Search results for "Meri"
showing 10 items of 7596 documents
General Diastereoselective Synthetic Approach toward Isospongian Diterpenes. Synthesis of (−)-Marginatafuran, (−)-Marginatone, and (−)-20-Acetoxymarg…
2012
This work describes a synthetic approach to the carbocyclic skeleton of isospongian diterpenes that uses the commercially available monoterpene (S)-carvone as a C-ring synthon, which is incorporated into the tetracyclic isospongian framework via a C -> ABC -> ABCD ring annulation strategy using intramolecular Diels-Alder and ring-closing metathesis reactions. This approach has been successfully used to prepare both the title natural isospongians and several nonnatural oxygenated analogues. A preliminary evaluation of the inhibitory activity of the small collection of synthesized isospongians on the mammalian mitochondrial respiratory chain revealed that most were able to inhibit the integra…
Understanding the cooperative NHC/LA catalysis for stereoselective annulation reactions with homoenolates. A DFT study.
2011
The role of Ti(Oi-Pr)(4) Lewis acid (LA) in the cooperative N-heterocyclic carbene (NHC)/LA catalyzed addition of enals to enones to yield cis-cyclopentenes has been investigated using DFT methods at the B3LYP/6-31G** computational level. Ti(IV) effectively catalyzes the reaction by formation of a complex with cinnamaldehyde 1, which favors the nucleophilic attack of NHC 5 on 1, and the subsequent proton abstraction to yield the extended Ti(IV)-Breslow intermediate 21. The nature of the metal involved in the LA catalyst plays a relevant role due to the more basic character of NHCs than aldehydes. Thus, strong LAs, such as Zn(OTf)(2), prevent the catalytic behavior of NHCs to form a very sta…
Recent advances in electrochemical meso- and β-functionalization of porphyrins and electrografting of diazonium porphyrins
2020
Abstract Recent studies on electrochemical meso- and β-functionalization of porphyrins and electrografting of diazonium porphyrin are presented. First, the electrochemical oxidative C–C coupling between porphyrins will be presented, followed by the intermolecular and intramolecular meso- and β-substitutions of porphyrins. Then, the latest results on diazonium porphyrin electrografting will be reviewed.
Tin(IV) catalyzed D-galacturonic acid anomerization
2008
A comparative kinetic study of anomerization between the four main forms (alpha/beta-pyranosides and alpha/beta-furanosides) of D-galacturonic acid (GalAH(2), 1) and tin(IV) complexed D-galacturonate ([Sn(CH(3))(2)(GalA)(H(2)O)(2)], 2) in D(2)O, is reported. Important differences are shown by pD vs. mutarotation rate profiles of 1 and 2. Coordination, in fact, causes: i) a spectacular increase of the rate due to kinetic template assistance of the carboxylic tail holding the metal in close proximity to the reactive centre and ii) a change in the dependence of the rate on the pD. Entropic and enthalpic factors contribute to the huge activation energy decrease in 2. Anomerization rates of the …
Towards a multilevel ant colony optimization
2014
Masteroppgave i Informasjons- og kommunikasjonsteknologi IKT590 Universitetet i Agder 2014 Ant colony optimization is a metaheuristic approach for solving combinatorial optimization problems which belongs to swarm intelligence techniques. Ant colony optimization algorithms are one of the most successful strands of swarm intelligence which has already shown very good performance in many combinatorial problems and for some real applications. This thesis introduces a new multilevel approach for ant colony optimization to solve the NP-hard problems shortest path and traveling salesman. We have reviewed different elements of multilevel algorithm which helped us in construction of our proposed mu…
A circular mesh scheme for the non-orthogonal finite difference time domain method
2002
Beam forming networks (BFN) are an important component of a complex satellite antenna system because they are used to provide accurate amplitude and phase excitation to the elements of the feed network. The need for handling high power and the need for a high degree of integrability, often leads one to choose square coaxial metal lines for constructing BFNs. BFNs usually require variable power dividers such as the rat-race (or ring) couplers with constant or variable divider ratios in order to deliver a prescribed amount of power to a certain element of an antenna array to steer the beam in a desired direction. However, modeling of such circular structures in square coaxial form is not an e…
Biomimetic diels–alder cyclizations for the construction of the brevianamide, paraherquamide, sclerotamide, asperparaline and VM55599 ring systems
1998
Abstract A potentially bio-mimetic Diels–Alder cyclization to construct the bicyclo[2.2.2] ring system common to the paraherquamides, marcfortines, sclerotamides, brevianamides, VM55599, and asperparaline is reported. Epi-deoxybrevianamide E (22) is converted into the corresponding lactim ether (23) and then oxidized with DDQ to provide an azadiene (24) which is tautomerized in the presence of base to azadiene 25 which, spontaneously cyclizes to give a 2:1 mixture of cycloadducts 26 and 27. These cycloadducts are each in turn, converted into d , l -C-19-epi-brevianamide A (20) and d , l -brevianamide B (6). The stereochemical implications of the [4+2] cycloaddition is discussed in the conte…
Development and validation of a procedure for estimating the hydrophobicity of structurally unrelated compounds by micellar liquid chromatography
1999
Reversed-phase liquid chromatography has been used most often to estimate values of log P, but despite years of study, there is no universally accepted method of performing these estimations. The main problem has to do with the fact that the hydrophobic parameter, log k w , depends on the hydrogen bond acceptor-donor character of the compounds. The use of micellar mobile phases to perform these estimations is evaluated here, and the influence of the nature of the surfactant (anionic, cationic, and nonionic) on the log k-log P relationships is studied. The use of a nonionic surfactant, such as Brij35, to prepare the mobile phases provided adequate results regardeless of the hydrogen bond acc…
Electronic Interactions in a New π-Extended Tetrathiafulvalene Dimer
2006
The first pi-extended tetrathiafulvalene (exTTF) dimer in which the two exTTF units are covalently connected by 1,3-dithiole rings has been obtained in a multistep synthetic procedure involving the Ullmann cross-coupling reaction by using copper(I) thiophene-2-carboxylate (CuTC). The electronic spectrum reveals a significant electronic interaction between the exTTF units. The electrochemical study carried out by cyclic voltammetry in solution and in thin-layer conditions, and the electrochemical simulation and spectroelectrochemical (SEC) measurements confirm the electronic communication and show that the oxidation of dimer 14 occurs as two consecutive 2 e(-) processes D(0)-D(0)-->D(2+)-D(0…
Dendron to central core S1-S1 and S2-S(n) (n1) energy transfers in artificial special pairs containing dendrimers with limited numbers of conformatio…
2013
Two dendrimers consisting of a cofacial free-base bisporphyrin held by a biphenylene spacer and function- alized with 4-benzeneoxomethane (5-(4-benzene)tri-10,15,20-(4-n-octyl- benzene)zinc(II)porphyrin) using either five or six of the six available meso-positions, have been synthesized and characterized as models for the an- tenna effect in Photosystems I and II. The presence of the short linkers, -CH2O-, and long C8H17 soluble side chains substantially reduces the number of conformers (foldamers) compared with classic dendrimers built with longer flexible chains. This simpli- fication assists in their spectroscopic and photophysical analysis, notably with respect to fluorescence resonance…