz David A. , Harris John F. , Carver Jeremy P. , Krepinsky Jiri J.
Published: May 2006

Synthesis of methoxycarbonyloctyl 2-acetamido-4-O-[(2′-acetamido-2′-deoxy-β-D-glucopyranosyl)-6-O-α-L-fucopyranosyl]-2-deoxy-(β-D-glucopyranoside as shown in Scheme 1 is described together with coupling it with bovine serum albumin and poly-L-lysine. Mouse and rabbit antisera to the BSA conjugate were prepared and partially characterized.
A 1H and 13C nuclear magnetic resonance study of carnosine
Authors: Friedrich Jan O. , Wasylishen Roderick E.
Published: May 2006

The proton and carbon-13 resonance signals of carnosine (β-alanyl-L-histidine) were unambiguously assigned using a variety of nmr techniques including proton–carbon chemical shift correlations, titrations of nmr chemical shifts, coupling constants, and isotope shifts. From the 13C nmr titration, carnosine's three pKa values were estimated to be 2.7, 7.1, and 10.6, and it was found that the imidazole ring existed predominantly as the 3-H tautomer in basic solution. Conformational information about the Cα—Cβ bond and about the N—Cα bond was deduced from observed 3J(C,H) and 3J(H,H) values. The 13C nmr spectrum of carnosine in solution is also compared with that obtained for a solid sample.
Squaraine chemistry. Synthesis of bis(4-dimethylaminophenyl)squaraine from dialkyl squarates. Mechanism and scope of the synthesis
Authors: Law Kock-Yee , Bailey F. Court
Published: May 2006

The mechanism and the scope of the synthesis of bis(4-dialkylaminoaryl)squaraines from di-n-butyl squarate and N,N-dialkylanilines have been studied. Results show that water and acid are key factors of the synthesis, and these two factors have been optimized. Yields of squaraine are also found to be sensitive to the steric effect provided by the alkyl chain in dialkyl squarates as well as to the concentration of the aniline reagent used in the synthesis. Mechanistic results suggest that alkyl squarate is the precursor of the synthesis, and that squaraine is formed by diarylation of alkyl squarate with an N,N-dialkylaniline derivative. Under optimized conditions, a number of squaraines have been synthesized. Yields, which are comparable to those synthesized from squaric acid, are obtained. Product analysis with good material balance has been achieved, and results show that arylation and diarylation of the starting di-n-butyl squarate are major side reactions in the squaraine synthesis.
Stereochemistry of dopaminergic ergoline derivatives: structures of pergolide and pergolide mesylate
Authors: Ma Lilian Y. Y. , Camerman Norman , Swartzendruber John K. , Jones Noel D. , Camerman Arthur
Published: May 2006

The crystal and molecular structures of the dopaminergic ergoline derivative pergolide have been determined both as the free base and as the mesylate salt. Despite differences in protonation at the nitrogen atom, the two structures are extremely similar in all aspects. Stereochemical comparisons are made with bromocriptine, and possible dopaminergic pharmacophores in ergolines are discussed.
Kinetics and mechanisms of oxidations by metal ions. Part VIII. Oxidation of n-propanol by aquomanganese(III) ions
Authors: Mehta Mahendra , Nagori Ratan R. , Mehrotra Raj N.
Published: May 2006

The oxidation of n-propanol by aquomanganese(III) ions in perchlorate medium of constant ionic strength (4 mol dm−3, NaClO4) is described by the rate law [i] which is based on the consideration of the reactions [ii]–[ix].[Formula: see text]A further analysis of the rate law in terms of the results indicated that MnOH2+ (aq) is the major oxidant. The values of the equilibrium constant (β1) and rate determining constant (k1) are reported together with the associated thermodynamic parameters. The values of the other constants, k, Ka, and β, could not be determined though an assumption was made that β ≈ β1.
Organometallic aspects of cytochrome P-450 metabolism
Authors: Setsune J.-I. , Dolphin D.
Published: May 2006

Carbeneiron porphyrins, (N,Fe)-bridged methyleneiron porphyrins, N-alkyliron porphyrins and σ-alkyliron porphyrins have all been shown to occur in nature and to be interconvertible via redox reactions. The chemistry of the naturally occurring iron porphyrins and model iron, cobalt, and ruthenium porphyrins is reviewed emphasizing their organometallic chemistry.
Time-resolved quenched phosphorescence detection of chromate in water samples using paired-ion reversed phase high performance liquid chromatography
Authors: Baumann Robert A. , Schreurs Marry , Gooijer Cees , Velthorst Nel H. , Frei Roland W.
Published: May 2006

The feasibility of time-resolved quenched phosphorescence as a detection method for chromate based on paired-ion reversed phase high performance liquid chromatography is shown. After separation detection is achieved by monitoring the time-resolved phosphorescence signal of biacetyl at 515 nm. Optimization of the time-resolution parameters of the luminescence detector results in a limit of detection of 1.4 × 10−7 M (2 ng on column) for chromate standard solutions. The linearity of the method is three orders of magnitude. Chromatograms of tap water and surface water samples spiked with chromate show the potential applicability of the method for real samples.
Basicity of 1,8-bis(dimethylamino)naphthalene and 1,4-diazabicyclo[2.2.2]octane in water and dimethylsulfoxide
Authors: Benoit Robert L. , Lefebvre Diane , Fréchette Monique
Published: May 2006

The ionization constants of the conjugate acids [Formula: see text] of two bases widely used in synthesis, 1,8-bis(dimethylamino)naphthalene (PS), and 1,4-diazabicyclo[2.2.2]octane (DABCO), have been obtained in Me2SO and water. The partition coefficients of both bases B and their heats of solution and protonation have also been determined. The basicity of PS is dramatically reduced when passing from water to Me2SO while the basicity of DABCO remains unchanged, thus making DABCO the stronger base in Me2SO. The solvent effect on the base strength is analyzed in terms of free energy and enthalpy of transfer for B and H+, and for BH+, from water to Me2SO. The entropy-controlled much stronger solvation of PS in Me2SO is in line with that of aromatic compounds while the increased solvation of DABCO in water is due to H-bonding of its two nitrogens. PSH+, like H+, is better solvated by Me2SO while DABCOH+ does not show a definite preference for one solvent. The calculated enthalpy of solution of gaseous PSH+ is abnormally low when compared to that of DABCOH+ and of related BH+ ions, thus indicating that the proton of PSH+, in a strong intramolecular H-bond, does not normally H-bond to the solvent. Our results suggest that the proton sponge acid-base system could be used as a convenient reference to determine the free energy and enthalpy of transfer of H+ between solvents.
Isolation and structure of rolliniastatin 1 from the South American tree Rolliniamucosa
Authors: Pettit George R. , Cragg Gordon M. , Polonsky Judith , Herald Delbert L. , Goswami Animesh , Smith Cecil R. , Moretti Christian , Schmidt Jean M. , Weisleder David
Published: May 2006

Rolliniamucosa (Annonaceae), known in primitive medical practices of Indonesia and the West Indies as a treatment for tumors, has been investigated employing the seeds from a South American variety and the murine P388 lymphocytic leukemia (PS) for bioassay. The principal antineoplastic constituent (PS, 28% life extension, at 0.25 mg/kg and ED504.5 × 10−5 μg/mL) was found to be a new bis-tetrahydrofuran designated rolliniastatin 1. Structural elucidation of rolliniastatin 1 was accomplished by a combination of high resolution nuclear magnetic resonance (300 MHz), mass spectral, and X-ray crystal structure techniques.
Resonance Raman spectroscopic evidence for the formation of covalent complexes between N-acylamino acid dithioesters and mercury papain
Authors: Lee Ho-Hi , Angus Ronald H. , Carey Paul R. , Storer Andrew C.
Published: May 2006

Ethyl dithioesters of N-acylglycine and N-acylalanine are shown to bind tightly to the active site of papain in which the side chain of cysteine-25 is in the mercurated form. Although both classes of dithioester bind to the active site mercury via their thiono (C=S) sulfur atoms, the resulting adducts are quite different. Absorption and resonance Raman data, supported by comparison with model compounds and by 15N and 13C=S isotopic substitutions, show that the glycine and alanine dithioesters are converted to enethiolates and 4-methyloxazolin-5-thiones, respectively. The difference in product formation is explained by a mechanism in which stereospecific proton removal is brought about by the -COO− side chain of Asp-158, which is situated in the vicinity of the active site. For glycine dithioesters the Asp-158 catalyses the removal of a Cα proton whereas for alanine dithioesters the alanine's NH proton is removed.
Chemical and electrochemical reduction of pyrazino[2,3-g]quinoxalines and of their benzo and dibenzo derivatives; the structure of fluorindine and the formation of tetraanion
Authors: Armand Joseph , Boulares Line , Bellec Christian , Pinson Jean
Published: May 2006

The structure of fluorindine is established by nmr as the 5,14-dihydroquinoxalino[2,3-b]phenazine. The catalytic hydrogenation of 2,3-di(p-methoxyphenyl)pyrazino[2,3-b]phenazine 2a leads to the 6,11-dihydro derivative 4a. The electrochemical reduction in an hydroorganic medium furnishes 4a and then the 1,4,6,11-tetrahydro derivative 8a. In dry DMSO the voltammogram shows four monoelectronic reversible systems corresponding to the successive formation of a radical anion, dianion, radical trianion, and tetraanion. Thus 2a appears as a new example of the very restricted class of compounds leading to tetraanions upon electrochemical reduction. The catalytic hydrogenation of 2,7-diphenylpyrazino[2,3-g]quinoxaline 1a or the reaction of LiAlH4 with 1,2,7,8-tetramethylpyrazino[2,3-g]quinoxaline 1b leads to 1,2,3,4-tetrahydro compounds. The electrochemical reduction of 1a and 1b in hydroorganic medium leads successively to 1,4-dihydro and then to 1,4,6,9-tetrahydro compounds which undergo a further rearrangement. In dry DMSO 1a and 1b behave differently from 2a: one only observes two reversible monoelectronic systems.
Growing interstellar cyanopolyacetylenes with ion/molecule reactions
Authors: Bohme Diethard K. , Włodek Stanisław , Raksit Asit B.
Published: May 2006

New and recent results of laboratory measurements are presented which allow an assessment of four ion/molecule reaction schemes which have been proposed for the gas-phase synthesis of interstellar cyanopolyacetylenes. The schemes are scrutinized in terms of the manner of growth of the carbon backbone and the manner in which nitrogen is entrained in the formation of ions of the type H2C2n+1N+ which may neutralize by proton transfer or electron–ion recombination to form cyanopolyacetylenes.
pKa values for substituted acetophenones: values determined by study of rates of halogenation
Authors: Guthrie J. Peter , Cossar John , Klym Alex
Published: May 2006

pKa values in aqueous solution have been determined for seven p-substituted acetophenones. The determinations are based on the diffusion-controlled reaction of an enolate with hypochlorous acid. The values determined for the acetophenones are as follows: substituent, value; H, 18.4; p-MeO, 19.0; p-F, 18.5; p-Cl, 18.1; p-Br, 18.0; p-NO2, 16.7; p-Me3N+, 17.1. ρ for these pKa values is −1.95. Using literature values for the enol content we show that most of this ρ value reflects substituent effects upon enolization. With relatively high hydroxide and low hypochlorite concentrations the major products from chlorination of acetophenones are the corresponding mandelic acids. Under similar conditions bromination of acetophenone gives benzoic acid.
The oxidation of stannous halides by tetrahalogeno-ortho-quinones, and the crystal structure of bis[2-(dimethylamino)ethyldimethylammonium] tris(tetrachlorocatecholato)tin(IV), (Me2NC2H4NMe2H)2[Sn(O2C6Cl4)3]
Authors: Annan Theodore A. , Chadha Raj K. , Tuck Dennis G. , Watson Katherine D.
Published: May 2006

The reaction between SnX2 (X = Cl, Br, I), 1,10-phenanthroline (phen) and Y4C6O2 (Y = Cl, Br) yields Y4C6O2SnX2•phen with the o-quinone being converted to the corresponding catecholate anion. Reactions in which phen is replaced by Me2NC2H4NMe2 (tmen) are more complex, and the products depend in part on the solvent. These products include Cl4C6O2SnCl2•tmen, and products arising from solvation by ROH (R = CH3, C2H5). An X-ray crystallographic study of (tmenH)2[Sn(O2C6Cl4)3] shows that the anion contains a six-coordinate tin atom bonded to substituted catecholate anions. Hydrogen-bonding between cation and anion appears to be an important factor in the stability of the lattice.
Synthesis of optically active forms of the δ-lactone of 3,5-dihydroxydecanoic acid
Authors: Satō Tsuguo
Published: May 2006

The δ-lactone of 3,5-dihydroxydecanoic acid was synthesized from methyl-3-oxooctanoate by utilizing its baker's yeast reduction.
The biosynthesis of caerulomycins in Streptomyces caeruleus. Isolation of a new caerulomycin and incorporation of picolinic acid and glycerol into caerulomycin A
Authors: Vining Leo C. , McInnes A. Gavin , McCulloch Archibald W. , Smith Donald G. , Walter John A.
Published: May 2006

Oxidation with permanganate of caerulomycin A labeled biosynthetically from [U-14C]lysine in cultures of Streptomyces caeruleus yielded picolinic acid with the same specific activity as the antibiotic. [3H]Picolinic acid was incorporated into caerulomycin A with high efficiency and the label was also found exclusively in picolinic acid after oxidation, implicating free picolinic acid as an intermediate in the pathway from lysine. The substituted pyridyl ring of caerulomycin was labeled by [2-13C, 1,3-14C]glycerol. 13C nuclear magnetic resonance analysis located the 13C predominantly at C-6, suggesting that C-5, C-6, and C-7 were derived from glycerol as an intact three-carbon precursor. Since no incorporation of [1-13C]glyceric acid was detected and [3-13C, 3-14C]methylglyoxal labeled the antibiotic only marginally, these compounds are probably not biosynthetic intermediates en route from glycerol. A new caerulomycin possessing an aldehyde in place of the aldoxime substituent at C-6 was isolated from culture broths and is postulated to be a late intermediate in the formation of caerulomycin A.
Anodic and cathodic reactions in acetonitrile/tetra-n-butylammonium tetrafluoroborate: an electrochemical and infrared spectroelectrochemical study
Authors: Foley John K. , Korzeniewski Carol , Pons Stanley
Published: May 2006

In situ infrared spectroelectrochemistry has been used to observe electrolyte/solvent breakdown processes in TBAF/acetonitrile at a platinum electrode. Anodic breakdown gives the adduct CH3CNBF3 as the major product. Cathodic breakdown gives the anion of 3-aminocrotonitrile as the major product in the absence of water, and the formation of this product is a consequence of reduction of hydrogen dissolved in the metal. In the presence of water the anion of acetamide is also formed, and breakdown commences at much less negative potentials.
Magnetism and structure in a series of halogen and hydroxo-bridged binuclear copper(II) diazine complexes. Structure of a dibromobridged derivative [Cu2(C14H10N4S2)Br4]
Authors: Thompson Laurence K. , Mandal Sanat K. , Charland Jean-Pierre , Gabe Eric J.
Published: May 2006

The ligand PTP (3,6-bis(2-pyridylthio)pyridazine) generates both hydroxo- and halogeno-bridged, square-pyramidal, binuclear copper(II) complexes ([Cu2(PTP)X4] (X = Cl (I), Br (II)); [Cu2(PTP)(OH)X3] (X = Cl (IV), Br (V)), which are antiferromagnetically coupled. The hydroxo-bridged derivatives are more strongly coupled than those with just halogen bridges and within each pair of complexes substantially increased spin exchange is observed for the bromo-derivatives (−2J = 130.5 ± 0.4 (I), 243.5 ± 0.3 (II), 294 ± 3 (IV), 326 ± 6 (V) cm−1). A related halogen-bridged derivative [Cu2(PAP46Me)Cl4] (III) (PAP46Me = 1,4-di(4,6-dimethyl-2-pyridylamino)phthalazine) exhibits much weaker spin exchange associated with the diminished superexchange capacity of the phthalazine bridge compared with pyridazine (−2J = 55 ± 0.1 cm−1). An X-ray study on [Cu2(PTP)Br4] reveals an asymmetric tribridged structure involving a pyridazine and two bromo-bridges with a copper–copper separation of 3.318(3) Å, one relatively short terminal copper–bromine bond (2.375(3), 2.398(3) Å), and a short and long copper–bromine bridging contact (2.380(3), 2.801(3); 2.414(3), 2.766(3) Å) per copper. The complex formed green crystals with a = 11.1042(18), b = 15.6554(10), c = 12.6058(14) Å, β = 113.485(11)°, P21/c, Z = 4, Rf = 0.045.
Structural studies of a hemicellulose B fraction (B-2) from the cork of Quercus suber
Authors: Asensio Amparo
Published: May 2006

Polysaccharide B-2 contains xylose, arabinose, glucose, galactose, 4-O-methylglucuronic acid, and rhamnose in the molar ratio of 17:12:12:6:4:1. The polysaccharide has a backbone of β(1 → 4)- and β(1 → 3)-linked glucopyranosyl, β(1→ 4)-linked xylopyranosyl, and (1 → 4)- and (1 → 6)-linked galactopyranosyl residues. Arabinofuranosyl, rhamnopyranosyl, and some xylopyranosyl and galactopyranosyl groups are present as nonreducing end units. Glucopyranosyluronic acid side chains are attached at O-2 of some xylose residues. The anomeric configurations of the different glycosyl groups were determined by a study of the chromium trioxide oxidation of the acetylated polysaccharide and by nuclear magnetic reasonance spectra of the methylated polysaccharide.
Heat-of-mixing for the partially miscible system N2/C2H6
Authors: Calado Jorge C. G. , Gopal Prakash , Zollweg John A. , Thompson W. Reid
Published: May 2006

Heats-of-mixing for the liquid system nitrogen/ethane have been measured at 92.3 K and 0.6309 MPa using a cryogenic flow calorimeter. The heat-of-mixing versus composition curve shows that the system is only partially miscible; the compositions between which liquid–liquid separation occurs have been estimated.
The structures and dynamics of SF6: new insights
Authors: Powell B. M. , Dolling G.
Published: May 2006

The structural and dynamical properties of the orientationally disordered solid SF6 have been studied. The present understanding of this state is discussed in terms of the microscopic nature of the orientational disorder, the influence of steric repulsion, and the concept of orientational frustration. A possible mechanism is considered for the phase transition to an ordered structure.
Quelles procédures utiliser en polarisation diélectrique pour mettre en évidence, avec certitude, les interactions entre des dérivés chlorés et bromés de l'éthane et la N,N-diméthylacétamide
Authors: Guérin M. , Benaziz L.
Published: May 2006

Numerous studies have shown the particular usefulness of dielectric methods for observing low energy specific molecular interactions in solution. Before applying these methods, it is necessary to evaluate the non-specific interactions between species in solution (in this paper, these interactions are termed the "normal" effect of the solvent). However, in the particular case of halogenated derivatives of ethane, which can exist in solution as several conformers of different polarities, this possibility was not available. The object of our work has been to study all the procedures that might be acceptable in evaluating this solvent effect. [Journal translation]
Micellar catalysis of organic reactions. 22. A comparison of the basic hydrolysis of benzodiazepinones in the presence of reactive counterion micelles and vesicles
Authors: Broxton Trevor J. , Sango Xenia , Wright Sallyanne
Published: May 2006

The basic hydrolysis of diazepam and several N-alkyl nitrazepam derivatives has been studied in the presence of reactive counterion micelles of cetyltrimethylammonium hydroxide (CTAOH) and vesicles of didodecyldimethylammonium hydroxide (DDAOH). In both surfactants, the rate of hydrolysis of all compounds was found to be dependent on the hydroxide concentration at constant surfactant concentration and this was interpreted as evidence for initial amide hydrolysis. The hydrolysis in CTAOH was inhibited by added salts in the order Br− < NO3− < SO42−. At concentrations above 3 mM surfactant, the rate of hydrolysis of each compound was similar in CTAOH and in DDAOH. At lower concentrations of CTAOH, however, the rate of hydrolysis was significantly lower than that in DDAOH. On the basis of this evidence, it was concluded that the cmc of CTAOH was between 2–3 mM, which is in good agreement with the value of 1.8 mM obtained by Zana from conductivity measurements. For diazepam, a mechanistic change is indicated on transfer from water to either micelles or vesicles and since vesicles are considered good models of biological membranes, this suggests that conclusions concerning the bioavailability of diazepam should not be based on studies in water but rather on studies in either micelles or vesicles.
Anisotropy of indirect spin–spin coupling constants from nuclear magnetic resonance powder patterns of rigid solids. 1J(31P,199Hg) in [HgP(o-tolyl)3(NO3)2]2
Authors: Penner Glenn H. , Power William P. , Wasylishen Roderick E.
Published: May 2006

The anisotropy of the indirect 31P,199Hg spin–spin coupling constant, ΔJ, in solid [HgP(o-tolyl)3(NO3)2]2 is obtained from an analysis of the 31P nuclear magnetic resonance powder pattern. The value of ΔJ, 5170 ± 250 Hz, is large and indicates that mechanisms other than the Fermi contact mechanism are important for this spin–spin coupling. The powder spectrum also indicates that the absolute sign of 1J(31P,199Hg) is positive.
Photoelectrochemical water splitting by tandem type and heterojunction amorphous silicon electrodes
Authors: Sakai Yuichi , Sugahara Satoshi , Matsumura Michio , Nakato Yoshihiro , Tsubomura Hiroshi
Published: May 2006

The tandem type hydrogenated amorphous silicon (a-Si) electrode having an [n–i–p–n–i–p] structure and a similar tandem a-Si electrode having [n–i–p–n–i–p–n] layers deposited on p-type crystalline Si showed cathodic photocurrents accompanied by hydrogen evolution starting at potentials 1.67 and 2.08 V, respectively, more positive than the thermodynamic hydrogen evolution potential in a sulfuric acid solution. These electrodes, when connected to an RuO2 counterelectrode, caused sustained water splitting without external bias, giving solar-to-chemical conversion efficiencies of 1.98% and 2.93%, respectively, under simulated AM 1, 100 mW cm−2 solar radiation. These efficiencies are critically compared with the efficiency of another type of solar photoelectrolysis of water, namely, the electrolysis of water by the electrical output from solid-state solar cells.
The radial charge distribution and the shell structure of atoms and ions
Authors: Simas Alfred M. , Sagar Robin P. , Ku Andrew C. T. , Smith Jr. Vedene H.
Published: May 2006

The radial charge distribution, D(r), for the neutral atoms, hydrogen through uranium, and the singly positive ions, helium through barium and lutetium through radium, are computed from the non-relativistic SCF wavefunctions of Clementi and Roetti and McLean and McLean. The radial charge distribution is examined to see how effective it is for the determination of the shell structure of an atom or an ion with the results indicating that the maxima are the topological features of D(r) most indicative of shells. At most five shells are apparent based on the criterion that the maxima in D (r) correspond to the shells. The positions of the maxima in D(r) are shown to correlate well with the shell radii from the Bohr–Schrödinger theory of an atom. The small deviations are consistent with the shielding of the outer electrons from the nucleus, by the inner electrons. Furthermore, it is shown that the effects of shielding are more pronounced on the values of the radial distribution function at the extremal points. The spherically averaged charge density, [Formula: see text] is seen to be monotonically decreasing for all these ground states, a result which extends the previously known range of atomic systems for which this is true.
Constant composition study of crystal growth of dicalcium phosphate dihydrate. The influence of polyphosphates, phosphonates, and phytate
Authors: Amjad Zahid
Published: May 2006

The influence of inhibitors of varying functional groups on the crystal growth of calcium phosphate dihydrate (CaHPO4•2H2O, DCPD) on DCPD seed crystals at pH 6.00, 37 °C, has been studied using the constant composition technique. The inhibitors studied include: (a) polyphosphate (sodium pyrophosphate, sodium hexametaphosphate, SHMP), (b) phosphonates (nitrilotris(methylene phosphonic acid), NTMP; 1-hydroxyethylidene-1,1-diphosphonic acid, HEDP; ethylenediaminetetra (methylene phosphonic acid), ETMP; hexamethylenediaminetetra(methylene phosphonic acid), HMDP); and (c) myoinositol hexaphosphoric acid (phytic acid). The effect of these inhibitors on the growth kinetics have been studied at several inhibitor concentrations. The retarding effect of these inhibitors is discussed in relation to calcium–inhibitor complex formation on the DCPD seed crystals and the structural features of the inhibitor molecules. The fit of the Langmuir adsorption to experimental data supports a mechanism of inhibition through molecular adsorption of the inhibitor ions on the surface of growing crystals. Based upon the kinetic data, the overall order of inhibitors effectiveness is[Formula: see text]
Iodination and deuteration of catechin derivatives
Authors: Kiehlmann E. , Lehto N. , Cherniwchan D.
Published: May 2006

The preparation and 1H and 13C nuclear magnetic resonance spectra of ten monoiodocatechin derivatives are described. The iodination of catechin with N-iodosuccinimide (NIS) takes place regiospecifically at C-6 in acetone but preferentially at C-8 in dimethylformamide; both products can be derivatized at oxygen in alkaline medium but lose iodine in the presence of acid. 3′,4′,5,7-Tetra-O-methylcatechin reacts with NIS regiospecifically at C-8 while catechin pentaacetate resists iodination. The debromination of 6-bromo- and 6,8-dibromocatechin with sodium sulfite in CD3CN/D2O is accompanied by H/D-exchange at C-6 and C-8. The synthesis of catechin pentaacetate from partially deuterated catechin and its conversion to tetra-O-methylcatechin proceed without H/D-exchange and permit distinction between the H-6 and H-8 chemical shifts.
The intermediates in the interaction of phenols with N-bromosuccinimide
Authors: Chow Yuan L. , Zhao Da-Chuan , Johansson Carl I.
Published: May 2006

Phenols react with N-bromosuccinimide to generate 2-bromocyclohexadienone-type intermediates that enolize to 2-bromophenols thermally or photolytically with a wide range of rate constants depending on the parent phenol. A trace amount of acids not only accelerates the rates of the formation and decomposition of the 2-bromodienones, it also promotes the rearrangement of the 2-bromodienones to cross-conjugated 4-bromocyclohexadienones. The enolization of these last compounds affords 4-bromophenols. The occurrence of autocatalysis in dienone decompositions arising from more acidic bromophenols is also established. The reaction is assumed to occur by a bromophilic attack of phenols on the N—Br bond to form unstable hypobromites that rapidly rearrange to the 2-bromocyclohexadienone intermediates. The quantum yield of the photodecomposition of 2-bromo-(2H)-naphthalene-1-one in the vicinity of 365 nm was determined to be unity.
A tensammetric study of polyacrylamides, including their determination
Authors: Smith-Palmer Truis , Wentzell Byron R. , Donini John C. , Jerrard Robert J.
Published: May 2006

Phase-sensitive tensammetry is shown to be a convenient and useful way to analyse polyacrylamides and associated derivatives (pams) in dilute aqueous solutions. Standard curves were obtained with ranges varying from 9 to 25 ppm and limits of detection varying from 0.7 to 2 ppm. Ways to change the range and sensitivity are discussed and demonstrated. Typical interferences and their effects are tabulated, and the analysis of pams in coal washings is discussed. The analysis of certain mixtures of polyacrylamides is shown to be possible.
Solvent effects in organic chemistry — recent developments
Authors: Abraham Michael H. , Grellier Priscilla L. , Abboud Jose-Luis M. , Doherty Ruth M. , Taft Robert W.
Published: May 2006

Solvent effects on a number of different processes have been surveyed, and results of the application of multiple linear regression analysis are discussed. The processes examined include examples of solubility of gases or vapours, distribution coefficients of solutes between water and a series of solvents, and solvent effects on conformational equilibria, on keto–enol tautomerism, and on reaction rates. It is shown that two particular equations, that due to Koppel and Palm and extended by Makitra and Pirig, and that due to Abraham, Kamlet, and Taft, can cope quite satisfactorily with solvent effects on these various processes. It is pointed out that interpretation of parameters obtained from equations that involve macroscopic quantities such as ΔG≠ or ΔG0 is not necessarily straightforward, and that some model is needed in order to interpret these macroscopic quantities in terms of microscopic quantities that can characterise, for example, solute–solvent interactions.
On chemical relaxation modes in homogeneous chemical systems
Authors: Palomares F. , Veguillas J. , Diaz M. A.
Published: May 2006

We study the compatibility between the linearized phenomenological equations of an homogeneous chemical gaseous system, and the theoretical ones obtained with the aid of the kinetic theory of gases. Our discussion is based on the concept of chemical relaxation mode. The theoretical description is provided by a reactive Boltzmann equation for each component of the system. Then, we show the conditions under which the linearized phenomenological equations can be reproduced.
Crenuladial, an antimicrobial diterpenoid from the brown alga Dilophus ligulatus
Authors: Tringali C. , Oriente G. , Piattelli M. , Geraci C. , Nicolosi G. , Breitmaier E.
Published: May 2006

From the brown alga Dilophus ligulatus a new secondary metabolite with antimicrobial activity, crenuladial 4, has been isolated and its structure determined.
A new dihydrofusarubin O-ethyl ether produced by the fungus Nectria haematococca (Berk, and Br.) Wr.
Authors: Barbier Michel , Devys Michel , Parisot Denise
Published: May 2006

A dihydrofusarubin O-ethyl ether from a strain of the fungus Nectria haematococca (Berk, and Br.) Wr. is reported. On the basis of spectrographic data, the stereochemistry 4 is proposed for the pyran ring of this new substance, which thus represents an isomeric form of the previously reported O-ethyl ether 3b.
Computational study of heterocyclic thione ligands. Part I. Five-membered heteroatomic thiones possessing an α-nitrogen heteroatom
Authors: Kapsomenos George S. , Akrivos Pericles D.
Published: May 2006

A SCF-MNDO study is made of the equilibrium between the "thiol" and "thione" tautomers of four species possessing a five-membered heterocyclic ring, where nitrogen and oxygen or sulfur are the α-heteroatoms. Based on the computationally derived Δ(ΔHf) values for each pair of forms, evidence is presented for the possibility of co-occurrence of the two forms in solution. The tautomerism is particularly interesting in this class of compounds, since it offers the possibility for a sulfur, nitrogen, or sulfur and nitrogen coordination. The nucleophilicity of each specific site in both forms is discussed in terms of net atomic charges and the HOMO constitution of the molecules. Results concerning the study of metal complexes are generally in accordance with these considerations, since the preference for a specific donation scheme is governed by an HSAB criterion.
Light-promoted catalytic hydrogenation of olefins with nickel complexes: the formation of nickel hydrides from the reaction of Ni(I) complexes with hydrogen
Authors: Chow Yuan L. , Li Huali , Yang Meng-Su
Published: May 2006

The conditions pertaining to triplet excited state ketone-sensitized photoreduction of Ni(acac)2 under hydrogen were shown to serve as a method for light-promoted hydrogenation of olefins with clean chemospecificity. For example, the double bond in the bicyclic system was preferentially hydrogenated over the other double bond in dicyclopentadiene and in 5-methylenebicyclo-[2.2.1]heptene. This photohydrogenation could be run under a wide range of conditions and was apparently a homogeneous catalytic process; in a late stage of photohydrogenation, concurrence of heterogeneous catalytic processes was not ruled out. The Ni(I) complexes of tetrahydrofuran and olefins generated from the photoreduction of Ni(acac)2 were shown to react with hydrogen to give nickel hydride complexes. By analogy with the generally accepted reaction pattern, coordinated olefins in these nickel hydride complexes probably spontaneously undergo intramolecular addition to give Ni–alkyl complexes. It is suggested that these Ni–alkyl complexes are photoexcited to generate a vacant coordination site so that the reaction with hydrogen can proceed to give products and regenerate a nickel hydride catalyst.
The chemistry of platinum hydrides. Part XXX. The chemical and structural effects of steric overcrowding in the compounds cis- and trans-H[R3Sn]Pt(PCy3)2, R = Ph, Cl
Authors: Clark Howard C. , Ferguson George , Hampden-Smith Mark J. , Ruegger Heinz , Ruhl Barbara L.
Published: May 2006

The reaction of Pt0(PCy3)2 with triphenyltin hydride yields cis-H(Ph3Sn)Pt(PCy3)2 as the initial product; isomerization to the trans isomer is accelerated by light, heat, and the presence of free phosphine. Both isomers have been characterized by multinuclear nmr spectroscopy; in particular, the 31P {1H} spectra in solution are correlated with the solid state CP/MAS 31P nmr spectra and the crystal structure. The crystal structures of trans-H(X3Sn)Pt(PCy3)2 X = Ph or Cl have been determined and the nature of the distortions produced by steric overcrowding is identified and discussed.
Comparison of muonium and positronium with hydrogen atoms in their reactions towards solutes containing amide and peptide linkages in water and micelle solutions
Authors: Barnabas Mary V. , Venkateswaran Krishnan , Walker David C.
Published: May 2006

Rate constants have been sought for the reaction of muonium (Mu) and o-positronium (Ps) with solutions of thirteen solutes containing [Formula: see text] the groups. Values of k range from <105 M−1 s−1 to 3 × 1010 M−1 s−1 and show a variety of trends. For instance, Mu adds across the carbonyl group much faster than does H, but abstracts from an adjacent methyl group more slowly. Mu adds exceptionally efficiently to the thiocarbonyl group. Abstraction reactions are identified by large enhancements in reaction rates when localized in micelles. Ps behaves quite differently to the others in neither abstracting nor adding to these compounds, consistent with it not being a pseudo-isotope of hydrogen. Keywords: muonium, positronium, hydrogen, hydrated electrons, micelles.