Search results for "Isomerism"

Microheterogeneous electrocatalytic chiral recognition at monoclinic vanadium-doped zirconias: enantioselective detection of glucose.

2007

Synthetic tetragonal and monoclinic vanadium-doped zirconias (t- and m-VxZr1-xO2, 0.005x0.150) exert an effective catalytic effect toward the electrochemical oxidation of glucose in aqueous alkaline media. The catalytic effect of monoclinic specimens attached to carbon and fluorine-doped tin oxide electrodes exhibits a remarkable enantioselectivity, so that catalytic currents for the oxidation of L-glucose at +0.92 V vs AgCl/Ag are considerably larger than those obtained for the oxidation of D-glucose. This enantioselectivity can be associated with the existence of a noncentrosymmetric coordination of vanadium centers in the monoclinic crystalline form of zirconia. From the electrochemical …

Conformational investigations of bis(α-aminoalkyl)phosphinic acids and studies of the stability of their complexes with Cu(II)

2008

Abstract Conformational investigations of a group of bis(α-aminoalkyl)phosphinic acids were performed by use of NMR spectroscopy and theoretical calculations. In the case of one of the studied compounds, substituted with aminobenzyl and amino(p-chlorobenzyl) moieties, a pH-dependent equilibrium between conformers, stabilized by intermolecular hydrogen bonds was observed. Potentiometric studies proved that these molecules formed stable complexes with copper(II) ions, where stoichiometry was 1:1 and 1:2 depending on pH of their aqueous solution.

Unusual oleanane-type saponins from Arenaria montana

2010

Three oleanane-type saponins, 3-O-β-d-glucopyranosylechinocystic acid 28-O-β-d-xylopyranosyl-(1→4)-[α-l-rhamnopyranosyl-(1→2)]-α-l-rhamnopyranosyl ester (1), 3-O-β-d-glucopyranosylechinocystic acid 28-O-α-l-arabinopyranosyl-(1→3)-β-d-xylopyranosyl-(1→4)-[α-l-rhamnopyranosyl-(1→2)]-α-l-rhamnopyranosyl ester (2), 3-O-β-d-glucopyranosylcaulophyllogenin 28-O-β-d-apiofuranosyl-(1→3)-β-d-xylopyranosyl-(1→4)-[β-d-apiofuranosyl-(1→3)]-α-l-rhamnopyranosyl-(1→2)-α-l-rhamnopyranosyl ester (3) were isolated from the whole plant of Arenaria montana. Their unusual structures for the Caryophyllaceae family were established mainly by 2D NMR techniques and mass spectrometry.

Light-induced opening and closing of the intramolecular hydrogen bond in glyoxylic acid.

2013

The isomerization process of glyoxylic acid (GA) conformers and their complexes with a water molecule were studied in a low temperature argon matrix. The research target was to understand how starting conformation and complexation affects the near-IR (NIR) induced conformer interconversion. The most stable GA conformer (Tc) is characterized by an intramolecular hydrogen bond, and it is found to undergo light-induced conformer interconversion slower than the open (Tt) conformer. Upon complexation with water, the isomerization processes slow down in the case of the Tc conformer, whereas for the Tt-based complex the influence of water is negligible on the isomerization process.

2019

Structural changes of glycolic acid (GA) complex with nitrogen induced by selective overtone excitation of the νOH mode were followed in argon matrices using FTIR spectroscopy. For the most stable SSC1 complex present in different trapping sites directly upon deposition site, selective changes in the νOH region were achieved upon near-infrared irradiation. Simultaneously, new conformers of the GA…N2 complex were formed, giving rise to several sets of bands in the νOH and νC=O regions of the spectra. Both position and intensity of new absorptions appeared to be highly sensitive on the wavelength of radiation used, as well as on the annealing of the matrix. Based on theoretical calculations a…

Raman spectroscopy and high-overtone driven isomerization of glyoxylic acid in solid argon

2013

High-overtone induced chemistry of glyoxylic acid isolated in a low-temperature argon matrix was investigated using Raman spectroscopy. The Raman spectra of two most stable conformers of glyoxylic acid are presented. Upon excitation in high overtone vibrational bands by 532 nm irradiation of the lowest energy conformer most abundant in neat deposited sample, the isomerization of glyoxylic acid was observed. The process most plausible proceeds via absorption into the fifth vibrational overtone state of the OH group or its combination with the torsional vibrational transition. The assignment of the fundamental vibrational spectra was assisted by quantum chemical harmonic and anharmonic vibrat…

Total Synthesis and Biological Investigation of (−)‐Artemisinin: The Antimalarial Activity of Artemisinin Is not Stereospecific

2018

Here, we describe an efficient and diversity-oriented entry to both (-)-artemisinin (1) and its natural antipode (+)-artemisinin, starting from commercially and readily available S-(+)- and R-(-)-citronellene, respectively. Subsequently, we answered the still open question regarding the specificity of artemisinins action. By using a drug-sensitive Plasmodium falciparum NF54 strain, we showed that the antimalarial activity of artemisinin is not stereospecific. Our straightforward and biomimetic approach to this natural endoperoxide enables the synthesis of artemisinin derivatives that are not accessible through applying current methods and may help to address the problem of emerging resistan…

Storage and release of false transmitters after infusion of (+)- and (?)-?-methyldopamine

1971

Rabbits were given an infusion of 10 mg/kg (−)- or 30 mg/kg (+)-α-methyldopamine and killed after 135 min. The noradrenaline content of the heart was decreased to 26±5 and 34±2%, respectively, of the control value. After infusion of the (+)-isomer the missing noradrenaline was replaced by (−)-α-methylnoradrenaline. Electrical stimulation of the sympathetic nerves or infusion of acetylcholine plus atropine caused an output of noradrenaline and (−)-α-methylnoradrenaline from the isolated heart. The two amines were released in the same proportion as they were stored in the heart and the total output of both amines equalled the output of noradrenaline from control hearts. Nerve stimulation caus…

Microwave-Assisted Organocatalytic Enantioselective Intramolecular aza-Michael Reaction with α,β-Unsaturated Ketones

2011

An organocatalytic enantioselective intramolecular aza-Michael reaction of carbamates bearing conjugated ketones as Michael acceptors is described. By using 9-amino-9-deoxy-epi-hydroquinine as the catalyst and pentafluoropropionic acid as a co-catalyst, a series of piperidines, pyrrolidines, and the corresponding benzo-fused derivatives (indolines, isoindolines, tetrahydroquinolines, and tetrahydroisoquinolines) can be obtained in excellent yields and enantioselectivities. In addition, the use of microwave irradiation at 60 °C improves the efficiency of the process giving rise to the final products with comparable yields and enantiomeric excesses. Some mechanistic insights are also consider…

Synthesis of aryl azides: A probe reaction to study the synergetic action of ultrasounds and ionic liquids

2011

Abstract The combined effect of ultrasounds and ionic liquids was used to perform the synthesis of aryl azides by nucleophilic aromatic substitution in ionic liquid/[1-butyl-3-methylimidazolium][N 3 ] binary mixtures. The ultrasounds efficiency was analyzed as a function of the substrate and of the ionic liquid structure. In the first case, both 6π and 10π electrons aryl halides were considered. As far as the ionic liquid structure is concerned, both aromatic and aliphatic ionic liquids were taken into account. Among aromatic cations, the effects due to different ability in giving hydrogen bond or π–π interactions were considered. The use of a geminal ionic liquid having an aromatic spacer …