Mechanism and scope of salen bifunctional catalysts in asymmetric aldehyde and α-ketoester alkylation

Metal complexes of C₂-symmetric Lewis acid/Lewis base salen ligands provide bifunctional activation resulting in rapid rates in the enantioselective addition of diethylzinc to aldehydes (up to 92% ee). Further experiments probed the reactivity of the individual Lewis acid and Lewis base components of the catalyst and established that both moieties are essential for asymmetric catalysis. These catalysts are also effective in the asymmetric addition of diethylzinc to α-ketoesters. This finding is significant because α-ketoesters alone serve as their own ligands to accelerate racemic 1,2-carbonyl addition of Et₂Zn and racemic carbonyl reduction. The latter proceeds via a metalloene pathway, and often accounts for the predominant product. Singular Lewis acid catalysts do not accelerate enantioselective 1,2-addition over these two competing paths. The bifunctional amino salen catalysts, however, rapidly provide enantioenriched 1,2-addition products in excellent yield, complete chemoselectivity, and good enantioselectivity (up to 88% ee). A library of the bifunctional amino salens was synthesized and evaluated in this reaction. The utility of the α-ketoester method has been demonstrated in the synthesis of an opiate antagonist.     

Carbohydrate analysis prepares to enter the "Omics" era

In this issue, Houseman and Mrksich describe a carbohydrate array preparation method that can be used to analyze protein-carbohydrate interactions and to characterize the substrate specificity of a carbohydrate-modifying enzyme. Carbohydrate chips were prepared by a novel procedure that allows the covalent attachment of carbohydrate-diene conjugates to a specially engineered monolayer surface. The surface presents a precisely controllable ratio of reactive benzoquinone and inert ethylene glycol groups. Nonspecific adsorption of proteins to the surface is extremely low, and the surface is compatible with popular detection techniques. The immobilization technique was demonstrated to be compatible with recently developed automated solid phase carbohydrate synthesis methods, paving the way for the development of highly complex carbohydrate arrays.     

Electron-capture negative ionization calibrants for magnetic sector mass spectrometers

Fomblin [poly(perfluoropropylene oxide)], PFK (perfluorokerosene) and FC-43 (perfluorotributylamine) are investigated as mass calibrants in electron-capture negative ionization mass spectrometry on a magnetic sector hybrod mass spectrometer. This work provides exact negative ion mass tables of these three calibrants obtained using high-resolution peak matching of representative ions of the calibrants against ions of known mass. The suitability of these calibrants for full data system control in the negative ionization mode is demonstrated for the techniques of high-resolution scanning, selected ion recording and calibration of the quadrupole sector of a hybrid instrument.     

Quantification of fg-pg amounts by electron capture negative ion mass spectrometry — Parameter optimisation and practical advices

Electron capture negative ion mass spectrometry (ECNI-MS) is a very suitable and popular technique for the identification and quantification of fg-pg amounts of compounds with a sufficiently high electron affinity. Many users of the ECNI mode have faced a lot of frustrating problems due to instrument contamination and wrong recommendations concerning instrument optimisation. This article summarises 14 years of experience with ECNI-MS using a large number of different instruments. Recommendations are given concerning optimisation procedures of important parameters such as ion source pressure and temperature as well as electron energy. An ion pressure optimisation method is proposed using a gas chromatograph. ECNI-MS is very sensitive against trace amounts of contaminants in the mass spectrometer and requires very clean components in the reagent gas line. Recommendations are given concerning suitable parts. Different other contamination sources are also discussed. The construction of a simple and clean gas inlet system is presented. Furthermore, contamination-free cleaning methods for the ion source are suggested. A test method based on the detection of hexachlorobenzene in the full scan mode (m/z 34–300) is proposed. It allows to evaluate both the background level in the mass spectrometer and the overall system performance. Clean instruments should show a signal-to-noise ratio of the total ion current GC signal of at least 20:1 without applying any mass signal area reject threshold. A linearity test procedure is also suggested. It shows that the linear range of a clean and optimised instrument is at least 3 orders of magnitude in the ECNI mode.     

EVIDENCE FOR THE PHOTOPROTECTIVE EFFECTS OF VITAMIN E

Abstract— The antioxidant vitamin E (α-tocopherol) may protect both animal and plant cell membranes from light-induced damage. The various biochemical and biophysical modes of protection are considered. An examination is made of the evidence that vitamin E plays an important prophylactic role against a number of serious light-induced diseases and conditions of the eye (cataractogenesis and retinal photodeterioration) and skin (erythrocyte photohem-olysis, photoerythema, photoaging and photocarcinogenesis) that are mediated by photooxidative damage to cell membranes.     

Divergence Criterion for a Class of Random Series Related to the Partial Sums of I.I.D. Random Variables

Journal of Theoretical Probability - Let $$ \{X, X_{n};~n \ge 1 \}$$ be a sequence of independent and identically distributed Banach space valued random variables. This paper is devoted to...     

A note on pseudocompactifications

There exists a countable spaceV _ω such that:

1. V _ω has a single non-isolated point,
2. V _ω has no pseudocompactificationX witht(X)=ω.