bluuebunny/crossref_metadata_2025 huggingface dataset:
A theoretical study of adiabatic proton transfer to simple substituted acetylenes In Their Ground And Excited States
Authors: Martin Peter S. , Yates Keith , Csizmadia Imre G.
Published: May 2006

RHF SCF 3-21G calculations are reported for the 1Σg+, 1A1, 3A′, and 1,3A″ states of simple substituted acetylenes (Y—C≡C—H, where Y = H, F, Cl,CH3, andCF3), the 1A1, 1A′, and 1.3A″ states of their Markovnikov (M) vinyl cations (Y—C+ = CH2), the 1A′ and 1.3A″ states of their anti-Markovnikov (aM) vinyl cations (YCH=C+H), and the corresponding hydrated vinyl cations. Equilibrium electronic structures and the mechanism of adiabatic protonation are described qualitatively via Lewis/resonance schematic representations of the species involved. Calculated proton affinities (PA) suggest that relative to ground state Y—C≡C—H (1Σ+/1A1), Y—C≡C—H* (1.3A″) is of greatly enhanced basicity with respect to protonation of both regiocenters. A graphical representation of the ordered pairs PA(M) versus PA(aM) as a function of substituent Y and electronic state, leads to the conclusions: (1) irrespective of both regiocenter (M/aM) and state (1Σ1+/1A1, 3A, 1.3A″) the PA's for Y—C≡C—H decrease in the order CH3 > H > Cl> F > CF3; (2) in proceeding from CH3C≡CH to CF3C≡CH, a change in protonation regiospecificity (M → aM) is experienced to approximately the same extent for both S0 and S1; (3) T2 exhibits no significant protonation regioselectivity. Critical comparison of the calculated results is made with available experimental data. An approximate picture of the energy profiles for the adiabatic hydrations of Y—C≡C—H via its ground, triplet and singlet states has been developed, based on the fixed points acetylene, vinyl cation and hydrated vinyl cation. Predicted relative reactivities of these three states are in excellent accord with available experimental data on rates of hydration. Keywords: excited states, proton transfer, photohydration.
An approach to the synthesis of bruceantin. The synthesis of a tetracyclic intermediate
Authors: Darvesh Sultan , Grant Andrew S. , Magee David I. , Valenta Zdenek
Published: May 2006

An intermediate (20) containing four of the five rings and seven of the ten chiral centers of bruceantin (1) was prepared. In a key reaction, 3-iodo-1-trimethylsilyl-5-hexen-1-yne (9) was found to add chemoselectively and stereoselectively to a dianion. The sequence also includes a selective attack by a sulfinate (17) on the terminal acetylene C-atom of the dianion of a keto acetylene, a concomitant cyclization in which the acetylene acts as an electrophile, and the use of the resulting allyl sulfoxide for the introduction of oxygen functionality by a reductive rearrangement. Keywords: bruceantin, sigmatropic rearrangement, stereoselective alkylation.
Total synthesis of 12-O-benzoyl-6,7,14,15-tetradehydro leukotriene B4 methyl ester: precursor of labeled LTB4
Authors: Pontikis Renée , Randrianasolo Lalatiana R. , Merrer Yves Le , Nam Nguyen Hoang , Azerad Robert , Depezay Jean-Claude
Published: May 2006

12-O-Benzoyl-6,7,14,15-tetradehydroleukotriene B4 methyl ester 1, the direct precursor of isotopically labeled LTB4's, has been prepared. This synthesis was accomplished by the assembly of two key chirons: α-hydroxyaldehyde of R configuration, 2, and propargylic alcohol of S configuration, (S)-7. The first was obtained from D-mannitol whereas the second was obtained by enzymatic resolution. Keywords: leukotriene B4, tetradehydroleukotriene B4, labeling, chiral α-hydroxyaldehyde, enzymatic resolution.
Frondoside A. A novel triterpene glycoside from the holothurian Cucumariafrondosa
Authors: Girard Michel , Bélanger Jacqueline , ApSimon John W. , Garneau François-Xavier , Harvey Christian , Brisson Jean-Robert
Published: May 2006

The isolation and characterization of a novel triterpene glycoside from the sea cucumber Cucumariafrondosa, collected from the Gulf of St. Lawrence, are described. Physico-chemical data obtained on the underivatized saponin are presented in support of structure 1 for frondoside A. The structure is characterized by the presence of a branched oligosaccharidic chain composed of five units. The holostane-type aglycone features an endocyclic double bond at position C-7,8 and a β-acetoxy group at C-16. Keywords: triterpene glycoside; Cucumariafrondosa saponins; saponins, triterpene from holothurians; holothurian saponins; echinoderm triterpene saponins.
Crystal structures of 7-azaindole, an unusual hydrogen-bonded tetramer, and of two of its methylmercury(II) complexes
Authors: Dufour Pascal , Dartiguenave Yves , Dartiguenave Michèle , Dufour Nathalie , Lebuis Anne-Marie , Bélanger-Gariépy Francine , Beauchamp André L.
Published: May 2006

Crystals of 7-azaindole ([Formula: see text], a = 11.312(4), b = 14.960(6), c = 15.509(5) Å, α = 102.86(3), β = 108.78(3), γ = 90.71(3)°, Z = 16, R = 0.052) contain tetrameric units of approximate S4 symmetry, in which the molecules are associated by means of four complementary N—H … N hydrogen bonds. [CH3Hg(7-azaindole)]NO3 ([Formula: see text], a = 7.818(3), b = 7.884(3), c = 9.135(4) Å, α = 97.89(3), β = 109.13(3), γ = 103.28(3)°, Z = 2, R = 0.039) contains well-separated nitrate ions and complex cations in which the methylmercury group is linearly bonded to the pyridine nitrogen atom, whereas the five-membered ring remains protonated. In the neutral [CH3Hg(azaindolate)] complex ([Formula: see text], a = 10.926(10), b = 11.333(8), c = 11.647(10) Å, α = 92.13(8), β = 104.83(9), γ = 111.86(7)°, Z = 6, R = 0.048), methylmercury groups have substituted the N—H proton in the five-membered ring for the three symmetry-independent molecules. Intermolecular secondary Hg … N bonds are found with pyridine nitrogens. Keywords: azaindole, methylmercury, crystal structure.
The microwave and far-infrared spectra and the geometry of 8-oxabicyclo[3.2.1]octane
Authors: Eggimann Thomas , Smithson Tracy L. , Wieser Hal , Lorenčak Primož , Bergquist Pamela , Badawi Hassan , Sibley Scott P. , Kuczkowski Robert L.
Published: May 2006

The structure of 8-oxabicyclo[3.2.1]octane was calculated by abinitio means at the STO-3G level, and the structural data used to assign the microwave spectrum of the title compound in the range of 26.5 to 40 GHz fitting 144 lines up to J = 48 by adjusting the rotational and three centrifugal distortion constants in the Watson A reduction. Structural parameters were deduced from the rotational constants by allowing all bond lengths and angles to vary in a least-squares manner. These parameters were in good agreement with those calculated at the STO-3G and 3-21G levels. Of many satellite lines observed, only the most intense could be utilized for locating the two lowest fundamental vibrations, estimated to occur at 190 and 240 cm−1. The far-infrared spectrum is reported in the region of 80 to 300 cm−1, exhibiting only one distinct sequence of Q branches at 264 cm−1. On the basis of unsealed abinitio force fields at both levels of the theory the corresponding fundamentals are assigned, respectively, to a twisting (A″) and a bending (A′) of the cycloheptane ring. Keywords: 8-oxabicyclo[3.2.1]octane, microwave and far-infrared spectra, abinitio STO-3G and 3-21G geometries.
Hydroxylation of prostanoids by fungi. Synthesis of (−)-15-deoxy-19-(R)-hydroxy-PGE1 and (−)-15-deoxy-18-(S)-hydroxy-PGE1
Authors: Holland Herbert L. , Brown Frances M. , Chenchaiah P. Chinna , Rao J. Appa
Published: May 2006

A series of racemic substituted cyclopentanones, with alkyl groups corresponding to the upper prostanoid side chain and (or) the lower prostanoid side chain without the C-15 alcohol, has been synthesized. Using a steroid template for the prostanoid molecule as a basis for selection, fungi capable of hydroxylating steroids have been used to biotransform the prostanoid substrates. The predominant products were hydroxylated at the prostanoid C-18 and C-19 positions. The hydroxylations were enantioselective, with excesses in the range 10–60%, and in most cases the predominant configuration corresponded to that of the natural prostanoids. The stereochemistry of the C-19 hydroxyl group was found to be R by degradation of products to methyl 6-acetoxyheptanoate and comparison of that material with a resolved sample, obtained via crystallization of the brucine salt of ethyl 6-phthaloxyheptanoate. Hydroxylation at C-18 gave the S configuration of alcohol. Hydroxylation at prostanoid C-15 was observed, but in all cases this was accompanied by other reactions. Hydroxylation of Rhizopusarrhizus has been used in a preparation of (−)-15-deoxy-19-(R)-hydroxy-PGE1 and (−)-15-deoxy-18-(S)-hydroxy-PGE1. Keywords: biotransformation, hydroxylation, prostaglandins, prostanoids.
Synthesis and characterization of a new crystalline tin(IV) arsenophosphate ion exchanger
Authors: Nath K. V. Surendra , Tandon S. N.
Published: May 2006

A new crystalline layered inorganic ion exchanger tin(IV) arsenophosphate with the formula Sn(HAsO4)(HPO4)•H2O has been prepared by refluxing the amorphous precursor. This exchanger has been characterized by X-ray powder pattern, chemical analysis, IR spectra, and thermal dehydration. Its ion exchange behaviour towards sodium ions is reported. The exchanger has an exchange capacity of 5.36 mequiv./g for Na+. The performance of tin(IV) arsenophosphate is compared with crystalline tin(IV) phosphate and arsenate. Keywords: cation exchanger, crystalline tin(IV) arsenophosphate, ion exchange, thermal behaviour, X-ray studies.
Excess volumes, isentropic compressibilities, and viscosities of binary mixtures containing cyclohexene
Authors: Sekar P. Raja , Venkateswarlu R. , Reddy Kalluru S.
Published: May 2006

Excess volumes, excess isentropic compressibilities, and excess viscosities have been reported for the binary liquid mixtures of cyclohexene with n-hexane, cyclohexane, benzene, trichloromethane, tetrachloromethane, and 1,4-dioxane at 303.15 K. VE results are negative for mixtures of cyclohexene with n-hexane and tetrachloromethane and are positive for the remaining systems. [Formula: see text] values are negative for mixtures of cyclohexene with n-hexane and positive for all other systems. The data of Δ ln η are positive for cyclohexene with cyclohexane and tetrachloromethane, and negative for the remaining systems. Prigogine–Patterson–Flory equation of state theory has been applied to predict excess volumes and excess enthalpies, and the viscosity relations proposed by Bloomfield are used to calculate free energy and free volume contributions to excess viscosity. Keywords: excess volumes, excess isentropic compressibilities, excess viscosities.
Synthesis and transannular Diels–Alder reaction of a 13-membered macrocyclic triene having a tetrasubstituted enol ether as dienophile
Authors: Bérubé Gervais , Deslongchamps Pierre
Published: May 2006

The syntheses of the acyclic triene trans–trans–cis27 and trans–trans–trans31 are described. Macrocyclization and concomitant transannular Diels–Alder reaction were performed with the chloride derivative obtained from the trans–trans–cis triene alcohol 27 yielding a mixture of the tricyclic compounds trans–syn–trans33 and cis–syn–cis34. On the other hand, macrocyclization of the chloride derived from trans–trans–trans triene 31 was not successful. Keywords: transannular process, Diels–Alder reaction, macrocyclic triene, macrocyclization, tricyclic compounds, organic synthesis.
Bromation régiosélective en série aromatique. II. Approche théorique du mécanisme de la substitution électrophile par l'ion tribromure Br3−
Authors: Berthelot Jacques , Guette Catherine , Desbène Paul-Louis , Basselier Jean-Jacques , Chaquin Patrick , Masure Daniel
Published: May 2006

A theoretical study (SCF PS-HONDO + CI) of the stability of the tribromide anion Br3− has been carried out. On the basis of qualitative MO arguments and MNDO calculations, we propose a first approach to the mechanism of electrophilic aromatic substitution by Br3−. It involves an axial attack of linear Br3−, the most electrophilic sites of which are terminal bromine atoms, although they bear a negative charge. This model suggests some arguments to explain the para versus ortho selectivity experimentally observed. Calculations performed on a series of MBr3 systems emphasize the importance of the countercation for the rate of the reaction as well as for its selectivity. Keywords: bromination, tribromide, mechanism, parabromophenol.
Redistribution reactions of alkoxy- and siloxysilanes, catalyzed by dimethyltitanocene
Authors: Xin Shixuan , Aitken Clare , Harrod John F. , Mu Ying , Samuel E.
Published: May 2006

Dimethyltitanocene is an excellent catalyst for the redistribution of alkoxy- and siloxyhydrosilanes. The redistribution reactions of triethoxysilane, diethoxymethylsilane, 1,3,5,7-tetramethylcyclotetrasiloxane, pentamethyldisiloxane, and H—(SiMe2O)n—SiMe2H (n = 1 to 3) are described. The mechanism of these reactions is discussed in terms of TiH mediated displacements. The possibility of both Ti(III) and Ti(IV) mediated displacements are considered and a mechanism involving the former, which fits all of the experimental data, is proposed. Keywords: siloxanes, alkoxysilanes, redistribution, catalysis, dimethyltitanocene.
Fluorination of sulfur-containing amino acids: Reaction of xenon difluoride with cysteine derivatives
Authors: Huang Xiaoling , Blackburn Barry J. , Au-Yeung Steve C. F. , Janzen Alexander F.
Published: May 2006

The reaction of XeF2 with cysteine derivatives RSCH2CH(NHR)COOR, i.e., S-benzyl-N-trifluoroacetyl-L-cysteine methyl ester, S-benzyl-N-carbobenzoxy-L-cysteine p-nitrophenyl ester, and S-methyl-N-trifluoroacetyl-L-cysteine methyl ester is described. The fluorinated products PhCHFSCH2CH(NHR)COOR, PhCH2SCHFCH(NHR)COOR, and FCH2SCH2CH(NHR)COOR were identified by 1H and 19F nuclear magnetic resonance spectroscopy. PhCH2SCHFCH(NHR)COOR loses HF within 24 hours to give Z- and E-PhCH2SCH=C(NHR)COOR. If the reaction with XeF2 is carried out in the presence of moisture the sulfoxide RS(O)CH2CH(NHR)COOR and sulfone RSO2CH2CH(NHR)COOR are produced. Keywords: fluorination, XeF2, amino acids, cysteine.
Enzymic hydroxylation and sulfoxidation of cyclopropyl compounds by fungal biotransformation
Authors: Holland Herbert L. , Chernishenko Michael J. , Conn Morgan , Munoz Anthony , Manoharan T. Samuel , Zawadski Michael A.
Published: May 2006

A series of compounds containing a cyclopropyl ring adjacent to the position of oxidation during fungal biotransformation has been prepared and subjected to enzymic oxidation using Mortierellaisabellina or Rhizopusarrhizus. Carbon hydroxylation and sulfoxidation reactions were observed, but in neither case did opening of the cyclopropyl ring occur. Both these reactions were subject to inhibition by carbon monoxide but not by cyanide ion, properties characteristic of cytochrome P-450 dependent mono-oxygenase enzymes. Hydroxylation at a cyclopropyl C—H bond has been studied by the use of phenylcyclopropane and phenylthiirane as substrates for the mono-oxygenase of M. isabellina. The former was not oxidized, but the latter was converted to phenyl glyoxylic acid in moderate yield. Keywords: biotransformation, cyclopropanes, fungi, metabolism.
The hemiacetal–cis-chalcone equilibrium of malvin, a natural anthocyanin
Authors: Brouillard Raymond , Lang Jacques
Published: May 2006

In slightly acidic aqueous solutions, malvin essentially exists, in a stable form, as a fast equilibrating reaction between a ring structure, the hemiacetal B, and a chain structure, the cis-chalcone CE. The temperature-jump technique has been used for the determination of the relaxation time τ characterizing this equilibrium. At 20 °C τ-values were found to be in the range 1s (for the more acidic solutions) to 2 × 10−4 s (for the neutral solutions). Both buffered and unbuffered solutions were investigated. It was found that τ is largely dependent upon pH and upon the concentration of the buffer. For instance, at constant pH, a plot of the observed first-order rate constant τ−1 versus the acetic acid – sodium acetate buffer concentration shows a nonlinear variation that can only be interpreted as a change in the proton transfer rate-limiting step with the buffer concentration. At 20 °C, the constant K = [CE]/[B] is 0.25 (±0.05) and the enthalpy change ΔH associated with this equilibrium is 20.7 (±2) Kj mol−1. Therefore, increasing the temperature greatly enhances the stability of the open cis-chalcone isomer. Possible implications arising from the existence of a fast chemical ring-closure reaction, in the case of natural anthocyanins, for the biosynthesis of flavonoids are discussed. In particular, it is suggested that, along with the main biosynthetic pathway to flavonoids, a minor pathway involving the anthocyanin cis-chalcone CE could also occur under favorable circumstances. Physico-chemical conditions in the plant organelles containing flavonoids (vacuoles) make this alternative plausible. Keywords: flavonoids, malvin, chalcone, catalysis, temperature jump.
Conformations and charge distributions in fluoronitrosoethylenes and nitrosoethylene
Authors: Sarasola Cecilia , Cossío Fernando P. , Ugalde Jesus M.
Published: May 2006

The optimized structures, bond orders, and electrostatic potentials for both transoid and cisoid fluoronitrosoethylenes and nitrosoethylene have been calculated by abinitio self-consistent-field molecular orbital procedure. The transoid structures are found to be of lower energy than the cisoids. Analyses of the calculated bond orders show that the C—C bond energy compares well with that of ethylene and the C—N bond energy with that of ethylamine, and therefore the contributions from ionic resonance structures are predicted to be small. The calculated electrostatic potential contour maps show that the presence of the nitroso group completely eliminates the negative potential region associated with the C—C double bond, thus rendering it less susceptible to electrophilic attack. Analyses of the values and relative locations of positive electrostatic potentials correctly predict that the nucleophilic addition to β-carbon is favoured when in the transoid conformation. Keywords: conformation, charge distribution, fluoronitrosoethylenes, nitrosoethylenes.
Cycloaddition reactions of dichlorodithionitronium hexafluoroarsenate(V) with alkynes and alkenes; the preparation and X-ray crystal structure of 1,3-dichloro-1,3,2-dithiazolidinium hexafluoroarsenate(V)
Authors: Parsons Simon , Passmore Jack , Schriver Melbourne J. , White Peter S.
Published: May 2006

The dichlorodithionitronium cation, ClSNSCl+, undergoes symmetry-allowed, concerted, cycloaddition reactions with alkynes and alkenes in liquid sulphur dioxide. With ethyne and propyne it gives corresponding 1,3,2-dithiazolium salts in quantitative yield. Cycloaddition with alkenes yields (hitherto unknown) 1,3-dichloro-1,3,2-dithiazolidinium cations. The reactions are sensitive to substituents, and only the cycloaddition with ethene occurs quantitatively on a preparative scale. Cycloadditions with propyne and E- and Z-2-butene have been observed in solution. The preparation, characterization, and X-ray crystal structure of the product of the reaction of CISNSCl+ with ethene, 1,3-dichloro-1,3,2-dithiazolidinium hexafluoroarsenate(V), are reported. Crystals of 1,3-dichloro-1,3,2-dithiazolidinium hexafluoroarsenate(V) are monoclinic, of space group P21/c, a = 6.3161(6) Å, b = 17.1724(23) Å, c = 19.1558(18) Å, and β = 98.143(8)°; Z = 4 and R = 0.061. The 1,3-dichloro-1,3,2-dithiazolidinium cation consists of a five-membered [Formula: see text]ring, axially substituted at the sulphurs by chlorine (average bond lengths: S—N 1.603(8) Å, S—Cl 2.025(4) Å, S—C 1.822(10) Å, and C—C 1.504(13) Å). Two crystallographically unique cations are linked by weak S..N contacts into pseudo-dimers. These data are used, together with structural comparisons with related SNS-containing cations and the sulphoxides and the 14N chemical shift, to show that the bonding in the SNS region of the cation is best represented by the valence canonicals [Formula: see text] and [Formula: see text], such that the sulphur and nitrogen atoms respectively carry absolute positive and negative charges. This picture is also supported by the fluoride contacts to the cation from the AsF6−anion, which are exclusively to sulphur. Nuclear magnetic resonance data were consistent with the retention of the solid state structure of [Formula: see text] in solution. Keywords: sulphur-nitrogen chloride, sulphur-nitrogen cations, 1,3-dichloro-1,3,2-dithiazolidinium, cycloaddition reactions, X-ray crystallography.
Intrinsic gramicidin fluorescence lifetimes in bilayer lipid membranes and in vesicles
Authors: Camaleńo-Delgado Jose-Miguel , Zhao Xiao Kang , Fendler Janos H.
Published: May 2006

Intrinsic Gramicidin A′ tryptophan steady-state fluorescence anisotropies and fluorescence lifetimes have been determined in bilayer lipid membranes (BLMs) prepared from glyceryl monooleate (GMO). In GMO BLMs, fluorescence anisotropy, the r value, was found to be 0.05 ± 0.02. Decays of Gramicidin A′ fluorescence intensities were fitted to the sum of three exponentials (τ1, τ2, and τ3) and appropriate pre-exponentials (A1, A2, and A3). These values allowed for the assessment of average fluorescence lifetimes, [Formula: see text]. These values related to those determined in vesicles prepared from dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine (DOPC), distearoylphosphatidylcholine (DSPC), and diphytanoylphosphatidylcholine (DPhPC). In BLMs, [Formula: see text], 3.6 ns, and 2.3 ns were determined for vertically, horizontally, and unpolarized average fluorescence lifetimes, respectively. Increasing the applied potential across the BLM from 0 to 80 mV increased [Formula: see text] from 2.2 ns to 4.9 ns and τ1 from 0.43 ± 0.05 to 0.73 ± 0.06 ns, as well as the contributions and lifetimes of the longer lived fluorescence (A2 and A3, τ2 and τ3). The emission maximum of Gramicidin A′ (334 nm in DPPC) and the absence of quenching by iodide ions indicated complete incorporation of the polypeptide into vesicles. The r values were of the order of 0.10 in vesicles prepared from DPPC and DSPC, both in the absence and in the presence of added 1.2 × 10−4 M CsCl. In vesicles prepared from DOPC and DPhPC, r values increased to 0.13 and 0.14 in water and to 0.15 and 0.20 in 1.2 × 10−4 M CsCl, respectively. At 25.0 °C, the temperature of the measurements, DPPC and DSPC are in their "solid" states, but DOPC and DPhPC are in their "liquid" states, [Formula: see text] values for Gramicidin A′ in vesicles prepared from DPPC, DOPC, and DSPC were all in the 3.0 ± 0.3 ns range. In DPhPC vesicles, [Formula: see text] was determined. Time-dependent anisotropics became observable in DOPC and DPhPC vesicles, particularly in the presence of 1.2 × 10−4 M CsCl. Keywords: gramicidin, fluorescence lifetimes, vesicles, bilayer lipid membranes, time-dependent anisotropics.
Syntheses of model oligosaccharides of biological significance. X.•Syntheses and NMR and mass spectral analysis of trideuteriomethyl di-3,6-O-(4-O-β-D-galactopyranosyl-2-acetamido-2-deoxy-β-D-glucopyranosyl)-β-D-galactopyranoside: the I antigen branchpoint penta- and tetrasaccharides and a related trisaccharide
Authors: Whitfield Dennis M. , Pang Henrianna , Carver Jeremy P. , Krepinsky Jiri J.
Published: May 2006

As part of our studies of complex oligosaccharides, in particular their three-dimensional structure, we have synthesized the antigen I branchpoint penta- and tetrasaccharides. The unbranched trisaccharide 3D was also synthesized, and its derivative 3B served as the intermediate for the synthesis of higher oligosaccharides. NMR spectra of major intermediates as well as of the final oligosaccharides were completely assigned. Mass spectra of the synthetic intermediates and the final oligosaccharides were analyzed and compared with those of a similar group of oligosaccharides containing L-fucose, N-acetyl-D-glucosamine, and D-galactose. Certain observations were made that could be utilized in the interpretation of mass spectra for the structural determination of protected oligosaccharides during the synthesis. Keywords: oligosaccharide synthesis, I antigen, carbohydrate mass spectrometry, carbohydrate NMR spectrometry, Gal-GlcNAc oligomers.
Resolution of β-hydroxy-α-amino acids by the action of proteases on their N-acyl methyl esters
Authors: Chênevert Robert , Létourneau Martin , Thiboutot Sonia
Published: May 2006

Methyl N-acetyl phenylserinates (1, 2), methyl N-acetyl nitrophenylserinates (3, 4), and methyl N-benzoyl threoninates (5, 6) were resolved conveniently into the enantiomers with high optical purities by enzymatic hydrolysis in the presence of α-chymotrypsin, subtilisin, or bromelain. The threo and erythro forms were treated separately and the latter form was usually more reactive. Chymotrypsin and subtilisin are highly specific for the enantiomer having the same configuration on carbon-2 (S) as the natural standard amino acids. In contrast, bromelain shows the reverse specificity on phenylserine derivatives. This is a notable exception to the usual S-stereospecificity pattern of proteases. Keywords: enzymatic hydrolysis, resolution of amino acids.
Structural characterization, by two-dimensional NMR spectroscopy and fast-atom-bombardment mass spectrometry, of a highly acylated trirhamnoside from Mezzettialeptopoda (Annonaceae)
Authors: Powell Donald A. , York William S. , Halbeek Herman van , Etse Joseph T. , Gray Alexander I. , Waterman Peter G.
Published: May 2006

The structure of Mezzettiaside 3, a highly acylated trisaccharide obtained from the stem bark of Mezzettialeptopoda, was determined by a combination of double-quantum-filtered {1H,1H} correlation spectroscopy, two-dimensional homonuclear Hartmann–Hahn, and 1H-detected {1H,13C} one-bond and multiple-bond shift correlation nuclear magnetic resonance experiments, in conjunction with fast-atom-bombardment mass spectrometry. Mezzettiaside 3 was found to be the α-(n-octyl)glycoside of 3,4-di-O-acetyl-L-rhamnopyranosyl-α(1 → 3)-2,4-di-O-acetyl-L-rhamnopyranosyl-α(1 → 3)-4-O-hexanoyl-L-rhamnopyranose. The heteronuclear multiple-bond correlation NMR technique proved to be invaluable in establishing the location of the O-acyl groups in this complex carbohydrate. Keywords: oligosaccharide, HMBC, HMQC, HOHAHA, FAB-MS.
Relative reactivity of three oxygen lone pairs of an α-nucleophile in SN2 reactions
Authors: Wolk J. L. , Hajnal M. R. , Hoz S. , Tarkka R. M. , Buncel E.
Published: May 2006

The SN2 reaction of H2C=NO− with CH3Cl was investigated at the AM1, STO-3G, and 6-31 + G* levels. Three modes of approach of CH3Cl to the oxygen of the nucleophile were examined: syn and anti to the lone pair on the nitrogen in the molecular plane, and an approach along the π system of the oximate anion. The AM1 method differed markedly from the two abinitio methods, both in the reactivity order of the three geometries as well as in the relative stabilities of the products. The reactivity order calculated at the STO-3G and the 6-31 + G* levels was syn > π > anti (AM1 order: anti > syn > π). Product stability order calculated by the two abinitio methods was syn > anti > π (AMI order: anti > syn > π). Excellent agreement was obtained between percent bond elongations at the transition state predicted by the equation suggested by Shaik, Schlegel, and Wolfe, and the activation barriers calculated at the 6-31 + G* level. The relevance of the calculations with respect to the α-effect is also discussed. Keywords: oxygen lone pairs, aximate, SN2, abinitio.
Photocyclisations de cétoacétals aromatiques en vue d'une approche synthétique de la crombénine
Authors: Cottet Françoise , Cottier Louis , Descotes Gérard
Published: May 2006

Crombenin 1 is a spiropeltogynoid compound with a spiroacetal moiety. A similar structure was synthesized starting from an orthocarbonylphenoxyisochroman by a Norrish type II photocyclisation. With an isochroman compound, the aromatic ring induces benzyl radical formation to give chiefly a bicyclic [4.3.1] derivative. However, with the introduction of electron-withdrawing or bulky substituents at C-4′, this side reaction is limited and the spiranic product becomes preponderant. Keywords: acetals, isochroman, photocyclisation, spiroacetals.
Dodecylammonium-glutamate Schiff bases of retinal
Authors: Singh Anil K. , Sandorfy Camille , Fendler Janos H.
Published: May 2006

Dodecylammonium glutamate (4) has been prepared from dodecylamine (2) and l-glutamic acid (3) in ethanol and in aqueous ethanol. The obtained crystalline glutamic acid salt of dodecylamine (4) has been characterized by absorption, FTIR, 1H NMR, 13C NMR, and mass spectroscopy. Reaction of all-trans retinal (5) with the α-amino group of 4 gave a Schiff base (6), which was protonated by the α-COOH group of the glutamyl residue. However, the protonated Schiff base underwent acid-catalyzed hydrolysis to 2 + 3 + 5. Some of 5 reacted with 2 to give the corresponding Schiff base. The proposed structures have been substantiated by insitu absorption, FTIR, and 1H NMR spectroscopy. Keywords: retinal Schiff base, retinal, rhodopsin model, dodecylammonium glutamate, Schiff-base protonation.
Thermal diffusion of weak electrolytes: aqueous phosphoric and iodic acids
Authors: Hui Lü , Leaist Derek G.
Published: May 2006

Soret coefficients of aqueous phosphoric acid and aqueous iodic acid are determined conductimetrically at 25 °C. As the molality of phosphoric acid drops from 0.05 toward 0.00 mol kg−1, the enthalpy of transport jumps from 2 to 15 kJ mol−1. For iodic acid, a stronger electrolyte, the corresponding increase is smaller, from 12 to 17 kJ mol−1. Equations are developed for the enthalpy of transport of 1:1 weak electrolytes. The equations are used to evaluate the reactive enthalpy of transport and the intrinsic enthalpies of transport of the ionized and molecular forms of phosphoric and iodic acids. Equating the enthalpies of transport of the acid molecules and the acid anions provides estimates of single-ion enthalpies of transport. Thermal diffusion measurements are reported for potassium dihydrogen phosphate and potassium iodate to help interpret the results. Keywords: thermal diffusion, enthalpy of transport, Soret coefficients, weak electrolytes, phosphoric acid, iodic acid.
1H and 13C NMR studies of the conformational mobility of 1,2-dimethoxybenzene in solution
Authors: Schaefer Ted , Sebastian Rudy , Lemire Alberta , Penner Glenn H.
Published: May 2006

1H NMR and 13C NMR spectral data are presented for 1,2-dimethoxybenzene-α,α′-13C, as well as for a number of other anisole derivatives. For the title molecule, the coupling constants between the 13C nucleus in the side chain and the para ring proton or 13C nucleus in the benzene ring show that the expectation value of sin2 θ is very near 0.2 at 300 K, where θ is the angle by which the methoxy groups twist out of the aromatic plane. This value of [Formula: see text] is much larger than that of 0.05 for anisole in solution, emphasizing the greater conformational mobility of the methoxy groups in 1,2-dimethoxybenzene (DMB). Long-range coupling constants between the methyl and ring protons in DMB are also discussed and compared with those in anisole and some of its derivatives. The preponderance of conformations with large values of θ, thought to occur in the vapor, disappears in solution at ambient temperatures. Under these conditions, the molecule is perhaps best described as preferring a planar conformation in which, however, the methoxy groups undergo excursions in θ whose average value is near 25°. Keywords: 1,2-dimethoxybenzene, conformational behaviour, internal mobility, long-range coupling constants, 1H and 13C NMR.
The homogeneous thermal conversion of methane to higher hydrocarbons in the presence of ethylene
Authors: Bossard Alain R. , Back Margaret H.
Published: May 2006

The pyrolysis of mixtures of methane and ethylene has been studied over the temperature range 774–853 K at total pressures below atmospheric to explore the effectiveness of ethylene as a catalyst for conversion of methane to higher hydrocarbons. Pyrolysis of ethylene alone at these temperatures yields ethane, propylene, butene-1, and butadiene as the major volatile products. In the lower temperature region the addition of methane increased the rate of production of ethane through the chain propagation reaction,[Formula: see text]thus effecting the conversion of methane to ethane. The rates of formation of butene-1 and butadiene were not appreciably altered and that of propylene was increased only at low pressures of ethylene. At higher temperatures methane caused a substantial increase in the rate of production of propylene, the result of the increasing importance of the methyl radical in the propagation reactions. Thus ethylene can efficiently convert methane to ethane or to propylene depending on the temperature of the reaction. Addition of small quantities of butene-1, which acted as a secondary initiator, to the mixtures increased considerably the overall rate of the conversion of methane. A mechanism is discussed which accounts for the main features of the reactions. Keywords: methane, ethylene, kinetics, pyrolysis, catalysis.
Sulfonyl esters. 2. CS cleavage in some substitution reactions of nitrobenzenesulfonates
Authors: Baum James Clayton , Bolhassan Jimat , Langler Richard Francis , Pujol Paul Joseph , Raheja Raj Kumar
Published: May 2006

An attempt to explore aromatic sulfonate esters as agents for the condensation of alcohols with mercaptans revealed an unusual process for sulfonate esters: CS bond rupture. Two mechanistic possibilities for CS bond rupture are explored: (i) radical anion intermediacy via single electron transfer and (ii) nucleophilic aromatic substitution. Both experiments and molecular orbital computations are presented to support the conclusion that nucleophilic aromatic substitutions are occurring. Keywords: sulfonyl esters, nitrobenzenesulfonates, CS bond rupture.
Thione–thiol tautomerism and stability of 2- and 4-mercaptopyridines and 2-mercaptopyrimidines
Authors: Stoyanov Stefan , Petkov Ivan , Antonov Liudmil , Stoyanova Tatyana , Karagiannidis Petros , Aslanidis Paraskevas
Published: May 2006

The possible thione–thiol tautomerism of 2- and 4-mercaptopyridines, 2-mercaptopyrimidine, and 4,6-dimethyl-2-mercaptopyrimidine in solution is studied by means of absorption (UV–VIS) spectroscopy. In accordance with earlier observations, polar solvents and self-association shift the apparent tautomeric equilibrium significantly towards the thione form. In dilute solutions of nonpolar solvents the thiol form predominates. On standing, significant changes are observed in the absorption spectra of these tautomeric compounds in ethanol, dioxane, and water. The time course of the tautomerization, followed spectrophotometrically, reveals quantitative transformation of the thiol form to the corresponding symmetrical disulfides. The influence of concentration, temperature, and irradiation with indirect sunlight are discussed. This thione–disulfide process is reversible in water, starting either from the tautomeric thione or from its symmetrical disulfide, implying a possible importance in biological systems. Keywords: thione–thiol tautomerism, absorption spectra, thiol–disulfide oxidation, mercaptopyridines and pyrimidines.
Kinetics and mechanism of oxidation of iodide ion by the molybdenum (VI) – hydrogen peroxide system
Authors: Arias Conchita , Mata Fernando , Perez-Benito Joaquin F.
Published: May 2006

The kinetics of oxidation of potassium iodide by hydrogen peroxide in aqueous perchloric acid has been studied both in the absence and in the presence of sodium molybdate by means of the initial-rates method. The law found for the total initial reaction rate is[Formula: see text]The activation energies associated with rate constants k1, k2, and k3 are 52 ± 1, 49 ± 1, and 42 ± 3 kJ mol−1, respectively. A mechanism in agreement with the experimental kinetic data is proposed, according to which rate constants k1, k2, and k3 correspond to the oxidations of iodide ion by H2O2, H3O2+ and H2MoO5, respectively. Keywords: catalysis, hydrogen peroxide, iodide ion, kinetics, molybdate ion.
Density effects on ion mobilities in electron attaching fluids: CS2, SF6, and C6F6
Authors: Gee Norman , Ramanan G. , Freeman Gordon R.
Published: May 2006

Ion mobilities μ were measured at densities n ranging from those of the low density gas (n < 0.01 nc, where nc = critical fluid density) through the dense gas and low-density liquid transition region (~0.5nc to 2nc) to the normal liquid (~3nc). The mobility at a given density was constant over a 10-fold variation of electric field strength, typically in the range 0.1 < E(MV/m) < 5. In the gas phase changes in mobility were dominated by changes in gas density. Comparison with neutral molecule transport in SF6 was used to illustrate the change in the effect of the ion on its local environment from that of molecular cluster formation to that of liquid electrostriction as density was increased. In the liquid, changes in ion mobility are dominated by changes in free volume. Mobilities in the present liquids could be described without reference to the activation energies suggested by the Arrhenius model. Keywords: ion, mobility, fluid, density, electron attaching.
Liquid–liquid equilibria for the water + ethanol + 2-ethyl-1-hexanol ternary system at several temperatures
Authors: Solimo Horacio N.
Published: May 2006

Liquid–liquid equilibrium data for the system water + ethanol + 2-ethyl-1-hexanol were obtained at 298.15, 308.15, and 318.15 K. Data for the binodal curves have been determined by cloud-point method and conjugate points on tie-line were obtained by correlating the refractive index of the binodal curves as a function of composition. The tie-line data at each temperatures were satisfactorily correlated using the method of Othmer and Tobias and the plait points coordinates were estimated. The experimental data were also fitted with the UNIFAC group contribution method for the activity coefficients using the isoactivity conditions as restraint equations. From the experimental results, ethanol extraction by 2-ethyl-1-hexanol appears not to depend on temperature and 2-ethyl-1-hexanol could be selected as a promissory solvent for extraction of ethanol from aqueous solutions. Keywords: liquid–liquid equilibria, ethanol extraction, water + ethanol + 2-ethyl-1-hexanol ternary system.
Optically pure (S)-cyclopent-2-en-1-ol and (S)-3-methoxycyclopentene
Authors: Wahhab Amal , Tavares Donald F. , Rauk Arvi
Published: May 2006

Optically pure (S)-cyclopent-2-en-1-ol, [Formula: see text] (c 1.1, CHCl3), and (S)-3-methoxycyclopentene, [Formula: see text] (c 3.86, n-hexane), were prepared from ethyl (1R,2S)-2-hydroxycyclopentanecarboxylate, which is readily available by yeast reduction of ethyl 2-oxocyclopentanecarboxylate. The optical purity of the alcohol was determined from its Mosher ester, and that of the ether by gas chromatography using a Ni-4-Pin column. Keywords: (S)-cyclopent-2-en-1-ol, (S)-3-methoxycyclopentene.
Concertedness and E2 elimination reactions: prediction of transition state position and reaction rates using two-dimensional reaction surfaces based on quadratic and quartic approximations
Authors: Guthrie J. Peter
Published: May 2006

An analytical expression for the two-dimensional reaction surface appropriate for discussing a concerted reaction has been derived for the quartic approximation to a reaction coordinate, and shown to be consistent with Marcus theory. By analysis of literature data for SN1 solvolyses and carbanion formation we have been able to reproduce the qualitative behavior of eliminations involving tBuBr, iPrBr, and EtBr in EtOH/EtO−, and to predict the rates of these reactions within the uncertainties of the input data. Keywords: concerted reactions, Marcus theory, eliminations, reaction surfaces.
β-Ureido acids and dihydrouracils. The kinetics and mechanism of the reversible ring closure of 3-(3'-methylureido)-propanoic acid and 3-(3-phenylureido)-2-methylpropanoic acid in sulfuric acid solutions
Authors: Rachina Vetka , Blagoeva Iva B. , Pojarlieff Ivan G. , Yates Keith
Published: May 2006

The pKBH+ values for protonation of the ureido groups of the title acids, and for protonation at the 2-carbonyl groups of the product hexahydropyrimidine-2,4-diones, have been determined by standard UV and NMR methods. A bell-shaped dependence of the observed rate constants on H2SO4 concentration was found for both the ring-closure and ring-opening directions. This medium dependence is satisfactorily reproduced by means of rate equations based on the excess acidity treatment, providing account is taken of unproductive protonation equilibria. The derived slope parameters (m≠) for the ring-closure direction indicate that the slow step involves proton transfer from nitrogen rather than to oxygen. For the ring-opening or hydrolysis reactions, the corresponding slope parameters (m≠m*) are characteristic of similar reactions known to proceed with slow C—N scission concerted with proton transfer. Keywords: mechanism, hydrolysis, cyclization, acid catalysis.
Observations on cation-binding characteristics and nuclear magnetic resonance spectra of the N-carboxymethyl and N-acetyl analogs of heparin
Authors: Rej Rabindra N. , Holme Kevin R. , Perlin Arthurs. S.
Published: May 2006

Based on the observed paramagnetic relaxation effects of Cu2+ and Fe3+ ions and the chemical shift displacements induced by Ca2+ ions, nuclear resonance spectroscopy has been used to compare cation-binding characteristics of heparin with those of its N-carboxymethyl and N-acetyl analogs and related polyanions. Heparin is unique in exhibiting a high affinity for all three cations. Among the other polymers, binding selectivities differ notably. Such factors as the incidence and distribution of anionic charges are discussed. The preparation of the N-carboxymethyl analog of heparin is described, and 1H and 13C nuclear magnetic resonance spectroscopic data are reported for it, as well as for the N-acetyl analog. Keywords: heparin, N-carboxymethyl analog, N-acetyl analog, cation binding, ion-selectivity, anionic charge distribution.
Acid-catalyzed hydrolysis of tetramethoxyethene in aqueous solution. Initial state stabilization by the methoxy groups
Authors: Kresge A. J. , Leibovitch M. , Kopecky K. R.
Published: May 2006

The acid-catalyzed hydrolysis of tetramethoxyethene to methyl dimethoxyacetate in aqueous solution at 25 °C was found to occur with the hydronium-ion catalytic coefficient [Formula: see text], to give the solvent isotope effect [Formula: see text], and to provide a Brønsted relation based upon six carboxylic acids with the exponent α = 0.42. These data indicate that the reaction proceeds via rate-determining proton transfer from the catalyzing acid to an olefinic carbon atom of the substrate. They also show tetramethoxyethene to be 1.0 × 106 times less reactive than 1,1-dimethoxyethene (ketene dimethyl acetal), a rate retardation 600 times greater than that expected from initial state stabilization by the two additional methoxy groups in tetramethoxyethene; possible causes of this disparity are discussed. Keywords: tetramethoxyethene, carbon–carbon double bond reactivity, ketene acetal, vinyl ether hydrolysis.
Structural studies of organoboron compounds. XXXIX.1 1,4-Bis(2,2-diethoxyethyl)-6,8-diphenyl-5,7,9-trioxa-4-aza-1-azonia-8- bora-6-boratabicyclo[4.3.0]nonane
Authors: Amt Henning , Kliegel Wolfgang , Rettig Steven J. , Trotter James
Published: May 2006

Reaction of N,N′-1,2-ethanediylidenebis(2,2-diethoxyethanamine)- N,N′-dioxide and phenylboronic acid yields the title compound. Crystals of 1,4-bis(2,2-diethoxyethyl)-6,8-diphenyl-5,7,9-trioxa-4-aza-1-azonia-8-bora-6-boratabicyclo[4.3.0]-nonane are triclinic, a = 12.625(2), b = 13.962(2), c = 9.273(1) Å, α = 104.81(1), β = 98.61(1), γ = 106.52(1)°, 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 = 0.058 and Rw = 0.081 for 3677 reflections with I ≥ 3σ(I). The molecule has a bicyclo[4.3.0]nonane structure containing one transannular N—B bond. Important bond lengths are sp3-bomn, N—B = 1.759(3), O—B = 1.435(3) and 1.454(3), C—B = 1.579(4); sp2-boron, O—B = 1.335(3) and 1.404(3), C—B = 1.557(4) Å. The (sp3)B—C(phenyl) bond is the shortest yet reported for this type of compound.Keywords: crystal structure, boron compound, organoboron compound.
A study on the backbone/side-chain interaction in N-formyl-(L)serineamide
Authors: Perczel Andrås , Daudel Raymond , Ångyån Jånos G. , Csizmadia Imre G.
Published: May 2006

The N-formyl-serineamide (For-Ser-NH2), a model diamide for the conformational behaviour of the protein backbone at serine residue, has been gradient optimized for selected conformations at the abinitio 3-21G level. Previous molecular mechanics ECEPP/2 calculations suggested that the optimal side-chain (χ1, χ2) and backbone [Formula: see text] conformations are strongly coupled, due to intramolecular H-bonding formed between the side-chain hydroxyl group and the amide moiety. Four different side-chain geometries (I–IV) were considered at each of the four backbone conformations (α, β, β′, γ) giving a total 16 different critical points on the coupled [Formula: see text] surface. The very fact that upon changing the [Formula: see text] values the global minimum of (χ1, χ2) surface was shifted indicated clearly that the two subspaces are strongly coupled. The present theoretical results were compared to experimental peptide and protein geometries taken from the Cambridge Structure Database and from the Brookhaven Protein Data Bank, respectively. Keywords: conformation of a serinediamide, backbone side-chain interactions, topological analysis of potential energy surfaces, abinitio calculations.
A study of the observable axial- and equatorial-2,4-dimethyl-2-adamantyl cations. Further examples of equilibrating nonclassical structures in "ordinary" tertiary carbocations
Authors: Buffam D. J. , Sorensen T. S. , Whitworth S. M.
Published: May 2006

The title cations have been prepared insitu from the corresponding tertiary alcohols, using three different superacid systems. The 13C NMR spectra were measured at a number of different temperatures, using the structurally related 13C peaks of internal 2-methyl-2-adamantyl cation as a reference. Using this protocol, one finds that the 13C chemical shifts of the four β carbons are quite temperature dependent, two of them shifting to higher field and the other two on the opposite face shifting to lower field, or vice versa, as the temperature is increased or decreased. This behavior is uniquely ascribed to a rapid equilibration between two "structures" for each cation system. The two structures in question involve selective C–C hyperconjugative delocalization on one face or the other of the 2-adamantyl skeleton. In the case of the 4-equatorial-methyl isomer, a structure with C–C hyperconjugation on the methyl-substituted face is most stable, while for the 4-axial-methyl cation, the structure with delocalization from the face opposite the substituent is favored. The former preference is regarded as stereoelectronic in origin while the latter is ascribed to steric effects. Keywords: non-planar carbocations, temperature dependent NMR shifts, stereoelectronic effects.
Vinyl ether hydrolysis. XXIII. Solvent isotope effect on the reaction of divinyl ether catalyzed by the hydronium ion
Authors: Kresge A. J. , Leibovitch M.
Published: May 2006

Rates of hydrolysis of divinyl ether (CH2=CHOCH=CH2), measured in dilute H2O and D2O solutions of perchloric acid at 25 °C, provide the catalytic coefficients kH+ = 0.0084 M−1 s−1 and kD+ = 0.0028 M−1 s−1, and these lead to the isotope effect kH/kD = 3.0. The magnitude of this isotope effect indicates that this reaction occurs by rate-determining hydron transfer from catalyst to substrate and thus follows the conventional mechanism for vinyl ether hydrolysis. Keywords: divinyl ether, vinyl ether hydrolysis, solvent isotope effect.