Thermolysis of N-allylic 1,2,4-triazoles

A number of 4-allylic substituted 3,5-diphenyl-4H-1,2,4-triazoles were thermolyzed at 315–320° in evacuated glass ampoules. The main reaction in the melt was rearrangement to the corresponding 1-substituted triazoles, which appeared to proceed via competing S_N2 and S_N2′ mechanisms. The allylic systems were observed to undergo [2,3]-allyl walk reactions between the 1- and 2-ring positions. Allyl to vinyl isomerization also took place. Substitution of the allylic moiety increased the rate of reaction but decreased the rate of isomerization of allylic to the vinylic substituted triazoles. The 4-vinyl substituted triazoles were inert under the reaction conditions. Some triazoles were converted into substituted pyridines. This was proposed to proceed via nitrogen extrusion and formation of a 1,3-dipolar intermediate (nitrile ylide) which added intramolecularly to the allyl moiety and subsequently aromatized to the pyridine.     

Model studies of the chemisorption of hydrogen and oxygen on nickel surfaces

Atomic chemisorption of hydrogen and oxygen on the Ni(100) surface has been studied using an Effective Core Potential (ECP) approach described in a previous paper. Clusters of up to 50 nickel atoms have been used to model the surface. The computed chemisorption energies are 62 kcal/mol (exp. 63 kcal/mol) for hydrogen and 106 kcal/mol (exp. 115–130 kcal/mol) for oxygen. Correlating the adsorbate and the cluster-adsorbate bonds is extremely important for obtaining accceptable results, particularly for oxygen. Reasonable convergence of chemisorption energies is obtained with 40–50 cluster atoms for both hydrogen and oxygen. For hydrogen the addition of a third cluster layer stabilizes the results considerably. Both hydrogen and oxygen are adsorbed at (or close to) the four-fold hollow site. The calculated barriers for surface migration are also in good agreement with the experimental estimates. The calculated equilibrium heights above the surface are on the other hand too high compared with experiments. This disagreement is believed to be due to core-valence correlation effects, which are not incorporated in the present ECP. The cluster convergence for the height above the surface is much slower than for the chemisorption energy.     

Synthesis of part of a proposed insulin second messenger glycosylinositol phosphate and the inner core of glycosylphosphatidylinositol anchors

Synthesis of 6-O-(2-amino-2-deoxy--D-glucopyranosyl)-D-myo-inositol 1-phosphate, an inner core structure found in various glycosylphosphatidylinositols, and the corresponding 1,2-cyclic phosphate, proposed as part of an insulin second messenger glycosylinositol phosphate, is described. Chirality in the inositol part of the molecule was achieved by the use of a known D-camphor acetal intermediate. The glycosylation used 4-O-allyl-2-azido-3,6-di-O-benzyl-2-deoxy--D-glucopyranosyl fluoride as glycosyl donor. The allyl group can be chemoselectively removed, opening a route to oligosaccharides bound to the 4-position of the glucosamine unit. The phosphorylation was accomplished by the phosphoramidate procedure.     

The bonding in second row transition metal dihydrides,difluorides and dichlorides

Summary The dihydrides, the difluorides and the dichlorides of the second row transition metal atoms from yttrium to palladium have been studied with methods including electron correlation of all valence electrons. Comparisons are made to the previously studied corresponding diatomic systems. It is found that the general trends of the binding energies of the second hydride and halide remain the same as in the diatomic hydrides and halides. The second ligand binding energies for the dihalides thus vary much more than for the dihydrides. This is due to important attractive effects between the halide lone-pairs and empty 4d-orbitals to the left and strong repulsions towards occupied 4d-orbitals to the right. For some systems the second ligand binds much more than the first ligand, as for RuF₂ where the difference is 34.3 kcal/mol, whereas for other systems the reverse is true, as for PdCl₂ where the first ligand binds more than the second with 20.4 kcal/mol. The results can be explained by strong ligand field effects and differences in the atomic spectra.     

ABSORPTION SPECTRAL SHIFTS OF CAROTENOIDS RELATED TO MEDIUM POLARIZABILITY

Abstract–Solvent induced absorption spectral shifts of the electronic transition from ground 1 A_g state to the excited 1B_u state in carotenoids have been studied. It is shown that the shift depends only on dispersion interactions in non-polar solvents. In polar media there is just a small extra contribution to the red-shift, due to other forms of interactions. The spectral shifts are well described by the theory, which expresses the shift relative to the gas phase value, as a function of solvent polarizability. The main conclusion is that the dominating mechanism behind the large red-shifted absorbance of carotenoids in the proteinacous environment, in vivo, is the mutual polarizability interactions between the carotenoids and the surrounding medium. The solution-phase values of the dipole moments of the lA_g to 1B_u transitions and the differences of isotropic polarizability between 1B_u and lA_g states of carotenoids in non-polar solvents are calculated and found to be around 13 D and 360 ų respectively. From the great overlap of absorption spectra between carotenoids in quinoline and carotenoids in vivo in purple bacterial antenna complexes, it can be expected that the carotenoids are surrounded by several aromatic amino acids in vivo. Comparisons have been done between the exicted states in carotenoids and in linear conjugated polyenes.     

Ligand-protein docking using a quantum stochastic tunneling optimization method

A novel hybrid optimization method called quantum stochastic tunneling has been recently introduced. Here, we report its implementation within a new docking program called EasyDock and a validation with the CCDC/Astex data set of ligand-protein complexes using the PLP score to represent the ligand-protein potential energy surface and ScreenScore to score the ligand-protein binding energies. When taking the top energy-ranked ligand binding mode pose, we were able to predict the correct crystallographic ligand binding mode in up to 75% of the cases. By using this novel optimization method run times for typical docking simulations are significantly shortened.     

Regiospecific alkylation of tetronic acids : Formation of 4-alkoxy-5H-furan-2-ones and 2-alkoxy-5H-furan-4-ones

2-Alkoxy-5$\text{H}$-furan-4-ones (7,8) and 4-alkoxy-5$\text{H}$-furan-2-ones (4,5) were prepared regiospecifically and in high yields from tetronic acids (4-hydroxy-5$\text{H}$-furan-2-ones) (2) in the first case by acetylating the 4-OH group and then reacting with trialkyloxonium tetrafluoroborate, and in the second case by alkylating tetrabutylammonium tetronates with dialkyl sulfate, respectively. Direct alkylation of tetronic acids with trialkyloxonium tetrafluoroborate gave in four cases regiospecific 2-O-alkylation, in one case 4-O-alkylation and in two other cases mixtures of 2- and 4-alkoxy derivatives.     

Importance of the choice of assumptions and models in the estimation of analytical quality specifications

The validity of any model depends on its ability to imagine the situation or problem to which it is applied. Further, the assumptions made in relation to the model are determining for the actual outcome. Within the field of clinical biochemistry a lot of models for analytical quality specifications, based on a variety of concepts and ’clinical settings’, have been proposed. A hierarchical structure for application of these approaches and models has been agreed on at several occasions in 1999. In this hierarchy, the highest rank is given to evaluation of analytical quality specifications based on ’clinical settings’/’clinical outcome’ models, followed by specifications based on biological variation and on ’clinicians opinions’. This contribution, deals with the problems of combining random and systematic errors and the implications of application of different models to a variety of clinical settings. Received: 1 June, 2002 Accepted: 17 July 2002 Presented at the European Conference on Quality in the Spotlight in Medical Laboratories, 7–9 October 2001, Antwerp, Belgium     

Point-splitting regularization for gauge theories: Quantum electrodynamics

As a method of regularization, point splitting has played an essential role in the recent theoretical determination of the masses of the Higgs boson and the top quark. It is the purpose of this paper to put this pointsplitting regularization on a firm basis. The result turns out to be extremely simple: replace the usual vertex factor-ieγ ^µ in quantum electrodynamics by $$ - ie(\gamma _\mu - \frac{{\not p}}{{p \cdot \delta }}\delta _\mu ),$$ wherep is the momentum of the photon line, andδ ^µ is the distance for point splitting. No additional vertices are needed.     

Norm inequalities for spherical means

The regularity and integrability of spherical means of functions inL ^p ( ⁿ ,n2, are studied. Sharp results are obtained.