Diastereoselective mukaiyama and free radical processes for the synthesis of polypropionate units

[reaction: see text] Reported herein is the synthesis of 8 out of 16 polypropionates derived from our propionate units. A new strategy involving a stereoselective Mukaiyama aldol reaction followed by a stereoselective free-radical-based hydrogen transfer, both under Lewis acid control, is used. Of particular interest is the remarkable reactivity of (i-PrO)TiCl(3) in this context to give only the 3,4-anti bromoesters.     

Incomplete factorizations of singular linear systems

Recent works have shown that, whenA is a Stieltjes matrix, its so-called modified incomplete factorizations provide effective preconditioning matrices for solvingAx=b by polynomially accelerated iterative methods. We extend here these results to the singular case with the conclusion that the latter techniques are able to solve singular systems at the same speed as regular systems.Research supported by the Fonds National de la Recherche Scientifique (Belgium) — Aspirant.     

Comparative enantioseparations of pharmaceuticals in capillary electrochromatography on polysaccharide-based chiral stationary phases containing selectors with or without chlorinated derivatives

The screening conditions of an existing chiral strategy in CEC were tested for their applicability on four chlorine-containing polysaccharide-based stationary phases. The selectors of these phases are cellulose tris(3-chloro-4-methylphenylcarbamate), amylose tris(5-chloro-2-methylphenylcarbamate), cellulose tris(4-chloro-3-methylphenylcarbamate) and cellulose tris(3,5-dichlorophenylcarbamate). The enantioselectivity of these phases was compared with those of the four phases without chlorine (Chiralpak? AD-RH, Chiralcel? OD-RH, Chiralpak? AS-RH and Chiralcel? OJ-RH) used in the earlier defined strategy. A test set of 48 structurally diverse drug compounds was analyzed using the screening conditions of the strategy. These results led to possibilities to upgrade the current screening strategy so that improved success rates are obtained. The chlorine-containing chiral stationary phases demonstrated an added value to the screening process since they showed enantioresolution for compounds not resolved by the chiral stationary phases not containing chlorine in their structure.     

Evaluation of different warping methods for the analysis of CE profiles of urinary nucleosides

Nowadays, numerous metabolite concentrations can readily be determined in a given biological sample by high-throughput analytical methods. However, such raw analytical data comprise noninformative components due to many disturbances normally occurring in the analyses of biological material. To eliminate those unwanted original analytical data components, advanced chemometric data preprocessing methods might be of help. Here, such methods are applied to electrophoretic nucleoside profiles in urine samples of cancer patients and healthy volunteers. In this study, three warping methods: dynamic time warping (DTW), correlation optimized warping (COW), and parametric time warping (PTW) were examined on two sets of electrophoretic data by means of quality of peaks alignment, time of preprocessing, and way of customization. The application of warping methods helped to limit shifting of peaks and enabled differentiation between whole electropherograms of healthy and cancer patients objectively by a principal component analysis (PCA). The evaluation of preprocessed data and raw data by PC analysis confirms differences between the applied warping tools and proves their suitability in metabonomic data interpretation.     

Preparation and preliminary characterization of poly(ethylene glycol)-pepstatin conjugate

The carboxyl function of pepstatin has been coupled, through an amide bond, to methoxypoly(ethylene glycol) (5 kDa), to which an amino function had been previously grafted. The mPEG-pepstatin conjugate inhibits hog pepsin (aspartic proteinase) in vitro as pepstatin itself, however, with a 400 times higher apparent K_i. The conjugate apparently does not inhibit proteinases belonging to other proteinase families such as serine (trypsin, carboxypeptidase Y), cysteine (Papaya proteinase III), or metallo (collagenase) proteinases.     

Endohedral condensation and higher exohedral coverage of Kr on open single-walled carbon nanotubes at 77 K

An isotherm quasi-discontinuity, or a near-vertical step, at 177 microTorr indicative of Kr condensation inside open single-walled carbon nanotubes (SWNT) has been observed at 77 K. The isotherm shows double adsorption-branch structure attributed to the existence of two endohedral phases of confined Kr. Three well-pronounced steps corresponding to the formation of various exohedral phases are present in the high-density (low-pressure) branch. The desorption branch exhibits three rounded steps assigned to higher order exohedral coverage.     

The Decomposition of cis-Fused Cyclopenteno-1,2,4-Trioxanes induced by Ferrous Salts and some oxophilic reagents

Two cis-fused cyclopenteno-1,2,4-trioxanes, 1a and 1b , were subjected to Zn in AcOH or FeCl₂ · 4H₂O in MeCN. In the first case, the main course was deoxygenation to give cyclopentanone ( 18 ) and the 1,4-diphenyl- or 1,4-bis(4-fluorophenyl)cyclopent-3-ene-1,2-diol 10 (Scheme 5). In the second case, isomerization chiefly occurred resulting in the formation of a dimer 9 of the respective 3,5-diaryl-5-hydroxycyclopent-2-enyl 5-hydroxypentanoates 8 (Scheme 3).     

New [4.4]cyclophanes: molecular parallelograms, triangles, rhombuses, pentagons, and supramolecular constructions

The fair or good yield synthesis of new [(4.4)(n)]cyclophanes (n = 1-5), starting from 1,4-bis(2-hydroxymethyl-5,5-dimethyl-1,3-dioxan-2-yl)benzene and several diacid-dichlorides, based on monomer and oligomer formation reactions (from 1 + 1 to 5 + 5), is reported. The structure and the complex architectures of the lattices for these cyclophanes are revealed by the X-ray molecular structure for five compounds, NMR investigations, and mass spectrometry measurements. Intramolecular and intermolecular CH-pi, p-pi, and pi-pi interactions are observed, both in solid state and solution.     

Recognition and stabilization of peptide alpha-helices using templatable nanoparticle receptors

alpha-Helices are important structural elements in proteins. To provide a scaffold for the facial recognition of peptides, we have explored the interaction of cationic mixed monolayer protected clusters (MMPCs) with a tetra-aspartate peptide in water. In these studies, substantial enhancement of peptide helicity was observed upon addition of the MMPC. Significantly, this stabilization increased with time, demonstrating templation of the monolayer to the peptide helix.     

Simultaneous flow measurement of specific heats and thermal expansion coefficients of liquids: Aqueoust-BuOH mixtures and neat alkanols and alkanediols at 25°C

A flow method is described for the simultaneous measurement of volumetric specific heat capacities c _p ^v and thermal expansion coefficients of liquids, using a Picker heat capacity microcalorimeter. the method involves a simple sequence of operations in which the calorimeter unit is used alternately to measure a difference in volumetric specific heats at constant flow rate and temperature, or a flow rate variation resulting from a temperature scan on a flow cell. The precision of the method in measurements is close to 1% which is comparable to that of current dilatometric methods; the overall time for the combined c _p ^v - measurement is less than 30 minutes. Typical uses and reliability of the method are illustrated through results for aqueous NaCl solutions, t-BuOH-water mixtures, and ,-diols (C₂–C₅) at 25°C. The and c _p ^v values in homologous series of normal alkanes, alcohols and diols appear well represented by equations comprising two contributions, one related to the hydrocarbon chain length, the other dependent on the number density of –OH groups. The general trends in these data and comparison with similar results for H₂O₂ and H₂O illustrate the magnitude of the c _p ^v and anomalies in liquid water.