Hydrogen‐bonding patterns in rac‐1‐acetyl‐5‐methyl‐2‐thioxoimidazolidin‐4‐one

In the title compound, C₆H₈N₂O₂S, also known as N‐acetyl‐2‐thiohydantoin–alanine, the molecules are joined by N—H...O hydrogen bonds, forming centrosymmetric R₂²(8) dimers; these dimers are linked by C—H...O interactions to form R₂²(10) rings, thus forming C₂²(10) chains that run along the [101] direction.     

Analysis of B3LYP and MP2 conformational population distributions in trans‐nicotine,acetylcholine, and ABT‐594

This work presents an analysis of the equivalence of MP2 and DFT (B3LYP functional) conformational populations. As a test case, we select three cholinergic agents (trans‐nicotine, acetylcholine, and the nicotinic analgesic ABT‐594), where the minima on the conformational energy hypersurfaces expand a large range of energies (～0–30 kJ mol^?1). From energetic and structural data obtained in vacuo at the MP2 and B3LYP/cc‐pVDZ levels, we build conformational partition functions, including the effect of the conformational kinetic energy and the rotovibrational coupling. Our results at a physiological temperature (37°C) show qualitative agreement in all cases. Quantitative agreement, however, is only found for trans‐nicotine and ABT‐594. In the first case, energy minima differ by <0.2 kJ mol^?1. Therefore, the equivalence of structural results translates in the equivalence of the conformational distribution. For ABT‐594, the minima are separated by as much as 8.0 kJ mol^?1, and the conformational energy determines the conformational distribution. In this case, the slight relative variation of conformational energy, between B3LYP and MP2, does not affect the population, since the secondary minima are high in energy and very low in population. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Three isomorphous Ln complexes: {[Ln2(bt)6(bno)(H2O)4]·bno}n (bt is but‐2‐enoate and bno is 4,4′‐bipyridyl N,N′‐dioxide), with Ln = Nd,Er and Y

The polymeric title compounds, namely catena‐poly[[[di‐μ‐but‐2‐enoato‐κ³O:O,O′;κ³O,O′:O′‐bis[diaquadibut‐2‐enoato‐κO;κ²O,O′‐neodymium(III)]]‐μ‐4,4′‐bipyridyl N,N′‐dioxide‐κ²O:O′] 4,4′‐bipyridyl N,N′‐dioxide solvate] and the erbium(III) and yttrium(III) analogues, {[Ln₂(C₄H₅O₂)₆(C₁₀H₈N₂O₂)(H₂O)₄]·C₁₀H₈N₂O₂}_n (Ln = Nd, Er and Y), form from [Ln₂(bt)₆(H₂O)₄] dimers (bt is but‐2‐enoate) bridged by 4,4′‐bipyridyl dioxide (bno) spacers into sets of parallel chains; these linear arrays are interconnected by aqua‐mediated hydrogen bonds into broad two‐dimensional structures, which in turn interact with each other though the hydrogen‐bonded bridged bno solvent units. Both independent bno units in the structures are bisected by symmetry centres.     

Influence of the 4,6-O-benzylidene, 4,6-O-phenylboronate, and 4,6-O-polystyrylboronate protecting groups on the stereochemical outcome of thioglycoside-based glycosylations mediated by 1-benzenesulfinyl piperidine/triflic anhydride and N-iodosuccinimide/trimethylsilyl triflate

The effect of 4,6-O-benzylidene acetals, 4,6-O-phenylboronate esters, and 4,6-O-polystyrylboronate esters on the stereoselectivity of couplings to galacto-, gluco-, and mannopyranosyl thioglycosides, otherwise protected with benzyl ethers, has been investigated by the benzenesulfinyl piperidine/trifluoromethanesulfonic anhydride (BSP), diphenyl sulfoxide/trifluoromethanesulfonic anhydride (Ph(2)SO), and N-iodosuccinimide/trimethylsilyl trifluoromethanesulfonate (NIS/TMSOTf) methods. The BSP and Ph(2)SO methods give comparable results in all three systems whereas the NIS method affords significantly different stereoselectivities in both the gluco and manno, but not the galacto series. The benzylidene acetal and boronate esters influence the stereochemistry in a similar manner in the beta-selective manno series and the alpha-selective galacto series but show significant differences with the glucose donors. The differences between the glucose, galactose, and mannose series reflect the established differences in reactivity and, especially for mannose, those in the anomeric effect and are best interpreted in terms of changes in the relative energetics between the alpha- and beta-covalent triflate intermediates and the various contact ion pairs with which they are necessarily in equilibrium.     

Approaches to the Synthesis of Perfluoroalkyl-Modified Carbohydrates and Derivatives: Thiosugars,Iminosugars, and Tetrahydro(thio)pyrans

Fluoroalkyl-substituted carbohydrates play relevant roles in diverse areas such as supramolecular chemistry, glycoconjugation, liquid crystals, and surfactants, with direct applications as wetting, antifreeze, and coating agents. In light of these promising applications, new methodologies for the late-stage incorporation of fluoroalkyl R_F groups into carbohydrates and derivatives are herein presented as they are relevant to the synthetic carbohydrate community. Previously reviewed protocols for the installation of R_F groups onto carbohydrates and derivatives will be succinctly summarized in the light of the new achievements. Fluoroalkyl-substituted iminosugars, on the other hand, are also interesting glycomimetic derivatives with prominent roles as glycosidases and glycosyltransferases inhibitors, as has recently been demonstrated. Also, they positively contribute to the study of sugar–protein interactions and enzyme mechanisms. New advances in the syntheses of fluoroalkyl-substituted iminosugars will also be presented here.     

Activation energies for nucleation and growth and critical cluster size dependence in JMAK analyses of kinetic Monte-Carlo simulations of precipitation

Kinetic Monte-Carlo (KMC) methods are used as an approach to simulate precipitation in Cu-alloyed bcc Fe. In order to characterize the process, transformed fractions, that is, the precipitated atoms, are related to Johnson-Mehl-Avrami-Kolmogorov theory. However, simulated data often deviate from corresponding fit curves and so does the resulting growth exponent when compared to theoretical expectations. Furthermore, some data may suggest the development of a metastable phase. In our study, we show that the characteristics of the transformed fraction and, as a consequence, the derived growth exponents sensitively depend on the number of atoms that are considered to form a particle and hence contribute to the transformed fraction. With a temperature dependence of the critical cluster size and additionally accounting for severe impingement of the particles, we obtain growth exponents which lie close to the expected range between n = 1.5 and n = 2.5 for pre-existing nuclei or continuous nucleation, respectively. From these, we obtain activation energies for nucleation and growth of precipitates. In this way, atomistic KMC simulations yield thermodynamical quantities, which can be valuable input parameters for larger length scale simulation methods, for example, for Phase Field Method simulations.     

The Chromatographic Analysis of Reduced Sulfur Gases in Antarctic Waters Following Pre-Concentration onto Tenax

Abstract

A technique was developed using sparging and pre-concentration onto a Tenax trap at ambient temperature allowing field measurements to be made of reduced sulfur gases in Antarctic marine and glacial melt waters. Following thermal desorption, gases were determined by gas chromatography using a flame photometric detector. Detection limits in ng 1^?1 were: H₂S 50, OCS 8, SO₂ 160, CH₃SH 6, CH₃SCH₃ 20 and CS₂ 2. Storage of melt water samples for more than 24 hours resulted in concentration changes of the reduced sulfur gases present. While the Antarctic environment imposed limitations on the analytical method, an investigation of various pond and marine waters was undertaken.     

Synthesis and characterization of Pd(II) complexes of 2‐ and 3‐thiophenecarbaldehyde immobilized on silica obtained from sepiolite

Silica obtained by acid treatment of sepiolite was used as a support for catalysts consisting of palladium complexes of 2‐ and 3‐thiophenecarbaldehyde. The support and the catalysts were characterized by X‐ray diffraction, and solid‐state ²⁹Si and ¹³C nuclear magnetic resonance spectroscopies. The supported palladium catalysts were used in the Suzuki reaction of bromobenzene with phenylboronic acid. Copyright © 2013 John Wiley & Sons, Ltd.