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Electronic Features (electronic + feature)
Selected AbstractsA MESSAGE FROM THE EDITOR: NEW ELECTRONIC FEATURESJOURNAL OF PHYCOLOGY, Issue 6 2003Article first published online: 24 NOV 200 No abstract is available for this article. [source] A Bowl-Shaped ortho -Semiquinone Radical Anion: Quantitative Evaluation of the Dynamic Behavior of Structural and Electronic Features,ANGEWANDTE CHEMIE, Issue 36 2010Akira Ueda Dr. Elegant aufgetischt: Das erste ,schüsselförmige" o -Semichinonradikal (siehe Bild; mit einem Na+ -Ion) wurde synthetisiert und bezüglich seiner elektronischen Eigenschaften charakterisiert. Das dynamische Verhalten (konkav,konvex) wurde mit EPR- und ENDOR/TRIPLE-Messungen sowie DFT-Rechnungen ebenfalls quantitativ untersucht. [source] ChemInform Abstract: 1,2-Azaborolyls, Isoelectronic Analogues of the Ubiquitous Cyclopentadienyl Ligand: Synthesis of B-Heteroatom-Substituted 1,2-Azaborolyl Complexes and an Assessment of Their Electronic Features.CHEMINFORM, Issue 25 2002Shih-Yuan Liu Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] N -Phenyl-1,2,4-triazoline-3,5-dione (PTAD) as a Dienophilic Dinitrogen Equivalent: A Simple Synthesis of 3-Amino-1,2,4-benzotriazines from Arylcarbodiimides,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 4 2010Mateo Alajarin Abstract N -Arylcarbodiimides react with PTAD to provide [1,2,4]triazolo[1,2- a][1,2,4]benzotriazines by a [4+2] cycloaddition reaction. When asymmetrically substituted diarylcarbodiimides are used, the cycloaddition proceeds with total chemoselectivity because only the more electron-rich aryl nucleus is involved. This observation has been rationalized by a computational study using DFT methods that shows that, in the reaction of HTAD with arylcarbodiimides, the magnitude of the energy barriers depend on the electronic features of the substituents at the aryl nucleus; the calculations also indicate that the reaction proceeds through asynchronous states with polar characteristics. The treatment of the final cycloadducts with potassium hydroxide affords 3-aryl(alkyl)amino-1,2,4-benzotriazines. [source] Alternative Mechanistic Paths in the Hetero-Diels,Alder Reaction of ,-Oxothiones: A Theoretical StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 21 2007Laura Legnani Abstract DFT calculations at the B3LYP/6-311+G(d,p) level for the C, H, and O atoms and at the 6-311+G(2df,p) level for the S atom were used to study the hetero-Diels,Alder reactions between several ,-oxothiones and ethylene or methyl vinyl ether (MVE). All the transition states and the intermediates along the reaction pathways, as well as the reaction products, were located. The reactions with ethylene are all concerted though asynchronous whereas in the reactions with MVE the electron-releasing character of the methoxy substituent lowers the energy barriers and enhances the asynchronicity and the charge transfer process to such an extent that the reaction may become unconcerted and exhibit a two-step mechanism with a zwitterionic intermediate derived from nucleophilic attack of electron-rich MVE to the sulfur atom of the strongly electrophilically activated ,-oxothiones. The reactions are also favored by the conjugation of the newly formed C=C bond. Moreover, the geometric features of the diene exert a nonnegligible role, as dienes that are planar or almost planar in their ground state show a lower energy barrier. Thus, both geometric and electronic features of the dienes as well as of the dienophiles play a significant role in the easiness of the reactions and in their mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] A joint study based on the electron localization function and catastrophe theory of the chameleonic and centauric models for the Cope rearrangement of 1,5-hexadiene and its cyano derivativesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 14 2005Victor Polo Abstract A novel interpretation of the chameleonic and centauric models for the Cope rearrangements of 1,5-hexadiene (A) and different cyano derivatives (B: 2,5-dicyano, C: 1,3,4,6-tetracyano, and D: 1,3,5-tricyano) is presented by using the topological analysis of the electron localization function (ELF) and Thom's catastrophe theory (CT) on the reaction paths calculated at the B3LYP/6-31G(d,p) level. The progress of the reaction is monitorized by the changes of the ELF structural stability domains (SSD), each being change controlled by a turning point derived from CT. The reaction mechanism of the parent reaction A is characterized by nine ELF SSDs. All processes occur in the vicinity of the transition structure and corresponding to a concerted formation/breaking of C1C6 and C3C4 bonds, respectively, together with an accumulation of charge density onto C2 and C5 atoms. Reaction B presents the same number of ELF SSDs as A, but a different order appears; the presence of 2,5-dicyano substituents favors the formation of C1C6 bonds over the breaking of C3C4 bond process, changing the reaction mechanism from a concerted towards a stepwise, via a cyclohexane biradical intermediate. On the other side, reaction C presents the same type of turning points but two ELF SSD less than A or B; there is an enhancement of the C3C4 bond breaking process at an earlier stage of the reaction by delocalizing the electrons from the C3C4 bond among the cyano groups. In the case of competitive effects of cyano subsituents on each moiety, as it is for reaction D, seven different ELF SSDs have been identified separated by eight turning points (two of them occur simultaneously). Both processes, formation/breaking of C1C6 and C3C4 bonds, are slightly favored with respect to the parent reaction (A), and the TS presents mixed electronic features of both B and C. The employed methodology provides theoretical support for the centauric nature (half-allyl, half-radical) for the TS of D. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1427,1437, 2005 [source] Current trends in QSAR on NO donors and inhibitors of nitric oxide synthase (NOS),,MEDICINAL RESEARCH REVIEWS, Issue 4 2002Christos A. Kontogiorgis Abstract This article evaluates the quantitative structure-activity relationships (QSAR) of nitric oxide (NO) radical donors and nitric oxide synthases (NOS) inhibitors, using the C-QSAR program of Biobyte. Furoxans, triazines, amidoximes, tetrazoles, imidazoles and N,,2-nitroarylamino acid analogues were included in this survey. In nine out of seventeen cases, the clog P plays a significant part in the QSAR of the NO radical donors and of the NOS inhibition. Many of the compounds must be interacting with a hydrophobic space in a non-specific way. In some cases molecular refractivity CMR/MR as well as sterimol parameters (B1 and L) are important. Electronic effects, with the exception of the Hammett's constant , and the Swain,Lupton parameter F, are not found to govern the biological activity. Stereochemical and electronic features are also found to be important. Indicator variables were used after the best model was found to account for the usual structural features. © 2002 Wiley Periodicals, Inc. Med Res Rev, 22, No. 4, 385,418, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/med.10012 [source] CP-MLR/PLS Directed Structure-Activity Modeling of the HIV-1 RT Inhibitory Activity of 2,3-Diaryl-1,3-thiazolidin-4-onesMOLECULAR INFORMATICS, Issue 4 2004Yenamandra Abstract A detailed structure-activity relationship study of the HIV-1 Reverse Transcriptase (RT) inhibitory activity of two series of compounds, 2-(2,6-dihalo phenyl)-3-(substituted pyridin-2-yl)-thiazolidin-4-ones and 2-(2,6-Dihalophenyl)-3-(substituted phenyl)-thiazolidin-4-ones, belonging to 2,3-diaryl-thiazolidin-4-ones in terms of physicochemical and structural descriptors have been carried out using combinatorial protocol interfaced multiple linear regression (CP-MLR) and partial least squares (PLS) analysis. The models developed in the study indicate a preference for hydrophobic compounds for better inhibitory activity. Also, a positive regression coefficient of I2,3, an indicator descriptor meant for the joint disposition of substituents of 2,3-diaryl moieties of thiazolidinones to address their ability in taking a butterfly like conformation, is in agreement with all earlier observations that compounds having the ability to take butterfly like conformation will be showing better inhibitory activity. The identified models suggest that the meta-positions of 3-aryl moiety has the ability to accommodate hydrophobic/ steric groups. The replacement of C2, of 3-phenyl by nitrogen resulted in 3-pyridyl variants of these analogues. Even though from geometry point of view, the phenyls and pyridyls span almost the same structural space and steric features, presence of nitrogen in pyridyls gave them a blend of polarity, electronic features and a different hydrophobicity profile when compared to corresponding phenyl analogue. Furthermore the PLS models, developed from those descriptors took part in CP-MLR models, indicate that most of the descriptors have almost equal influence in accounting for the variation in the activity of these compounds. This suggests that the factors responsible for the variation in the activity have been uniformly distributed across the varying centers of the molecule. The study suggests that thiazolidinones with 3-(pyridin-2-yl) moiety provide scope for further substituent variation to modulate the HIV-1 RT inhibitory activity. [source] Comparative QSAR Studies on Toxicity of Phenol Derivatives Using Quantum Topological Molecular Similarity IndicesCHEMICAL BIOLOGY & DRUG DESIGN, Issue 5 2010Bahram Hemmateenejad Quantitative structure activity relationship (QSAR) analyses using a novel type of electronic descriptors called quantum topological molecular similarity (QTMS) indices were operated to describe and compare the mechanisms of toxicity of phenols toward five different strains (i.e., Tetrahymena pyriformis, L1210 Leukemia, Pseudomonas putida, Raja japonica and Cucumis sativus). The appropriate QSAR models for the toxicity data were obtained separately employing partial least squares (PLS) regression combined with genetic algorithms (GA), as a variable selection method. The resulting QSAR models were used to identify molecular fragments of phenol derivatives whose electronic properties contribute significantly to the observed toxicities. Using this information, it was feasible to discriminate between the mechanisms of action of phenol toxicity to the studied strains. It was found that toxicities of phenols to all strains, except with L1210 Leukemia, are significantly affected by electronic features of the phenolic hydroxyl group (C-O-H). Meanwhile, the resulting models can describe the inductive and resonance effects of substituents on various toxicities. [source] Novel Hypervalent Complexes of Main-Group Metals by Intramolecular Ligand,Metal Electron Transfer*CHEMISTRY - A EUROPEAN JOURNAL, Issue 23 2004Matthias Driess Prof. Dr. Abstract New fascinating electronic features of the simple diketoamine chelate ligand HN[CH2C(tBu)O]2 (1) are described. Unexpectedly, the corresponding trianionic amido-dienolate form of 1 is capable of reducing main-group metal atoms M after initial coordination and intramolecular L,M two-electron transfer and of stabilizing main-group elements in unusual low oxidation states. This is impressively shown by the synthesis and structural characterization of the novel Ge and Sn complexes 4,6 by redox reactions of lithiated 1 with the corresponding metal halides GeCl4 and MCl2 (M=Ge, Sn). Surprisingly, conversion of tris-lithiated 1 with GeCl4 readily consumes two molar equivalents of GeCl4 and results in the formation of the neutral GeCl3 complex 4 and GeCl2. The former represents the second example of a structurally characterized neutral octahedrally coordinated germanium compound. Reaction of dilithiated 1 with GeCl2 does not lead to the expected ClGe(+2) complex but affords the novel dimeric germylene 5, whereas similar reaction using SnCl2 furnishes the monomeric stannylene (ClSn(+2) complex) 2 and elemental tin due to the higher oxidation potential of Sn(+2). Unexpectedly, a similar redox reaction of dilithiated 1 with PbCl2 furnishes the first air- and water-stable lithium 1,2-diketoimine,enolate 7 and elemental lead. Compound 7 is tetrameric in the solid state and consists of a strongly distorted Li4O4 cubic core with trigonal-bipyramidal coordinated Li+ ions. [source] Structure,Activity Relationships of SSAO/VAP-1 Arylalkylamine-Based SubstratesCHEMMEDCHEM, Issue 4 2009Francesc Yraola Dr. Abstract SSAO/VAP-1 substrates may be valuable for the treatment or prevention of diabetes mellitus, as they show insulin-mimetic properties. This review highlights the importance of studying the relevant steric and electronic features in the development of new ligands with better SSAO/VAP-1 recognition, enhanced selectivity over other amine oxidases, and improved metabolic behavior. Semicarbazide-sensitive amine oxidase/vascular adhesion protein-1 (SSAO/VAP-1) substrates show insulin-mimetic effects and are therefore potentially valuable molecules for the treatment of diabetes mellitus. Herein we review several structural and electronic aspects of SSAO arylalkylamine-based substrates. Two main modifications directly affect amine oxidase (AO) activity: 1),variation in ring substitution modulates the biological activity of the arylalkylamine ligand by converting a substrate into a substrate-like inhibitor, and 2),variation in the number of methylene units between the aromatic ring and the ammonium groups of the arylalkylamine substrates dramatically alters the oxidation rate between species. Furthermore, we review relevant information about mammalian SSAO/VAP-1 substrate selectivity and specificity over monoamine oxidases (MAOs). [source] | |||||||||||||||||||||||||