Insights into the mechanism of the site-selective sequential palladium-catalyzed cross-coupling reactions of dibromothiophenes/dibromothiazoles and arylboronic acids. Synthesis of PPARbeta/delta agonists

A reactivity study, aided by NMR spectroscopy, allowed a mechanistic rationale to be postulated for the palladium-catalyzed regioselective coupling of arylboronic acid (and arylstannane where feasible) at the position next to the sulfur atom in functionalized dibromothiophenes and dibromothiazoles. The analysis of the NMR spectra (using 19F from the boronic acid CF3 group and 31P from the phosphine of the catalyst as probes) of the entire reaction starting from the dibromoheterocycles allowed the qualitative proposal that the transmetalation is the rate-limiting step for both sequential substitution processes. The extremely facile oxidative addition at the C-Br bond next to the sulfur atom of the heterocycle instead determines the positional selectivity. An additional Stille reaction then replaced the second halogen, providing the trisubstituted heterocyclic scaffolds of PPAR ligands, which displayed PPARbeta/delta agonist activity, as revealed by reporter assays in living cells.     

The nonlinear Schrödinger equation with a potential

We consider the cubic nonlinear Schrödinger equation with a potential in one space dimension. Under the assumptions that the potential is generic, sufficiently localized, with no bound states, we obtain the long-time asymptotic behavior of small solutions. In particular, we prove that, as time goes to infinity, solutions exhibit nonlinear phase corrections that depend on the scattering matrix associated to the potential. The proof of our result is based on the use of the distorted Fourier transform – the so-called Weyl–Kodaira–Titchmarsh theory – a precise understanding of the “nonlinear spectral measure” associated to the equation, and nonlinear stationary phase arguments and multilinear estimates in this distorted setting.     

Calorimetric Behaviour of Anhydrous β-Cyclodextrin at Very Low Temperature

Heat capacities of anhydrous -cyclodextrin were measured between 10 K to 300 K using an adiabatic low-temperature calorimeter. Two distinct types of thermal behaviour were observed in the temperature range 50–130 K, depending on the way the sample was cooled. In the absence of hydration water molecules, we assumed that the two types of thermal behaviour could be attributed to freezing in more or less ordered states of the hydroxyl groups engaged in the intramolecular interglucose flip-flop-type interactions.     

Cyan fluorescent protein: molecular dynamics, simulations, and electronic absorption spectrum

The dynamics and electronic absorption spectrum of enhanced cyan fluorescent protein (ECFP), a mutant of green fluorescent protein (GFP), have been studied by means of a 1 ns molecular dynamics (MD) simulation. The two X-ray conformations A' and B' of ECFP were considered. The chromophore was assumed to be neutral, and all titratable residues were taken in their standard protonation state at neutral pH. The protein was embedded in a box of water molecules (and counterions). The first result is that the two conformations A' and B' are found to be stable all along the simulation. Then, an analysis of the hydrogen-bond networks shows strong differences between the two conformations in the surroundings of the nitrogen atom of the indolic part of the chromophore. This is partly due to the imperfection in the beta barrel near the His148 residue, which allows the access of one solvent molecule inside the protein in conformation A'. Finally, quantum mechanical calculations of the electronic transition energies of the chromophore in the charge cloud of the protein and solvent water molecules were performed using the TDDFT method on 160 snapshots extracted every 5 ps of the MD trajectories. It is found that conformations A' and B' exhibit very similar spectra despite different H-bond networks involving the chromophore. This similarity is related to the weak charge transfer involved in the electronic transition and the weak electrostatic field created by ECFP near the chromophore, within the hypotheses made in the present simulation.     

Mesostructures of cobalt nanocrystals. 1. Experiment and theory

Solid mesostructures made of cylinders are produced by the slow evaporation of cobalt nanocrystals dispersed in hexane and subjected to an applied field perpendicular to the substrate. Varying the initial nanocrystal concentration is found to be an efficient method for changing the pattern size. The experimental structures and the theoretical predictions based on the minimization of the total free energy are in good agreement. A comparison of experiment with theory allowed us to conclude that the mesostructures form as a result of a liquid-gas phase transition during the evaporation process. Within the theoretical model and the experimental data, it is concluded that the phase ratio of the magnetic to the total volume and the height of the cylinders govern the pattern geometry. In contrast, because of the saturation of the magnetization curve, the mesostructures are not influenced by the field strength.     

Structure de benzoates de fluoroalkyles

The crystal and molecular structure of 2,2,3,3-tetrafluoropropyl-parachlorobenzoate (C₁₀H₇ClF₄O₂) has been determined by X-ray diffraction. The crystals are monoclinic, P2₁, a = 4.826(1), b = 9.036(2), c = 12.442(4)Å, β = 94.64(2)°, V = 540.8 ų, Z = 2, D_x = 1.66 Mg m^?3. Starting from the ¹³C data of this compound, the identification of five other fluoroalkylbenzoates has been achieved by N.M.R. spectroscopy.     

A novel type of bis-quinolizidine alkaloid from the sponge: Petrosia seriata

The structure of petrosin, the major alkaloid isolated from the sponge Petrosia seriata, has been determined by X-ray diffraction analysis. Petrosin is the first member of a new class of bis-quinolizidine alkaloids containing a C-16 macrocycle.     

2-Aminothiazole derivatives. II. Mass spectra of ethyl N-(2-thiazolyl)-malonamates and ethyl 2-thiazolylaminomethylenemalonates

The fragmentation pattern of the title compounds under electron impact are discussed. During fragmentation in the mass spectrometer, these compounds undergo a rare type of rearrangement involving a long distance transfer of hydrogen.