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排序方式: 共有177条查询结果,搜索用时 203 毫秒
31.
Joncour A Décor A Liu JM Dau ME Baudoin O 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(19):5450-5465
The asymmetric synthesis of novel axially chiral biaryl compounds 5 a-f containing a seven- or eight-membered heterocyclic medium ring is described. These molecules can be considered to be structural hybrids of allocolchicine- and steganacin-type natural products. The synthesis featured an atropo-diastereoselective biaryl Suzuki coupling in which a benzylic stereocenter efficiently transferred its stereochemical information to the biaryl axis. The coupling conditions were optimized, and two biphenylphosphane ligands (DavePhos and S-Phos) were found to give the highest yields and diastereoselectivities. A three-element stereochemical model was proposed to explain the observed diastereoselectivities. In a second key step, the medium ring of the target molecules was formed by a stereoselective S(N)1-type cyclodehydration that probably involved a configurationally stable carbocationic intermediate, as supported by calculations. Alternatively, S(N)2-type cyclizations were employed on the same Suzuki coupling products to give the target molecules in a stereodivergent or stereoconvergent manner. These cyclization methods furnished the target hybrid analogues 5 a-f with ee values above 94 %. All analogues were evaluated as antimicrotubule agents and against a panel of cancer-cell lines using colchicine (1) and N-acetylcolchinol (3) as references. Promising activities were found for R,aR-configured compounds 5 a, b and 5 f; in particular, ethyl analogue 5 b showed a twofold antimicrotubule activity relative to colchicine. 相似文献
32.
33.
Jean-Philippe Chancelier Marouen Messaoud Agnès Sulem 《Mathematical Methods of Operations Research》2007,65(2):239-259
The aim of this paper is to solve the fixed point problems:
where
is a finite set, L is contractive and B is a nonexpansive operator and
where
and
are general control sets, the operators L
w
are contractive and operators B
z
are nonexpansive. For these two problems, we give conditions which imply existence and uniqueness of a solution and provide
a policy iteration algorithm which converges to the solution. The proofs are slightly different for the two problems since
the set of controls is finite for (1) while it is not necessary the case for problem (2). Equation (2) typically arises in
numerical analysis of quasi variational inequalities and variational inequalities associated to impulse or singular stochastic
control. 相似文献
34.
Laurent David Alexandra Montembault Nadège Vizio Agnès Crépet Christophe Viton Alain Domard Isabelle Morfin Cyrille Rochas 《Macromolecular Symposia》2005,222(1):281-286
The scaling of the polyelectrolyte scattering peak in chitosan solutions, as deduced from the relation qmax∼cpα was studied by synchrotron SAXS as a function of the charge density of the polymer. We observe a variation in the α exponent corresponding to the limit of the ionic condensation, by varying the degree of acetylation of the polymer. The nature of the solution medium also affects the polyelectrolyte peak, and it is shown that in alcoholic/water mixtures, the lower dissociation of the acid induces a lower charge density, thus influencing the polyelectrolyte ordering. 相似文献
35.
Andrius Garbaras Justina Šapolaitė Inga Garbarienė Žilvinas Ežerinskis Agnė Mašalaitė-Nalivaikė Raminta Skipitytė 《Isotopes in environmental and health studies》2018,54(5):463-474
In the present study, a combination of the stable carbon isotope ratio (13C/12C) with radiocarbon data (14C) allowed us to perform the aerosol source apportionment. Filter samples of PM1 were collected during the warm and cold periods in rural and urban sites in Lithuania. The 14C/12C ratio of total carbon (TC) was measured using the single stage accelerator mass spectrometer quantifying of fossil and non-fossil derived aerosol emissions. The δ13C value was measured using an elemental analyser interfaced with an isotope ratio mass spectrometer. We have found that the highest fraction of contemporary carbon (fc?=?0.82) was measured during a warm period in a rural location. A higher fraction of fossil fuel-derived carbon was observed for air masses transported from highly industrialized Western European regions during both seasons. Isotope mass balance calculations revealed that the traffic emissions composed 15 and 25?% in rural and urban sites, respectively, and did not change during either season. Input from coal-derived aerosol particles was estimated to be 15?% at an urban site during the cold period. The combination of the stable carbon isotope ratio with the radiocarbon data allowed us to distinguish coal, liquid fossil fuel combustion, and non-fossil derived aerosol particle emissions. 相似文献
36.
Compain G Martin-Mingot A Frapper G Bachmann C Jouannetaud MP Thibaudeau S 《Chemical communications (Cambridge, England)》2012,48(47):5877-5879
Anti-Markovnikov additions to non-conjugated unsaturated amines in superacid are reported. In situ NMR studies, DFT calculations and labelled substrates reactions support the involvement of new ammonium-carbenium superelectrophiles in this original process. 相似文献
37.
Gaulier C Hospital A Legeret B Delmas AF Aucagne V Cisnetti F Gautier A 《Chemical communications (Cambridge, England)》2012,48(33):4005-4007
A reducing agent-free version of CuAAC able to operate under open air conditions is reported. A readily-synthesizable, hydrophilic and highly stable Cu(I)-NHC allows the clean ligations of unprotected peptides comprising sensitive side chains, at millimolar concentrations. 相似文献
38.
Martin-Mingot A De Oliveira Vigier K Jérôme F Thibaudeau S 《Organic & biomolecular chemistry》2012,10(13):2521-2524
Herein, we show that after polyprotonation, superacid HF-SbF(5) is able to selectively depolymerise cellulose to water-soluble carbohydrates with 68 wt% yield of glucose. This process is efficient at low temperature, thus avoiding the formation of side products as commonly observed with conventional acids. 相似文献
39.
Agnès Traverse 《Hyperfine Interactions》1997,110(1-2):159-164
Characterization of the local environment of metallic ions (Cu, Fe, Ti, Ni and Er) implanted in the ceramic matrix AlN have
led us to conclude that the heat of formation (\Delta H) of the implanted ion nitride (I-N) is the parameter which has to
be considered in order to predict the final system. When ΔHI-N is negative, bonds between the implanted ion and nitrogen atoms are formed, when ΔHI-N is positive, clusters of the implanted ion are formed. Moreover, we give some physical characteristics of the Cu and Ni clusters
in the AlN matrix and show the differences between imbedded and deposited clusters of the same size.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
40.
S. D. Jones S. Paul Blundell Agnès M. A. Bodin 《Accreditation and quality assurance》1997,2(5):250-257
A strategy for ensuring that measurements of ten common contaminants found in soils from former gasworks are of optimum quality
over long timescales is described. In particular, the performance of laboratories in applying validated measurement methods
in both small- and large-scale inter-laboratory trials over a 5-year period is assessed. It is shown that a production engineering
concept, the experience curve, has a significant influence on the quality of some of the measurements made and that this concept
can be used to model the performance of laboratories. Complexity of the measurement procedure and familiarity of analysts
with the measurement operations are suggested as the main drivers of method error. The implications of the achievable quality
of measurement on the costs of land re-development and on confidence in the quality of the rejuvenated land are discussed.
Received: 31 August 1996 Accepted: 5 February 1997 相似文献