bluuebunny/crossref_metadata_2025_split huggingface dataset:
Syntheses of Segmental Heteroleptic Ligands for the Self‐Assembly of Heteronuclear Helical Supramolecular Complexes
Authors: Piguet Claude , Bocquet Bernard , Hopfgartner Gérard 
Published: December 2004

AbstractThe development of a modified Phillips reaction allows the synthesis of complicated heteroleptic ligands based on pyridine and 1H‐benzimidazole moieties. The key‐step formation of aromatic 1H‐benzimidazole rings from N‐(2‐nitroaryl)arenecarboxamides significantly improves the possibilities of the classical intermolecular Phillips reactions and can be applied to the synthesis of large oligo‐multidentate units. Segmental heteroleptic ligands containing two different bidentate coordinating units (see 14) or bidentate and terdentate sites connected by ‘diphenylmethane’ ( = methylenebis(phenylene)) spacers (see 15 and 16) were obtained in fair to good yield by using multistep syntheses following this strategy. Heterotrileptic ligands (bidentate‐terdentate‐bidentate, see 19 and 20; terdentate‐terdentate‐terdentate, see 23) requiring the simultaneous formation of four 1H‐benzimidazole rings in one step were prepared and demonstrate the extended possibilities of this methodology. Data are presented to show that the milder conditions used for this modified Phillips reaction together with its flexibility are essential for the development of ligands adapted for the self‐assembly of heteronuclear helical complexes.
Synthesis of Linear Oligomers of (R)‐3‐Hydroxybutyrate and Solid‐State Structural Investigations by electron microscopy and X‐ray scattering
Authors: Seebach Dieter , Bürger H. Michael , Müller Hans‐Martin , Lengweiler Urs D. , Beck Albert K. , Sykes Kathy E. , Barker Phil A. , Barham Peter J. 
Published: December 2004

AbstractRepetitive treatment of the biopolymer P(3‐HB) (molecular weight > 105 Dalton, storage or s‐P(3‐HB)), with lithium hexamethyl disilazanid (LHMDS) at −70° in THF leads to a mixture of oligomers with increasingly sharp distribution around a 15‐, 30‐, and 45mer. Discrete fragments are also isolated when P(3‐HB) is heated under reflux (89°) in neat Et3N. Linear oligo(3‐HB) derivatives (3‐7) containing up to 96 3‐HB units are synthesized using an exponential segment‐coupling strategy. These oligomers are used to calibrate size‐exclusion chromatography columns for the analysis of oligo(3‐HB) samples from the different sources. The linear oligo‐(3‐HB) derivatives also served as a model with respect to the physical properties of high molecular weight P(3‐HB) and were investigated as such by transmission electron microscopy (TEM) and by small‐ and wide‐angle X‐ray scattering (SAXS and WAXS). The thicknesses of the lamellar crystallites (long periods) formed by the 8mer, 16mer, and 32mer, are ca. 26, 52, and 53 Å, respectively, indicating that the 32mer molecules are folded once, very tightly, into a ‘hair‐pin’‐type conformation. High‐molecular‐weight P(3‐HB), which was crystallized in a similar way, also has a lamellar crystallite thickness of ca. 50–65 Å. Thus, the treatment of P(3‐HB) with LHMDS at low temperature causes etching of the amorphous regions, an effect well known in polymer science for studying the regularity of chain folding. The ca. 50‐Å packing within the tight folds of P(3‐HB) is discussed in view of its possible function in ion transport through cell membranes.
Glycosylidene Carbenes. Part 17. Glycosidation of benzyl β‐D‐ and β‐L‐ribopyranosides. Further evidence for the effect of stereoelectronic control on the regioselectivity of glycosidation
Authors: Uhlmann Peter , Vasella Andrea 
Published: December 2004

AbstractThe H‐bonds of the enantiomeric ribosides 4 and 5 and their glycosidation by the diazirine 1 are described. HO–C(2) and HOC(4) of 4 and 5 form a ‘flip‐flop’ H‐bonding system, with HOC(3) acting as a H‐bond donor to OC(2) or OC(4). HOC(2) and HOC(4) of monomeric 4 and 5 are thus the most strongly acidic OH groups. Glycosidation of 4 and 5 by 1 depends on the solvent, the temperature, and the concentration. It yields up to 91% of a mixture of anomeric pairs of the 1,2‐, 1,3‐, and 1,4‐linked disaccharides 8–13 and 20–25, respectively, which were characterized as their diacetates 14–19 and 26–31 (Scheme). Glycosidation in CH2Cl2 and in dioxane yielded mostly the 1,3‐linked disaccharides 10/11 and 22/23 (α/β ca. 4:1), while glycosidation in THF leads mostly to the 1,2‐ and 1,4‐linked regioisomers (β>α). There are small, but significant differences in the glycosidation of 4 and 5. These, the regio‐, and the stereoselectivities are rationalized as the consequences of the stereoelectronic control of both the H‐transfer from HOC(2) or HOC(4) to the intermediate carbene and of the formation of the glycosidic CO bond, and of the coordination of the intermediate oxycarbenium ion with THF.
Novel Cholic‐Acid‐Type Sterones of Deltocyathus magnificus, a deep‐water scleractinian coral from the Loyalty Islands, SW Pacific
Authors: Guerriero Antonio , D'Ambrosio Michele , Zibrowius Helmut , Pietra Francesco 
Published: December 2004

AbstractCholic‐acid‐type 3‐keto steroids 1–8, all isolated from Deltocyathus magnificus (2–8 after CH2N2 treatment), are the first secondary metabolites obtained from a deep‐water scleractinian. Steroids 1–3, 5, and 7 are new, their main structural oddities being loss of C(24) (see 5) or hydroxylation in the side chain (see 3 and 7). Steroids 4, 6, and 8, from the same coral, were previously known from other marine organisms.
Synthesis and Circular Dichroism of Both Enantiomers of 2‐deuterofluoroacetic acid ( Fluoro[2H1]acetic acid)
Authors: Gartz Dagmar , Reed Jennifer , Rétey János 
Published: December 2004

AbstractSharpless epoxidation of (E)‐1‐(trimethylsilyl)[1‐2H1]oct‐1‐en‐3‐o1 (3a) yielded (1S,2S,3S)‐ and (1R,2R,3R)‐1‐(trimethylsilyl)‐1,2‐epoxy[1‐2H1]octan‐3‐ols (4a and 4b, resp.) which were converted in three steps into (S)‐ and (R)‐fluoro[ 2H1]acetic acid (7a and 7b, resp.) in good yields. Their high isotopic and optical purity was established by 1H‐ and 19F‐NMR, mass, and circular‐dichroism spectroscopy.
Synthesis of 1,3,4,5‐Tetrahydro‐2‐benzoxepin Derivatives as Conformationally Restricted Analogues of cyclamenaldehyde‐type compounds and as intermediates for highly odour‐active homologues
Authors: Skouroumounis Georges , Winter Béat 
Published: December 2004

AbstractNine 1,3,4,5‐tetrahydro‐2‐benzoxepin derivatives have been prepared as mimics of the folded (gauche) conformation of cyclamenaldehyde (1) and related compounds, but none of them showed the typical lily‐of‐the valley (muguet) odour activity. However, conversion of these substances to previously unknown analogues of 1 having a Me substituent on the aromatic ring in an ortho position to the side chain led to new fragrance substances with remarkable properties. The results indicate that the extended (anti) conformation is more likely to be the ‘bioactive’ one at the receptor site(s).
A Ring Enlargement from Seven‐ to Ten‐Membered‐Ring sulfonamide derivatives
Authors: Orahovats Alexander S. , Bratovanov Svetoslav S. , Linden Anthony , Heimgartner Heinz 
Published: December 2004

AbstractThe reaction of 3‐(dimethylamino)‐2,2‐dimethyl‐2H‐azirine (1a) with 4,5‐dihydro‐7,8‐dimethoxy‐1,2‐benzothiazepin‐3‐one 1,1‐dioxide (4) in dioxane at room temperature gave the correspondingly substituted 4H‐1,2,5‐benzothiadiazecin‐6‐one 1,1‐dioxide 5a in 64% yield (Scheme 2). The structure of this novel ten‐membered ring‐enlargement product was established by X‐ray crystallography (Fig.). Under more vigorous conditions (refluxing dichloroethane), 5a was formed together with the isomeric 6a, both in low yield. The 3‐(dimethylamino)‐2H‐azirines 1b and 1c reacted sluggishly to give the two isomeric ring‐enlargement products of type 5 and 6 in yields of 24–29% and 2–4%, respectively (Table 1). Even less reactive is 2,2‐dimethyl‐3‐(N‐methyl‐N‐phenylamino)‐2H‐azirine (1d), which reacted with 4 in MeCN only at 65°. Under these conditions, besides numerous decomposition products, only traces of 5d and 6d were formed. No ring enlargement was observed with the sterically crowded 1e, which bears an isopropyl group at C(2).
Electrochemical Behaviour and Antioxidant Activity of Some Natural Polyphenols
Authors: Born Markus , Carrupt Pierre‐Alain , Zini Roland , Brée Françoise , Tillement Jean‐Paul , Hostettmann Kurt , Testa Bernard 
Published: December 2004

AbstractA number of natural polyphenols (chlorogenic acid (9), cordigol (11), cordigone (12), danthrone (1), 1,5‐dihydroxy‐3‐methoxyxanthone (2), eriosematin (7), flemichin D (8), frutinone A (6), mangiferin (4), quercetin (5), 1,3,6,7‐tetrahydroxyxanthone (3) and verbascoside (10)) were investigated for their redox properties using cyclic voltammetry. The antioxidant properties of these compounds were also examined in two models, namely lipid peroxidation in rat synaptosomes and AAPH‐mediated oxidation of serum albumin. Compounds with a catechol group (9, 4, 5, 3 and 10) were oxidized below 0.4 V and inhibited lipid peroxidation with IC50 values between 2 and 8 μM. Compounds having one or more isolated phenolic groups and showing an oxidation potential between 0.45 and 0.8 V (11, 12 and 8) inhibited lipid peroxidation with IC50 between 7 and 9 μM, except 2 (0.45 V), danthrone (0.96 V) and eriosematin which showed no or modest antioxidant activity. Some of the investigated compounds also protected albumin from oxidation, but no structure‐activity relationship was apparent, suggesting that other factors beside redox potential influence this activity.
Preparation of N‐Fmoc‐Protected β2‐ and β3‐Amino Acids and their use as building blocks for the solid‐phase synthesis of β‐peptides
Authors: Guichard Gilles , Abele Stefan , Seebach Dieter 
Published: December 2004

AbstractN‐Fmoc‐Protected (Fmoc = (9H‐fluoren‐9‐ylmethoxy)carbonyl) β‐amino acids are required for an efficient synthesis of β‐oligopeptides on solid support. Enantiomerically pure Fmoc‐β3‐amino acids β3: side chain and NH2 at C(3)(= C(β)) were prepared from Fmoc‐protected (S)‐ and (R)‐α‐amino acids with aliphatic, aromatic, and functionalized side chains, using the standard or an optimized Arndt‐Eistert reaction sequence. Fmoc‐β2‐ Amino acids (β2 side chain at C(2), NH2 at C(3)(= C(β))) configuration bearing the side chain of Ala, Val, Leu, and Phe were synthesized via the Evans' chiral auxiliary methodology. The target β3‐heptapeptides 5–8, a β3‐ pentadecapeptide 9 and a β2‐heptapeptide 10 were synthesized on a manual solid‐phase synthesis apparatus using conventional solid‐phase peptide synthesis procedures (Scheme 3). In the case of β3‐peptides, two methods were used to anchor the first β‐amino acid: esterification of the ortho‐chlorotrityl chloride resin with the first Fmoc‐β‐amino acid 2 (Method I, Scheme 2) or acylation of the 4‐(benzyloxy)benzyl alcohol resin (Wang resin) with the ketene intermediates from the Wolff rearrangement of amino‐acid‐derived diazo ketone 1 (Method II, Scheme 2). The former technique provided better results, as exemplified by the synthesis of the heptapeptides 5 and 6 (Table 2). The intermediate from the Wolff rearrangement of diazo ketones 1 was also used for sequential peptide‐bond formation on solid support (synthesis of the tetrapeptides 11 and 12). The CD spectra of the β2‐ and β3‐peptides 5, 9, and 10 show the typical pattern previously assigned to an (M) 31 helical secondary structure (Fig.). The most intense CD absorption was observed with the pentadecapeptide 9 (strong broad negative Cotton effect at ca. 213 nm); compared to the analogous heptapeptide 5, this corresponds to a 2.5 fold increase in the molar ellipticity per residue!
Regioselectivity of the Base‐Induced Ring Cleavage of 1‐Oxygenated Derivatives of Cyclobutabenzene
Authors: Gokhale Abha , Schiess Peter 
Published: December 2004

AbstractOxy anions 3 generated from 1,2‐dihydrocyclobutabenzen‐1‐ones 1 through addition of a charged nucleophile or from 1‐hydroxy‐1,2‐dihydrocyclobutabenzenes 2 by deprotonation with base lead to stable products through distal and/or proximal cleavage of the strained four‐membered ring via benzyl carbanion 4 and/or aryl carbanion 5. A systematic study of this process reveals the relative stability of the two isomeric carbanions 4 and 5 as a key factor in determining the course of the ring‐cleavage reaction. While benzyl carbanions 4 can be trapped with carbon electrophiles, attempts at trapping aryl carbanions 5 with electrophiles other than H+ failed. In protic solvents, the magnesium salt of the tertiary alcohol 2 shows an increased rate of proximal cleavage as compared to its alkali salts. From this, we conclude that, in contrast to benzyl carbanions 4, free aryl carbanions 5 are of transient existence only. Proximal C,C‐bond cleavage seems to occur either through protonation of 5 from a fast, reversible equilibrium 3⇌5 in which 3 strongly predominates, or in protic solvents possibly even through a rate‐limiting protonation of 3 at the aromatic C‐atom, bypassing free anion 5 altogether. Thus, additional factors other than just the relative stability of isomeric carbanions 4 and 5 are of importance in determining the regiochemistry of the base‐induced C,C‐bond cleavage in ketones 1 and in alcohols 2.
Über Vinylchlorid und seine Polymerisationsprodukte. III. Zur Kenntnis der Polymerisation von Vinylchlorid in Lösung
Authors: Fierz‐David H. E. , Zollinger Heh. 
Published: December 2004

Abstract1. Zur Lösungspolymerisation von Vinylchlorid mit Peroxydkatalysatoren eignen sich besonders jene Lösungsmittel, welche das Polymere praktisch überhaupt nicht lösen.
Etudes sur les matières végétales volatiles XXXV. Sur la constitution du soi‐disant „linaloloxyde”︁ (époxylinalol)
Authors: Naves Y. R. , Bachmann P. 
Published: December 2004

AbstractL'époxylinalol est l'époxy‐2,3‐diméthyl‐2,6‐octène‐7‐ol(6). Il est hydrogéné par le sodium et l'alcool en un mélange de diméthyl‐2,6‐octène‐7‐ol‐(3,6) et d'un oxyfurane ou oxypyrane. Le spectre Raman de l'époxyde a été déterminé.
Mikrobestimmung von Alkylimid‐Gruppen
Authors: Fierz‐David H. E. , Pfanner E. , Oppliger F. 
Published: December 2004

AbstractAbgeänderte Apparatur zur Bestimmung kleiner Mengen Alkoxyl, Alkylimid, Alkoxyl neben Alkylimid und Methoxyl neben Äthoxyl. An Stelle des üblichen Kupferoxydheizbades wurde ein elektrisch geheiztes Luftbad verwendet. Durch die Anbringung eines Kühlmantels, eines mit Natronasbest gefüllten Absorptionsröhrchens und durch die dadurch bedingte Verwendung von Stickstoff statt Kohlendioxyd zum Durchleiten durch die Apparatur konnte ein restloses Zurückhalten der Jodwasserstoffdämpfe erreicht werden. Dieses war bei den früher bekannten Apparaturen nicht möglich, da nur eine Waschvorrichtung angebracht war, deren Inhalt sehr leicht verbraucht wurde, was sich vor allem ungünstig bei den Alkylimid‐Bestimmungen auswirkte. Ferner wurde eine neue Absorptionsvorlage verwendet, die ein besseres Auffangen der Alkyljodide gewährleistet.
Die Bildung von Furan‐2,5‐dicarbonsäure aus d‐Glucuron und d‐Galakturonsäure beim Menschen. Ein neuer Abbauweg für Kohlenhydrate
Authors: Flaschenträger B. , Cagianut B. , Meier F. 
Published: December 2004

Abstract1. Die Ausscheidung der im normalen menschlichen Harn vorkommenden Furan‐2,5‐dicarbonsäure wird nach Einnahme von 5 g d‐Glucuron und d‐Galakturonsäure um das 5–15 fache gesteigert. Da nur ein Bruchteil der gebildeten Dicarbonsäure zur Ausscheidung gelangt, muss ein grösserer Umsatz im Organismus angenommen werden. Sie lässt sich als Abbauprodukt der Zuckerreihe deuten.
Über eine neue l‐Aminosäure‐oxydase (Ophio‐l‐aminosäure‐oxydase). 3. Mitteilung
Authors: Zeller E. A. , Maritz A. , Iselin B. 
Published: December 2004

Abstract1. Die Oxydation von l‐Prolin, l‐Oxyprolin, N‐Methyl‐l‐leucin, N‐Methyl‐l‐phenylalanin wird durch die l‐Aminosäure‐oxydase der Schlangengifte (Ophio‐l‐aminosäure‐oxydase) auch unter extremen Versuchsbedingungen nicht katalysiert.
Elektrochemisch‐konstitutive Beziehungen beim Haemoglobin
Authors: Pauli Wolfgang , Schwarzacher Walther 
Published: December 2004

Abstract1. Krystallisiertes Oxyhämoglobin wird durch Elektrodialyse ohne jede Denaturierung bis x = 2,3–5 × 10−6 r. O. und zu der Eigenreaktion pH = 7,048–6,92 in 4–5‐proz. Lösung gereinigt. Die maximale Protonaufnahme (aus HCl) bzw. Abgabe (an Alkalilauge) wird mit 156,5 × 10−5‐n. bzw. 126 × 1−5‐n. je g Hb bestimmt. Das entspricht einer maximalen Ladungszahl (Lz) von 108 positiven bzw. 87 negativen Ladungen je Teilchen. Hb wandert auch in sehr starkem elektrischen Feld nur spurenweise kathodisch.
Scaled AMO calculations on the hydrogen molecule II. The 1s σ 2p σ 1∑ state
Authors: Brändas Erkki 
Published: December 2004

AbstractA scaled version of the AMO method is applied to the 1sσ2pσ 1∑ state of the hydrogen molecule. A method to extend the domain of the mixing parameter λ to the whole complex plane is described and applied in the present calculation. All the parameters introduced have been varied completely. A considerable improvement in the computed energy values is found for large internuclear separations R. The comparison between our potential energy curve and the accurate curve calcualted by Kolos and Wolniewicz is studied and, for example, for R = 12 a.u. the energy difference is only 18% of that for R = 2.43 a.u. The equilibrium separation is found to be 2.140 a.u. in poor agreement with 2.429 a.u. obtained by the previously mentioned authors.In the separated atom limit, the state under consideration does not dissociate into H+ + H−, although the ionic character of the wave function is dominating in the region 3 ≦ R ≦ 8 a.u. The connections with earlier calculations and methods, especially the scaled version of the MO–LCAO approximation, are also pointed out and discussed.
Improved lower bounds for the convergence radius of RS‐perturbation theory
Authors: Bingel W. A. 
Published: December 2004

AbstractF. Rellich and T. Kato have given lower bounds for the convergence radius of Rayleigh–Schrödinger perturbation theory, if the perturbation operator is bounded relative to the unperturbed Hamiltonian. It is shown here that these lower bounds can usually be improved.
Molecular‐orbital calculations on some mesoionic compounds
Authors: Sundaram K. , Purcell William P. 
Published: December 2004

AbstractMolecular‐orbital calculations have been performed on sydnone, sydnone imine, ψ‐oxatriazole, and isosydnone rings and their positive ionized forms by the Pariser–Parr–Pople–Brown–Heffernan method. The importance of elaborate open‐shell calculations and the effect of introducing nonneighbor core resonance integrals in this order of approximation are investigated. The available experimental data on closely related compounds are qualitatively in agreement with the calculated values. The computed charges and bond orders demonstrate that certain structural features are common to all five‐membered mesoionic ring systems and in agreement with recent experimental findings.
Théorie quantique du pouvoir rotatoire magnétique des molécules diamagnétiques. II. Applications
Authors: Smet P. 
Published: December 2004

AbstractThe general formalism of the variational theory of the Faraday effect established in the first part of this paper is now applied to the actual computation of the Verdet constant of the hydrogen molecule, of water and of some saturated hydrocarbons. The numerical results have a correct order of magnitude and even, for hydrogen, are rather close to the experimental value.
Convergent iterative solutions for the lowest state of a system with large perturbations
Authors: Weltin E. 
Published: December 2004

AbstractIterative methods for the calculation of the lowest eigenvalue and the corresponding eigenvector are derived from the partitioned Schrödinger equation in matrix form. While they have a formal similarity to the Brillouin‐Wigner perturbation expansion, they converge for a much wider class of matrices. The double iteration methods (DIM and DIMA) lead to a series of upper and lower bounds to the desired lowest eigenvalue in accordance with Löwdin's bracketing theorem and can be used in a strategy which insures convergence to the solution for arbitrarily large perturbations. A much more efficient single iteration method (SIMA) produces the proper solution even for numerically very ill‐conditioned matrices (large offdiagonal elements and small separation of the diagonal elements from the lowest). An important feature of the iterative methods is their insensitivity toward roundoff errors which makes them especially suited for very large matrices, where direct diagonalisation, for example by the Jacobi method, is impracticable.
The variation method and the algebraic eigenvalue problem
Authors: Wallis A. , McElwain D. L. S. , Pritchard H. O. 
Published: December 2004

AbstractThe generalized algebraic eigenvalue problem (A − λB)x = 0 arises in the use of the variation method in quantum mechanics. If, within the limitations of the computer word‐length, the basis set used to expand the trial wave function is linearly dependent, the matrix B becomes singular. Three different algorithms designed to deal with this difficulty have been investigated, paying special attention to the problem of identifying which members of the basis set are effectively linearly dependent. The advantages and limitations of each method are discussed.
Correlation problems in atomic and molecular systems III. Rederivation of the coupled‐pair many‐electron theory using the traditional quantum chemical methodst
Authors: Čižek J. , Paldus J. 
Published: December 2004

AbstractThe equations of the coupled‐pair many‐electron theory (CPMET) for the closed shell systems are rederived both in the spin‐orbital and orbital forms without the use of second quantization, Wick's theorem or the technique of Feynman‐like diagrams. Only the Slater rules are used for the calculation of necessary matrix elements. A comparison with earlier papers shows clearly the usefulness and conceptual simplicity of the mathematical methods of quantum field theory both in the derivation of the CPMET, in spin‐orbital form, and in the process of excluding spin variables.
A perturbed generalized eigenvalue equation for the helium atom. II
Authors: Woodward M. R. , Amos A. T. , Laughlin C. 
Published: December 2004

AbstractThe Schrödinger equation for helium is written as a generalized eigenvalue equation which is solved for the ground state using perturbation theory. Two zero order equations are used and defined implicitly by specifying their eigenvalues and eigenfunctions. In both cases the eigenfunctions are taken to be the complete discrete set of products of eigenfunctions of the generalized eigenvalue equation for the hydrogen atom. To complete the definition the zero order eigenvalues are needed and two different intuitive choices are made. In the better of our two calculations an energy of –2.9014 a.u. is obtained for helium using the perturbation theory to second order.
High orders corrections to the Van der, Waals–London forces. I. A model problem: Two interacting hydrogen molecules is the minimal basis set
Authors: Malrieu J. P. 
Published: December 2004

AbstractThe classical intermolecular Rayleigh‐Schrödinger perturbation expansion is used with a finite basis of simple products of single determinants. For two hydrogen molecules with a minimal basis set, one shows that the ring and ladder diagrams dominate the perturbation series. The contributions of the purely intermolecular convex and concave ring diagrams are summed at all orders. The mixed ladder‐ring diagrams are also included. The series converges if the norm of the first order perturbed wave function is smaller than ½. The summation multiplies the Van der Waals–London forces by an explicit factor.