Search results for "LEAN"
showing 10 items of 3611 documents
Reactivity of lithium β‑ketocarboxylates: the role of lithium salts
2017
Lithium beta-ketocarboxylates 1(COOLi), prepared by the reaction of lithium enolates 2(Li+) with carbon dioxide, readily decarboxylate in THF solution unless in the presence of lithium salts, in which case they are indefinitely stable at room temperature in inert atmosphere. The availability of stable THF solutions of lithium beta-ketocarboxylates 1(COOLi) in the absence of carbon dioxide allowed reactions to take place with nitrogen bases and alkyl halides 3 to give alpha-alkyl ketones 1(R) after acidic hydrolysis. The sequence thus represents the use of carbon dioxide as a removable directing group for the selective monoalkylation of lithium enolates 2(Li+). The roles of lithium salts in …
Unveiling the high reactivity of benzyne in the formal [3+2] cycloaddition reactions towards thioamides through the Molecular Electron Density Theory
2020
Abstract The domino reaction of benzyne with thioamide has been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G(d) level. This domino reaction takes place through i) a formal [3 + 2] cycloaddition (32CA) reaction affording an ammonium ylide, and ii) an extrusion of ethylene from this species yielding a dihydrothiazole. Topological analysis of the electron density of benzyne shows its pseudodiradical structure, that is, without any energy cost, changes to a carbenoid one, allowing its participation as electrophile in polar reactions. As a consequence, the formal 32CA reaction does not have an activation enthalpy. Analysis of the changes of electron density al…
Rapid Synthesis of Unsymmetrical Sulforhodamines Through Nucleophilic Amination of a Monobrominated Sulfoxanthene Dye
2014
A transition-metal-free method for the synthesis of N-substituted unsymmetrical sulforhodamine fluorophores from an unusual monobrominated sulfoxanthene dye and primary or secondary amines by direct SNAr-type reactions is presented. The simplicity and effectiveness of this “postamination” procedure were demonstrated through the rapid preparation of a library of multifunctional red-emitting rhodamine analogues. Some of these analogues are equipped with a reactive handle and retain the two water-solubilizing sulfonic acid moieties of the starting halogenated derivative; this makes them ideal candidates for biolabeling applications. The potential utility of this expeditious strategy to finely …
Triterpene saponins from Eryngium kotschyi
2015
Four new oleanane-type saponins 3-O-alpha-L-rhamnopyranosyl-(1 -> 4)-beta-D-glucuronopyranosyl-22-O-beta, beta-dimethylacryloylA1-barrigenol (1), 3-O-alpha-L-rhamnopyranosyl-(1 -> 4)-beta-D-glucuronopyranosyl-22-O-angeloylA1-barrigenol (2), 3-O-beta-D-glucopyranosyl-(1 -> 2)-[beta-D-glucopyranosyl-(1 -> 6)]-beta-D-glucopyranosyl-21,22,28-O-triacetyl-(3 beta,21 beta,22 alpha)-olean-12-en-16-one (3), and 3-O-beta-D-glucopyranosyl-(1 -> 2)-glucopyranosyl-22-O-beta-D-glucopyranosylsteganogenin (4), along with the known 3-O-beta-D-galactopyranosyl-(1 -> 2)-[alpha-L-arabinopyranosyl-(1 -> 3)]-beta-D-glucuronopyranosyl-22-O-angeloylA1-barrigenol and 3-O-alpha-L-rhamnopyranosyl-(1 -> 4)-beta-D-gluc…
The effect of iron on the biodegradation of natural dissolved organic matter
2016
Iron (Fe) may alter the biodegradation of dissolved organic matter (DOM), by interacting with (DOM), phosphorus (P) and microbes. We isolated DOM and a bacterial community from boreal lake water and examined bacterial growth on DOM in laboratory experiments. Fe was introduced either together with DOM (DOM-Fe) or into bacterial suspension, which led to the formation of insoluble Fe-precipitates on bacterial surfaces (Fe coating). In the latter case, the density of planktonic bacteria was an order of magnitude lower than that in the corresponding treatment without introduced Fe. The association of Fe with DOM decreased bacterial growth, respiration, and growth efficiency compared with DOM alo…
Persistent photovoltage in methylammonium lead iodide perovskite solar cells
2014
Open circuit voltage decay measurements are performed on methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells to investigate the charge carrier recombination dynamics. The measurements are compared to the two reference polymer-fullerene bulk heterojunction solar cells based on P3HT:PC60BM and PTB7:PC70BM blends. In the perovskite devices, two very different time domains of the voltage decay are found, with a first drop on a short time scale that is similar to the organic solar cells. However, two major differences are also observed. 65-70% of the maximum photovoltage persists on much longer timescales, and the recombination dynamics are dependent on the illumination intensity.
Polymer solar cells based on diphenylmethanofullerenes with reduced sidechain length
2011
Diphenylmethanofullerenes (DPMs) show interesting properties as acceptors in polymer bulk heterojunction solar cells due to the high open circuit voltages they generate compared to their energy levels. Here we investigate the effect of reducing the alkane sidechain length of the DPMs from C12 to C6 in the properties of the solar cell. This change leads to an increase in the electron mobility, thus allowing for a lower fullerene content, which in turn results in an increase in the short circuit current and, finally, in an increase in the efficiency of the device (from 2.3 to 2.6%) due to the higher concentration of the more absorbing polymer in the film. Atomic force microscopy images and ex…
New Triterpenoid and Ergostane Glycosides from the Leaves of Hydrocotyle umbellata L.
2011
Two new triterpenoid glycosides, together with two new ergostane glycosides, umbellatosides A–D (1–4, resp.), have been isolated from the leaves of Hydrocotyle umbellata L. Their structures were established by 2D-NMR spectroscopic techniques (1H,1H-COSY, TOCSY, NOESY, HSQC, and HMBC) and mass spectrometry as 3β,22β-dihydroxy-3-O-[α-L-rhamnopyranosyl-(12)-β-D-glucuronopyranosyl]olean-12-en-28-oic acid 28-O-β-D-glucopyranosyl ester (1), 3-O-[α-L-rhamnopyranosyl-(12)-β-D-glucuronopyranosyl]oleanolic acid 28-O-β-D-glucopyranosyl ester (2), (3β,11α,26)-ergosta-5,24(28)-diene-3,11,26-triol 3-O-(β-D-glucopyranosyl)-11-O-(α-L-rhamnopyranosyl)-26-O-β-D-glucopyranoside (3), and (3β,11α,21,26)-ergosta…
A new oleanane glycoside from the roots ofAstragalus caprinus
2006
A novel oleanane-type triterpene saponin (1) together with two known molecules, soyasapogenol B and astragaloside VIII were isolated from the roots of Astragalus caprinus. Their structural elucidation was performed mainly by 2D NMR techniques (COSY, TOCSY, NOESY, HSQC, HMBC) and mass spectrometry. Compound 1 was determined as 3-O-[alpha-L-rhamnopyranosyl-(1 --> 2)-beta-D-glucuronopyranosyl]-22-O-beta-D-apiofuranosyl-soyasapogenol B.
Fast and continuous synthesis of nanostructured iron spinel in supercritical water: influence of cations and citrates
2014
International audience; Spinel iron oxide nanoparticles were obtained under supercritical water conditions in a continuous and fast (less than 10s) way by modifying the initial stoichiometric Fe II /Fe III molar ratio from (1/2) to (3/0), without base solution, and using citrates directly with iron precursors. This result opens the way of an economical and environmentally benign approach to synthesize superparamagnetic iron oxide nanoparticles (SPIONs) in important yields.