Asymmetric base-mediated epoxide isomerisation

The base-mediated rearrangement of epoxides into allylic alcohols is a well-known synthetic transformation. The first enantioselective version of the reaction using a chiral base was reported in 1980. Since then, the reaction has received a lot of attention mostly due to the great usefulness of chiral allylic alcohols in organic synthesis. Major breakthroughs in the area were the first report on using a sub-stoichiometric amount of chiral base, and the development of chiral bases for a true catalytic reaction protocol. The present review covers the time from when the first asymmetric epoxide isomerisation reaction was reported (1980) up to now, focusing on the period 1997-2001.     

Novel fluoro-substituted benzo- and benzothieno fused pyrano[2,3-c]pyrazol-4(1H)-ones

A straightforward, two-step synthesis of fluoro substituted chromeno[2,3-c]pyrazol- and [1]benzothieno[2′,3′:5,6]pyrano[2,3-c]pyrazol-4(1H)-ones, respectively, is presented. Hence, treatment of 1-substituted or 1,3-disubstituted 2-pyrazolin-5-ones with fluoro substituted 2-fluorobenzoyl chlorides or 3-chloro-6-fluoro-1-benzothiophene-2-carbonyl chloride using calcium hydroxide in refluxing 1,4-dioxane gave the corresponding 4-aroylpyrazol-5-ols, which were cyclized into the fused ring systems. 5-Fluorochromeno[2,3-c]pyrazol-4(1H)-one was obtained upon treatment of the 1-(4-methoxybenzyl) protected congener with trifluoroacetic acid. Treatment of 5-fluorochromeno[2,3-c]pyrazol-4(1H)-ones with methylhydrazine afforded novel tetracyclic ring systems such as 2-methyl-7-phenyl-2,7-dihydropyrazolo[4′,3′:5,6]pyrano[4,3,2-cd]indazole. Detailed NMR spectroscopic investigations (¹H, ¹³C, ¹⁵N, ¹⁹F) with the obtained compounds were undertaken.     

Investigation of Matrix Effects on the Determination of Carbadox and Olaquindox in Feed by LC–MS/MS

A comprehensive survey of matrix effects on the LC–MS/MS analysis of the banned antibiotic growth promoters carbadox and olaquindox in feed was carried out. Various factors of sample preparation procedure and measurement were systematically investigated by pre- and post-extraction addition and postcolumn infusion experiments. In general, strong signal suppression up to 70 % for carbadox and up to 90 % for olaquindox was observed when using different extraction solvents and techniques as well as different chromatographic conditions. Reduction of matrix effects was achieved by SPE clean-up and dilution of sample extracts. Nevertheless, matrix effect profiles determined by postcolumn infusion revealed, that reduction of signal suppression at a respective retention time cannot guarantee improvement of the methods performance. If high variability of matrix effects is present along the chromatographic run, accuracy might decrease despite reduced signal suppression. Besides method parameters, different feedingstuffs were investigated and showed similar matrix effects.

     

Avoiding Pitfalls in Comparison of Activity and Selectivity of Solid Catalysts for Electrochemical HMF Oxidation

Electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) offers a renewable approach to produce the value-added platform chemical 2,5-furandicarboxylic acid (FDCA). The key for the economic viability of this approach is to develop active and selective electrocatalysts. Nevertheless, a reliable catalyst evaluation protocol is still missing, leading to elusive conclusions on criteria for a high-performing catalyst. Herein, we demonstrate that besides the catalyst identity, secondary parameters such as materials of conductive substrates for the working electrode, concentration of the supporting electrolyte, and electrolyzer configurations have profound impact on the catalyst performance and thus need to be optimized before assessing the true activity of a catalyst. Moreover, we highlight the importance of those secondary parameters in suppressing side reactions, which has long been overlooked. The protocol is validated by evaluating the performance of free-standing Cu-foam, and CuCoO modified with NaPO₂H₂ and Ni, which were immobilized on boron-doped diamond (BDD) electrodes. Recommended practices and figure of merits in carefully evaluating the catalyst performance are proposed.     

Accurate Modelling of Group Electrostatic Potential and Distributed Polarizability in Dipeptides

Within the scope of accurate structure-property correlations in biomolecules, this work investigates how conformations and electronic configurations of biologically relevant macromolecules affect their intermolecular potentials. With the purpose of testing the suitability of a simple and universal model, the dipeptides are made from the assembly of their building blocks, namely the amino acid residuals or, more finely tuned, the individual functional groups. The model makes use of functional-group electrostatic potentials (GEP) and distributed polarizabilities (GDP), which enable an in depth analysis of the correlation between structural features and property build-up. GEPs and GDPs are calculated for various conformers and protonation states of L-alanyl-L-alanine, glycyl-L-alanine, L-alanylglycine, and glycylglycine, which are prototypic molecules to model the pertinent functional groups. The model provides GEPs that reproduce the exact potential to an average accuracy of ca. 0.05 au. The good agreement between the properties estimated with the simple model and those calculated with state-of-the-art quantum chemical methods encourages further testing of the predictive power of this model, simulating for example interaction energies and optoelectronic properties.