Enantioselective conjugate addition of nitro compounds to α,β-unsaturated ketones: an experimental and computational study

A series of chiral thioureas derived from easily available diamines, prepared from α-amino acids, have been tested as catalysts in the enantioselective Michael additions of nitroalkanes to α,β-unsaturated ketones. The best results are obtained with the bifunctional catalyst prepared from L-valine. This thiourea promotes the reaction with high enantioselectivities and chemical yields for aryl/vinyl ketones, but the enantiomeric ratio for alkyl/vinyl derivatives is very modest. The addition of substituted nitromethanes led to the corresponding adducts with excellent enantioselectivity but very poor diastereoselectivity. Evidence for the isomerization of the addition products has been obtained from the reaction of chalcone with [D(3)]nitromethane, which shows that the final addition products epimerize under the reaction conditions. The epimerization explains the low diastereoselectivity observed in the formation of adducts with two adjacent tertiary stereocenters. Density functional studies of the transition structures corresponding to two alternative activation modes of the nitroalkanes and α,β-unsaturated ketones by the bifunctional organocatalyst have been carried out at the B3LYP/3-21G* level. The computations are consistent with a reaction model involving the Michael addition of the thiourea-activated nitronate to the ketone activated by the protonated amine of the organocatalyst. The enantioselectivities predicted by the computations are consistent with the experimental values obtained for aryl- and alkyl-substituted α,β-unsaturated ketones.     

Highly diastereo- and enantioselective direct Barbas-List aldol reactions promoted by novel benzamidoethyl and benzamidopropyl prolinamides in water

Four novel benzamido-functionalized prolinamides have been prepared and tested as organocatalysts for enantioselective aldol reaction of aldehydes and cyclic ketones in water. In particular, prolinamide derived from achiral ethylene diamine was the best catalyst leading to anti aldols in excellent diastereomeric (up to 98/2) and enantiomeric (up to 99/1) ratios, thereby showing that lateral amide functionalities might be a key issue for facilitating "in water" chemistry. These catalysts are cheaper and easier to prepare than those previously described.     

Crystal structures and voltammetric behavior of three copper(II) complexes derived from bulky Schiff bases

Three Schiff base copper(II) complexes have been prepared and characterized by elemental analysis, mass spectra, i.r., electronic spectra, _eff and X-ray crystal structures. Cyclic voltammetry studies on the complexes indicate a dependence of the cathodic potentials upon electronic effects, but independence on the solid state structure.     

Highly Enantioselective Construction of Tricyclic Derivatives by the Desymmetrization of Cyclohexadienones

The asymmetric synthesis of tricyclic compounds by the desymmetrization of cyclohexadienones is presented. The reaction tolerated a large variety of substituents at different positions of the cyclohexadienone, and heterocyclic rings of different sizes were accessible. Density functional theory calculations showed that the reaction proceeds through an asynchronous [4+2] cycloaddition.     

Formation of halogenated by-products of parabens in chlorinated water

Chemical transformations of four alkyl esters of p-hydroxybenzoic acid, parabens, in chlorinated water samples are investigated. Quantification of the parent species and identification of their reaction by-products were performed using gas chromatography in combination with mass spectrometry. Experiments were accomplished considering free chlorine and paraben concentrations at the mg L⁻¹ and μg L⁻¹ level, respectively. Concentration of water samples, using solid-phase extraction, and silylation of the target species were carried out in order to improve the detectability of parent species and their possible transformation products, achieving quantification limits at the low ng L⁻¹ level. Under employed experimental conditions, the decrease in the concentrations of parabens followed pseudo-first-order kinetics. Half-lives values obtained for model ultrapure water solutions were in good agreement with those observed in tap water samples. For the first type of sample, only two by-products were detected for each paraben. They corresponded to chlorination of the aromatic ring in one or two carbons situated in ortho-positions to the hydroxyl group. Both species were also generated after the addition of parabens to chlorinated tap water. Moreover, three new transformation products were noticed for each parent compound. They were identified as bromo- and bromochloro-parabens, formed due to the existence of traces of bromide in tap water sources. Experiments carried out by mixing paraben-containing personal care products with tap water, containing free chlorine, confirmed the formation of all above described halogenated by-products. In addition, the presence of the di-chlorinated forms of methyl and propyl paraben has been detected for first time in raw sewage water samples.     

Quantum modified Regge–Teitelboim cosmology

In this paper we study the late evolution of Relic Gravitational Waves in coupled dark energy models, where dark energy interacts with cold dark matter. Relic Gravitational Waves are second tensorial order perturbations of the Lemaitre–Friedman–Robertson–Walker metric and experiment an evolution ruled by the scale factor of the metric. We find that the amplitude of Relic Gravitational Waves is smaller in coupled dark energy models than in models with non interacting dark energy. We also find that the amplitude of the waves predicted by the models with coupling term proportional to the dark energy density is smaller than those of the models with coupling term proportional to dark matter density.     

Abelian varieties with finite abelian group action

An automorphism of an abelian variety induces a decomposition of the variety up to isogeny. There are two such results, namely the isotypical decomposition and Roan’s decomposition theorem. We show that they are essentially the same. Moreover, we generalize in a sense this result to abelian varieties with action of an arbitrary finite abelian group. An early version of this article was inadvertently published before all the revisions had been completed and then retracted [https://doi.org/10.1007/s00013-018-1244-3]. This article is the final peer reviewed version.

     

Synthesis,structural characterization and in vitro cytotoxic activity of novel polymeric triorganotin(IV) complexes of urocanic acid

Two novel triorganotin carboxylate complexes of the biologically active urocanic acid have been synthesized and characterized. Elemental analysis, melting point, spectroscopic techniques – IR, ¹H, ¹³C and ¹¹⁹Sn NMR – mass spectrometry and X‐ray diffraction studies have been used for structural characterization. Crystal structures of the tin(IV) derivatives show that urocanic acid acts as a bridging bidentate ligand through its imidazole nitrogen atom and its carboxylic group, producing a polymeric one‐dimensional chain. The molecular structures of the complexes, catena‐poly‐tri(n‐butyl)tin(IV) 3‐(3H‐imidazol‐4‐yl)prop‐2‐enoate (1) and catena‐poly‐triphenyltin(IV) 3‐(3H‐imidazol‐4‐yl)prop‐2‐enoate (2), present a distorted trigonal–bipyramidal configuration. This is further confirmed by ¹¹⁹Sn NMR in the solid state. The tin(IV) derivatives form double‐stranded ribbons via N―H^…O―H bonds. Nevertheless, the compounds are essentially monomeric in solution, with a tetrahedral configuration as observed by ¹¹⁹Sn NMR in solution. The cytotoxic activity of the titled compounds has been tested against six human cell lines and the corresponding IC₅₀ values are reported. Both tin(IV) compounds have a high to very high in vitro cytotoxic activity against the tumor cell lines K562, HCT‐15 and MCF‐7. Compound 1 is 86 times more active than cisplatin in the HTC‐15 cell line. Copyright © 2012 John Wiley & Sons, Ltd.     

Interaction of Metal–Dipeptide Complex with Ninhydrin in the Absence and Presence of Conventional CTAB Surfactant

The present work is aimed at studying the interaction between copper-glycyltyrosine [(Cu(II)-Gly-Tyr)]⁺ and ninhydrin in water and in micelles formed by cetyltrimethylammonium bromide (CTAB) using spectrophometric measurements at 80°C and pH 5.0. The order of reaction remains the same in the two systems, that is, first- and fractional-order kinetics with respect to [Cu(II)-Gly-Tyr]⁺ and [ninhydrin], respectively, in the excess of ninhydrin over [Cu(II)-Gly-Tyr]⁺. It was observed that the product formed is same in both the media. The reaction is catalyzed by CTAB, and the maximum rate enhancement is about three fold. Quantitative kinetic analysis of k_ψ–[CTAB] data was explained in terms of pseudo-phase of the micelles (assuming the association/incorporation of both the reactants at the micellar surface).     

Impact of the Synergistic Collaboration of Oligothiophene Bridges and Ruthenium Complexes on the Optical Properties of Dumbbell‐Shaped Compounds

The linear and non‐linear optical properties of a family of dumbbell‐shaped dinuclear complexes, in which an oligothiophene chain with various numbers of rings (1, 3, and 6) acts as a bridge between two homoleptic tris(2,2′‐bipyridine)ruthenium(II) complexes, have been fully investigated by using a range of spectroscopic techniques (absorption and luminescence, transient absorption, Raman, and non‐linear absorption), together with density functional theory calculations. Our results shed light on the impact of the synergistic collaboration between the electronic structures of the two chemical moieties on the optical properties of these materials. Experiments on the linear optical properties of these compounds indicated that the length of the oligothiophene bridge was critical for luminescent behavior. Indeed, no emission was detected for compounds with long oligothiophene bridges (compounds 3 and 4 , with 3 and 6 thiophene rings, respectively), owing to the presence of the ³π? π* state of the conjugated bridge below the ³MLCT‐emitting states of the end‐capping Ru^II complexes. In contrast, the compound with the shortest bridge ( 2 , one thiophene ring) shows excellent photophysical features. Non‐linear optical experiments showed that the investigated compounds were strong non‐linear absorbers in wide energy ranges. Indeed, their non‐linear absorption was augmented upon increasing the length of the oligothiophene bridge. In particular, the compound with the longest oligothiophene bridge not only showed strong two‐photon absorption (TPA) but also noteworthy three‐photon‐absorption behavior, with a cross‐section value of 4×10^?78 cm⁶ s² at 1450 nm. This characteristic was complemented by the strong excited‐state absorption (ESA) that was observed for compounds 3 and 4 . As a matter of fact, the overlap between the non‐linear absorption and ESA establishes compounds 3 and 4 as good candidates for optical‐power‐limiting applications.