Browsing by Author "Vassilev N."

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    Chercher de l'eau: The switching mechanism of the rotary switch ethyl-2-(2-(quinolin-8-yl)hydrazono)-2-(pyridin-2-yl)acetate
    (2020-05-01) Deneva V.; Vassilev N.; Hristova S.; Yordanov D.; Hayashi Y.; Kawauchi S.; Fennel F.; Völzer T.; Lochbrunner S.; Antonov L.
    The E/Z switching mechanism of the rotary switch ethyl-2-(2-(quinolin-8-yl)hydrazono)-2-(pyridin-2-yl)acetate was studied by NMR, UV–Vis, and ultrafast spectroscopy and modeled by advanced quantum-chemical calculations. Three possible mechanisms were considered theoretically: out-of-plane rotation, in-plane inversion and proton transfer. Neither of them correctly describes the experimental data by using implicit solvation. Taking into account the existence of water in the used solvents, which influence the measured rate constants, an explicit solvation was attempted in the quantum-chemical calculations. According to a simplified model, the water molecules form a wire, being able to transfer the proton from Z to E form of the switch. This leads to substantially lower transition states and corresponds to the experimentally observed rate constants. This information shines new light on the mechanism of isomerization in the rotary switches and on the understanding of the mechanism of hydrazone tautomerism as a whole.
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    Hexa(1,1,3,3-tetramethyl-butyl)-hexakis(dimethylphosphinoyl-methoxy)-calix[6]arene: Synthesis, characterization and implementation as a synergistic agent in the solvent extraction of lanthanoids
    (2017-01-01) Varbanov S.; Tashev E.; Vassilev N.; Atanassova M.; Lachkova V.; Tosheva T.; Shenkov S.; Dukov I.
    A new lower rim phosphorus-containing calix[6]arene with six dimethyl-methylene phosphine oxide groups, namely 5,11,17,23,29,35-hexa(1,1,3,3-tetramethyl-butyl)-37,38,39,40,41,42-hexakis-(dimethylphosphinoylmethyleneoxy)-calix[6]arene (S), has been synthesized and characterized. The structure of this calix[6]arene was identified and confirmed by elemental analysis, IR, 1H and 31P{H} NMR spectroscopy and ESI-mass-spectrometry. The 1H and 31P{H} NMR spectra indicate that at room temperature in DMSO-d6 and 1,1,2,2-tetrachloroethane-d2 solutions, S exists as a mixture of conformers. The synergistic solvent extraction of selected lanthanoid ions (La, Nd, Eu, Ho, Lu) with 4-benzoyl-3-methyl-1-phenyl-5-pyrazolone (HP) and S in chloroform has been studied. It was found that in the presence of S the lanthanoid ions have been extracted as LnP3·S species. A very large synergistic enhancement (up to 105) was established. The stoichiometry and the structure of the solid complex of Eu(III) with HP and S (1:3:1) were proposed based on elemental analysis, IR, 1H NMR and 31P NMR data.
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    NEW DERIVATIVES OF GALANTHAMINE CONTAINING PEPTIDE FRAGMENT
    (2020-01-01) Vezenkov L.; Cena H.; Danalev D.; Karadjova V.; Tsekova D.; Bardarov V.; Vassilev N.
    New derivatives of galanthamine containing a peptide fragment with an antiaggregation activity are synthesized. Herein the process of synthesis, purification and characterization of the newly obtained compounds is described. Syntheses were performed by consecutive attachment of Boc-protected amino acids to either norgalanthamine or HGly-Ogal using TBTU/DIPEA and TCTU/DIPEA methods. The purification of final products was realized by treating with EtOAc or by HPLC. The newly compounds were characterized by TLC, NMR, m.p. and MS. Two pharmacological effects are expected to be combined in a single molecule: the anticholinesterase and the antiaggregation ones