bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical studies of peptidic structures. Conformation of the tetrapeptide N-acetyl-Asp-Glu-Lys-Ser-NH-CH3
Authors: Thornton S. E. , Fraga S.
Published: May 2006

The peptide Asp-Glu-Lys-Ser has been studied using the software package ALTA:maPS, with simulation of the pH conditions and the influence of the solvent, in order to investigate the possible stabilization of its structure by a hydrogen bond and a salt-bridge. The theoretical results confirm the existence of a salt-bridge, between the side chains of the Glu and Lys residues, in aqueous solution at neutral pH but not at acid pH, in agreement with the observation from NMR studies. On the other hand the calculations indicate that no hydrogen bond exists in aqueous solution at either pH. The fact that an in-vacuum simulation shows the existence of a hydrogen bond between the peptidic CO group of the Asp residue and the peptidic NH group of the Ser residue suggests that a molecular dynamics study might confirm that a small percentage (less than 25%) of the peptide molecules may exist as an H-bonded conformer, as postulated from the NMR results. Key words: peptides, conformation, theoretical prediction.
Metallometallation of allenes: an exploratory study
Authors: Singh Shankar M. , Oehlshlager Allan C.
Published: May 2006

Reaction of either n-Bu3SnCu(CN)Li (8) or (n-Bu3Sn)2Cu(CN)Li2 (9) with 1,2-undecadiene gave 71–92% of a (Z/E) mixture of 1-tri-n-butylstannyl-2-undecene (27) accompanied by 8–32% of a mixture of (E)-2-tri-n-butylstannyl-2-undecene (28) and 2-tri-n-butylstannyl-1-undecene (29). When the above reactions were warmed to 0 °C, 28 and 29 were the major products. The mixed cuprates, 10–14, gave the mixtures rich in vinylstannanes, 28 and 29, along with 27. Addition of Sn—Al reagents, 15 and 16, or Sn—B reagents, 18 and 19, to 1,2-undecadiene in the presence of 5 mol% of CuCN or CoCl2•(PPh3)2 provided mixtures rich in vinylstannanes, 28 and 29. However, with 1 equiv. of CuCN, mixtures rich in allylstannanes 27 were obtained. Reaction of 5-hydroxy-1,2-pentadiene with the Sn—Al reagents, 15 and 16, gave 53–61% of a single 5-hydroxy-2-tri-n-butylstannyl-1-pentene (32) in the presence of 5 mol% of CoCl2•(PPh3)2. The stannylcopper adducts generated in the reaction of 1,2-undecadiene and 8 or 9 were treated with iodomethane to give 55–78% of a (Z/E: 67/33) mixture of 1-tri-n-butylstannyl-2-methyl-2-undecene (33). Addition of mixed silylcuprates, 21, 22, and 23, to 1,2-undecadiene gave mixtures rich in (E)-2-dimethylphenylsilyl-2-undecene (35) and 2-dimethylphenylsilyl-1-undecene (36). A rationale for the putative adducts' formation is also presented. Key words: Sn—Cu, Sn—Al, Sn—B reagents; putative adducts.
Electrochemically oxidized carbon fibres as an adsorbent for the attachment of dissolved substances. Adsorption of nitro compounds and ion-exchange of heavy metals
Authors: Theodoridou E. , Jannakoudakis A. D. , Jannakoudakis P. D. , Antoniadou S.
Published: May 2006

The adsorption of several aromatic nitro compounds and the ion-exchange of heavy metal ions on electro-oxidized carbon fibres have been investigated using cyclic voltammetric and polarographic techniques. Electro-oxidation is performed by potentiostatic double pulse application. This procedure results in the generation of many functional —OH and —COOH groups with adsorptive and ion-exchanging properties.Multimolecular layers of adsorbed substances may be formed through a procedure of successive adsorption of the nitro-compound and electro-reduction to the corresponding amine, resulting in the attachment of considerable amounts of the nitro-compound to the carbon fibres.The ion-exchange capacity is estimated to be ca. 1 mequiv. g−1 and with slight deviations it follows the rank Ag, Cu, Cd, Pb, Hg. After the electro-reduction of the exchanged metal ions, the ion-exchange process can be repeated several times. This procedure is of importance for the removal of significant amounts of heavy and toxic metals from industrial waste waters. Key words: electro-oxidized carbon fibres, adsorption of aromatic nitro compounds, cation-exchange of heavy metals.
A molecular mechanics study of conformational isomerism in 1- and 3-aryl hydantoins and 3-aryl-2-thiohydantoins
Authors: Colebrook Lawrence D.
Published: May 2006

Energy profiles for internal rotation about the C—N pivot bond in a series of 1- and 3-aryl hydantoins and 3-aryl-2-thiohydantoins have been computed using the MMX molecular mechanics force field. Rotational ground and transition states have been identified and their energies calculated. Conformational preferences of diastereomeric rotamers have been investigated. Computed rotational energy barriers generally are within ± 2 kcal/mol of the experimentally determined values. Key words: conformational isomerism, hindered rotation, hydantoins, thiohydantoins, molecular mechanics.
The hydroxonium perchlorate complex of cis-syn-cis-dicyclohexano 18-crown-6 ether: conformational analysis in the solid state and in solution at low temperature as studied via 13C NMR methods
Authors: Buchanan G. W. , Morat C. , Kirby R. A. , Tse J. S.
Published: May 2006

From 100.6-MHz 13C NMR coalescence temperature measurements, the title complex is shown to undergo a single degenerate ring inversion process with a free energy barrier of [Formula: see text]. This is ca. 5 kJ/mol higher than the barrier in the free crown. The solid state 13C CPMAS spectrum is indicative of a totally unsymmetrical conformation and the shift trends in the solid are analyzed in terms of local torsional angle variations. Comparisons of experimental vs. calculated (GIAO) 13C chemical shift ranges for the CH2O carbons in the solid are made and satisfactory agreement with respect to the overall range and magnitude of the 13C shielding differences is obtained. Key words: crown ethers, stereochemistry, NMR.
Excess Gibbs energy for binary mixtures of acetonitrile with acetic acid, propionic acid, isobutyric acid, and trimethylacetic acid
Authors: Banipal T. S. , Lark B. S. , Singh S.
Published: May 2006

Total vapour pressures for binary mixtures containing acetic acid, propionic acid, isobutyric acid, and trimethylacetic acid with acetonitrile have been measured for the entire composition range at 298.15 and 318.15 K using a static manometric method. All systems show positive deviations from Raoult's law, enhanced by both an increase in temperature and an increase in the methylation of acetic acid. Activity coefficients have been calculated by taking into consideration the dimerization of these carboxylic acids in the vapour phase. TSE values obtained from GE and earlier reported HE values are found to be negative for acetic acid, about zero for propionic and isobutyric acids, and positive for trimethylacetic acid for the whole composition range. The results have been interpreted in terms of various contributions such as depolymerization, heteromolecular dipole–dipole interactions, and the increasing dimerization constant and steric hindrance with increase of complexity of the acid. Key words: excess Gibbs energy, carboxylic acids, acetonitrile, activity coefficients
Charge transfer and electron transfer processes in biologically significant systems. 1. Charge transfer complex formation between 1,3,5-trinitrobenzene and N,N-dimethyl-3,4-di-O-methyldopamine, a dopamine analogue
Authors: Dust Julian M.
Published: May 2006

The interactions of N,N-dimethyl-3,4-di-O-methyldopamine, 1, a structural analogue of the important neurochemical, dopamine, with 1,3,5-trinitrobenzene (TNB) were studied primarily by 1H nuclear magnetic resonance (nmr). The dopamine analogue, a donor, forms a charge transfer complex with TNB, a model acceptor, in CDCl3 and CD3CN. Equilibrium constants were determined from the 1H nmr charge transfer induced chemical shift changes. The results are discussed in terms of the probable type of donation from the amine, 1, to TNB (n → π* versus π → π*), comparison with dopamine, and with regard to possible charge transfer interactions in molecular receptors. Keywords: N,N-dimethyl-3,4-di-O-methyldopamine, charge transfer complex, equilibrium constant.
Binding of water in aqua 18-crown-6 dichloropicric acid complexes in the solid state and its reversible thermal removal
Authors: Kolthoff Izaak M. , Chantooni JR Miran K.
Published: May 2006

Thermogravimetry, differential scanning calorimetry, and FT-IR vibrational spectral studies on the 1:1:1 18-crown-6: dichloropicric acid: water complex reveal that it remains unaltered when heated to melting. On the other hand, one molecule of water is expelled from the 1:2:2 complex by heating prior to melting. This results in an ionic microcrystalline product of undetermined crystal structure, in which the conformation of the crown is lower than D3d in symmetry. The enthalpy accompanying this process, which involves rupturing of two water–crown hydrogen bonds, is −44 to −55 kJ mol−1. The original 1:2:2 complex is regenerated by exposure of the heated adduct to water vapor. It was found that H2O in the structure of the solid 1:2:2 adduct was partially replaced by D2O at room temperature upon exposure to D2O vapor. The crown–H2O OH stretch in the IR exhibits a small or negligible positive isotope effect on deuteration. Keywords: crown ether, hydrogen bonding, water, dichloropicric acid.
On the nature of the anomeric effect in XHn—CH2—OH systems, with X = F, O, N, C
Authors: Grein F. , Deslongchamps P.
Published: May 2006

Using geometry optimizations at the 6-31G** level, energies, bond distances, and charges were obtained for systems XHn—CH2—OH with X = F, O, N, C, for various rotational conformers of the OH group. Conformational analysis shows that systems with X = F and O receive anomeric stabilization due to a partial π-bond forming in the CH2—OH fragment of the molecule, amplified or diminished by lone pair repulsion (or dipole) energies. For the systems with N and C, no π-bond stabilization is found. However, the e-NH2—CH2—OH system is still stabilized due to favourable lone pair repulsion circumstances. Keywords: anomeric effect, abinitio studies, π-bonding model.
Long-range shielding and NMR chemical shifts in silicon carbide polytypes
Authors: Guo Deqi , Hartman J. Stephen , Richardson Mary Frances
Published: May 2006

Despite an apparently successful empirical correlation, it is incorrect to ascribe variations in 29Si and 13C chemical shifts in silicon carbide polytypes to diamagnetic anisotropy effects Keywords: silicon carbide, long-range shielding, polytypism, MAS NMR.
Sorption of magnesium–picolinate complexes on a macroporous styrene–divinylbenzene copolymer containing immobilized 8-hydroxyquinoline and on the copolymer itself
Authors: Persaud Gocool , Cantwell Frederick F.
Published: May 2006

The sorbent XAD-OXINE was prepared by covalently binding 8-hydroxyquinoline onto the surface of the macroporous copolymer Amberlite XAD-2 to give about 20% surface coverage. Sorption, onto both XAD-2 and XAD-OXINE, of the magnesium-containing species that are present in solutions of magnesium ion and picolinic acid was studied by the column equilibration/atomic absorption technique at solution pH of 6.00 and 7.50 and ionic strength of 0.25. By using solutions containing various amounts of added magnesium and picolinic acid, in which the ratio of the two metal–ligand complex species, MgP+/MgP2, varied greatly (P− is the ligand picolinate), it was shown that neither the species Mg2+ nor the species MgP+ sorb to an appreciable extent onto the parent polymer XAD-2, but the neutral species MgP2 sorbs strongly and exhibits a linear isotherm. Then, by measuring the sorption isotherm of the species Mg2+ on XAD-OXINE in the absence of picolinic acid, by measuring the overall sorption of magnesium-containing species on XAD-OXINE in the presence of picolinic acid, and by assuming that the sorption of the species MgP2 onto XAD-OXINE was about the same as it was on XAD-2, it was possible to show that the species MgP+ sorbs strongly onto XAD-OXINE, and exhibits linear sorption isotherms. Thus, XAD-OXINE sorbs all three species Mg2+, MgP+, and MgP2. Keywords: Amberlite XAD-2, immobilized oxine, magnesium ion.
The 18-crown-6.2 chloroacetonitrile complex. X-ray crystal structure and solid phase motions of guest and host as studied by variable temperature 13C CPMAS NMR spectroscopy
Authors: Buchanan G. W. , Rodrigue A. , Bensimon C. , Ratcliffe C. I.
Published: May 2006

The title complex has two distinct 18-crown-6 moieties of approximate D3d symmetry in the unit cell. The 45.3 MHz 13C CPMAS spectra have two regions of broadening for the crown ether carbons. At high temperatures a dipolar washout mechanism is operative, leading to broadening when molecular motion has a correlation time approximately equal to the inverse of the decoupling field. At lower temperatures broadening occurs when the motional correlation time is equal to the inverse of the chemical shift difference. The activation energy for 18-crown-6 reorientation in this solid is calculated to be ca. 48 kJ/mol. For the chloroacetonitrile, the CH2 carbon also shows dipolar washout, but at a temperature much lower than the 18-crown-6. The activation energy for reorientation for this "guest species" is ca. 42 kJ/mol.
A general synthesis of 5,7-diaminoimidazo[4,5-b]pyridine ribosides ("2-amino-1-deazaadenosines") from 5-amino-4-imidazolecarboxamide riboside (AICA riboside)
Authors: Francis John E. , Moskal Michael A.
Published: May 2006

A short general synthesis of 5-substituted 5,7-diaminoimidazo[4,5-b]pyridines from 5-amino-4-imidazolecarboxamide riboside (AICA riboside) was designed to prepare isosteres of substituted 2-aminoadenosines that are selective adenosine A2 receptor agonists. AICA riboside was converted to a hydroxyl-protected 5-amino-4-imidazolecarbonitrile riboside and reacted with an N,N-disubstituted acetamide in the presence of phosphoryl chloride. Sodium hydride treatment completed the ring closure and introduced the 7-amino group. The hydroxyl protecting groups were removed under standard conditions. N-Substitution of the acetamide by one benzyl moiety led to a 5-N-substituted derivative through hydrogenolysis whereas N,N-dibenzylacetamide led to the 5,7-diamino compound. A 6-methyl analog was obtained from an N,N-disubstituted propionamide. This synthesis may be adapted to other heterocyclic systems, as illustrated by the preparation of an example of the imidazo[4,5-b]pyrrolo[3,2-e]pyridine ring system.
Annulation sequences employing methyl (E)- and (Z)-6-iodo-3-trimethylstannyl-2-hexenoates and related bifunctional reagents. Preparation of cisoidcis and cisoidtrans bicyclic dienes
Authors: Piers Edward , Friesen Richard W. , Rettig Steven J.
Published: May 2006

A useful annulation method leading to the stereospecific synthesis of structurally novel, functionalized bicyclic dienes is described. Alkylation of 2-(methoxycarbonyl)cyclopentanone (34) and 2-(methoxycarbonyl)cyclohexanone (35) with methyl (E)- and (Z)-6-iodo-3-trimethylstannyl-2-hexenoates (27, 28) provided the keto diesters 36–39. Conversion of these substances into the corresponding enol triflates 47–50, followed by palladium(0)-catalyzed ring closure of the latter compounds, afforded the bicyclic dienes 55–58. In similar fashion, the keto esters 34 or 35 were transformed into the dienes 59–65. An X-ray crystallographic analysis of the p-nitrobenzoate 69, which was derived from the cisoidcis diene 64, showed that, in 69, the dihedral angle between the two carbon–carbon double bonds is −54.0(5)°.
Electron thermalization distances and free-ion yields in dense gaseous and liquid benzene
Authors: Gee Norman , Freeman Gordon R.
Published: May 2006

Electron thermalization has been studied in gaseous and liquid benzene at 4.1 ≤ d/kg m−3 ≤ 878 (temperatures 295–560 K) using measurements of the free-ion yield [Formula: see text] as a function of electric field strength E and temperature T. The measured [Formula: see text] values at each T were compared to those calculated using an extended Onsager model. Assumption of a three-dimensional Gaussian distribution of secondary electron thermalization distances YG resulted in too large a field dependence. The Gaussian with the small added tail, YGP, gave the correct dependence. Values of the yield extrapolated to zero field, [Formula: see text] and of the most probable thermalization distance bGP were obtained. Variation of the density-normalized distance bGPd with reduced density d/dc (dc = critical fluid density) was expected to be similar to that in ethene, due to the π-electrons in the two compounds. Instead, it was similar to that in ethane. Throughout the liquid range, epithermal electrons were de-energized less efficiently than in the gas at d < 0.5 dc where the benzene molecules are further apart. As the density increases above 2 dc the values of bGPd decreased as in other hydrocarbons, rather than like those in hexafluorobenzene, which increased sharply. Dielectric constants were also measured up to 560 K.
Synthesis of sugar-glutarimide combinations
Authors: Park Hee Moon , Piatak David M. , Peterson John R. , Clark Alice M.
Published: May 2006

Several sugar glutaramide combinations, wherein the aldehyde group of the sugar was incorporated as C-4 of the imide ring, were prepared to provide molecules with chemotherapeutic potential. The most interesting bioassay results were seen in the inhibition of the bacteria Mycobacteriumintracellularae by the glutarimide combinations with D- and L-xylose.
Chiral organosilicon compounds in synthesis. Preparation and stereoselective alkylations of silylcinnamyl carbanions
Authors: Lamothe S. , Cook K. L. , Chan T. H.
Published: May 2006

Silylallyl carbanions of type 13 bearing a chiral lithium complexing substituent remote from silicon can be alkylated regio- and stereoselectively at the α-position by small electrophiles in nonpolar solvents. The regiochemical outcome of the reaction was found to be highly dependent on the size of the incoming electrophile. Secondary alkyl halides react preferentially at the γ-center and good levels of stereoselectivity are still obtained in nonpolar solvents despite the rather long distance between the reacting center and the chiral auxiliary. Transformation of the alkylated allylsilanes into optically active alcohols and carboxylic acids may represent a potential synthetic application of the method.
Coordination chemistry of thioether–pyridazine macrocycles. I. Synthetic, structural, and spectroscopic studies of Cu(II) and Cu(I) complexes of novel thioether–pyridazine macrocycles that contain two or three pyridazine subunits
Authors: Chen Liqin , Thompson Laurence K. , Bridson John N.
Published: May 2006

The preparations of thioether–pyridazine macrocycles containing three (L1) and two (L2) pyridazine subunits and their copper complexes are described. The ligands are characterized by 1H nuclear magnetic resonance and mass spectrometry and in one case by X-ray crystallography, and the complexes by infrared, electronic, and electron spin resonance (esr) spectra and in some cases by X-ray crystallography. The complex [Cu3(L1)2Cl6]•2CHCl3 (1) crystallized in the triclinic system, space group [Formula: see text] with a = 13.661(2) Å, b = 14.174(3) Å, c = 9.412(2) Å, α = 101.08(2)°, β = 96.94(2)°, γ = 75.76(2)°, V = 1728.2(6) Å3, and Z = 2 (R = 0.056, Rw = 0.048 for 2080 reflections). Two monodentate pyridazine rings in each ligand bind to one square-planar copper centre with the third monodentate pyridazine in each ligand linking the two to the central square-planar copper. The complex [Cu(L2)Cl2] (2) crystallized in the orthorhombic system, space group Pnma, with a = 8.571(1) Å, b = 16.104(3) Å, c = 13.961(2) Å, V = 1927(1) Å3, and Z = 4 (R = 0.037, Rw = 0.033 for 1070 reflections). A cis square-planar structure exists for 2 with monodentate pyridazines. [Cu(L2)2]•(ClO4)2•CH3CN•CHCl3 (5) crystallized in the triclinic system, space group [Formula: see text] with a = 12.888(4) Å, b = 17.462(6) Å, c = 10.906(1) Å, α = 96.07(2)°, β = 104.18(2)°, γ = 94.51(2)°, V = 2352(1) Å3, and Z = 2 (R = 0.053, Rw = 0.044 for 2941 reflections). Two ligands involving monodentate pyridazine rings bind to a square-planar copper(II) centre. The protonated ligand salt [L2H](ClO4)•H2O (6) crystallized in the monoclinic system, space group P21/n, with a = 14.762(4) Å, b = 8.637(5) Å, c = 16.267(4) Å, β = 92.78(2)°, V = 2072(1) Å3, and Z = 4 (R = 0.064, Rw = 0.053 for 1456 reflections). No sulfur coordination is observed in these complexes and there is no apparent spin exchange in the trinuclear derivative.
Stereocontrolled synthesis of optically active 14β-hydroxy steroids and other steroid derivatives
Authors: Ruel Réjean , Deslongchamps Pierre
Published: May 2006

The synthesis of optically active 14β-hydroxy steroids via base-catalyzed reactions is reported.
Structural studies of organoboron compounds. LII. N-Ethylhydroxylamine- (O-B)1,7-diethyl-3,5-diphenyl-2,4,6-trioxa-7-aza-1-azonia-3-bora-5- boratabicyclo[3.3.0]octane
Authors: Kliegel Wolfgang , Lubkowitz Gottfried , Rettig Steven J. , Trotter James
Published: May 2006

The reaction of N,N′-diethyl- N,N′-dihydroxymethanediamine with oxybis(diphenylborane) in absolute ethanol yields N-ethylhydroxylamine(O-B)1,7-diethyl-3,5-diphenyl-2,4,6-trioxa-7-aza-1-azonia-3-bora-5-boratabicyclo[3.3.0]octane [3-(1-oxa-2-azoniabutyl)-1,7-diethyl-3,5-diphenyl-2,4,6-trioxa-7-aza-1-azonia-3,5-diboratabicyclo[3.3.0]octane], 7b, in good yield. Crystals of this material are monoclinic, a = 16.396(2), b = 11.303(2), c = 22.898(4) Å, β = 92.78(1)°, Z = 8 (two molecules per asymmetric unit), space group P21/a. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.041 and Rw = 0.050 for 3602 reflections with I ≥ 3σ(I). The product 7b is formed by O→B coordination between N-ethylhydroxylamine and the bicyclic bis-phenylboronate of N,N′-diethyl-N,N′-dihydroxymefhanediamine. Compound 7b is only the second structurally characterized example of an N-alkylhydroxylamine(O-B)boron complex having an open-chain B,N-betaine structure. Bond lengths about boron are: B—O(N) = 1.467(4)–1.527(4), B—O(B) = 1.433(4)–1.451(4), B—N = 1.688(4) and 1.707(4), B—C = 1.586(4)–1.606(5) Å.
Structural studies of organoboron compounds. LIII. N,N-Diethylhydroxylamine-(O-B)1,7-dimethyl-3,5-diphenyl-2,4,6-trioxa-7-aza-1-azonia-3-bora-5- boratabicyclo[3.3.0]octane
Authors: Kliegel Wolfgang , Lubkowitz Gottfried , Rettig Steven J. , Trotter James
Published: May 2006

Reaction of the condensation product of N,N′-dimethyl-N,N′-dihydroxymethanediamine and phenylboronic acid with N,N-diethylhydroxylamine gives N,N-diethylhydroxylamine(O-B)1,7-dimethyl-3,5-diphenyl-2,4,6-trioxa-7-aza-1-azonia-3-bora-5-boratabicyclo[3.3.0]octane [3-(2-ethyl-1-oxa-2-azoniabutyl)-1,7-dimethyl-3,5-diphenyl-2,4,6-trioxa-7-aza-1-azonia-3,5-diboratabicyclo[3.3.0]octane, 3] in high yield. Crystals of 3 are orthorhombic, a = 11.8132(11), b = 15.4768(11), c = 11.7325(16) Å, Z = 4, space group P212121. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.039 and Rw = 0.044 for 1729 reflections with I ≥ 2σ(I). The title compound is the second example of this recently characterized class of O-B coordinated hydroxylamine complexes, stabilized by an intramolecular [Formula: see text] hydrogen bond [Formula: see text]. Bond lengths about boron are: B—O(N) = 1.482(4)–1.526(4), B—O(B) = 1.412(4) and 1.420(4), B—N = 1.712(4), B—C = 1.591(5) and 1.602(5) Å.
Thermal cyclizations of protonated poly-unsaturated aldehydes
Authors: Elia George R. , Childs Ronald F. , Shaw Gary S.
Published: May 2006

Protonated 2,4-hexadienal (1H) and 2,4,6-octatrienal (2H), prepared by protonation of the analogous aldehydes in FSO3H, isomerized to give cyclized products 3H and 4H at 30 °C and −20 °C respectively. The rate constants for the cyclization of 1H were measured in both FSO3H and CF3SO3H. It was found that the rate constant for isomerization decreased when CF3SO3H was used as the reaction medium. It is suggested that the thermal cyclizations of 1H and 2H involve diprotonated species with protonation occurring on oxygen. 1H and 2H underwent photoisomerization at −78 °C in FSO3H to yield 5H and 7H respectively. It was found that 5H underwent a cyclization reaction at −40 °C to give 6H, which subsequently rearranged to give 3H at −10 °C. However, 7H was found to isomerize to 2H at −50 °C. The mechanisms of the thermal isomerization are discussed.
Reactions of hydriodic acid with aldonolactones and n-alkanolactones. Interconversions between lactones and iodocarboxylic acids
Authors: Liu Zhengchun , Granata Alessandro , Shen Xinhua , Perlin Arthur S.
Published: May 2006

Aldonolactones containing from four to eight carbon atoms, and lactones of the related monohydroxy-n-alkanoic acids, were subjected to reaction with 57% hydriodic acid at 125 °C. As in the classical studies of Kiliani, the reduction of D-glycero-D-ido-heptono-1,4-lactone yielded mainly γ-heptanolactone. Analogously, the corresponding γ-alkanolactones were obtained as major products from the 1,4-lactones of the D-xylono-, D-allono-, and D-erythro-L-talo-octono configuration. Monoiodo-n-alkanoic acids were also formed in admixture with the lactones in all of these reactions. D-Erythrono-1,4-lactone was unique among the aldonolactones in that it led only to an acid, i.e., 3-iodo-n-butanoic acid. The latter was also the product of the non-reductive reaction of hydriodic acid with β-butyrolactone whereas, by contrast, γ-butyrolactone afforded 4-iodobutanoic acid. Among compounds in the five to eight carbon series, it was found that under conditions close to equilibrium the ratio of lactone to iodoacid decreased progressively with the length of the carbon chain; e.g., in the 4 h reactions of γ-valero-, γ-capro-, γ-heptano-, and γ-octanolactone, the ratios were 2.4, 1.2, 0.2, and 0.1, respectively. An accompanying characteristic of these reactions is a progression in the number of isomeric iodoacids formed. Whereas γ-valerolactone was accompanied by 4-iodopentanoic acid, there were two isomers (4- and 5-) of iodohexanoic acid, three monoiodo- (including 6-iodo-) heptanoic acids, and four (including 7-iodo-) octanoic acids. In all instances, the isomer substituted at the penultimate carbon was major. An interplay of several individual reactions, including ring-opening displacements, eliminations–additions, and rearrangements, as well as a probable influence of entropy changes on the lactone-acid equilibria, appear to account largely for these observations.
Synthesis and characterization of chemical intermediates of acyl or alkyl phospholipids
Authors: Nasser Boubker , Morpain Claude , Laude Bernard , Latruffe Norbert
Published: May 2006

Precursors of phospholipids or ether lipids were synthesized from D-mannitol, i.e., 1-O-myristoyl-2-O-benzyl-sn-glycerol 11 and 1-O-hexadecyl-2-O-benzyl-sn-glycerol 13. The chemical pathways involved sequential protection reactions using allyl, triphenylmethyl, or benzyl groups via 3-O-allyl-sn-glycerol. Eleven intermediates were isolated and for the first time characterized with their spectrometric (especially 1H nmr) and physical parameters and yield. This work allowed preparation of chemically defined membrane phospholipids from compound 11 as well as ether lipids from compound 13 with cell mediator activities bearing different probes such as photoactivatable, fluorescent, or radioactive groups.
Tetramethylguanidino-substituted pyrylium and thiopyrylium salts
Authors: Galezowski Wlodzimierz , Leffek Kenneth T. , Pruszynski Przemyslaw
Published: May 2006

Four new tetramethylguanidino-substituted pyrylium and thiopyrylium salt dyes have been prepared and their absorption maxima in the visible region measured. The positions of the absorption maxima are similar to those observed for thiopyrylium salt dyes used in photoconductive film compositions. The λmax values for the new dyes, when compared to those of other pyrylium and thiopyrylium dyes in the literature, yield a σp value of −0.86 ± 0.04 and a σ+ value of −1.8 ± 0.1 for the tetramethylguanidino group, in good agreement with previous determinations.
Rearrangements in the acid-catalyzed formation of lactones from 2-hydroxynorbornane-2-acetic acid derivatives and related compounds
Authors: Yates Peter , Kaldas Magdy
Published: May 2006

A mixture of the epimeric ethyl 2-hydroxynorbornane-2-acetates (1) on treatment with concentrated sulfuric acid is converted in turn to the lactones of exo-2-hydroxynorbornane-1-acetic acid (4), endo-6-hydroxynorbornane-endo-2-acetic acid (5), and exo-3-hydroxynorbornane-exo-2-acetic acid (6). With trifluoroacetic acid or 50% sulfuric acid, 1 gives 4, but this does not react further. In concentrated sulfuric acid the parent acids of 1 (7) and (E)- and (Z)-(norborn-2-ylidene)acetic acids (8 and 9) and their ethyl esters (10 and 11) give 6 as the infinity product. A mixture of 5-norbornene-endo- and exo-2-acetic acid (30 and 31) on treatment with 50% sulfuric acid gives 4, 5, 6, and exo-2-hydroxynorbornane-syn-7-acetic acid lactone (33). Routes are proposed for the formation of the lactones that involve protonation and carbocation formation followed by rearrangement via Wagner–Meerwein, endo-6,2-hydride, and exo-3,2-hydride shifts in decreasing order of preference. It is postulated that the usual inhibition of the rearrangement of tertiary to secondary norbornyl carbocations is not operative when the third substituent is a carboxymethyl group or its derivatives because of the electron-withdrawing properties of such groups relative to simple alkyl groups. A preliminary study has shown that exo-5-acetyloxy-endo-2-hydroxynorbornane-exo-2-acetic acid (35) with 50% sulfuric acid gives four products that are considered to be the γ-lactones of endo-5-acetyloxy- and endo-5-hydroxynorbornane-1-acetic acid (38 and 39) and exo-2-acetyloxy-1-hydroxy-and 1,2-dihydroxynorbornane-syn-7-acetic acid (40 and 41). Protonation of either the hydroxyl or acetyloxyl group is postulated, giving two carbocations that undergo rearrangements as in the case of 1, together with 3,2-hydroxyl shifts. The structures of the lactones are assigned on the basis of spectroscopy, reactivity, and analogy. The reactions of the lactones, which lead to a variety of hydroxy- and oxonorbornaneacetic acids, illustrate their synthetic potential.
2-Chloro-2,4-dioxo-3-methyl-1,3,2-thiazaphospholidine. Comments on its phosphorylating properties
Authors: Modro Agnes M. , Modro Tomasz A.
Published: May 2006

2-Alkoxy-2,4-dioxo-3-methyl-1,3,2-thiazaphospholidines undergo facile ring-opening reactions with chloride ion, and, in particular, with water. This reactivity may limit their value as first intermediates in the sequential synthesis of phosphoric diesters from the parent 2-chloro-2,4-dioxo-3-methyl-1,3,2-thiazaphospholidine.
Lewis acid mediated Diels–Alder reactions of 2H-thiopyrans
Authors: Ward Dale E. , Gai Yuanzhu
Published: May 2006

Lewis acid mediated Diels–Alder reactions of 2H-thiopyrans with acrylate, crotonate, and methacrylate dienophiles have been investigated. Modest to good yields of predominantly exo adducts are obtained with 4-substituted 2H-thiopyrans. With 5-substituted 2H-thiopyrans, high endo selectivity is observed. No adducts were produced with methacrylate dienophiles. Relative to the thermal reactions, the exo selectivity is enhanced in the presence of Lewis acids. Unusually high exo selectivity is observed using "wet" Et2AlCl as the promoter.
The acidities of 4-arylsulfonylmethylpyridines and N-methyl and N-benzyl 4-arylsulfonylmethylpyridinium cations in aqueous solution
Authors: Wodzinski Stefan , Bunting John W.
Published: May 2006

The pKa values for the deprotonation of a series of 4-(X-phenylsulfonylmethyl)pyridines (6) (pKa = 19.89 (X = H); ρ = 3.0) were determined in aqueous dimethyl sulfoxide solutions at 25 °C using the H0q acidity function. The pKa values were also measured for the corresponding series of N-mefhyl 4-(X-phenylsulfonylmethyl)pyridinium cations (2) (pKa = 11.27 (X = H); ρ = 1.45) and also for a series of N-(X-benzyl) 4-phenylsulfonylmethylpyridinium cations (7) (pKa = 10.70 (X = H); ρ = 0.65) in aqueous solution (ionic strength 0.1 at 25 °C). Comparison of the substituent effects upon the pKa values of the sulfonyl-activated carbon acids 2, 6, and 7 with the substituent effects upon the pKa values of the corresponding three series of ketones gives insight into the electron-density distribution in the carbanionic conjugate bases of sulfone and ketone carbon acids. Extrapolation of a linear free energy relationship between the pKa values of neutral sulfones and ketones allows the estimation of pKa = 28.7 for the deprotonation of methyl phenyl sulfone in aqueous solution.
Influence of the acidity on the kinetics of diphenylamine oxidation by peroxodisulfate anions
Authors: Balón M. , Guardado P. , Carmona C. , Hidalgo J. , Munoz M. A.
Published: May 2006

A kinetic study is reported on the oxidation of diphenylamine (DPA) by peroxodisulfate (PDS) anions in H2SO4 and buffered acetic acid – acetate 20% v/v methanol–water media. The primary detectable product of the reaction is N-phenyl-p-benzoquinoneimine (PBQ), which undergoes acid-catalyzed hydrolysis giving different decomposition products. At pH > 4 (acetic acid – acetate buffers) the hydrolytic decomposition is very slow and it does not interfere with PBQ formation, but in H2SO4 media of pH < 2.5, PBQ decomposes as it is formed. In these media, the reaction obeys the kinetic rate law:[Formula: see text]a, the kinetic parameter, being acid dependent. Neither radical promoters (Ag+) nor radical traps (allyl alcohol) influence the reaction rate. In acetic acid – acetate buffers of pH > 4 the kinetic rate law is simple: first order in each one of the reactants, and acid independent. These results have been interpreted by assuming the formation, in a preequilibrium step, of an N-diphenylhydroxylamine-O-sulfonate intermediate, which further rearranges to yield PBQ. The rate-determining step of the reaction changes with the protonation state of the intermediate, whose protonation pKa has been kinetically estimated as 2.4.
Phosphonolipids 4. A phosphonic acid analogue of platelet activating factor
Authors: Liu Yue-jin , Tropp Burton E. , Engel Robert
Published: May 2006

A phosphonic acid analogue of platelet activating factor (PAF) bearing a backbone that is isosteric with and has stereochemistry corresponding to that of natural PAF has been synthesized. The synthesis uses a route maintaining stereochemical integrity.
Reactions of Ni(II) and its complexes with free radicals derived from thymine
Authors: Chakrabarti S. , Mandal P. C. , Bhattacharyya S. N.
Published: May 2006

The effects of nickel (II) ions and its complexes on the products arising from the γ-radiolysis of thymine were studied. The decomposition of the pyrimidine base and formation of different radiolytic products indicate that nickel(II) ions or its complexes have little effect on the radiosensitivity of thymine. The transient hydroxyl adduct of thymine reacts with Ni(II) with the formation of a complex having a nickel–carbon bond. The rate constants for the reactions are of the order of 106 dm3 mol−1 s−1. Possible mechanisms for the formation of different products in the radiolysis of thymine in the presence of nickel(II) compounds are discussed.
SO2 extrusion in 1,2,6-thiadiazine 1,1-dioxides: a novel synthesis of pyrazoles
Authors: Klein Jean F. , Pommelet Jean Claude , Chuche Josselin , Elguero José , Goya Pilar , Martinez Ana
Published: May 2006

A novel synthesis of the pyrazole ring involving SO2 extrusion by flow vacuum pyrolysis in the 1,2,6-thiadiazine 1,1-dioxide system is described. The reaction has potential industrial significance for routes to pyrazoles or functionalized pyrazoles that do not involve hydrazines.
Étude de la conformation de dérivés pyrroliques
Authors: Oussaid Boualem , Hubert Cathy , Moeini Leila , Garrigues Bernard , Fayet Jean-Pierre
Published: May 2006

The conformations of pyrrole derivatives (1, 2, 3a, b, 4a, b, 5, 6, and 7) have been elucidated from theoretical investigations, dipole moment measurements, use of stereospecific coupling through five bonds, as well as Overhauser experiments (6).
13C and 1H NMR investigation of pilocarpine hydrochloride in water solution
Authors: Gaggelli Elena , Gaggelli Nicola , Valensin Gianni , Vivi Antonio
Published: May 2006

Dynamic and geometric features of pilocarpine in water solution were delineated by interpreting 13C and 1H NMR spin-lattice relaxation rates and nuclear Overhauser effects. The motional correlation time was evaluated at 53 ± 5 ps at 298 K. 1H—1H distances, as calculated from double-selective 1H spin-lattice relaxation rates, and 13C—1H distances, as calculated from selective 13C–{1H} Overhauser effects, gave evidence of a C7 puckered conformation of the furanone ring and of an angle of ca. 50° between the two rings.
Electron paramagnetic resonance study of radicals derived from 2- and 3-fluoropropene
Authors: Smith Peter , Hougum Cynthia Ann , Hosseinian Art Aresh
Published: May 2006

Using the TiCl3–H2O2, –NH2OH, −Na2S2O8, and −H2O2/Na2SO3, continuous-flow, radical-generating methods at 25 °C, we have successfully characterized by electron paramagnetic resonance spectroscopy radicals formed by the reaction of each of the substrates 2- and 3-fluoropropene with the radicals •OH, •NH3+, •OSO3−, and •SO3−, respectively. With 2-and 3-fluoropropene, respectively, the radicals identified were of general formula CH2(X)ĊFCH3 and CH2(X)ĊHCH2F where -X is the added group, except that treatment of 3-fluoropropene with the TiCl3–H2O2 reaction system also yielded •CH2CH(X)CH2F. The results obtained are discussed in terms of the effect of the presence of the fluorine substituent on both the apparent reactivity of the substrate molecules towards the radical-generating systems and the spectroscopic properties of the radicals observed.
Substituent effects of rate constants for thermal [4π + 2π] cycloreversion of spiro-fused β-lactam oxadiazolines
Authors: Zoghbi Michel , Warkentin John
Published: May 2006

Twelve Δ3-1,3,4-oxadiazolines in which C-2 is also C-4 of a β-lactam moiety (spiro-fused β-lactam oxadiazoline system) were thermolyzed as solutions in benzene. Substituents in the β-lactam portion affect the rate constant for thermal decomposition of the oxadiazolines to N2, acetone, and a β-lactam-4-ylidene. The total spread of first-order rate constants at 100 °C was 47-fold and the average value was 6.7 × 10−4 s−1. A phenyl substituent at N-1 or at C-3 was found to be rate enhancing, relative to methyl. At C-3, H and Cl were also rate enhancing, relative to methyl. The data are interpreted in terms of the differential effects of substituents on the stabilities of the ground states, and on the stabilities of corresponding transition states for concerted, suprafacial, [4π + 2π] cycloreversion. The first products, presumably formed irreversibly, are N2 and a carbonyl ylide. The latter subsequently fragments to form acetone (quantitative) and a β-lactam-4-ylidene.
2-Carbomethoxy-1,3-butadiene: an electronically activated diene in [4 + 2] cycloadditions with electron-deficient dienophiles
Authors: Spino Claude , Crawford Jason
Published: May 2006

Cross Diels–Alder reactions between 2-carbomethoxy-1,3-butadiene 1 and electron-rich dienes were carried out. It was found that diene 1 had a higher reactivity than even the well-known Danishefsky diene in its Diels–Alder cycloaddition with electron-deficient dienophiles. In addition, (Z)-1-methylthio-3-carbomethoxy-1,3-butadiene 11 was found to have a higher reactivity than (Z)-1-methylthio-1,3-butadiene and (Z)-2-carbomethoxy-1,3-hexadiene, while both (E)- and (Z)-1,2-(bis)-carbomethoxy-1,3-butadiene 14 reacted efficiently with maleic anhydride to give the corresponding cy-cloadduct 15.
Inhibitors of endo-α-mannosidase. Part III. Congeners of 1-deoxy-3-O-(α-D-glucopyranosyl)-mannojirimycin modified in the glucose unit
Authors: Spohr Ulrike , Bach Mimi
Published: May 2006

The syntheses of congeners of 1-deoxy-3-O-(α-D-glucopyranosyl)-mannojirimycin (1), a strong inhibitor of the glycoprotein-processing endo-mannosidase, are described. The chemical modifications of 1 involved all monodeoxy-genations and mono-O-methylations of the glucose unit and the replacement of this unit by D-galactose, D-xylose, and 2-chloro-2-deoxy-D-glucose. As reported previously, none of the modifications of 1, including deoxygenations and O- and N-methylations of the deoxymannojirimycin unit, improved the inhibitory properties, but demonstrated the high specificity in the recognition of 1 by the enzyme and allowed the assignment of intermolecular hydrogen bonds of the inhibitor • enzyme complex. Essential for complex formation were found NH-5, OH-2, OH-4, and OH-6 of the DMJ unit, as well as OH-3′, OH-4′, and CH2-6′ of the glucose unit. The residual activities on deoxygenating the OH-2′ and OH-6′ groups of 1 suggest their involvement at the periphery of the binding site.
Styrylbenzodiazinones 1. Synthèse, structure et propriétés photophysiques
Authors: Cazaux Louis , Faher Mourad , Picard Claude , Tisnès Pierre
Published: May 2006

Several 3-styryl derivatives of 1,4-benzodiazine-2-one and of 1-methyl-1,4-benzodiazine-2-one were prepared. A structural study of these compounds by 1H and 13C NMR and by molecular modelling was carried out. The rotational isomerism around the bond connecting the imine and vinyl groups in these E styrylbenzodiazinones is discussed. The s-cis rotamer is shown to be the predominant isomer. The photophysical properties of these fluorescent dyes are reported.
Energetics of acetic acid enol in aqueous solution. A new method for thermodynamic estimation
Authors: Guthrie J. Peter
Published: May 2006

A new disproportionation calculation allows the estimation of the free energy of formation of the enol of acetic acid as 65 ± 2 kcal/mol. The value of pKE derived from this free energy, pKE = 21 ± 2, is in satisfactory agreement with information from the literature about rates of exchange. Analysis of the data on rates of exchange of the C-H protons of acetic acid using Marcus theory allows an independent estimate of the enol content. Exchange in acid and in base lead to internally consistent estimates, pKE = 19.3 ± 2.2, which are within the combined uncertainties of the values from the thermodynamic estimate.
Photolysis of ClNO adsorbed on MgO(100)
Authors: Hodgson L. , Ziegler G. , Ferkel H. , Reisler H. , Wittig C.
Published: May 2006

The 365 nm pulsed laser photolysis of nitrosyl chloride adsorbed on a rough MgO(100) surface at 90 K has been studied. Mass spectrometric detection was used to record time-of-flight (TOF) spectra by monitoring Cl+ and NO+. These ions can derive from parent ClNO, which fragments completely in the mass spectrometer, as well as from Cl and NO photofragments. The TOF distributions are considerably slower than for the corresponding gas phase photodissociation process. NO was also detected state selectively by using resonance enhanced multiphoton ionization (REMPI), and a channel corresponding to direct adsorbate photolysis was identified. The state selective detection of NO molecules that emerge from the surface following photolysis shows unambiguously that their rotational degrees of freedom reflect the surface temperature (Trot = 100−140 K), even at low coverages. At similar photolysis wavelengths, gas phase ClNO photodissociation is known to produce highly rotationally excited NO with a distinctive non-statistical distribution peaked at J″ = 46.5. Our studies suggest that, contrary to the gas-phase photolysis results, Cl and NO are not ejected rapidly following photolysis of surface-bound species on a repulsive potential energy surface. We postulate that ClNO grows in islands, with MgO defect sites serving as nucleation centers. Photofragment rotational and translational excitations are quenched efficiently due to strong attractive interactions that equilibrate NO to the surface temperature. Desorption of intact ClNO may also take place, but following (i.e., not during) the photolysis pulse. Such desorbed species can contribute to the TOF spectra, but not the REMPI spectra.
Reaction of unsaturated azlactones with sulfur nucleophiles: some observations
Authors: Mukerjee Arya K. , Rao Laxmi , Homami Seyed Saied , Joseph Kiran
Published: May 2006

Unsaturated azlactones undergo thiolysis with toluene-α-thiol, but 2-substituted (Z)-4-o-acetoxybenzylidene-4,5-dihydrooxazol-5-ones form 3-acylaminocoumarins in the presence of triethylamine. Sodium sulfide in ethanol brings about ethanolysis of the 1,5 bond and deacetylation of an acetoxy group. (Z)-4-Benzylidene-4,5-dihydrooxazol-5-one reacts with potassium thiocyanate to give (E)-4(5)-benzylidene-2-thiohydantoin.
An X-ray and conformational study of the α-chloro derivatives of 5,7-dihydro-1,11-dimethyl-6H-dibenzo[a,c]cyclohepten-6-one
Authors: Fraser Robert R. , Bensimon Corinne , Faibish Neil C. , Kong Fanzuo
Published: May 2006

The crystal structures for four α-chloro derivatives 2–5 of the titled ketone 1 have been determined, confirming their earlier assignments of stereochemistry. Variations in the conformations of 2–5 suggested some flexibility in the central seven-membered ring. Molecular mechanics calculations of the changes in energy as a function of the internal torsional angles at the carbonyl of these chloro derivatives indicated the ring flexibility to be comparable to that of cyclohexanone. In the three compounds containing an axial chlorine the distance between the chlorine atom and two of the carbon atoms of the more remote benzene ring was 3.2 ± 0.1 Å, suggesting the value for the thickness of a benzene ring may be significantly smaller than the accepted value when it is interacting with a single atom.
Crystal and solution structure of a pendant-armed macrocyclic complex of palladium(II)
Authors: Chak B. , McAuley A. , Whitcombe T.W.
Published: May 2006

The synthesis and X-ray crystal structure of [Pd)py2[9]aneN2S)](BF4)2 (py2[9]aneN2S = 1-thia-4,7-bis(pyridylmethyl)diaza-cyclononane) (P21/n,a = 10.233(4) Å, b = 11.484(5) Å, c = 19.913(6) Å, β = 94.03(4)°, V = 2334(2) Å3, R = 0.0705, Rw = 0.0759 ) are presented. The crystal structure indicates a long range interaction (2.92 Å) between the planar palladium and the apical sulphur atom. Although this distance is longer than the value for a Pd—S bond observed normally there is evidence for a net positive interaction provided by the solution structure as determined by NMR spectroscopy. Comparative analysis of the structural features exhibited by this complex with similar nine-membered chelating ligands emphasizes the presence of an axial Pd—S bond.
Senegalene, une nouvelle acétogénine oléfinique mono-tétrahydrofuranique des graines d'Annona senegalensis
Authors: Sahpaz Sevser , Laurens Alain , Hocquemiller Reynald , Cavé André , Cortes Diego
Published: May 2006

Using activity-directed fractionation, the chromatography of the bioactive methanol extract of Annona senegalensis seeds led to isolation of a new mono-tetrahydrofuranic γ-lactone acetogenin characterized by the presence of a double bond on the alkyl chain, senegalene 1. Its structure was elucidated on the basis of spectroscopic (2D-NMR and MS) and chemical methods. Four known acetogenins have been also isolated: molvizarin 2, asimicin 3, rolliniastatin-2 4, and squamocin 5. Comparative structure–activity relationships were studied using senegalene 1 and its derivatives in KB and VERO cell lines in in vitro tests.
Transposition en série tricyclique — synthèse et réactivité de bis-triméthylsilyl-bicyclo[n,1,0]2-alcène antioxydes
Authors: Grelier-Marly Marie-Catherine , Grignon-Dubois Micheline
Published: May 2006

The bis-trimethylsilyl-bicyclo[n,1,0]2-alkene antioxides (n = 4 and 6) have been synthesized from their 1,3-diene precursors by a three-step process (dichlorocyclopropanation, silylation, and epoxidation). Their rearrangement in the presence of solid and liquid Lewis acids was studied, and compared with the previously described reactivity of 2-carene oxide. With these silylated epoxides, the breaking of the C3—O bond is favoured while C2—O bond breaking had previously been observed with 2-carene oxide. All the resulting products were the three-membered ring-opened ones, but the opening mechanism differs according to the value of n, leading mainly to 7,8-bis-trimethylsilyl-9-hydroxy-bicyclo[4,3,0]non-6-ene when n = 6 and 2,2′-bis-trimethylsilyl-cyclohex-3-ene carboxaldehyde when n = 4.