bluuebunny/crossref_metadata_2025 huggingface dataset:
A selenosulfonation-based approach to (22S,23E)-6β-methoxy-3α,5-cyclo-5α-ergost-23-en-22-ol, a key intermediate for brassinolide synthesis
Authors: Back Thomas G. , Brunner Kurt , Vijaya Krishna M. , Lai Enoch K. Y.
Published: May 2006

(22S,23E)-6β-Methoxy-3α,5-cyclo-5α-ergost-23-en-22-ol (2), an intermediate in the synthesis of brassinolide (μ) and related compounds, was synthesized by a new approach. (20S)-6β-Methoxy-3α,5-cyclo-5α-pregnane-20-carboxaldehyde (5) was converted to the corresponding 22S and 22R-24-yn-22-ol derivatives 6a and 6b by treatment with 3-lithio-1 -trimethylsilylpropyne, followed by tetra-n-butylammonium fluoride. The products were subjected to selenosulfonation, base-catalyzed isomerization, and selenoxide elimination to afford the respective 22S and 22R allenic sulfones 9a and 9b. The 22S isomer 9a underwent addition with lithium diisopropylcuprate – dimethyl sulfide complex, followed by reductive desulfonylation with magnesium in methanol, to afford 2, as well as smaller amounts of its 24(28) isomer 11a and (22E)-6β-methoxy-3α,5-cyclo-5α-ergosta-22,24(28)-diene (12). Similar treatment of 9b afforded 13, the 22R isomer of 2, as the principal product, as well as its 24(28) isomer 11b, and diene 12. Keywords: brassinolide, (22S,23E)-6β-methoxy-3α,5-cyclo-5α-ergost-23-en-22-ol, selenosulfonation, allenic sulfones.
Structural analysis of the specific capsular polysaccharide of Streptococcuspneumoniae type 22F
Authors: Richards James C. , Perry Malcolm B. , Kniskern Peter J.
Published: May 2006

The structure of the specific capsular polysaccharide produced by Streptococcuspneumoniae type 22F (American type 22) was investigated by high-field 1H and 13C nuclear magnetic resonance spectroscopy, composition, methylation analysis, and periodate oxidation studies. The polysaccharide was found to be a high molecular weight acidic polymer composed of D-glucose, D-galactose, D-glucuronic acid, and L-rhamnose residues to form a regular repeating hexasaccharide unit having the structure[Formula: see text]in which the β-L-rhamnopyranosyl residues were substituted by O-acetyl groups in 80% of the repeating units. The 1H and 13C nmr resonances of the O-deacetylated type 22F polysaccharide were completely assigned by application of two-dimensional homo- and heteronuclear chemical shift correlation techniques. Keywords: Streptococcuspneumoniae polysaccharide, NMR analysis.
Une application du cycle pyrazoline à la lactamisation et à la formylation de 6H thiazines-1,3
Authors: Tuloup Rémy , Danion-Bougot Renée , Danion Daniel , Pradère Jean-Paul , Toupet Loïc
Published: May 2006

A pyrazoline ring is used as a masking, a stereodirecting, and a latent formyl group in the synthesis of 7-amino-4-formyl cephems from 6H-1,3-thiazines. Keywords: thiazine, cephem, pyrazoline, benzeneseleninic anhydride.
Tris-imidazole tripodands: synthesis and complexation
Authors: Potvin Pierre G. , Gau Roberto , Unrau Cathleen M. , Gehl Nikolaus K.
Published: May 2006

Simple syntheses of seven imidazole-terminated tripodands are described. Some are N-methylated and water-soluble and others are N-benzylated and only lipo-soluble. Pronounced and unusual selectivity for hydrazinium ion was found in complexation experiments using Cram's extraction methods. Keywords: imidazole, tripodands, extraction, selectivity, hydrazinium.
ipso Nitration in p-halophenyl ethers
Authors: Clewley Robin G. , Fischer Alfred , Henderson George N.
Published: May 2006

Addition of nitronium ion ipso to halogen occurs on nitration of the p-haloanisoles in acetic anhydride at −60 °C. In the cases of p-fluoro- and p-chloro-anisole, addition of the nitronium ion is reversible and only small amounts of ipso products are obtained. With p-bromoanisole nitrodebromination occurs. When p-halophenyl ethers containing a trapping substituent, e.g., 2-(4-chlorophenoxy)-2-methylpropanoic acid, are used as substrates, substantial amounts of the spiro diene with nitro ipso to halogen, e.g., 3,3-dimethyl-8-chloro-8-nitro-1,4-dioxaspiro[4.5]deca-6,9-dien-2-one, can be isolated. The results demonstrate that extensive ipso attack at the halogen-substituted position is general in the nitration of p-halophenyl ethers. Keywords: ipso nitration, ether, diene, p-haloanisole.
Characterization of metal complexes of 1,10-phenanthroline, 2,2′-bipyridine, and their derivatives by fast atom bombardment mass spectrometry
Authors: Miller Jack M. , Balasanmugam Kesagapillai
Published: May 2006

A series of 1,10-phenanthroline (phen), 5-nitro-1,10-phenanthroline (5-NO2-phen), 2,9-dimethyl- 1,10-phenanthroline (dmp), 2,2′-bipyridine (bpy), and 4,4′-dimethy-2,2′-bipyridine (dmb) metal complexes has been characterized using fast atom bombardment mass spectrometry (FAB MS). Intact cations are observed in the positive ion FAB mass spectra of all tetracoordinated complexes, [Ag(L)2]NO3, [Cu(L)2]2 SO4•5H2O, [Cu(L)2]SO4, and [Tl(L)2](ClO4), where L=bpy, phen, or dmp. Structurally significant fragment ions (ML2+, ML+, M+, and (L+H)+, where M+=metal ion) are also observed. Species such as Ag[Ag(L)2]NO3+, where L=bpy or phen, and Tl[Tl(phen)2](ClO4)+ are also seen. The hexacoordinate complexes, [Co(phen)3]Cl2, [Ru(bpy)Cl2], [Fe(phen)2(CN)2], [Fe(L)3](ClO4), where L=5-NO2-phen, dmb, or bpy, and [Fe(L)3]SO4, where L=bpy or phen, show intact cations in the positive ion spectra; [Ni(bpy)3]X2, where X=Cl−,Br−, NO3−, ClO4−, SCN−, and tartrate2−, and [Co(bpy)3](ClO4) do not. Generally, fragment ions such as ML3+, ML2X+, ML2+, MLX+, and (L+H)+ are observed, where X=singly charged intact anion or HSO4−. Comparison of FAB results with those of laser mass spectrometry (LMS) show that they are comparable, through reduction processes are more important for FAB. LMS always gave the intact cation while FAB was not consistent. Keywords: FAB mass spectrometry, phenanthroline complexes, bipyridine complexes, laser desorption mass spectrometry.
The solution structures of chiral Ti4+ alkoxides
Authors: Potvin Pierre G. , Gau Robert , Kwong Patrick C. C. , Bianchet Stephen
Published: May 2006

1,4-Di-O-methyl-L-threitol and 1,2,5,6-tetra-O-methyl-L-mannitol were prepared by new, more convenient, and higher yielding routes. These and 1,2;5,6-di-O-isopropylidene-D-mannitol formed 2:2 complexes with Ti(OiPr)4, but did not readily form 1:2 complexes in the presence of excess Ti4+. 1H and 13C nuclear magnetic resonance spectra showed that the structures in solution of the 2:2 complexes were analogous to solid-state structures of tartaric acid derivatives, i.e., possessing bridging diolate oxygens. In contrast, R,R- and meso-diisopropyl tartrates, Ν,Ν′-dibenzyl tartramide, and Ν,Ν′-di(2-phenyl)ethyl tartramide behaved very differently. The tartramides formed mixtures of complexes that became dominated by 2:3 complexes in the presence of excess Ti(OiPr)4. Two tertiary tartramides failed to provide well-defined complexes. The R,R-tartrate formed a monocyclic (nonbridged) 2:2 complex, but an equilibrating mixture of 1:2 and 2:2 complexes with excess Ti4+. The equilibrium shifted upon cooling toward a 1:2 complex possessing a bridging diolate oxygen, as did the meso 1:2 complex. Other temperature-dependent phenomena were also studied. Keywords: titanium(IV), chiral alkoxides, solution structures, NMR, Sharpless catalysts.
Reactions of α-phenylglycinamide with some carbonyl compounds. Formation of 5,7-diisopropyl-8,8-dimethyl-2-oxo-3-phenylimidazolidino-[1,2-c]-tetrahydro-[1,3]-oxazine, and determination of structure and stereochemistry by nuclear Overhauser effect difference measurements
Authors: Qing Ning , Colebrook Lawrence D. , Edward John T. , Kon Allan , Chubb Francis L.
Published: May 2006

α-Phenylglycinamide 5 reacts with ketones to give either the Schiff base 3 or the 4-imidazolidone 4, but reacts with boiling isobutyraldehyde to give the bicyclic compond 6. The structure and stereochemistry of 6 have been established by 1H nuclear Overhauser effect difference measurements, supported by molecular mechanics calculations. Keywords: structure, stereochemistry, nuclear Overhauser effect, molecular mechanics, α-phenylglycinamide reactions.
The dissociation constant and acid hydrolysis rate of hydroxysulfamic acid
Authors: Littlejohn David , Wizansky Abigail R. , Chang S. G.
Published: May 2006

The acid dissociation constant and hydrolysis rate of hydroxysulfamic acid (HSA) in acid aqueous solutions have been studied. The acid dissociation constant was determined to be K = 1.5 ± 0.5 M at 298 K. The hydrolysis rate, R, was found to be R = k[H+][HSA], where k = 6.2 × 1012 exp (−26 300/RT) M−1 s−1 at (μ = 1.0 M and [HSA] is the concentration of all forms of hydroxysulfamic acid. Keywords: hydroxysulfamic acid, hydrolysis, acid dissociation.
Structural studies of organoboron compounds. XXXIV. 4-(N-Methoxymethylimino)-2,2-diphenyl-1,3-dioxa-2-borata-1,2,3,4-tetrahydronaphthalene and 4-diethylimino-2,2-diphenyl-1,3-dioxa-2-borata-1,2,3,4-tetrahydronaphthalene
Authors: Kliegel Wolfgang , Tajerbashi Mahmood , Rettig Steven J. , Trotter James
Published: May 2006

Details of the syntheses, physical properties, and crystal structures of the title compounds are reported. Crystals of 4-(N-methoxymethylimino)-2,2-diphenyl-1,3-dioxa-2-borata-1,2,3,4-tetrahydronaphthalene, 4, are monoclinic, a = 9.8072(5), b = 9.6217(7), c = 19.6793(12) Å, β = 94.871(5)°, Z = 4, space group P21/c, and those of 4-diethylimino-2,2-diphenyl-1,3-dioxa-2-borata-1,2,3,4-tetrahydronaphthalene, 9, are orthorhombic, a = 17.0593(7), b = 16.1372(6), c = 14.3753(5) Å, Z = 8, space group Pbca. Both structures were solved by direct methods and were refined by full-matrix least-squares procedures to R = 0.046 and 0.038 for 2198 and 2969 reflections with I ≥ 3σ(I), respectively. In both structures the N,N-dialkylsalicylamidate ligands coordinate the diphenylboron moieties via the phenolate oxygen and amide oxygen atoms, forming six-membered O—B—O chelate rings. The O(ar)—B, O(amide)—B, and mean C—B bond lengths (corrected for libration) are as follows: 1.490(3), 1.549(4), and 1.612 Å for 4 and 1.494(2), 1.550(2), and 1.612 Å for 9. Keywords: crystal structures, boron compounds, organoboron compounds.
Chromyl fluoride in acidic and basic hydrogen fluoride solutions
Authors: Besida John , O'Donnell Thomas A. , Eller P. Gary
Published: May 2006

Chromyl fluoride provides a nonconducting solution in anhydrous hydrogen fluoride with Raman spectroscopic features indistinguishable from those of the solute in the gas phase, indicating no fluoride or proton transfer in solution. With increasing acidity in the SbF5–HF solvent system, CrO2F2 forms cationic species, the nature of which is discussed. The acidity-dependence of cationic speciation in HF–TaF5 and in HF–AsF5 is consistent with that in HF–SbF5. Keywords: chromyl fluoride, antimony fluoride, Lewis acid, Raman spectroscopy, hydrogen fluoride.
A dynamic NMR study of the nitrogen lone pair orientation in 2-benzoyl-5-methyl-5-aza-1,3-dithiacyclohexane
Authors: Pinto B. Mario , Johnston Blair D. , Nagelkerke Ruby , Juaristi Eusebio , González Eduardo A.
Published: May 2006

Nuclear Overhauser enhancement difference experiments and magnetization transfer experiments on 2-benzoyl-5-methyl-5-aza-1,3-dithiacyclohexane 1 at 186 K show conclusively that the N-methyl group adopts a preferred axial orientation. The result has implications for evaluation of the origin of the anomeric effect in this and related compounds. The free energy of activation for ring interconversion in 1 is estimated by spin saturation transfer experiments to be 10.7 ± 0.4 kcal mol−1. Keywords: D NMR, conformational analysis, nuclear Overhauser enhancement, saturation transfer.
4,5-Dihydro-4,5-dihydroxyimidazoles as products of the reduction of 2-nitroimidazoles. HPLC assay and demonstration of equilibrium transfer of glyoxal to guanine
Authors: Panicucci Rick , McClelland Robert A.
Published: May 2006

An HPLC method has been employed to study the electrochemical reduction (mercury cathode at −800 mV with respect to calomel electrode) of the 2-nitroimidazole benznidazole (N′-benzyl-(2′-nitro-1′-imidazoyl)acetamide). The principal product of this reduction is the cyclic guanidinium ion 3c (protonated N′-benzyl-(2′-amino-4′,5′-dihydro-4′,5′-dihydroxy-1-imidazoyl)acetamide), which forms in a linear fashion as the nitroimidazole is reduced and accounts for 75% of the product upon completion of the reduction. To perform the HPLC analysis quantitatively an authentic sample of this product (isolated as cis-trans isomers) was prepared as the sulfate salt through the reaction of N-benzyl-2-guanidinoacetamide sulfate with aqueous glyoxal. The two isomers of 3c arise through the nonreductive decomposition of the 2-hydroxylaminoimidazole, which is the product of a four-electron reduction of the nitroimidazole. Analysis of high field 1H NMR spectra also showed that the two isomers of 3c were the principal products following electrochemical reduction, neutral aqueous zinc reduction, and radiation chemical reduction. Previous investigations using NMR of the reductions of misonidazole (3-methoxy-1-(2′-nitro-1′-imidazoyl)-2-propanol) and 1-methyl-2-nitroimidazole have shown that the corresponding dihydroimidazoles 3a and 3b are the major products. The agreement of these various NMR and HPLC results suggests that the formation of dihydroimidazoles 3 is a general phenomenon for model reductions of 2-nitroimidazoles in neutral aqueous solution. Previous workers have shown that 2-nitroimidazole reduction mixtures, when treated with guanine derivatives, form the adduct 4 derived from the guanine and glyoxal. This work demonstrates that this adduct is also formed when authentic samples of 3a, 3b, and 3c are reacted with 2′-deoxyguanosine. A quantitative HPLC analysis, however, demonstrates that the reaction does not proceed to completion, and in fact the equilibrium for formation of 4 is unfavorable. This suggests that guanines are not useful derivatizing agents for the quantitative assay of "glyoxal-like" products formed in chemical or biological reductions of 2-nitroimidazoles. Keywords: nitroimidazole, reduction of nitroimidazoles, glyoxal from nitroimidazoles.
On the correlation of the excess surface heat capacity and the surface molar volume of liquid
Authors: Goldsack Douglas Eugene , Paquette Lise Jeanne
Published: May 2006

Excess surface heat capacity values [Formula: see text] have been calculated using several surface density models. It is expected that calaculated [Formula: see text] values would be negative to be thermodynamically consistent. Using bulk liquid volume data as a means of calculating first estimates of surface volumes yields positive [Formula: see text] for a group of compounds of wide polarity. However, a simple linear or non-linear surface density model relating bulk density to surface density and reduced temperatures has been shown to yield negative values of [Formula: see text] This new surface density model is applicable to both polar and non-polar compounds and thus should prove useful for calculating molar surface areas for polar compounds which are necessary for calculating the surface tensions of polar–polar liquid mixtures. Keywords: excess surface heat capacity, surface density, monolayer model.
Solubility of ammonia in chloroform: analysis in terms of Henry's law and the equilibrium constant for hydrogen-bonded complex formation
Authors: Smith John F. , Li Charles , Roth Myron , Hepler Loren G.
Published: May 2006

We have measured the solubility of ammonia in chloroform at several pressures and at three temperatures (25, 30, and 35 °C). Results of these measurements have been analysed in terms of the Henry's law constant that applies to "simple" dissolution of the type B(g) = B(dissolved) and the equilibrium constant K for the liquid phase complex-forming equilibrium represented by A + B = AB where A and B represent chloroform and ammonia, respectively. As expected on the basis of this analysis, ammonia is more soluble (same pressure and temperature) in chloroform than in carbon tetrachloride, where no hydrogen-bonded complex can be formed. Keywords: gas solubility, hydrogen-bonded complexes, Henry's law constant, ammonia–chloroform complex, chloroform-ammonia complex.
An example of conformational disorder in solids: X-ray crystallographic and 13C CPMAS NMR studies of dibenzo and dicyclohexano-13-crown-4 ethers and some lithium complexes
Authors: Buchanan G. W. , Kirby R. A. , Charland J. P.
Published: May 2006

For the cis–anti–cis isomer of dicyclohexano-13-crown-4 ether, disorder has been detected in the crystal structure, with two conformations being present in the ratio of ca. 3:2. Crystal structures for two Li+ complexes of the cis–syn–cis isomer have also been determined. Representative 13C solid phase NMR spectra are presented and discussed for these systems and the related dibenzo substituted compounds. Keywords: crown ethers, solids, NMR, conformation.
Inert films on electrodes. Electrochemical properties of carbon fibre electrodes modified by α- and β-naphthol electropolymerization
Authors: Karabinas Pantelis
Published: May 2006

Modification of carbon fibre electrodes has been achieved by electropolymerization of α- and β-naphthols. The formation of the film was followed spectrophotometrically by probing the decrease of the concentration of the corresponding monomer in dilute solutions. The electrochemical behaviour of several depolarizers on the modified electrodes showed that the charge transfer takes place at the electrode–film interface. The film structure can be satisfactorily described by the "pin-hole" model. Keywords: carbon fibres, inert film, electromodification, α-naphthol, β-naphthol.
Structure of an acidic polysaccharide elaborated by Bacilluspolymyxa-1231-ATCC 842
Authors: Abdel-Kader Salah M. , Issa Mohamady A. , El-Shafei Mohamed A.
Published: May 2006

An extracellular acidic polysaccharide, elaborated by Bacilluspolymyxa, was composed of D-glucose, D-mannose, D-galactose, and D-glucuronic acid (approximate molar ratio of 4:3:1:1). Methylation and fragmentation analysis by partial acid hydrolysis indicated that the polysaccharide had a complicated, highly branched structure, consisting mainly of β(1 → 3) and (1 → 4) D-glycosidic linkages. The backbone chain containing D-glucose, D-mannose, and D-glucuronic acid residues is attached at the C-4, C-3, and C-4 positions, respectively, one out of every three glucose residues being substituted at the C-2 position to form two kinds of branches at the C-2 and C-4 positions, with side chains of single or a few carbohydrate units that are terminated by D-glucose and (or) D-mannose residues. Keywords: acidic polysaccharide, Bacilluspolymyxa.
The preparation and reactions of ω-bromoacylmanganese pentacarbonyls
Authors: Masters Andrew P. , Sorensen Ted S.
Published: May 2006

The reaction of ω-bromoacyl chlorides Br—CH2—(CH2)n—COCl and Mn(CO)5− is reported. In the case of n = 0, one obtains ketene, Mn(CO)5Br, and chloride ion. In contrast, the corresponding chloro compound gives the known chloroacyl manganese complex. With n = 1, 2, or 3, one can isolate the corresponding ω-bromoacylmanganese complexes. On mild heating, the n = 1 complex produces only ethylene and Mn(CO)5Br. The n = 2 and 3 complexes are also relatively labile on heating in THF solution and produce the 1-alkene as the major organic product. In chloroform solution, the corresponding ω-bromoalkyl complex is produced initially on heating, but further heating again produces mainly the 1-alkene. However, the n = 2 complex also produces appreciable amounts of cyclopropane. Keywords: bromoacylmanganese pentacarbonyl, pentacarbonyl manganate, ethylene, cyclopropane.
The photoelectron spectra and electronic structure of 2-carbonyl furans
Authors: Klapstein Dieter , MacPherson Charles D. , O'Brien Robert T.
Published: May 2006

The He(I), λ = 58.4 nm, photoelectron spectra of a series of six 2-carbonyl furans and 2-furonitrile have been measured in the gas phase. The low ionization energy bands are assigned to photoionizations from π orbitals of the furan ring, orbitals localized on the carbonyl group, and orbitals of the carbonyl substituents. Changes in ionization energies within the series can be rationalized in terms of the relative conjugative and inductive effects of the substituents. Substituent inductive effects were found to be dominant. The relatively broad vibrational fine structure on the low IE bands of the carbonyl furans could result from the presence of rotational isomers of the species. Keywords: photoelectron spectra, 2-carbonyl furans, molecular orbitals.
Computer simulation studies of the solvation of oligosaccharides
Authors: Beierbeck Helmut , Lemieux Raymond U.
Published: May 2006

Computer simulations of the hydration of the tetrasaccharide αLFuc(1 → 2)βDGal(1 → 3) αLFuc(1 → 4) βDGlcNAc-OMe and one of its deoxy derivatives were carried out as a first step toward an understanding of the role of water in the recognition of oligosaccharides by proteins. The model consisted of one solute molecule in one of its low-energy conformations, surrounded by 250 water molecules. The solute–water and water–water interaction energies were evaluated with Clementi's potentials. Ensemble states for the derivation of average properties were generated by Monte Carlo methods. As expected, the strength of interaction between water and the amphiphilic solute was found to be considerably greater at hydrophilic than at hydrophobic sites. Furthermore, the average distance between the solvation shell and the solute surface was about 1 Å greater near hydrophobic than around hydrophilic groups. There is also a definitive alignment of the water dipole around the hydroxyl groups of the saccharide, which is not evident over the hydrophobic regions. Finally, in a parallel investigation the tetrasaccharides were solvated by adding water to the solute at sites of minimum energy of association, in order to explore less-CPU-time-consuming alternatives to Monte Carlo simulations. While the patterns of interaction energies, distances, and dipole orientations were found to be similar to Monte Carlo results, the time savings were not as significant as hoped. Keywords: Monte Carlo simulations, numerical solvation studies, solvation simulations of oligosaccharides, molecular recognition, carbohydrate solvation.
Synthesis, electronic spectroscopy, and evaluation of photoconducting oligomers with pendant bichromophores. Part 1. Influence of donor–acceptor orbital overlap on aggregate emission and photoactivity
Authors: Gravel D. , Gauthier S. , Brisse F. , Raymond S. , D'Amboise M. , Messier P. , Zelent B. , Durocher G.
Published: May 2006

The synthesis, electronic spectroscopy, and photoactivity of two polysebacate oligomers with pendant donor–acceptor bichromophores of the alkylcarbazole–dinitrofluorene type (P1) and the alkylcarbazole–trinitrofluorenone carboxylic acid ester type (P2) are reported. The crystal structure of the basic acceptor chromophore A2 and donor-acceptor bichromophore B2 of oligomer P2 are also reported. In bichromophore B2, the amount of overlap is evaluated at 50% while the distance between the donor and the acceptor is 3.42 Å. It is shown that while no ground state intramolecular charge-transfer interaction takes place in the bichromophore B1 of oligomer P1 itself (in solution or the solid state), aggregates (acceptor–acceptor) are present in 3MP and in the solid state. The spectrum of oligomer P1 in low temperature 3MP matrices indicates a small aggregation effect while aggregate fluorescence of bichromophore B1 and oligomer P1 is observed in the solid state. Absorption spectra show that bichromophores B2 and B2a and oligomer P2 give rise to charge-transfer bands with maxima at around 550 and 450 nm in CH3CN and 590 and 470 nm in the solid state. The fluorescence and the phosphorescence of the carbazolyl chromophore and the phosphorescence of the 2,4,7-trinitro-9-fluorenone chromophore are quenched for B2, B2a, and P2 in EPA but a structureless emission is observed between 500 and 680 nm in the solid state that can be assigned to aggregate fluorescence. The negative values of ΔGET indicate that intramolecular electron transfer in bichromophores B1, B2, and B2a should be highly probable on thermodynamic grounds if optimal orbital overlap is available. The spectroscopic properties of the bichromophoric compounds studied are discussed on the basis of conformational effects. The photoactivity evaluation of oligomers P1 and P2 shows that P2 is more photoactive than P1 due, mainly, to better orbital overlap between the donor and the acceptor moieties in oligomer P2 as compared to oligomer P1. Furthermore, a comparison of P1 and P2 to a standard PVK–TNF system reveals that the latter is much more photoactive due, again, mainly to better orbital overlap. Finally, the relative efficiencies of these systems as hole or electron conductors is discussed. Keywords: photoconducting polymers, donor–acceptor bichromophores, bichromophores, substituted carbazoles, poly-nitrofluorene derivatives.
On the relationship between thermodynamic pKa's of azoles and the oxidation potentials of their pentacyanoferrate(II) complexes
Authors: Lopez Vicenta , Catalan Javier , Claramunt Rosa Ma , Lopez Concepcion , Cayon Eduardo , Elguero Jose
Published: May 2006

The comparison between thermodynamic pKa values and oxidation potentials of the pentacyanoferrate(II) complexes of a series of azoles shows the existence of two parallel lines: one for pyrazoles and another one for benzazoles. Using the equation corresponding to the second one, the pKa value of 1-methylbenzotriazole (2.49) can be estimated from the E1/2 value of its complex. Keywords: azole basicity, oxidation potentials, pyrazoles, benzotriazoles.
The conformational equilibria of 3-methoxybenzaldehyde in polar and nonpolar solutions. 1H NMR and MO calculations
Authors: Schaefer Ted , Sebastian Rudy , Li Hua Yan , Quach Tinh
Published: May 2006

Analyses of the 1H nuclear magnetic resonance spectra of 3-methoxybenzaldehyde in CS2/C6D12 and in acetone-d6 solutions at 300 K yield the values of stereospecific long-range spin–spin coupling constants for the aldehydic and methyl protons. These coupling constants are used to deduce the conformational bias for both substituents. The conformers in which the carbonyl group lies trans to the methoxy group have a population Pt, which is 0.63(2)Pc for the CS2/C6D12 solution, Pc being the population of the conformers in which the carbonyl group lies cis to the methoxy substituent; in acetone-d6 solution, Pt = 0.80(2)Pc. Both of these numbers are substantially larger than that deduced from nuclear Overhauser enhancements for a CDCl3 solution. If Pt is the fractional population of the conformers in which the methoxy is oriented trans to the aldehyde substituent, then Pt = 0.53(1)Pc in CS2/C6D12 and Pt = 0.67(1)Pc in acetone-d6, both numbers again being rather larger than that deduced from polarization experiments. STO-3G and 4-21G MO computations of the conformer stabilities agree qualitatively with the relative stabilities of the four planar conformers. Extrapolation of the populational data in solution yields vapor phase estimates midway between the two theoretical estimates. Limits can be placed on the fractional population of each of the conformers in the vapor and in the solutions. The relative stabilities of the free conformers can be rationalized in terms of electrostatics and of preferences of the carbonyl and methoxy groups for certain orientations with respect to the ring C—C bonds of highest double bond character. Keywords: 3-methoxybenzaldehyde, NMR; 3-methoxybenzaldehyde, conformations for solutions and vapor; 3-methoxybenzaldehyde, MO calculations.
13C magnetic resonance studies. 142. The fate of highly substituted acyclic ketones in strong base: β-enolization, Haller–Bauer cleavage, and an alkyl to aryl acyl shift
Authors: Dyllick-Brenzinger Rainer A. , Patel Vijay , Rampersad Mollin B. , Stothers J. B. , Thomas Shirley E.
Published: May 2006

Di-tert-butyl ketone (2a), 5,5,7,7-tetramethyl-6-undecanone (2b), and 2,4-dimethyl-2,4-diphenyl-3-pentanone (2d) have been found to undergo slow rearrangement by β-proton abstraction from the α-methyl groups upon treatment with t-BuO−/t-BuOH/≥ 185 °C. This is in contrast to the absence of such rearrangement in cyclic systems. It is the only significant process observed for 2a and 2b, whereas with 2d and its monomethyl analog 22 Haller–Bauer type cleavage and an unprecedented 1,3-acyl shift from sp3 to sp2 carbon accompany the β-enolate rearrangement. The rates of 1H/2H exchange of the methyl protons in 2a, b were found to be similar to those reported for a variety of cyclic systems but this exchange does not occur in 2d, where only aryl proton exchange was observed. Keywords: β-enolization, acyclic ketones, acyl shift.
Spruce budworm (Choristoneurafumiferana) antifeedants 2. Structure–activity relationship among some functionalized decalin compounds
Authors: Chan T. H. , Guertin K. R. , Prasad C. V. C. , Thomas A. W. , Strunz G. M. , Salonius A.
Published: May 2006

Several compounds, each possessing a polyoxygenated decalin structure, were synthesized and tested for antifeedant activities against the spruce budworm, Choristoneurafumiferana. Definite biological activity was observed for some of these compounds. A structure–activity relationship was discussed on the basis of these results. Keywords: decalin, synthesis, antifeedant, spruce budworm.
Reactions of thiyl radicals with phenothiazines and other antioxidants in aqueous solutions. A pulse radiolysis study
Authors: Lal Manohar , Mahal H. S.
Published: May 2006

Reaction rate constants of thiyl radicals with phenothiazines, promethazine (PMZ), chlorpromazine (CPZ), prochlorperazine (PPZ), trimeperazine tarterate (TPZ), and other antioxidants, e.g., ascorbic acid and 2,2-azinobis(3-ethyl benzthiazoline-6 sulphonate) (ABTS) have been estimated using pulse radiolysis technique. In general they are quite high (108–109 M−1 s−1). The k values for cysteine, cysteamine, mercaptoethanol, and mercaptopropionic acid with similar structures and molecular weights are more or less similar for PMZ, CPZ, and PPZ.Rate constants are lower in value for [Formula: see text] radicals of phenothiazines showing that they are dependent upon the structure and the molecular weight of the compound.Rate constants are the same whether the thiyl radicals are generated from thiols or their corresponding disulphides. The k values for chlorine-containing phenothiazines, chlorpromazine, and prochlorperazine are higher than those of promothazine Keywords: pulse radiolysis, thiyl radicals, phenothiazines, ABTS, ascorbic acid.
Liquides de toupies symétriques: leur tenseur de polarisabilité optique intrinsèque via deux expériences sous champ statique: CH3CN
Authors: Brot Claude
Published: May 2006

NMR line splitting under a strong static electric field (EF-NMR) allows one to measure the average alignment (second spherical harmonic) of the molecule studied under the influence of the field. This average alignment (per unit squared field) can be formulated theoretically by an expression involving the permanent dipole moment, the anisotropy of the static polarizability, etc. and possibly orientational intercorrelations within the liquid. On the other hand the Kerr constant per molecule can be represented by the same expression, multiplified by a factor that is independent of any static electrical property of the molecule (for a gas this factor would be simply proportional to the anisotropy of the optical polarizability). Consequently the ratio of the above experimental results contains information bearing only on the optical properties. The formalism of the "tensor of polarizability increment" developed by the author for liquids made up of ellipsoidal molecules is employed. It is shown that by combining the above ratio with the constitutive equation for the refractivity of the liquid, the two distinct principal elements of the tensor of polarizability increment of the symmetric top molecules constituting the liquid are immediately obtained. Using then the molecular volume, a van der Waals estimate for the elongation of the molecule, and the index of refraction of the liquid, one can calculate the elements of the optical polarizability tensor invacuo. The method is illustrated by neat acetonitrile for which the necessary experimental data exist in the literature. One finds, in units 1040 C2 m2 J−1 5.06 for the mean polarizability and 2.52 for its anisotropy. A published experimental study using depolarized light scattering in the gas phase indicates that the corresponding figures are 4.96 and 2.49 respectively at λ = 633 nm. Finally and accessorily, it is suggested that dipolar intercorrelations are weak in acetonitrile. Indeed, using an analogous formalism for the static permittivity of the liquid, a value for the permanent moment of the molecule is deduced. This value is within 1% of the experimental gas value. Keywords: polarizability tensors, liquids, Kerr constant, NMR.
The formation of N-alkylporphyrins during epoxidation of ethylene catalyzed by iron(III) meso-tetrakis(2,6-dichlorophenyl)porphyrin
Authors: Nakano Taku , Traylor Teddy G. , Dolphin David
Published: May 2006

During the epoxidation of ethylene using iron meso-tetrakis(2,6-dichlorophenyl)porphyrin chloride and iodosopentafluorobenzene several N-alkylporphyrins were formed. The major product was 21-carboxymethyl-5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin. This was derived, by oxidation, from the corresponding 21-formylmethyl complex which in turn was obtained from the initially formed N-hydroxyethylporphyrin, a compound not isolated due to its ready oxidation. Keywords: N-alkylporphyrins, suicide labelling, cytochrome P-450, hemin catalysis, epoxidation, oxidation.
Radical clocks as probes of 1,4-biradical intermediates in the photochemical cycloaddition reactions of 2-cyclopentenone with alkenes
Authors: Rudolph Andreas , Weedon Alan C.
Published: May 2006

The products of the photochemical reaction of 2-cyclopentenone with 1,6-heptadiene and with vinylcyclopropane have been examined. With 1,6-heptadiene the products were cyclobutanes, which arise from 2 + 2 photocycloaddition between the cyclopentenone carbon–carbon double bond and one of the two terminal double bonds of the heptadiene. The 1,4-biradical that is an intermediate in this reaction contains a derivative of a 1-hexenyl radical; no products derived from cyclization of this intermediate to a cyclopentylmethyl radical were observed. With vinylcyclopropane some of the products isolated also arose from 2 + 2 photocycloaddition; in addition, comparable quantities of products resulting from rearrangement of the intermediate 1,4-biradical were obtained. In this case the initially produced intermediate 1,4-biradical contains a cyclopropylmethyl radical that can rearrange to a homoallylic radical; the products isolated were derived from intramolecular disproportionation in the new 1,7-biradical containing the homoallylic radical. The rate constant for the rearrangement of the cyclopropylmethyl radical to the homoallylic radical is known and was used as a clock to estimate the lifetime of the initially produced 1,4-biradical. The value estimated is of the order of 50 ns. The structures of the products indicate that the initial bonding between the excited cyclopentenone and vinylcyclopropane takes place at both the α-position and the β-position of the enone and not exclusively at the α-position as a recent report claims. Keywords: enone photocycloaddition, 1,4-biradicals, radical clocks.
The ketonization of acetophenone enol in concentrated aqueous sulphuric and perchloric acid solutions. Implication on X acidity function correlations of the enolization reaction and determination of the keto–enol equilibrium constant as a function of acidity
Authors: Chiang Y. , Kresge A. J. , O'ferrall R. A. More , Murray B. A. , Schepp N. P. , Wlrz J.
Published: May 2006

Rates of ketonization of the enol of acetophenone, generated by flash photolytic photohydration of phenylacetylene, were measured in aqueous sulfuric and perchloric acid solutions over the concentration range 1–50 wt.% acid; rates of enolization of acetophenone, monitored by bromine scavenging, were also measured in aqueous perchloric acid solutions over the same concentration range. The results suggest that the curvature observed in a previous X acidity function correlation of the rate of enolization in sulfuric acid solutions was an artifact produced by insufficiently efficient scavenging, and that introduction of the activity of water in the correlating expression, used previously to eliminate the curvature and believed to reflect covalent involvement of water in the enolization reaction, is unnecessary. The present results also show that the keto–enol equilibrium constant for acetophenone decreases with increasing acidity in these concentrated sulfuric and perchloric acid solutions. Key words: acetophenone, enolization, ketonization, keto–enol equilibrium, concentrated acid solutions.
Distorted amides as models for activated peptide N-C=O units. 2. The synthesis, hydrolytic profile, and molecular structure of 3,4-dihydro-2-oxo-1,4-propanoquinoline
Authors: Wang Qingping , Bennet A. J. , Brown R. S. , Santarsiero B.D.
Published: May 2006

A distorted anilide (3,4-dihydro-2-oxo-1,4-propanoquinoline (3)) has been synthesized, its structure determined by X-ray diffraction, and its hydrolysis profile from pH 0–13 investigated. The amide unit in 3 is distorted in that the N lone pair is twisted 35° and tilted 15° out of optimum conjugation with the C=O II system. Due to this distortion, 3 hydrolyses rapidly in H2O at 25 °C. The pH/rate constant profile exhibits domains consistent with OH− attack on 3[Formula: see text] and H2O attack on a protonated form of 3[Formula: see text]; pKa (3-H+) = 0.56). The kinetic pKa is most consistent with an N-protonated form. It is also shown that this mechanism of hydrolysis is common to the series of bridged anilides consisting of 3, 3,4-dihydro-2-oxo-1,4-ethanoquinoline (1), and 2,3,4,5-tetrahydro-2-oxo-1,5-ethanobenzazepine (2). Keywords: distorted amide, hydrolysis, X-ray structure.
Structural studies of organoboron compounds XL. 4,7-Dicyclohexyl-2,2,5,6-tetraphenyl- 1,3-dioxa-4-aza-7-azonia-2-boratacycloheptane
Authors: Amt Henning , Kliegel Wolfgang , Rettig Steven J. , Trotter James
Published: May 2006

The synthesis of 4,7-dicyclohexyl-2,2,5,6-tetraphenyl-1,3-dioxa-4-aza-7-azonia-2-boratacycloheptane is reported. Crystals of the compound are orthorhombic, a = 10.893(1), b = 28.072(1), c = 10.596(1) Å, Z = 4, space group P212121. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.036 and Rw = 0.043 for 2480 reflections with I ≥ 3σ(I). The molecule has a seven-membered chelate structure. The seven-membered ring, which has a twisted chair conformation, is stabilized by an intramolecular [Formula: see text] hydrogen bond. Bond lengths O—B = 1.468(4) and 1.554(4), C—B = 1.621(5) and 1.626(4) Å are consistent with moderately strong binding of the diphenylboron moiety by the O,O-chelating ligand. Keywords: crystal structure, organoboron compound, boron compound.
Benzoxathiete and related structures: experimental and quantum chemical studies
Authors: Naghipur Ali , Reszka Krzysztof , Lown J. William , Sapse Anne-Marie
Published: May 2006

The products, plausible intermediates, and their mechanisms of formation in the aprotic diazotization of 2-[(2-acetoxyethyl)-sulfinyl (and sulfonyl)]anilines with isoamyl nitrate have been investigated by experimental and quantum chemical methods. Oxidation of 1,2,3-benzthiadiazole with hydrogen peroxide in acetic acid affords 1,2,3-benzthiadiazole-1-oxide, 13. The thermal stability of 13 up to 135 °C, together with EPR evidence which disfavors an aryldiazenyl radical precursor, discount 13 as an intermediate in the formation of benzoxathiete 14 or its valence tautomer 17 during aprotic diazotization of the sulfinylanilines. Electron paramagnetic resonance evidence in conjunction with spin trapping indicates an intermediate arylaminyl radical. This evidence, taken together with abinitio calculations of optimized geometries energies and energy differences for biradical intermediate 15, favors a mechanism of formation of benzoxathiete 14 via aryldiazoate anhydride 5 thence to rapid ring closure of carbon-centered radical 8. Further oxidation of 1,2,3-benzthiadiazole-1-oxide, 13, with hydrogen peroxide in methanol – acetic acid affords biphenylene and dibenzo-1,4-oxathiane-S-oxide, 38. Rose bengal sensitized photooxidation of 1,2,3-benzthiadiazole affords 13, biphenylene, and 38. Formation of the latter, in which one of the original S—O bonds has been broken, requires the formation of benzoxathiete-S-oxide, 34, and its rapid valence tautomerism to ketosulfine 36 and (2 + 4) cycloaddition of 36 to the simultaneously generated dehydrobenzene to give 38. Both ring closure of singlet biradical 35 to 34 and valence tautomerism of the latter to 36 are predicted by abinitio calculations to be facile and exothermic. In contrast to the aprotic diazotization of 2-[(2-acetoxyethyl)sulfinyl]anilines, the reaction of isoamyl nitrite with the corresponding sulfonyl anilines may plausibly follow a mechanism via 1,2,3-benzthiadiazole-1,1-dioxide 33 owing to the thermal instability of the latter and supported by abinitio treatments of the energetics of the processes involved. In addition EPR evidence, in conjunction with spin trapping of carbon centered radicals, support the viability of the pathway via 23, 25, 28, and 16 to biphenylene 20. The abinitio calculations of the energy differences between the reaction intermediates and estimates of the activation energies elucidated several aspects of these novel reactions. Keywords: benzoxathiete, abinitio calculations, valence tautomerism.
Synthesis of protein conjugates and analogues of N-acetylneuraminic acid
Authors: Roy René , Laferrière Craig A.
Published: May 2006

N-Acetylneuraminic acid (1) was prepared from the salivary gland mucins of the Chinese swiftlet by an improved procedure using acidic resin hydrolysis. Compound 1 was transformed into the known acetochloroneuraminic acid (3) by a new two-step procedure. Koenigs–Knorr glycosylation of 3 followed by subsequent reductive ozonolysis of the 2-propenyl α-glycoside afforded the key aldehyde precursors 7 and 8, which were coupled to bovine serum albumin or tetanus toxoid by reductive amination. The factors influencing the extent of incorporation were investigated. A series of N-acetylneuraminic acid analogues modified at strategic functionalities were also synthesized. Keywords: sialic acid, N-acetylneuraminic acid, neoglycoproteins, bovine serum albumin, tetanus toxoid.
Conformational analysis and structure elucidation of 2,3-dimethyl- and 2,4-dimethyl-2,4-hexadienes by AM1 and He(I) ultraviolet photoelectron spectroscopy
Authors: Werstiuk N. H. , Clark K. B. , Leigh W. J.
Published: May 2006

A study of (E)-2,3-dimethyl-2,4-hexadiene (E-3) and (Z)-2,3-dimethyl-2,4-hexadiene (Z-3) by the semiempirical computational method AM1 and He(I) ultraviolet photoelectron spectroscopy (UPS) has established that this combination of AM1/UPS will be generally useful for detailing conformational and electronic properties of alkyl-substituted 1,3-dienes. This method was used to elucidate the structures of the stereoisomeric 2,4-dimefhyl-2,4-hexadienes obtained from the photolysis of 1,3,3,4-tetramethylcyclobutene (2). Keywords: 2,3- and 2,4-dimethyl-2,4-hexadienes, conformational analysis, AM1, He(I) photoelectron spectroscopy.
Catalysis by α-cyclodextrin of the reaction of formic acid with bromine
Authors: Tee Oswald S. , Javed Bushra , Mikkelsen Susan R.
Published: May 2006

α-Cyclodextrin (CD) modestly increases the rate of oxidation of formic acid by bromine. The variation of rate constants with [Br−] is consistent with dominance of the formation of a CD•Br3− complex in determining the fraction of free Br2 and the CD•Br2 complex, under the reaction conditions. Potentiometric measurements support a dissociation constant of about 0.2 mM for the CD•Br3− complex, in agreement with an earlier spectrophotometric value. Analysis of the rate increases with [CD] implies that CD•Br2 is more reactive (~10 times) than free Br2 towards formate ion, as was found for phenols. These conclusions differ from those of another recent study. Keywords: catalysis, α-cyclodextrin, oxidation, formic acid, bromine.
The photochemistry of 1-cyano-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene
Authors: Bender Christopher Owen , Clyne Dean Sutherland , Dolman Douglas
Published: May 2006

The title compound (10) was prepared, along with the 4-cyano isomer (11), by the direct irradiation of 5-cyanobenzocyclooctatetraene (9); the yields were 82% (Φ10 = 0.0030) and 12% (Φ11 = 0.0004) respectively. Triene 10 was thermally and photochemically reactive. Heating solutions of 10 at 150 °C for 1 h gave COT 9 quantitatively. On direct irradiation 10 forms 5-cyanobenzosemibullvalene (12; 5%, Φ = 0.019), COT 9 (70%, Φ = 0.56), and 1-cyanonaphthalene (14%, Φ = 0.078). Sensitized irradiation of 10 gave 9 exclusively (92%, Φ = 0.88). COT 9 was also produced by the direct irradiation of semibullvalene 12 (75%, Φ = 0.13). Studies with deuterium labelled 10 suggest that the photoformation of COT 9 involves a simple electrocyclic opening of the cyclobutene ring of the triene. Additionally, the labelling results indicate that the formation of semibullvalene 12 from 10 derives from the operation of two reaction pathways, the major one of which appears to be a Zimmerman di-π-methane rearrangement. The mechanism proposed for the minor pathway to 12 has not been observed in other bicyclo[4.2.0]octatrienes. Key words: mechanisms, rearrangements, photochemistry, di-π-methane.
π-Facial stereoselectivity in the Diels–Alder reactions of a rigid carbocyclic diene: spiro(bicyclo[2.2.1]heptane-2,1′-[2,4]cyclopentadiene)
Authors: Burnell D. Jean , Valenta Zdenek
Published: May 2006

The plane-nonsymmetrical diene spiro(bicyclo[2.2.1]heptane-2,1′-[2,4]cyclopentadiene) (2) was synthesized, and the π-facial stereoselectivity of its Diels–Alder cycloadditions with three cyclic dienophiles was examined. Adduct ratios were very similar, and the selectivity is surmised to be mainly the result of steric interactions at the transition state. Key words: facial stereoselectivity, Diels–Alder, cycloaddition.
Phenyltetramethylguanidines: substituent effect on rates and isotope effects in their reaction with bis(4-nitrophenyl)cyanomethane in acetonitrile
Authors: Pruszynski Przemyslaw , Leffek Kenneth T.
Published: May 2006

A series of 22 phenyl substituted derivatives of 2-phenyl-1,1′,3,3′-tetramethylguanidine were used as bases in the proton transfer reaction from bis(4-nitrophenyl)cyanomethane in acetonitrile solvent. Brønsted and Hammett type relationships are examined for this family of closely related substrates. The Brønsted relationship constructed from the pKa values determined in acetonitrile shows β = 0.55 ± 0.03. The best fit of the Hammett relationship, with special values for p-NO2 and p-CN groups, gives the reaction constant ρ = −1.39 ± 0.1. The kinetic isotope effect, kH/kD, increases from 9.6 to 12.4 at 25 °C with decreasing base strength, i.e., when the reaction becomes more thermoneutral. Key words: tetramethylguanidine, proton transfer, deuterium isotope effects.