Exo conformers of N‐(pyridin‐2‐yl)‐ and N‐(pyridin‐3‐yl)norbornene‐5,6‐dicarboximide crystals

Two isomeric pyridine‐substituted norbornenedicarboximide derivatives, namely N‐(pyridin‐2‐yl)‐exo‐norbornene‐5,6‐dicarboximide, (I), and N‐(pyridin‐3‐yl)‐exo‐norbornene‐5,6‐dicarboximide, (II), both C₁₄H₁₂N₂O₄, have been crystallized and their structures unequivocally determined by single‐crystal X‐ray diffraction. The molecules consist of norbornene moieties fused to a dicarboximide ring substituted at the N atom by either pyridin‐2‐yl or pyridin‐3‐yl in an anti configuration with respect to the double bond, thus affording exo isomers. In both compounds, the asymmetric unit consists of two independent molecules (Z′ = 2). In compound (I), the pyridine rings of the two independent molecules adopt different conformations, i.e. syn and anti, with respect to the methylene bridge. The intermolecular contacts of (I) are dominated by C—H...O interactions. In contrast, in compound (II), the pyridine rings of both molecules have an anti conformation and the two independent molecules are linked by carbonyl–carbonyl interactions, as well as by C—H...O and C—H...N contacts.     

Carbonyl and nitrosyl diruthenium compounds: Crystal structure of [Ru2(O2CMe)(DPhF)3(CO)]BF4 · CH2Cl2 and its isomorphous nitrosyl analogue

The reaction of [Ru₂(O₂CMe)(DPhF)₃(H₂O)]BF₄ (DPhF = N,N′-diphenylformamidinate) with CO gas leads to [Ru₂(O₂CMe)(DPhF)₃(CO)]BF₄ (1), that is the first isolated carbonyl complex containing the Ru₂⁵⁺ unit. The nitrosyl analogue [Ru₂(O₂CMe)(DPhF)₃(NO)]BF₄ (2) is prepared by reaction of Ru₂Cl(O₂CMe)(DPhF)₃ with NOBF₄. However, the attempts to obtain the cyanide derivative by reaction of Ru₂Cl(O₂CMe)(DPhF)₃ or [Ru₂(O₂CMe)(DPhF)₃(H₂O)]BF₄ with NaCN were unsuccessful. The structure of compounds 1 · CH₂Cl₂ and 2 · CH₂Cl₂ are described. Both compounds are isomorphous. The magnetic measurements at variable temperature demonstrate that 1 is paramagnetic with one unpaired electron in all range of temperature, in contrast to the three unpaired electrons usually present in Ru₂⁵⁺ complexes. The analogous nitrosyl compound 2 is diamagnetic.     

Valence-state analysis through spectroelectrochemistry in a series of quinonoid-bridged diruthenium complexes [(acac)(2)Ru(mu-L)Ru(acac)(2)](n) (n=+2, +1, 0, -1, -2)

The quinonoid ligand-bridged diruthenium compounds [(acac)(2)Ru(mu-L(2-))Ru(acac)(2)] (acac(-)=acetylacetonato=2,4-pentanedionato; L(2-)=2,5-dioxido-1,4-benzoquinone, 1; 3,6-dichloro-2,5-dioxido-1,4-benzoquinone, 2; 5,8-dioxido-1,4-naphthoquinone, 3; 2,3-dichloro-5,8-dioxido-1,4-naphthoquinone, 4; 1,5-dioxido-9,10-anthraquinone, 5; and 1,5-diimido-9,10-anthraquinone, 6) were prepared and characterized analytically. The crystal structure analysis of 5 in the rac configuration reveals two tris(2,4-pentanedionato)ruthenium moieties with an extended anthracenedione-derived bis(ketoenolate) pi-conjugated bridging ligand. The weakly antiferromagnetically coupled {Ru(III)(mu-L(2-))Ru(III)} configuration in 1-6 exhibits complicated overall magnetic and EPR responses. The simultaneous presence of highly redox-active quinonoid-bridging ligands and of two ruthenium centers capable of adopting the oxidation states +2, +3, and +4 creates a large variety of possible oxidation state combinations. Accordingly, the complexes 1-6 exhibit two reversible one-electron oxidation steps and at least two reversible reduction processes. Shifts to positive potentials were observed on introduction of Cl substituents (1-->2, 3-->4) or through replacement of NH by O (6-->5). The ligand-to-metal charge transfer (LMCT) absorptions in the visible region of the neutral molecules become more intense and shifted to lower energies on stepwise reduction with two electrons. On oxidation, the para-substituted systems 1-4 exhibit monocation intermediates with intervalence charge transfer (IVCT) transitions of Ru(III)Ru(IV) mixed-valent species. In contrast, the differently substituted systems 5 and 6 show no such near infrared (NIR) absorption. While the first reduction steps are thus assigned to largely ligand-centered processes, the oxidation appears to involve metal-ligand delocalized molecular orbitals with variable degrees of mixing.     

Organocatalytic asymmetric synthesis of alpha,alpha-disubstituted alpha-amino acids and derivatives

It is shown that racemic oxazolones are excellent reagents for the synthesis of chiral quaternary amino acids and its derivatives by the diastereo- and enantioselective nucleophilic addition to alpha,beta-unsaturated aldehydes catalyzed by diarylprolinol silyl ethers. The scope of this new organocatalytic reaction is demonstrated for different oxazolones having aromatic and alkyl groups at the reactive carbon atom and different aromatic and aliphatic substituted alpha,beta-unsaturated aldehydes, for which the stereoselective reaction proceeds with good yield, moderate to good to very high diastereoselectivity, and very high enantioselectivity. The potential of the reaction is shown for the synthesis of optically active alpha,alpha-disubstituted alpha-amino acids, alpha-quaternary proline derivatives, amino alcohols, lactams, and tetrahydropyranes. Furthermore, we have calculated by DFT-methods the transition-state structures that account for both the diastereo- and enantioselectivity observed for the addition of oxazolones to the alpha,beta-unsaturated aldehydes. For one class of compounds, the stereoselectivity is controlled by a hydrogen-bonding interaction of the enolate-form of the oxazolone with an ortho-hydroxy-phenyl substituent of the alpha,beta-unsaturated aldehyde, whereas the benzhydryl-protecting group in the oxazolone determines the diastereo- and enantioselectivity in a more general manner for both aromatic and aliphatic alpha,beta-unsaturated aldehydes.     

Three Bis‐BODIPY Analogous Diruthenium Redox Series: Characterization and Electronic Structure Analysis

The dianion derived from (2Z,6Z)‐3,7‐diphenyl‐N2,N6‐di(pyridin‐2‐yl)pyrrolo[2,3‐f]indole‐2,6(1H,5H)‐diimine (H₂BL), a modified BODIPY ligand precursor, is shown to be capable of bridging two metal complex fragments RuL₂, L=2,4‐pentanedionato (acac^?), 2,2’‐bipyridine (bpy) or 2‐phenylazopyridine (pap) in [Ru(acac)₂Ru(μ‐BL)Ru(acac)₂] ( 1 / 2 ), [Ru(bpy)₂Ru(μ‐BL)Ru(bpy)₂](ClO₄)₂ ([ 3 ](ClO₄)₂) and [Ru(pap)₂Ru(μ‐BL)Ru(pap)₂](ClO₄)₂ ([ 4 ](ClO₄)₂). The compounds, including a diastereoisomeric pair 1 (meso) and 2 (rac) were spectroscopically and structurally characterized. Reversible electron transfers as revealed by cyclic and differential pulse voltammetry allowed for an EPR and UV‐vis‐NIR spectroelectrochemical investigation of several neighboring charge states. Together with susceptibility measurements and TD‐DFT calculations the assignment of oxidation states reveals that 1 , 2 are diruthenium(III) species which can be oxidized or reduced by one electron whereas 3 ²⁺ and 4 ²⁺ contain ruthenium(II) and get reduced or oxidized mainly at the dianionic bridge ( 3 ²⁺) or are reduced at the ancillary ligands pap ( 4 ²⁺).     

Unravelling the Time Scale of Conformational Plasticity and Allostery in Glycan Recognition by Human Galectin-1

The interaction of human galectin-1 with a variety of oligosaccharides, from di-(N-acetyllactosamine) to tetra-saccharides (blood B type-II antigen) has been scrutinized by using a combined approach of different NMR experiments, molecular dynamics (MD) simulations, and isothermal titration calorimetry. Ligand- and receptor-based NMR experiments assisted by computational methods allowed proposing three-dimensional structures for the different complexes, which explained the lack of enthalpy gain when increasing the chemical complexity of the glycan. Interestingly, and independently of the glycan ligand, the entropy term does not oppose the binding event, a rather unusual feature for protein-sugar interactions. CLEANEX-PM and relaxation dispersion experiments revealed that sugar binding affected residues far from the binding site and described significant changes in the dynamics of the protein. In particular, motions in the microsecond-millisecond timescale in residues at the protein dimer interface were identified in the presence of high affinity ligands. The dynamic process was further explored by extensive MD simulations, which provided additional support for the existence of allostery in glycan recognition by human galectin-1.     

Complexes self‐associate by hydrogen bonding and metallophilic attraction: Theoretical study

Hydrogen bonding and metallophilic attractions are studied in the model systems: [(AuNH₃Cl)₂], [(AuNH(CH₃)₂Cl)₂], [{Au₂(μ‐SH)(PH₂O)(PH₂OH)}₂], [(CuNH₃Cl)₂], and [{Cu(NH₃)Cl}₄] at the Hartree–Fock (HF) and second‐order Møller–Plesset (MP2) levels. The two interactions are found to be comparable and prevailing in the final structure. It is determined that the aurophilic contact has a same magnitude that the hydrogen bonding, and is stronger than the cuprophilic interaction. The presence of hydrogen bond directs the growth of the crystal. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Microwave-assisted extraction through an aqueous medium and simultaneous cleanup by partition on hexane for determining pesticides in agricultural soils by gas chromatography: a critical study

A simple microwave-assisted extraction and partitioning method (MAEP) using water-acetonitrile and n-hexane for desorption and simultaneous partitioning, respectively, together with gas chromatography (GC) was studied to determine representative pesticides (trifluralin, metolachlor, chlorpyriphos and triadimefon) with a broad range of physico-chemical properties in agricultural soils. Three points were considered crucial in this study: instrumental and sample-associated factors affecting extraction of the target compounds were studied through experimental design; the spiking procedure at trace levels was carried out to reproduce the solute-soil sorption taking place in the environment as closely as possible; and results were analyzed taking into account the adsorption behaviour of the compounds on different kinds of soils. The complete analytical procedure proposed consisted of the MAEP of pesticides from 1.0 g of soil with 1 mL of 1:1 water/acetonitrile mixture, and 5 mL of hexane for trapping. The microwave heating program applied was 2 min at 250 W and 10 min at 900 W, and 130 °C maximum temperature. After extraction, the hexane layer was evaporated to dryness; the residue was re-dissolved and directly analyzed by gas chromatography electron capture detection (GC-ECD). Clean chromatograms were obtained without any additional cleanup step. Besides the four pesticides used to optimise MAEP, the method was applied to determine an additional group of pesticides (triallate, acetochlor, alachlor, endosulphan I and II, endrin, methoxychlor and tetradifon) in different soils. Most of the compounds studied were recovered in good yields with relative standard deviations (R.S.D.s) below 9% and detection limits ranged from 0.004 to 0.036 μg g⁻¹. The described method is efficient and fast to determine hydrophobic pesticides at ng g⁻¹ level in soil with different clay-to-organic matter ratios.     

Acerolide, a new cytotoxic cembranolide from the soft coral Pseudopterogorgia acerosa

A new cembranolide, acerolide (1) together with the known compound pseudopterolide (2) were isolated from the 2-propanol extract of the soft coral Pseudopterogorgia acerosa. The structure of 1 was determined on the basis of detailed spectroscopic analysis. Compound 1 showed moderate in vitro cytotoxicity against a panel of 14 tumor cell lines.     

Facilitating the culture of mammalian nerve cells with polyelectrolyte multilayers

When neuron-like cells (NLCs) derived from pluripotent embryonal carcinoma cells (P19) are cultured on bare tissue culture substrates, they require a monolayer of fibroblast cells to exhibit normal neurite outgrowth, behavior typical of neuronal cultures. However, substrate treatment with polyelectrolyte multilayers (PEMs) composed of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonic acid) (PSS) significantly improved these cultures. Cell morphology was more spread, indicative of healthy cells, and direct attachment of neuronal cell bodies to the treated surface was observed. Neuronal outgrowth across the surface was not dependent on an underlying fibroblast monolayer with the PEMs surface treatment. Additionally, the PEMs surface treatment can be used to condition various surfaces, facilitating neuronal cultures on surfaces which are natively hydrophilic (tissue culture polystyrene) or hydrophobic (poly(dimethylsiloxane), PDMS). Microfluidic networks were used to micropattern the PEMs onto PDMS, resulting in confined regions of cellular attachment and directed neuronal outgrowth. The ability of PEMs to encourage NLC attachment without supporting cells to a variety of surfaces and surface geometries greatly simplifies neuronal culture methodology and enables neuronal investigations in new environments.