(+)- and (-)-2-aminocyclobutane-1-carboxylic acids and their incorporation into highly rigid beta-peptides: stereoselective synthesis and a structural study

[Chemical reaction: See text] Several derivatives of (+)- and (-)-2-aminocyclobutane-1-carboxylic acid, 1, have been prepared through enantiodivergent synthetic sequences. The stereoselective synthesis of free amino acid (+)-1 has been achieved, and this product has been fully characterized for the first time. Stereocontrolled alternative synthetic methodologies have been developed for the preparation of bis(cyclobutane) beta-dipeptides in high yields. Among them, enantio and diastereomers have been synthesized. beta,beta- and beta,delta-Dimers resulting from the coupling of a cyclobutane residue and a linear amino acid have also been prepared. The ability of the cyclobutane ring as a structure-promoting unit both in the monomers and in the dimers has been manifested. The NMR structural study and DFT theoretical calculations evidence the formation of strong intramolecular hydrogen bonds giving rise to cis-fused [4.2.0]octane structural units that confer high rigidity on these molecules both in solution and in the gas phase. The contribution of a cis-trans conformational equilibrium derived from the rotation around the carbamate N-C(O) bond has also been observed, the trans form being the major conformer. In the solid state, this equilibrium does not exist, and moreover, intermolecular hydrogen bonds are present.     

Simultaneous recording of spin-state-selective NMR spectra for different InS spin systems

Heteronuclear magnetization transfer occurring during heteronuclear cross-polarization mixing processes in liquid-state NMR experiments can be easily monitored as a function of the involved in-phase, antiphase, and multiple-quantum magnetization components. The theoretical background on the simultaneous detection of E.COSY-type, TROSY-type, or spin-edited multiplet patterns for different IS and I(2)S spin systems in the same solution-state NMR spectrum is described. The proposed pulse scheme preserves high sensitivity levels and shows good tolerance to the presence of undesired cross-talk artifacts for both NH and NH(2) multiplicities providing an interesting NMR tool for biomolecular applications.     

Interdisciplinary study on pottery experimentally impregnated with wine

Experimentally developed ceramic pots, with two different sizes of grain, were half-filled with wine and subjected to thermal alteration at constant elevated temperature ((60 ± 2)°C) in darkness for 12 weeks. This work sought to characterise the samples thereby obtained from chemical and mineralogical perspectives using scanning electron microscopy and an energy-dispersive X-ray microanalysis system (SEM-EDX), Fourier transform infrared spectroscopy (FTIR) and capillary electrophoresis (CE) with UV detection as an alternative to chromatographic methods, due to its good resolution, automation, simplicity, high speed, low consumption of chemicals and short time required for sample preparation. The capillary electrophoresis method was used for the detection of five wine biomarkers: succinic acid, malic acid, tartaric acid, citric acid and lactic acid. In general, it was noted that the fine-grained ceramic assortment retained the organic material better than the coarser-grained ceramics. An interesting observation derived from this study was that not only could tartaric acid be considered as a biomarker for wine residues in archaeological pottery, but malic acid could also act similarly for white wine and lactic acid for red wine.     

An expansion formula for fractional derivatives of variable order

In this work we extend our previous results and derive an expansion formula for fractional derivatives of variable order. The formula is used to determine fractional derivatives of variable order of two elementary functions. Also we propose a constitutive equation describing a solidifying material and determine the corresponding stress relaxation function.     

Aryl‐Copper(III)‐Acetylides as Key Intermediates in CCsp Model Couplings under Mild Conditions

The mechanism of copper‐mediated Sonogashira couplings (so‐called Stephens–Castro and Miura couplings) is not well understood and lacks clear comprehension. In this work, the reactivity of a well‐defined aryl‐Cu^III species ( 1 ) with p‐R‐phenylacetylenes (R=NO₂, CF₃, H) is reported and it is found that facile reductive elimination from a putative aryl‐Cu^III‐acetylide species occurs at room temperature to afford the C_aryl?C_sp coupling species ( I_R ), which in turn undergo an intramolecular reorganisation to afford final heterocyclic products containing 2H‐isoindole ( P , P , P_Ha ) or 1,2‐dihydroisoquinoline ( P_Hb ) substructures. Density Functional Theory (DFT) studies support the postulated reductive elimination pathway that leads to the formation of C?C_sp bonds and provide the clue to understand the divergent intramolecular reorganisation when p‐H‐phenylacetylene is used. Mechanistic insights and the very mild experimental conditions to effect C_aryl?C_sp coupling in these model systems provide important insights for developing milder copper‐catalysed C_aryl?C_sp coupling reactions with standard substrates in the future.     

How to measure long-range proton-carbon coupling constants from 1H-selective HSQMBC experiments

Heteronuclear long-range scalar coupling constants (ⁿJ_CH) are a valuable tool for solving problems in organic chemistry and are especially suited for stereochemical and configurational analyses of small molecules and natural products. This tutorial will focus on the step-by-step implementation of several 2D ¹H frequency selective HSQMBC experiments for the easy and accurate measurement of either the magnitude or both the magnitude and the sign of long-range ⁿJ_CH couplings. The performance of these experiments will be showcased with several scenarios in a range of different experimental conditions.     

LR-selHSQMBC: Simultaneous Detection and Quantification of Very Weak Long-Range Heteronuclear NMR Correlations

The optimum detection and accurate measurement of longer-range (⁴J and higher) heteronuclear NMR correlations is described. The magnitude and/or the sign of a wide range of large and small long-range couplings can be simultaneously determined for protonated and non-protonated ¹³C and ¹⁵N nuclei using the LR-selHSQMBC experiment.     

Optimal shape of a strongest inverted column

By using Pontryagin's maximum principle we determine the shape of the strongest column positioned in a constant gravity field, simply supported at the lower end and clamped at upper end (with the possibility of axial sliding). It is shown that the cross-sectional area function is determined from the solution of a nonlinear boundary value problem. A variational principle for this boundary value problem is formulated and two first integrals are constructed. These integrals lead to an a priori estimate of the value of one the missing initial condition and to the reduction of the order of the system. The optimal shape of a column is determined by numerical integration.