Search results for "CONDENSATION"
showing 10 items of 468 documents
Head-to-tail connected double calix[4]arenes
1997
New macrotricyclic compounds consisting of two calix[4]arene substructures connected by aliphatic chains of various length (three to five carbon atoms) between two oppositep-positions and two distal phenolic oxygens have been synthesized. Starting withp-tert-butyl-calix[4]arene, two O-protected phenolic units are attachedvia ether links in 1,3-position by reaction with the corresponding tosylates. After deprotection, the new calix[4]arene is formed by fragment condensation with 2,6-bisbromomethylated 4-alkylphenols. The structure of one example (8c) has been confirmed by single crystal X-ray analysis. Both calixarene parts assume the cone conformation, a molecule of acetonitrile being inclu…
Synthesis of Areno-Condensed [24]Annulenes
2001
[24]Annulenes condensed with three phenanthrene units (11a−d) or with three chrysene ring systems (22) were prepared by multi-step syntheses. The cyclic condensation reaction in the final step led to highly symmetrical compounds. Long flexible alkoxy groups attached to the periphery enhance the solubility and give rise to a strong aggregation of the molecules which was observed in solution by NMR and fluorescence excitation spectroscopy, and in the pure state by the detection of liquid crystalline phases in differential scanning calorimetry and polarization microscopy.
Liquid crystals in the series of 2,4,6-tristyryl-1,3,5-triazines
2004
Abstract Alkaline condensation reactions of 2,4,6-trimethyl-1,3,5-triazine ( 1 ) and substituted benzaldehydes ( 2a – n ) yield 2,4,6-tristyryl-1,3,5-triazines ( 3a – n ). A sufficient number and length of the alkoxy chains at the benzene rings provide liquid crystalline phases Col hd . A special structure was found for compound 3i with 9 hexyloxy chains; it exists in the solid state in a helical columnar arrangement, which is transformed by heating to a hexagonal columnar mesophase. Irradiation of the mesophases of 3i – 3m leads to partial cyclodimerization reactions, which cause different textures and lower the clearing points. The border line between the irradiated and the unirradiated z…
Azolium and acetate ions in DMF: Formation of free N-heterocyclic carbene. A voltammetric analysis
2016
In order to reveal the possible formation of free N-heterocyclic carbene (NHC) in DMF-azolium and acetate solutions, the proton exchange equilibrium between azolium cations and CH3COO− was investigated (by cyclic voltammetry) by adding CH3COOH or tetrabutylammonium acetate to DMF solutions of imidazolium or thiazolium salts of different acidity.The voltammetric analysis confirms that the deprotonation of the azolium cation by CH3COO− (with the formation of free NHC) is significant in the case of the more acidic thiazolium cations, while it is not effective with the less acidic imidazolium ones.Accordingly, the NHC-catalyzed benzoin condensation was carried out in DMF solutions of azolium sa…
The chemistry of acetone at extreme conditions by density functional molecular dynamics simulations
2011
Density functional molecular dynamics simulations have been performed in the NVT ensemble (moles (N), volume (V) and temperature (T)) on a system formed by ten acetone molecules at a temperature of 2000 K and density ρ = 1.322 g cm(-3). These conditions resemble closely those realized at the interface of an acetone vapor bubble in the early stages of supercompression experiments and result in an average pressure of 5 GPa. Two relevant reactive events occur during the simulation: the condensation of two acetone molecules to give hexane-2,5-dione and dihydrogen and the isomerization to the enolic propen-2-ol form. The mechanisms of these events are discussed in detail.
Compromised repair of radiation-induced DNA double-strand breaks in Fanconi anemia fibroblasts in G2
2020
Fanconi anemia (FA) is a rare chromosomal instability syndrome with various clinical features and high cancer incidence. Despite being a DNA repair disorder syndrome and a frequently observed clinical hypersensitivity of FA patients towards ionizing radiation, the experimental evidence regarding the efficiency of radiation-induced DNA double-strand break (DSB) repair in FA is very controversial. Here, we performed a thorough analysis of the repair of radiation-induced DSBs in G1 and G2 in FA fibroblasts of complementation groups A, C, D1 (BRCA2), D2, E, F, G and P (SLX4) in comparison to normal human lung and skin fibroblasts. γH2AX, 53BP1, or RPA foci quantification after X-irradiation was…
A comparative study of the sealing ability of two root canal obturation techniques
1995
A comparison was made of the ability of two root canal obturating techniques to prevent dye microleakage: gutta-percha lateral condensation and mechanically plasticized gutta-percha (JS Quick-fill). Twenty central incisors were prepared and obturated by each technique. After rendering the teeth transparent, linear dye penetration was found to be 0.48 and 0.52 mm, respectively. The difference between the two techniques was not significant. As for the distribution of the sealing cement (AH26) in the teeth obturated with JS Quickfill, the cement was located in the most peripheral zone of the obturation alongside the dentinal walls, whereas the gutta-percha was found in the central part of the …
2.14 Selected Diastereoselective Reactions: Enolate Alkylation
2012
This chapter discusses processes in which enolates are subjected to reaction with alkylating reagents. In all the processes examined here, at least one of the two reaction components is chiral and the reaction gives rise to one or more diastereomeric compounds. In most cases, the enolate is generated through deprotonation of the corresponding carbonyl precursor with a suitable base, but in several cases, the enolate is generated differently, for example, through Michael addition of a nucleophile to a conjugated carbonyl compound. The various factors that have been found to influence the stereochemical outcome of the alkylation reaction in a measurable way are also discussed. The material ha…
Thermodynamic approach of supercontinuum generation
2009
International audience; This paper is aimed at providing an overview on recent theoretical and experimental works in which a thermodynamic description of the incoherent regime of supercontinuum generation has been formulated. On the basis of the wave turbulence theory, we show that this highly nonlinear and quasi-continuous-wave regime of supercontinuum generation is characterized by two different phenomena. (i) A process of optical wave thermalization ruled by the four-wave mixing effects: The spectral broadening inherent to supercontinuum generation is shown to result from the natural tendency of the optical field to reach its thermodynamic equilibrium state, i. e., the state of maximum n…
Water Sorption on Mesoporous Aluminosilicate MCM-41
1995
Characterization of the interaction of water with the highly ordered mesoporous solid MCM-41 (pore diameter ∼2.5 nm) is undertaken with the aid of several techniques (adsorption gravimetry, X-ray diffraction, Fourier transform infrared spectroscopy, and controlled rate-evolved gas analysis). The relatively complex water-MCM-41 interactions are characterized by a type V isotherm indicating an initial repulsive character followed by a capillary condensation step of the adsorbate. This highlights both hydrophobic and hydrophilic properties of this potential model mesoporous adsorbent.