Tandem [1,5]-H shift/6π-electrocyclizations of ketenimines bearing 1,3-oxathiane units. Computational assessment of the experimental diastereoselection

N-Aryl ketenimines bearing a 1,3-oxathiane function at the ortho position of the keteniminic nitrogen atom convert into spiro[1,3-oxathiane-2,4′(3′H)quinolines] under mild thermal treatment. These cyclization processes are interpreted in terms of a two-step tandem sequence involving a [1,5]-H migration followed by a 6π-electrocyclic ring closure. Moreover, the cyclization of 1,3-oxathiane-ketenimines having two different substituents at the terminal carbon atom of the ketenimine moiety provided spiroquinolines bearing two stereocenters, the C3 and C4 atoms, with moderate diastereoselectivity. A DFT study support that the mechanism of these conversions consists of a [1,5]-H shift/6π-electrocyclization sequence, in which the [1,5]-H shift is the rate-limiting step. A quantitative kinetic analysis of the cyclization of an oxathiane-ketenimine with a prochiral ketenimine function explains the sense and degree of the experimentally observed diastereoselectivity.     

Diels–Alder reactions of perfluoroketene dithioacetals with electron-rich 1,3-dienes: a new access to polysubstituted aromatic sulfides

The present paper describes the Diels–Alder reactions of perfluoroketene dithioacetals with electron-rich 1,3-dienes (2,3-dimethylbuta-1,3-diene, isoprene, penta-1,3-diene) followed by spontaneous HF and thiol elimination, leading to polysubstituted aromatic sulfides in moderate to good yields. Reactions seem to be dependent on the substitution patterns of perfluoroketene dithioacetals; the best results were obtained from trifluoromethyl or pentafluoroethyl and ethylsulfanyl derivatives. Theoretical calculations performed at the DFT level are in good agreement with the experimental results and show that the overall process is controlled by the cycloaddition step.     

Oxyfold: A Simple and Efficient Solid‐Supported Reagent for Disulfide Bond Formation

The synthesis and use of novel polymer‐supported reagents for disulfide bond formation is described. This family of supported reagents consists of a series of oxidized methionines grafted onto a solid support. Their cost and the simplicity of their preparation through N‐carboxyanhydride polymerization on beads make them reactants of choice for the formation of disulfide bridges in peptides.     

Nano-chemistry and scanning probe nanolithographies

The development of nanometer-scale lithographies is the focus of an intense research activity because progress on nanotechnology depends on the capability to fabricate, position and interconnect nanometer-scale structures. The unique imaging and manipulation properties of atomic force microscopes have prompted the emergence of several scanning probe-based nanolithographies. In this tutorial review we present the most promising probe-based nanolithographies that are based on the spatial confinement of a chemical reaction within a nanometer-size region of the sample surface. The potential of local chemical nanolithography in nanometer-scale science and technology is illustrated by describing a range of applications such as the fabrication of conjugated molecular wires, optical microlenses, complex quantum devices or tailored chemical surfaces for controlling biorecognition processes.     

Thermogravimetric measurement of amorphous cellulose content in flax fibre and flax pulp

Usually the raw material for flax pulp production is a blend which contains fibres and shives. In order to better understanding the structure of these materials and the effects of flax pulping, X-ray diffraction and thermogravimetry analysis under air atmosphere have been used. There was a significant effect of the fibre size on the composition, crystallinity, and thermal behaviour of the flax pulps. On the other hand, data obtained from thermogravimetric analysis have been modelled on the basis of two cellulose types characterized by different crystallinity levels, using kinetics equations based on the nucleation concept. As a result of these simulations, composition of the samples, pulp crystallinity and the proportion of amorphous cellulose are calculated.     

3,2-Hydroxypyridinone (3,2-HOPO) vinyl sulfonamide and acrylamide linkers: aza-Michael addition reactions and the preparation of poly-HOPO chelators

The HOPO vinyl sulfonamide 3 and the corresponding HOPO acrylamide 10 were easily prepared by short synthetic sequences. Investigation of the aza-Michael reactions of these linkers showed that they proceed at a higher rate in solvent systems containing water. The scope and limits of the aza-Michael reactions of 3 and 10 were examined. Reagents 3 and 10 reacted cleanly with piperazine to give the corresponding adducts which were deprotected to give the di-HOPO ligands 7 and 16. Reaction of HOPO acrylamide 10 with 1,4,7-triazacyclononane gave the tris-adduct 17 which was deprotected to give the desired tris-HOPO ligand 18. Overall, the aza-Michael reactions of 3 and 10 appear to be governed not only by the solvent but also by the nature of the amine and the solubility of the reaction intermediates.     

Beyond the navier-stokes regime in hydrodynamics

In this letter we show that if one suitably modifies the conventional linear relationship between forces and fluxes in irreversible thermodynamics, then all the higher-order hydrodynamic equations, namely, the Burnett, super-Burnett, etc. equations, are consistent with both the entropy balance equation and the local equilibrium assumption. Some implications of this result are also pointed out.     

Dynamics and bifurcations of a modified Leslie‐Gower–type model considering a Beddington‐DeAngelis functional response

In this paper, a planar system of ordinary differential equations is considered, which is a modified Leslie‐Gower model, considering a Beddington‐DeAngelis functional response. It generates a complex dynamics of the predator‐prey interactions according to the associated parameters. From the system obtained, we characterize all the equilibria and its local behavior, and the existence of a trapping set is proved. We describe different types of bifurcations (such as Hopf, Bogdanov‐Takens, and homoclinic bifurcation), and the existence of limit cycles is shown. Analytic proofs are provided for all results. Ecological implications and a set of numerical simulations supporting the mathematical results are also presented.     

Solubility of sildenafil citrate in propylene glycol + water mixtures at various temperatures

The solubility of sildenafil citrate (SC) in aqueous mixtures of propylene glycol (PG) was investigated. This study was carried out at different mass fractions of PG ranging from 0.1 to 0.9 at T = 293.2–313.2 K. The solubility of SC in the mixed solutions increased with increasing temperature and PG mass fraction.

The solubility values were correlated by two co-solvency models (Jouyban–Acree model and van’t Hoff–Jouyban–Acree model). The mean relative deviations (MRD) were 5.7% and 5.5%, respectively. Density of the SC-saturated solutions over the entire solvent composition and temperature range was also measured and the results correlated with the Jouyban–Acree model. Furthermore, the apparent thermodynamic properties, dissolution enthalpy, dissolution entropy and Gibbs free energy change of dissolution process of SC in all the mixed solvents were calculated according to van’t Hoff and Gibbs equations. Dissolution of SC in these mixed solvents is an endothermic process.     

Further calculations on solubility of dipyrone in some binary solvent mixtures at various temperatures

The recently reported solubility data of dipyrone in binary solvent mixtures of {ethanol + water}, {methanol + ethanol} and {methanol + 1-propanol} at various temperatures have been used to report further numerical results based on the Jouyban–Acree model.