bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectator catalysis in the cleavage of p-nitrophenyl acetate and p-nitrophenyl hexanoate by "hydroxypropyl-β-cyclodextrin"
Authors: Gadosy Timothy A. , Tee Oswald S.
Published: May 2006

Aliphatic alcohols that form host–guest complexes with "hydroxypropyl-β-cyclodextrin" retard the cleavage of m-nitrophenyl acetate by hydroxypropyl-β-cyclodextrin in basic aqueous solution, due to competitive inhibition. By contrast, these same species do not inhibit the reaction of p-nitrophenyl acetate and p-nitrophenyl hexanoate to the same extent and, in some cases, the addition of alcohols serves to increase the rate of reaction. The observed reaction kinetics require the presence of a process that has one molecule of the "potential inhibitor" in the transition state for ester cleavage. Rate constants, ka, for the reaction of the {ester•hydroxypropyl-β-cyclodextrin} complexes with a series of potential inhibitors show a strong dependence on the ability of the potential inhibitor to bind to the cyclodextrin. On the other hand, rate constants for the kinetically equivalent reaction of the ester with the {cyclodextrin•potential inhibitor} complex show little dependence on the alcohol structure and they vary over a very limited range. The negative logarithms of the apparent dissociation constant of the potential inhibitor from the transition state show a strong dependence on the ability of the potential inhibitor to bind to hydroxypropyl-β-cyclodextrin, indicating that the binding of the potential inhibitor in the initial state and the transition state is similar. It is concluded that the cleavage of p-nitrophenyl acetate and p-nitrophenyl hexanoate by hydroxypropyl-β-cyclodextrin in the presence of 14 potential inhibitors can occur with the ester largely outside of the hydroxypropyl-β-cyclodextrin cavity during the transition state, allowing the cavity to be occupied by a molecule of potential inhibitor. Key words: cyclodextrin, spectator catalysis, esterolysis.
Propargyl calix[4]arenes and their complexes with silver(I) and gold(I)
Authors: Xu Wei , Vittal Jagadese J. , Puddephatt Richard J.
Published: May 2006

A calix[4]arene 1b has been substituted at the lower rim by reaction with propargyl bromide to give either the 1,3- bis(propargyl) derivative 2b, which is formed regioselectively, or the tetrakis(O-propargyl)calix[4]arene 3 under different reaction conditions. The molecular structure of 2b has been characterized by X-ray analysis (monoclinic, space group P21/c, Z = 4, a = 10.960(1), b = 24.302(3), c = 10.673(1) Å, β = 112.99(8)°, R = 0.0516). The molecules of 2b in the crystal are arranged in a "head-to-head, tail-to-tail" manner with a phenyl substituent of one molecule partly enclosed in the bowlic cavity of another. Since the two OCH2C≡CH groups are accessible, 2b can be polymerized by heating to 250 °C and the polymerized propargyl calix[4]arene derivatives are stable up to 460 °C. Compound 3 exists as a mixture of conformers, the ratio of partial cone to 1,3-alternate conformations being 4:1 or 2:1, dependent on the preparative conditions. These propargyl calix[4]arene derivatives can be further derivatized to form silver(I) or gold(I) alkynyl units by reaction with AgNO3 or [AuCl(SMe2)], respectively, and base. Further reaction of these transition metal complexes with phosphines yields calixarene derivatives with two metal-phosphine units at the lower rim of the calix[4]arene bowl. Key words: calix[4]arene, propargyl, gold, silver, polymer.
Dispersion formulas for real- and imaginary-frequency-dependent hyperpolarizabilities
Authors: Bishop David M.
Published: May 2006

The dynamic second hyperpolarizability for real frequencies, [Formula: see text] in the limit ωi → 0 can be expressed as [Formula: see text] where ωL2 = ωσ2 + ω12 + ω22 + ω32 and [Formula: see text] is the frequency-independent (static) quantity; the parallel sutsscript [Formula: see text] indicates that the polarization and electric fields all lie along the same axis. In this paper the coefficient [Formula: see text] is evaluated exactly for the H atom and very accurately for H−, He, and Li+. A similar analysis is carried out for [Formula: see text] in the limit ω → ∞. Key words: nonlinear optics, hyperpolarizabilities, dispersion formulas.On peut exprimer la deuxième hyperpolarisabilité dynamique de fréquences réelles, [Formula: see text] dans la limiteωi → 0, peut être exprimée sous la forme [Formula: see text], dans laquelle ωL2 = ωσ2 + ω12 + ω22 + ω32 et [Formula: see text] est la quantité (statique) indépendant de la fréquence; l'indice parallèle [Formula: see text] indique que la polarisation et les champs électriques sont situés dans le même axe. Dans ce travail, on a fait une évaluation exacte de [Formula: see text] pour l'atome d'hydrogène et d'une façon précise pour H−, He et Li+. On a effectué une analyse semble pour [Formula: see text]dans la limite ω → ∞. Mots clés : optique non linéaire, hyperpolarisabilités, formules de dispersion. [Traduit par la rédaction]
Strongly bound doubly excited states of Be2
Authors: Bruna Pablo J. , Wright James S.
Published: May 2006

The triplet and singlet potential curves of Be2 generated by single and double excitations from 2σu into 3σg and (or) 1πu are studied with a multireference configuration interaction (MRD-CI) method. Relative to X1Σg+(2σg2 σu2) with Re = 4.72 bohr and ωe = 258 cm−1 (calculated here), these antibonding MO → bonding MO excitations lead to average decreases in bond distance (in bohr) of 0.55 (2σu → 3σg), 0.88 (2σu → 1πu), 0.93 (2σu2 → 3σg1πu), and 1.22 (2σu2 → 1πu2). The increase in vibrational frequencies ranges from 240 to 600 cm−1. The 3σg MO is found to be less bonding than 1πu, confirming predictions made by Bader et al. The experimental states A1Πu and B1Σu+ correspond to doubly excited 11Πu (2σu2 → 3σg1πu) and singly excited 11Σu+(2σu → 3σg), respectively. The 13Σg− and 11Δg states, both 2σu2 → 1πu2, preserve their doubly excited structure up to dissociation. Within the Franck–Condon region, 13Πu changes from bound (2σu2 → 3σg1πu) to repulsive (mixed 2σu → 1πg/2σg → 1πu), thereby creating the unusual situation of a strongly bound potential (short Re, high ωe) with an adiabatic dissociation energy near zero. The singlet counterpart 11Πu, however, behaves regularly as its doubly excited character is maintained up to large R(Be—Be). Key words: ab initio calculations, beryllium dimer, doubly excited states, electronic transitions, potential curves.
Atomic origins of molecular polarizabilities
Authors: Laidig Keith E.
Published: May 2006

We demonstrate that the multipole polarizability tensors of a molecule are expressible as a sum of atomic contributions, each of which is based upon the change in the spatial distribution of electronic charge within each atom resulting from the application of electric fields and field gradients. The use of the spatially defined atoms of subsystem quantum mechanics correctly partitions molecular polarizabilities into physically meaningful atomic contributions. It is shown that the origin of any molecular polarizability tensor may be understood from an investigation of these contributions and the general expressions for the summation of atomic components to yield molecular polarizabilities are presented. Their use is demonstrated by the construction and investigation of the axial components of the dipole–dipole dipole–quadrupole, and quadrupole–quadrupole polarizability tensors of carbon monoxide and carbon sulfide. Key words: atomic polarizabilities, atoms-in-molecules, molecular polarizabilities.
The anomeric effect in first-row/second-row acetal-like systems
Authors: Scott Camilla , Grein Friedrich
Published: May 2006

Ab initio calculations were performed at the HF/6-31+G* level on first-row/second-row neutral, protonated, and anionic acetal-like systems. The neutral and anionic systems are of the general form XHmCH2YHn(−) with XHm = F, a-OH, s-NH2, S-CH3; YHn = SH, PH2; and YHn = PH−, SiH2−. For the protonated systems the general form is XHm+CH2YHn, with XHm+ = a-FH+, s-OH2+, s-NH3+; and YHn = SH, PH2. In all cases the hydrogens on Y were rotated about the CY bond, and geometries were optimized for each setting of the YHn dihedral angles. Following previously developed methods, the electronic part e of the anomeric stabilization was evaluated. For PH2 rotation, −ep (e is always negative) is smaller than for corresponding nitrogen systems, whereas for SH rotation the −es parameters are larger than for corresponding oxygen systems. In the case of protonated molecules, the energy associated with the reverse anomeric effect was estimated to be −4 kcal/mol for s-OH2+CH2SH and −3 kcal/mol for s-NH3+CH2SH. For corresponding phosphorus systems they are close to −2 and −5 kcal/mol, respectively. The π-bonding model applied to neutral species shows that in the case of SH rotation the F and O systems clearly follow the trend of the π-bonded cation CH2=SH+. For PH2 rotation, however, only the F system can be considered to correlate with such a model. Key words: anomeric effect, reverse anomeric effect, first-row/second-row atoms, phosphorus, sulphur.
Methoxy-(2-trimethylsilyl)ethoxycarbene. Reactions with Michael acceptors, with hydroxylic compounds, and with miscellaneous functional groups
Authors: Pole David L. , Sharma Pradeep K. , Warkentin John
Published: May 2006

Methoxy-(2-trimethylsilyl)ethoxycarbene reacts with two equivalents of dimethyl acetylene dicarboxylate, methyl propiolate, phenyl acetylene, or phenyl isocyanate without rearrangement of the carbene group. N-Phenylmaleimide captures the carbene with 1:1 -stoichiometry. The structure of the product implies that a migration of the trimethylsilylethyl group from oxygen to carbon accompanies that reaction. A mechanism for that complex rearrangement is proposed. Phenol and tert-butyl alcohol afford the orthoformates expected from overall insertion of the carbene into the OH bond. Key words: dialkoxycarbene, insertion into O—H, C—H bonds, rearrangement, N-phenylmaleimide, dialkoxyoxadiazoline.
Carbon protonation of some phenylynamines in concentrated aqueous perchloric acid solution. Lowering of the upper limit for the pKa of phenylynammonium ions
Authors: Kresge A.J. , Paine S.W.
Published: May 2006

Rates of carbon protonation of five phenylynamines (PhC≡CNH2, PhC≡CNHiPr, PhC=CNHC6F5, PhC≡CN(CH2CH2CN)2, and PhC≡CNMeC6F5) were determined in concentrated aqueous perchloric acid solution and the data were analyzed by the Cox–Yates method using the X0 acidity function. The extrapolated hydronium-ion catalytic coefficients so obtained are consistent with values measured directly in dilute acid solution, and the slopes of the Cox-Yates plots are similar to predictions made with the aid of Marcus rate theory for reactions originating from free ynamine initial states but unlike those predicted for reactions starting from nitrogen-protonated ynammonium ion initial states. This shows that none of these phenylynamines are protonated in even the most acidic solutions used (4 M) and sets new upper limits as low as pKa ≤ −3.1 for the conjugate acids of these ynamines. Comparison of the pKa limit for PhC≡CNH3+ with a literature value for the corresponding saturated compound, PhCH2CH2NH3+, gives a base-weakening effect for the phenylethynyl group of at least 12.5 pK units. Key words: acetylenic amines, concentrated acids, X0 excess acidity scale, Cox-Yates method, Marcus rate theory.
Molecular motion in crown ethers. Application of 13C and 2H NMR to the study of 4-carboxybenzo-24-crown-8 ether and its KNCS complex in solution and in the solid phase
Authors: Buchanan G.W. , Moghimi A. , Ratcliffe C.I.
Published: May 2006

Large-amplitude solid phase molecular motion has been detected in the macrocyclic ring of the title crown either via 13C CPMAS NMR. To study the details of the dynamic processes, two selectively deuterated d4 derivatives have been prepared and examined via 2H NMR as a function of temperature. A phase change occurring around 277 K has been verified by differential scanning calorimetry (DSC) and a model for the motional processes has been developed involving equivalent two-site flips of the CD2 groups. The amplitude of the CD2 motions apparently decreases the closer the group is to the aromatic ring. The influence of KNCS complexation on the 13C CPMAS spectrum and on 13C spin lattice relaxation times in solution has been explored. Key words: macrocyclic ethers, solid phase dynamics.
Sulfonyl esters 7. The second and third sequences in the Trithioorthoformate Reaction
Authors: Ginige Kashyapa Ananda , Goehl John Edward , Langler Richard Francis
Published: May 2006

A previous report has established the intermediacy of a sulfide-sulfonate ester in the one-pot conversion of an aryl mercaptan and a sulfonate ester into the corresponding trithioorthoformate. This report describes evidence for the sequential intermediacy of a bissulfide-sulfonate ester and a dithiosulfène in the conversion of the sulfide-sulfonate ester into the trithioorthoformate. A new sulfonate ester is shown to give the highest yield of trithioorthoformate. Key words: single electron transfer, sulfonate esters, sulfenes.
Hydrogen vs. electron transfer mechanisms in the chain decomposition of phenacyl bromides. Use of isotopic labeling as a mechanistic probe
Authors: Renaud Jocelyn , Scaiano J. C.
Published: May 2006

Ring-substituted α-bromoacetophenones react with alcohols in a chain reaction leading to the corresponding acetophenone, HBr, and the carbonyl compound from oxidation of the alcohol. Two different mechanisms, involving hydrogen or electron transfer by ketyl radicals, have been proposed in order to accommodate the unusual selectivities of these reactions. By studying the efficiency of isotope incorporation from deuterated alcohols, it has been possible to determine the relative contributions from both mechanisms. For example, electron transfer dominates in the case of 2-propanol, while hydrogen transfer is more important for methanol. The results demonstrate that ring substitution in the starting ketone is not a main contributing factor in the discrimination between the two mechanisms. The only parameter that seems to be playing a major role is the nature (reducing strength) of the ketyl radicals. Key words: dehydrobromination, charge transfer, isotope effect, ketyl radicals.
The acid-catalyzed decomposition of nitramide
Authors: Cox Robin A.
Published: May 2006

Much attention has been paid to the base-catalyzed decomposition of nitramide, but despite this it was not certain that a corresponding acid-catalyzed reaction even existed before the work described in this paper. The excess acidity method has been applied to nitramide decomposition rate constants obtained in aqueous perchloric acid media, and to a lesser extent in aqueous hydrochloric and sulfuric acids. This analysis shows that the decomposition in dilute acid solution is actually a base-catalyzed reaction, with water acting as the base (perhaps in a cyclic process involving two water molecules), with rate constants in good agreement with previous estimates. However, in more concentrated acid media a true acid-catalyzed reaction can be dissected out. Activation parameters are given for both mechanisms, and it is shown that the acid-catalyzed process is similar to the acid-catalyzed decomposition of alkylnitramines. In sulfuric acid the reaction is slightly faster than it is in the other acids, due to the presence of a base-catalyzed process involving sulfate. Key words: nitramide, excess acidity, acid-catalyzed, base-catalyzed, kinetics.
On the dissociation energy of Ti(OH2)+. An MCSCF, CCSD(T), and DFT study
Authors: Irigoras A. , Ugalde J.M. , Lopez X. , Sarasola C.
Published: May 2006

The dissociation energy of the Ti(OH2)+ ion–molecule complex was calculated by the multiconfigurational self-consistent field theory, coupled cluster theory, and two density functional theory based methods, using both all-electron basis sets and effective core potentials. The calculations show that approximate density functional theory gives results in better agreement with experiment than either the multiconfigurational self-consistent field theory or the coupled cluster theory, with both all-electron basis sets and effective core potentials. Nevertheless, the optimized geometries and harmonic vibration frequencies are very similar, irrespective of the level of theory used. The interconfigurational energy ordering of the two valence electronic configurations dn−1s and dn−2s2 of the 4F electronic state of the titanium cation were also calculated and are discussed. Key words: ab initio, dissociation energy, ion–molecule complex, effective core potentials, transition metals.
Studies on the interaction of selenite and selenium with sulphur donors. Part 5. Thiocyanate
Authors: Milne Christopher , Milne John
Published: May 2006

The oxidation of thiocyanic acid by selenous acid has been shown to take place in an analogous way to that found for selenous acid oxidation of most thiols and thiolates to give the corresponding selenotrisulfide and disulfide products[Formula: see text]The products of this reaction have been studied by IR, Raman, and Se-77, C-13, and N-15 NMR spectroscopy. The vibrational spectra of the solid Se(SCN)2have been assigned based on S-bonded thiocyanate ligands and, through NMR and vibrational spectroscopy, both this linkage isomer and the isomer with N- and S-bonded thiocyanate have been identified and characterized in solutions of Se(SCN)2. A complex combination of scalar relaxation processes, involving chemical exchange and quadrupolar effects, accounts for the line broadening observed in the carbon-13 NMR spectra. Key words: selenite, thiocyanate, selenium dithiocyanate, Se-77, N-15, and C-13 NMR; Raman spectroscopy; linkage isomerism
Ionically conducting polyether composites
Authors: Stevens J.R. , Wieczorek W.
Published: May 2006

Ionic conductivity in polymer–salt electrolytes occurs in the amorphous regions of the complex. Poly(ethylene oxide) (PEO) is the best polyether for complexing salts. Unfortunately, it is partially crystalline at ambient temperatures. With inorganic (i.e., alumina) or organic (i.e., poly(acrylamide) (PAAM)) fillers the crystallization of PEO is inhibited and the room temperature conductivity is enhanced in these mixed phase systems by over two orders of magnitude (to ~ 10−4 S/cm) above the base PEO–salt system (<10− S/cm). Even adding PAAM to an initially amorphous system (oxymethylene-linked PEO–LiClO4) increases the room temperature conductivity by 2 to 3 times. Various alkali metal salts (Li, Na) and NH4SCN are used with α-Al2O3, θ-Al2O3, PAAM′ and poly(N,N′-dimethyl acrylamide) as fillers. The aluminas stiffen the complex and increase Tg. The addition of the organic fillers lowers Tg, as is to be preferred. It is suggested that changes in the conductivity with changes in salt and filler concentration are due to changes in the ultrastructure and morphology and are the result of an equilibrium between various Lewis acid – Lewis base reactions. Qualified success has been achieved in modelling ionic conductivity in these composite electrolyte systems using an effective medium approach. In this approach it has been assumed that the main conductivity enhancement takes place in thin amorphous layers of the polyether that coat the dispersed polyacrylamide particles separated in a microphase. In the best complexes this layer is identified by a second Tg. Key words: polyethers, composites, ionic conductivity, phase structure, Lewis acids and bases.
Exploratory study of the quenching of photosensitizers by initiators of free radical "living" polymerization
Authors: Scaiano J.C. , Connolly Terrence J. , Mohtat Nadereh , Pliva Claudette N.
Published: May 2006

Several compounds of the type PhCR1R2-TEMPO (where TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxide) have been prepared and examined as potential photosensitized initiators for "living" free radical polymerizations. For example, these compounds are excellent quenchers of triplet xanthone (rate constants in the range (8.0–13) × 108 M−1 s−1) and result in the formation of benzylic and TEMPO radicals. The importance of excited state energy and nature of the excited triplet state have also been explored. We also tested pyrene as a possible singlet sensitizer. While pyrene fluorescence is efficiently quenched, the process does not lead to cleavage of the C—O bond. A model is proposed to account for this anomaly in the photosensitized decomposition of these TEMPO adducts. Keywords: TEMPO, laser flash photolysis, time resolved emission spectroscopy, sensitizers, quenching, photoinitiator.
Synthesis, high-field NMR, X-ray structure, and conformational analysis of a 10-membered diamide disulfide ring
Authors: Maharajh Rabindranath B. , Snyder James P. , Britten James F. , Bell Russell A.
Published: May 2006

N,N′-[Dimethyl-(2,2′-dithiobisacetyl)]ethylenediamine (1) has been synthesized in 30% overall yield from N,N′-dimethylethylenediamine and thioacetic acid by an improved procedure involving simultaneous deprotection and oxidative cyclization with iodine. This cyclic diamide disulfide exists in solution as a mixture of two Z,Z and one Z,E disulfide, and amide ring conformers and has been characterized by nuclear Overhauser effect (NOE), 1H–1H, 1H–13C shift-correlated 2D-NMR and molecular modelling studies. Among the Z,Z ring conformers Z,Z1 and Z,Z2, the former predominates and interconverts with the latter isomer by rotation about the S—S bond with an activation energy of 14.5 ± 1.3 kcal/mol. Coalescence of N-CH3 signals occurred at ca. 127 °C (500 MHz), which corresponded to an approximate barrier to amide rotation of 19.3 kcal/mol. Aromatic solvent-induced shifts in C6D6 corroborated molecular mechanics and NOE predictions of amide stereochemistry. The structure of the Z,E stereoisomer of 1 has been determined by single-crystal X-ray diffraction at 296 K. A large geminal N-CH2 inequivalence (>2 ppm in CDCl3) was observed for the Z,Z conformers. Proton chemical shifts have been calculated for the conformers of 1 and related molecular fragments with DFT/GIAO theory. Absolute chemical shifts are modelled within 0.2 ppm of experiment. The unusual nonequivalence of geminal N-CH2 and S-CH2 protons can be understood as a combination of shielding mechanisms derived from short N-methyl contacts, amide group orientation, and sulfur lone-pair disposition. An implication of these results is the possibility of using α-CH (and eventually α-CH) shifts to probe the local conformational space in cyclic peptides and other conformationally constrained rings. Keywords: amide/disulfide rotamers, conformational analysis, density functional theory, DFT/GIAO NMR shift calculations, methylene nonequivalence, molecular modelling.
Chemical modification of chlorophyll a: synthesis of new regiochemically pure benzoporphyrin and dibenzoporphyrin derivatives
Authors: Ma Lifu , Dolphin David
Published: May 2006

Starting from pheophorbide a methyl ester (5), a degradation product of chlorophyll a, routes for the preparation of 3-vinylporphyrins 6 and 9 and of an [A,C]-divinylporphyrin 16 are described. These porphyrins were obtained as a result of chemical modification of 5 on ring D and the exocyclic ring E. Diels–Alder reactions of these (di)vinylporphyrins with dimethyl acetylenedicarboxylate, to afford new regiochemically pure benzoporphyrin derivatives 29 and 30 and dibenzoporphyrin derivative 33 for potential use as sensitizers in photodynamic therapy of tumors, were also investigated. Keywords: Diels–Alder, (di)vinylporphyrins, (di)benzoporphyrins, photodynamic therapy, chlorophyll.
Photolysis of 5,5-dibenzyl-Δ3-1,3,4-oxadiazolines
Authors: Warkentin John , Woollard John Mck.R.
Published: May 2006

Photolysis of dibenzyl-Δ3-1,3,4-oxadiazolines (3) in the presence of dimethyl acetylenedicarboxylate (DMAD) gives only modest yields of the expected symmetrical 3,3-dibenzylcyclopropenes (4), but these are accompanied by more than six by-products, including unsymmetrical cyclopropenes, methylenecyclopropanes, and various pyrazoles. The origin of this array of products can be explained by a series of steps starting with photolysis of 3 to form a diazoalkane that undergoes 1,3-dipolar cycloaddition to DMAD, generating a 3H-pyrazole as initial product. The latter is further photolyzed to a symmetrical cyclopropene in competition with benzyl group migration by thermal 1,5-sigmatropic or ion-pair rearrangement to afford a 4H-pyrazole. The 4H-pyrazole in turn undergoes photolysis to an unsymmetrical cyclopropene, which rearranges to a methylenecyclopropane. The 4H-pyrazole also undergoes autoxidation, in the presence of air, to afford a benzoyl-4H-pyrazole. Additionally, in competition with rearrangement, the various pyrazoles lose a benzyl group or a methoxycarbonyl group to afford pyrazoles with one less substituent. Keywords: 5,5-dibenzyl-Δ3-1,3,4-oxadiazolines, photolysis of; 3,3-dibenzyl-3H-pyrazoles, rearrangement of; 3,4-dibenzyl-4H-pyrazoles, autoxidation of; 3,4-dibenzyl-4H-pyrazoles, photolysis of; cyclopropenes, rearrangement to methylenecyclopropanes.
Ketene acetals from thermolysis of aryloxy methoxy oxadiazolines. Evidence for carbonyl ylide intermediates
Authors: Couture Philippe , El-Saidi Manal , Warkentin John
Published: May 2006

Thermolysis of oxadiazolines (5) in benzene at 110 °C leads to ketene acetals (11) as minor products. Carbonyl ylide intermediates (6), and oxiranes (7), presumably in equilibrium with those ylides, are implicated as unstable precursors of the ketene acetals although none of the oxiranes (carbonyl protected α-lactones) were isolable and only one of the ketene acetals was isolable in pure form. The evidence points to the two-step sequence of thermolysis of oxadiazolines, namely, initial cycloreversion to N2 and carbonyl ylide (6), rather than concerted fragmentation to N2, acetone, and carbene (12). The first-formed ylide does fragment to carbene and acetone in a second step that competes with oxirane formation. A tentative mechanism for reaction of 7 with 12, to afford 11, is advanced. Keywords: carbonyl ylide, dioxy oxirane, ketene acetal, oxadiazoline.
A study of formation and fragmentation of ionic metal complexes of substituted benzoic and cinnamic acids with Al(III)–glycerol by fast atom bombardment mass spectrometry
Authors: Saraswathi Mandapati , Miller Jack M.
Published: May 2006

Substituted benzoic and cinnamic acids were studied by FAB mass spectrometry using matrix-based Al(III) ions. Complex ions derived from these acids, such as ( M + 117 )+ ( 117 = (Al + glycerol − 2H)+ ), ( M + 159 )+ ( 159 = [2Al + 2glycerol − 5H − 74(epoxypropanol)]+), and ( M + 233)+ ( 233 = (2Al + 2glycerol − 5H)+), when subjected to collisionally activated dissociation (CAD), gave an ion corresponding to ( M − OH)+. Formation of this ion was explained by the transfer of the carboxylic -OH group to the ( Al + glycerol − 2H)+, ( 2Al + 2glycerol − 5H − 74 )+, and ( 2Al + 2glycerol − 5H )+ portion of the complexes. This transfer is more pronounced in the case of meta- and para-substituted acids. The CAD of the ( M + 117 )+ ions yielded ( M + Al(III) )+ ions resulting from the insertion of Al(III) into the carboxylic acid, which was followed by the elimination of CO to produce ( M + Al(III) − CO)+ ions. Interpretation of spectral data was clarified by study of the formation and fragmentation of ions produced from deuterated benzoic and cinnamic acids. Keywords: substituted benzoic and trans-cinnamic acids, Al(III)–glycerol, FAB mass spectrometry, ionic metal complexes, collisionally induced dissociation, Al(III) insertion, carbonyl cations.
An examination of sodium alginate from Sargassum by NMR spectroscopy
Authors: Llanes Francisco , Sauriol Françoise , Morin Frederick G. , Perlin Arthur S.
Published: May 2006

Alginic acid, obtained from Sargassum brown algae that drift from the Sargasso Sea to beaches in Cuba, has been examined as an aqueous solution by 1H and 13C NMR spectroscopy, and also in the solid state by MAS 13C NMR spectroscopy. Its composition is distinctive, relative to that for alginic acid from most other brown algae, in having a lower content of β-D-mannopyranosyluronic acid residues (M) than of α-L-gulopyranosyluronic acid residues (G). That is, M/G is ~0.6, whereas ratios of >1 have been reported for many other species, including Macrocystispyrifera, which was examined here as well. In acquiring the high-resolution spectra, partial depolymerization was used to give solutions of reduced viscosity; as is well known, this procedure may cause separation into fractions and the formation of sediments, examples of which are cited. Proton resonances of the reducing end units of chain-shortened segments of the polymers exhibited unusual, marked, line broadening at elevated temperature, which may reflect thermal dehydration. MAS 13C experiments offered the advantage of information about the intact alginate samples. Although the spectra consisted of broad, heavily overlapping peaks, curve-fitting analysis afforded qualitative and quantitative data that supplement those obtained with solutions of the polymers. Keywords: Sargassum alginate, 1H NMR of fractions, M/G constituent ratios, MAS 13C NMR, curve-fitting analysis.
New terpenoid metabolites from the skin extracts, an egg mass, and dietary sponges of the Northeastern Pacific dorid nudibranch Cadlinaluteomarginata
Authors: Dumdei Eric J. , Kubanek Julia , Coleman John E. , Pika Jana , Andersen Raymond J. , Steiner Jorge Rios , Clardy Jon
Published: May 2006

Chemical investigations of Cadlinaluteomarginata skin extracts, egg masses, and dietary sponges have led to the identification of the novel terpenoids cadlinaldehyde (30), spongian 32, seco-spongian 35, 20-acetoxy-12-marginatone (38), and lutenolide (39) from the nudibranch skin extracts, the new drimane sesquiterpenoid 1α,2α-diacetoxyalbicanyl acetate (40) from the nudibranch's egg mass, and the new sesquiterpenoids O-methyl-9-oxofurodysininlactone (47), 2-oxomicro-cionin-2-lactone (48), and O-methyl-2-oxomicrocionin-2-lactone (49), from the dietary sponge Pleraplysilla sp. The known terpenoids furodysinin (1), furodysin (16), marginatafuran (21), and 9,11-dihydrogracillin A (37), which have been frequently isolated from C. luteomarginata skin extracts, were found for the first time in extracts of the dietary sponges Pleraplysilla sp. and Aplysilla sp. One of the new terpenoids, cadlinaldehyde (30), has an unprecedented degraded sesterterpenoid skeleton. Keywords: nudibranch, sponge, terpenoids, structure elucidation.
Synthesis and evaluation of thio-trisaccharides as acceptors for N-acetylglucos-aminyltransferase-V
Authors: Lu Pu-Ping , Hindsgaul Ole , Li Hong , Palcic Monica M.
Published: May 2006

N-Acetylglucosaminyltransferase-V (GlcNAcT-V) transfers a β-linked GlcNAc residue from UDP-GlcNAc to the 6-OH group of the αMan residue in oligosaccharides terminating in the sequence β-D-GlcpNAc-(1 → 2)-α-D-Manp-(1 → 6)-β-D-Glcp-OR (5, R = (CH2)7CH3). The terminal GlcNAc moiety may be replaced by a Glc residue to produce trisaccharide 6. Two thio analogs (7, 8) of trisaccharide 6, where the oxygen atoms in the glycosidic linkages between sugar residues were replaced by sulfur, were prepared by multistep chemical synthesis that made use of the key intermediates 1,2-anhydro-3,4,6-tri-O-benzyl-α-D-glucose (10) and 1,2-anhydro-3,4,6-tri-O-benzyl-β-D-mannose (13) as donors for the glycosylations. The thio analogs (7,8) were kinetically evaluated as substrates for GlcNAcT-V and found to be acceptors with two- to three-fold increase in Vmax but higher Km values (7, Km = 376 μM; 8, Km = 300 μM) than their parent compound 6 (Km = 111 μM), which has the natural oxygen linkage. The thio analogs 7 and 8 could be quantitatively converted into the expected product tetrasaccharides (27, 28) by incubation with GlcNAcT-V and UDP-GlcNAc. The enzymatic results indicate that GlcNAcT-V tolerates the substitution of the natural oxygen linkage of the acceptor by a sulfur linkage. Keywords: N-acetylglucosaminyltransferase-V, enzyme acceptors, trisaccharide analogs, thioglycosides.
Oxygen insertion in Sesamumindicum furanofuran lignans. Diastereoselective syntheses of enzyme substrate analogues
Authors: Marchand Patrice A. , Lewis Norman G. , Zajicek Jaroslav
Published: May 2006

The furofuran lignans in sesame seed have an unusual oxygen insertion between their furan and aryl rings. In our continuing investigations on the isolation and characterization of the enzyme(s) involved, the diastereoselective syntheses of various substrate analogues for the oxygen insertion step were developed for future substrate specificity and inhibitor studies. This synthetic strategy also provided entry to so-called furofuranone epoxy-lignans, such as salicifoliol from Bupleurum sp., and acuminatolide from Helichrysum sp. Keywords: furofuran lignans, sesame, epimers, salicifoliol, acuminatolide.
β-Cyclodextrin effects on photo-Claisen rearrangement of allyl phenyl ether
Authors: Sanchez Ana M. , Veglia Alicia V. , Rossi Rita H. de
Published: May 2006

The photo rearrangement of allyl phenyl ether (1) in water and in solutions containing β-cyclodextrin (β-CD) leads to the para (2) and ortho (3) allyl phenols and phenol (4) as principal products. The substrate associates with β-CD forming a 1:CD complex with a K1-CD = (4.6 ± 0.6) × 102 M−1. When the photolysis is carried out in presence of oxygen there is a decrease in the total yields of products, but the inhibition is less marked in the presence of β-CD. The product quantum yields (Φ) for the complexed substrate are different for the free substrate, being higher for the ortho product, almost the same for the para product, and lower for phenol. The ortho selectivity was confirmed in the photolysis of the solid complex; these results are in agreement with those reported by this laboratory for the photo-Fries rearrangement of acetanilide and phenyl acetate. Keywords: cyclodextrin, photo-Claisen rearrangement, selectivity.
1996 Merck Frosst Award Lecture Synthetic adventures along a Rocky Mountain road
Authors: Keay Brian A. , Maddaford Shawn P. , Cristofoli Walter A. , Andersen Neil G. , Passafaro Marco S. , Wilson Noel S. , Nieman James A.
Published: May 2006

This paper describes the chemistry presented during the Merck Frosst Centre for Therapeutic Research Lecture Award given at the 79th Chemistry in Canada Conference held in St. John's, Newfoundland in June 1996. The first section describes the synthesis of (+)-xestoquinone using an asymmetric palladium-catalyzed polyene cyclization as the key step that creates the C and D rings and the stereogenic centre (68% ee) in one step. Extensions of the work involving an in situ Suzuki reaction are presented. The synthesis of C2-symmetric biaryls and the synthesis of a recently isolated binaphthyl natural product is described using this new method. A new one-pot desilylation–oxidation procedure of silyl ethers is described in detail for the preparation of aldehydes and ketones directly without the need for the isolation of the alcohol intermediate. Finally, a highly diastereoselective (>97%) Diels–Alder reaction is presented using (+)-cis,cis-spiro[4.4]nonane-1,6-diol as a new chiral auxiliary. One of the alcohols is attached to a pivalate, the other to an acrylate, and the Diels–Alder reaction with cyclopentadiene provides only one adduct (by 1H NMR and HPLC) with the endo stereochemistry. Keywords: (+)-xestoquinone, asymmetric palladium-catalyzed polyene cyclization, in situ Suzuki reaction, desilylation–oxidation reaction, spirodiols, chiral auxiliaries.
Synthesis of thaxtomin C
Authors: King Russell R.
Published: May 2006

N-Methyl-L-4-nitrotryptophan methyl ester (2) and t-Boc- L-phenylalanine (3) were condensed to form the dipeptide 4, which was then cyclized to give 12-N-methylcyclo-(L-4-nitrotryptophyl-L-phenylalanyl) (1), a.k.a. thaxtomin C, the principle phytotoxin associated with Streptomycesipomoeae. Keywords: Streptomycesipomoeae, phytotoxin, thaxtomin C.
Évaluation de l'effet électroattracteur du motif 4-nitrobenzofurazanyle : ionisation de 7-anilino 4-nitrobenzofurazanes
Authors: Hallé Jean-Claude , Soulié Pascale , Pouet Marie-José , Mokhtari Malika
Published: May 2006

The ionization of a series of 7-(4′-X anilino) 4-nitrobenzofurazans (4H) (X = NO2, CN, CO2Me, F, H, Me, OMe) to give the conjugate amide anions 4− has been studied by 1H and 13C NMR in Me2SO-d6. Concomitantly, their acidity (pKa) has been measured spectrophotometrically in a H2O–Me2SO mixture with a molar fraction [Formula: see text] Although the measured pKa values for the whole series of compounds 4H lie in a relatively narrow range (6.40, X = NO2/7.98, X = OMe), they reveal that each derivative is about 3 times more acidic than the similarly X-substituted 2,4,6-trinitrodiphenylamines. These results, together with an analysis of substituent effects on the proton and carbon chemical shifts [Formula: see text] and the related deprotonation parameters (Δδ), which are specially significant for H6, C4, C6, and C7, emphasize the very strong electron-withdrawing character of the 4-nitrobenzofurazanyl moiety. In addition, the observation of nice linear correlations of the type [Formula: see text] has enabled us to control and sometimes to firmly assign the various chemical shifts. Keywords: 7-anilino 4-nitrobenzofurazans, acidities, 1H and 13C NMR deprotonation shifts, water – dimethyl sulfoxide mixtures.
Stabilized haloylides: synthesis and reactivity
Authors: Kayser Margaret M. , Zhu Jun , Hooper Donald L.
Published: May 2006

This paper describes simple and successful methods for the preparation of stabilized halogenated phosphoylides and discusses their stability, reactivity, and utility in organic synthesis. Preparations of several halogenated compounds including pharmaceutically interesting enol lactones are reported. Keywords: halogenated stabilized ylides, preparation, structure, stability, reactivity, applications; vinylic halides, halo enol lactones.
Nucleophilic aromatic substitution with dialkoxycarbenes
Authors: Ross Joseph P. , Couture Philippe , Warkentin John
Published: May 2006

Dimethoxycarbene, generated at 110 °C by thermolysis of 2,2-dimethoxy-5,5-dimethyl-Δ3-1,3,4-oxadiazoline, displaces fluoride from aromatic rings that are activated with electron-withdrawing groups. Intermolecular substitution on Sanger's reagent and on hexafluorobenzene are reported, together with intramolecular substitution by a dioxycarbene with a tethered aryl group. Keywords: aromatic substitution, aryl(dimethoxy)fluoromethanes, aryl fluoride, dialkoxycarbene, nucleophilic substitution.
Substituent effects on the reactivity of the silicon–carbon double bond. Arrhenius parameters for the reaction of 1,1 -diarylsilenes with alcohols and acetic acid
Authors: Bradaric Christine J. , Leigh William J.
Published: May 2006

Absolute rate constants for the reaction of a series of ring-substituted 1,1 -diphenylsilene derivatives with methanol, tert-butanol, and acetic acid in acetonitrile solution have been determined using nanosecond laser flash photolysis techniques. The three reactions exhibit small positive Hammett ρ-values at 23 °C, consistent with a mechanism involving initial, reversible nucleophilic attack at silicon to form a σ-bonded complex that collapses to product via rate-limiting proton transfer. Deuterium kinetic isotope effects and Arrhenius parameters have been determined for the reactions of 1,1-di-(4-methylphenyl)silene and 1,1-di-(4-trifluoromethylphenyl)silene with methanol, and are compared to those for the parent compound. Proton transfer within the complex is dominated by entropic factors, resulting in negative activation energies for reaction. The trends in the data can be rationalized in terms of variations in the relative rate constants for reversion to reactants and proton transfer as a function of temperature and substituent. A comparison of the Arrhenius activation energies for reaction of acetic acid with 1,1-diphenylsilene (Ea = +1.9 ± 0.3 kcal/mol) and the more reactive di-trifluoromethyl analogue (Ea = +3.6 ± 0.5 kcal/mol) suggests that carboxylic acids also add by a stepwise mechanism, but with formation of the complex being rate determining. Keywords: silene, substituent effects, kinetics, Arrhenius, flash photolysis.
Temperature-dependent oxygen electrochemistry on platinum low-index single crystal surfaces in acid solutions
Authors: Grgur B.N. , Marković N.M. , Ross P.N.
Published: May 2006

Using the rotating ring-disk technique (RRDPt(hkl)E), the oxygen-reduction reaction (ORR) was studied in sulfuric acid solution over the temperature range 298–333 K At the same temperature, the exchange current density increased in the order, [Formula: see text] which gives the order of absolute kinetic activities of Pt(hkl) in 0.05 mol/L H2SO4: Pt(111) < Pt(100) < Pt(110). We found that at high current densities every crystal face has an ideally temperature-dependent Tafel slope, i.e., −2 × 2.3(RT/F). The activation energy for the ORR is independent of the surface geometry, [Formula: see text]. The insensitivity of the activation energy to surface structure implies that the reaction pathway for the ORR is the same on all three single-crystal faces. A "series" pathway for the ORR was proposed, with the first electron transfer being the rate-determining step. The structure sensitivity in the kinetics of the ORR on Pt(hkl) is attributed to the structure-sensitive adsorption of bisulfate and hydroxyl anions and a simple site-blocking effect of these adsorbed anions on the rate of the ORR. Keywords: platinum single crystals, oxygen reduction, peroxide reduction, temperature effects, activation energy.
LORENTZ-SYMMETRY VIOLATION AND ELECTRICALLY CHARGED VORTICES IN THE PLANAR REGIME
Authors: BELICH H. , COSTA-SOARES T. , FERREIRA M. M. , HELAYËL-NETO J. A. , ORLANDO M. T. D.
Published: May 2006

We start from a Lorentz noninvariant Abelian-Higgs model in 1+3 dimensions, and carry out its dimensional reduction to D = 1+2. The planar model resulting thereof is composed by a Maxwell—Chern—Simons–Proca gauge sector, a massive scalar sector, and a mixing term (involving the fixed background, vμ) that realizes Lorentz violation for the reduced model. Vortex-type solutions of the planar model are investigated, revealing charged vortex configurations that recover the usual Nielsen–Olesen configuration in the asymptotic regime. The Aharonov–Casher Effect in layered superconductors, that shows interference of neutral particles with a magnetic moment moving around a line charge, is also studied. Our charged vortex solutions exhibit a screened electric field that induces the same phase shift as the one caused by the charged wire.
Satellite cell activation in stretched skeletal muscle and the role of nitric oxide and hepatocyte growth factor
Authors: Tatsumi Ryuichi , Liu Xiaosong , Pulido Antonio , Morales Mark , Sakata Tomowa , Dial Sharon , Hattori Akihito , Ikeuchi Yoshihide , Allen Ronald E.
Published: May 2006

In the present study, we examined the roles of hepatocyte growth factor (HGF) and nitric oxide (NO) in the activation of satellite cells in passively stretched rat skeletal muscle. A hindlimb suspension model was developed in which the vastus, adductor, and gracilis muscles were subjected to stretch for 1 h. Satellite cells were activated by stretch determined on the basis of 5-bromo-2′-deoxyuridine (BrdU) incorporation in vivo. Extracts from stretched muscles stimulated BrdU incorporation in freshly isolated control rat satellite cells in a concentration-dependent manner. Extracts from stretched muscles contained the active form of HGF, and the satellite cell-activating activity could be neutralized by incubation with anti-HGF antibody. The involvement of NO was investigated by administering nitro-l-arginine methyl ester (l-NAME) or the inactive enantiomer NG-nitro-d-arginine methyl ester HCl (d-NAME) before stretch treatment. In vivo activation of satellite cells in stretched muscle was not inhibited by d-NAME but was inhibited by l-NAME. The activity of stretched muscle extract was abolished by l-NAME treatment but could be restored by the addition of HGF, indicating that the extract was not inhibitory. Finally, NO synthase activity in stretched and unstretched muscles was assayed in muscle extracts immediately after 2-h stretch treatment and was found to be elevated in stretched muscle but not in stretched muscle from l-NAME-treated rats. The results of these experiments demonstrate that stretching muscle liberates HGF in a NO-dependent manner, which can activate satellite cells.
Predictive Factors of Restenosis After Coronary Implantation of Sirolimus- or Paclitaxel-Eluting Stents
Authors: Kastrati Adnan , Dibra Alban , Mehilli Julinda , Mayer Sandra , Pinieck Susanne , Pache Jürgen , Dirschinger Josef , Schömig Albert
Published: May 2006

Background—

The efficacy of drug-eluting stents in reducing restenosis risk has not been uniform across patient subsets. Identifying predictive factors of restenosis may help improve outcomes after percutaneous coronary interventions.

Methods and Results—

All patients who underwent successful implantation of sirolimus- or paclitaxel-eluting stents in native vessels for de novo lesions between August 2002 and December 2004 were eligible for this study. All data were prospectively collected. Angiographic restenosis was defined as diameter stenosis ≥50% at follow-up in the in-segment area. Target lesion revascularization was defined as any revascularization procedure involving the target lesion. Included in this study were 1845 patients with 2093 target lesions. Multivariable analysis showed that vessel size, final diameter stenosis, and drug-eluting stent type were the strongest predictors of restenosis. A 0.5-mm decrease in vessel size was associated with adjusted odds ratios (ORs) of 1.74 (95% CI, 1.31 to 2.32) for angiographic restenosis and 1.65 (95% CI, 1.22 to 2.23) for target lesion revascularization. A 5% increase in final diameter stenosis was associated with adjusted ORs of 1.30 (95% CI, 1.15 to 1.47) for angiographic restenosis and 1.18 (95% CI, 1.03 to 1.35) for target lesion revascularization. Compared with paclitaxel-eluting stent, sirolimus-eluting stent was associated with adjusted ORs of 0.60 (95% CI, 0.44 to 0.81) for angiographic restenosis and 0.67 (95% CI, 0.49 to 0.91) for target lesion revascularization.

Conclusions—

Vessel size and drug-eluting stent type are the most important predictors of angiographic and clinical restenosis, with drug-eluting stent type having a particular impact on restenosis of small coronary vessels.
The Satiric Method of Ibn Dāniyāl: Morality and Anti-morality in Tayf al-Khayāl
Authors: Zargar Cyrus Ali
Published: May 2006

Ibn Dāniyāl's (d. 1311) shadow play Tayf al-Khayāl takes as its setting Cairo in the early Mamlūk period, a city depicted in the play as decadent and yet subjected to ineffective moral reforms. The play's complex satiric response to such reforms is the subject of this study. Contemporary theories of satire, especially Frank Palmeri's approach to narrative satire, are used to shed light on Ibn Dāniyāl's method of constructing a sense of satiric ambiguity. Special attention is given to the playwright's use of misplaced religious language and his ironic portrayal of the relationship between power and morality.