Exploiting C3-symmetry in the dynamic coordination of a chiral trisoxazoline to copper(II): improved enantioselectivity, and catalyst stability in asymmetric lewis acid catalysis

Chiral C3-symmetric trisoxazolines are highly efficient stereodirecting ligands in enantioselective Cu(II) Lewis acid catalysis which is based on the concept of a stereoelectronic hemilability of the divalent copper; in direct comparison with the analogous bisoxazoline systems they are more efficient in the enantioselective alpha-amination as well as the enantioselective Mannich reaction of prochiral beta-ketoesters.     

Atropisomerism in 1,5-bridged calix[8]arenes

[structure: see text] The first example of two discrete calix[8]arene conformational isomers, 2 and 2a, has been obtained by exhaustive benzylation of 1,5-tetramethylene-bridged calix[8]arene 1. It is demonstrated, with the aid of X-ray crystallography, that these atropisomers have two 3/4-cone halves oriented syn or anti with respect to the bridge/bridgeheads moiety. VT NMR studies indicate that the tert-butyl-through-the-annulus inversion is inhibited in 1, while groups larger than n-hexyl or benzyl are required for curtailing the O-through-the-annulus route.     

EXAFS Spectroscopy of the Alkoxide Precursor Zr(O n Bu)4 and its Modification in Solution

Summary. The molecular structure of the transition metal alkoxide Zr(OⁿBu)₄ in toluene and its modification by addition of i-propanol, tetrahydrofurane, and the coordinating ligand pentane-1,3-dione (Hacac) were investigated by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopy. Zr(OⁿBu)₄ dissolved in toluene forms dimers. It was proved that cluster size is a function of the number of added equivalents ligand. In contrast, the addition of i-propanol or tetrahydrofurane caused no structural changes observable by EXAFS spectroscopy. A detailed discussion of the structural models is given in terms of possible alternatives and errors within the EXAFS analysis.     

The potential of composite pyrolysis gas chromatographic mass spectra in the study of lignocellulosics

The objective of this work was to evaluate the use of composite mass spectral (CMS) data from pyrolysis gas chromatography/mass spectrometry (PY–GC/MS) for lignocellulosic materials. Various forages, by-products and fiber fractions derived from them were examined as CMS by PY–GC/MS. The PY–GC/MS system consisted of a heated platinum filament, a capillary gas chromatograph and an ion trap detector (ITD) mass spectrometer operated under electron impact conditions. Mass spectra were then composited in several ways by summing all the mass spectra acquired within retention times corresponding to major product classes. CMS data were entered in a dedicated library and compared using the ITD library editor software. The usefulness of such a simple procedure for studies related to lignocellulose analysis, such as forage recognition, development of analytical methods and digestibility/maturity correlation, is discussed.     

Nitrite increases the enantioselectivity of sulfoxidation catalyzed by myoglobin derivatives in the presence of hydrogen peroxide

The effect of nitrite in the sulfoxidation of organic sulfides catalyzed by myoglobin (Mb) in the presence of hydrogen peroxide has been investigated. A general improvement in enantioselectivity was found for the reaction catalyzed by horse heart metMb and a series of sperm whale metMb derivatives including the wild type protein, the active site mutants T67K Mb, T67R Mb, T67R/S92D Mb, and the T67K Mb derivative reconstituted with the modified prosthetic group protohemin-l- histidine methyl ester.     

Cyanidverbrückte Koordinationspolymere aus cis‐ oder trans‐[Ru(tBuNC)4(CN)2] und MnCl2: Zum Einfluß unterschiedlicher Topologien auf die magnetischen Eigenschaften von Materialien

Cyanide Bridged Coordination Polymers from cis‐ or trans‐[Ru(^tBuNC)₄(CN)₂] and MnCl₂: About the Influence of Different Topologies on the Magnetic Properties of Materials The reaction of cis‐ or trans‐[Ru(^tBuNC)₄(CN)₂] with MnCl₂ as an additional transition metal fragment yields the one dimensional coordination polymers {cis‐[Ru(CN)₂(^tBuNC)₄] MnCl₂}_n, ( 1 ), and {trans‐[Ru(CN)₂(^tBuNC)₄]MnCl₂}_n, ( 2 ), with a different arrangement of the metal centers caused by the different stereochemistry of the starting compounds. The variation of the Ru‐C‐N‐Mn geometry nevertheless leads to significant differences in the magnetic properties of 1 and 2 . The coordination polymer derived from trans‐[Ru(^tBuNC)₄(CN)₂] shows a more efficient antiferromagnetic intrachain interaction between the manganese centers compared to the cis‐derivative.     

Orientation and order in thin films of a combined liquid crystalline polymer

The order in thin films of a combined liquid crystalline polymer is studied by X-ray reflection. Films of thicknesses of less than 200 nm on float glass are investigated as a function of temperature. The polymer with mesogenic groups in the main and side-chains exhibits smectic and cholesteric mesophases. Measurements in the smectic phases show a Bragg peak and smectic layers are oriented parallel to the substrate. The sample is thus macroscopically ordered by the influence of substrate and free surface. The film surface is very smooth after spincoating; surface roughness is typically 0.8 nm. First annealing of samples leads to a significant roughening of the free surface; roughness increases to 2.1 nm. Order as a function of film thickness depends on the interaction of the polymer with the substrate and free surface. These interactions give rise to a typical correlation length of perturbations in smectic ordering.     

Environmental effects on a prion's helix II domain: copper(II) and membrane interactions with PrP180-193 and its analogues

An abnormal interaction between copper and the prion protein is believed to play a pivotal role in the pathogenesis of prion diseases. Copper binding has been mainly attributed to the N-terminal domain of the prion protein, but this hypothesis has recently been challenged in some papers which suggest that the C-terminal domain might also compete for metal anchoring. In particular, the segment corresponding to the helix II region of the prion protein, namely PrP180-193, has been shown both to bind copper and to exhibit a copper-enhanced cytotoxicity, as well as to interact with artificial membranes. The present work is aimed at extending these results by choosing the most representative model of this domain and by determining its copper affinity. With this aim, the different role played by the electrostatic properties of the C- and N-termini of PrP180-193 (VNITIKQHTVTTTT) in determining its conformational behaviour, copper coordination and ability to perturb model membranes was investigated. Owing to the low solubility of PrP180-193, its copper affinity was evaluated by using the shorter PrPAc184-188NH2 (IKQHT) analogue as a model. ESI-MS, ESR, UV/Vis, and CD measurements were carried out on the copper(II)/PrPAc184-188NH2 and copper(II)/PrP180-193NH2 systems, and showed that PrPAc184-188NH2 is a reliable model for the metal interaction with the helix II domain. The affinity of copper(II) for the helix II fragment is higher than that for the octarepeat and PrP106-126 peptides. Finally, the different ability of PrP180-193 analogues to perturb the DPPC model membrane was assessed by DSC measurements. The possible biological consequences of these findings are also discussed briefly.     

Application of nanosized zeolites in methanol conversion processes: A short review

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