Search results for "element"
showing 10 items of 13601 documents
ChemInform Abstract: A Catalytic Highly Enantioselective Direct Synthesis of 2-Bromo-2-nitroalkan-1-ols Through a Henry Reaction.
2009
Highly enantiomerically enriched 2-bromo-2-nitroalkan-1-ols are prepared by direct condensation of aliphatic and aromatic aldehydes with bromonitromethane in the presence of a catalytic amount of copper(II) acetate and a C1-symmetric camphor-derived amino pyridine ligand.
Enantioselective synthesis of 2-substituted-1,4-diketones from (S)-mandelic acid enolate and α,β-enones
2006
[EN] An approach for the synthesis of chiral non-racemic 2-substituted-1,4-diketones from (S)-mandelic acid and ¿,ß-enones has been developed. The reaction of lithium enolate of the 1,3-dioxolan-4-one derived from optically active (S)-mandelic acid and pivalaldehyde with ¿,ß-unsaturated carbonyl compounds proceeds readily to give the corresponding Michael adducts in good yields and with high diastereoselectivities. The addition of HMPA (3 equiv) reverses and strongly enhances the diastereoselectivity of the reaction. A change in the reaction mechanism from a lithium catalyzed to the one where catalysis has been suppressed by coordination of HMPA to lithium is proposed to explain these resul…
Ferrocenylbis(ylene) phosphoranes
1997
The ferrocenylbis(methylene)phosphorane Fc-P(=CTms 2 ) 2 ( 1 ) (Fc = −C 5 H 4 FeC 5 H 5 ) can be synthesized starting from ferrocenyldichlorophosphane FcPCl 2 or ferrocenyldilithiophosphane FcPLl 2 . The X-ray structure of ( 1 ) shows some unusual structural features, which indicate considerable electronic interaction of the ferrocenyl group and the σ 3 λ 5 -phosphorane unit. As a consequence of this interaction, the rotational barrier of the methylene bonds is extraordinary low. Therefore, in contrast to other bis(methylene)phosphoranes, the endo - and exo -Tms groups are indistinguishable on the NMR time scale even at −100°C. The analogous ferrocenylbis(imino)phosphorane Fc-P(=NTms) 2 ( 9…
ChemInform Abstract: Enantioselective Zinc-Mediated Conjugate Addition of Terminal Alkynes to Enones.
2013
Zinc for conjugate alkynylation: The enantioselective conjugate addition of terminal alkynes to 2-arylidene-1,3-diketones in the presence of diethylzinc and a catalytic amount of (R)-VANOL has been developed. The reaction can be applied to different aromatic and heteroaromatic alkynes and enones, giving the expected products in good yield and with enantiomeric excesses up to 91%. The products can be enantiomerically enriched up to 99% ee by crystallization (see scheme).
Model interpretation from the additive elements of the PWRSS in GLMMs
2013
Generalized Linear Mixed models(GLMMs)have rapidly become a widely used tool for modelling clustered and longitudinal data with non-Normal responses. Although a large amount of work has been done in the literature on likelihood-based inference on GLMMs,little seems to have been done on the decomposition of the total variability associated to the different components of a mixed model.In this work we try to generalize the idea of likelihood additive elements Whittaker,1984), proposed in the context of GLMs,to the case of GLMMs by using the Penalized Weighted Residual Sum of Squares(PWRSS). The proposal is illustrated by means of areal application.
Contribution of thallium-201-SPECT to the grading of tumorous alterations of the brain
1992
Single photon emission computed tomography (SPECT) with thallium-201-chloride (201Tl) was used in 22 patients to assess the grade of malignancy of brain tumors. Low- and high-grade malignant gliomas could be well differentiated by calculating the Grade Index (GI), i.e., 201Tl uptake in the tumor area relative to a contralateral brain region. Low-grade gliomas (WHO-grade I-II) usually showed a GI of1.5. Tumors classified histologically as high-grade malignant (WHO-grade III-IV) had GI values greater than 1.42 and a mean value of 1.89. Until labelled amino-acid tracers for gamma-cameras become commercially available, thallium-201 brain-SPECT can provide an independent and complementary method…
An in vitro model for the study of human parathyroid gland tissue: Single cell suspensions and monolayer cultures
1982
An in vitro model for studies of parathyroid physiology is described using single cell suspensions and adherent monolayer cultures of human parathyroid tissue. The isolated cells were viable and maintained functional properties tested by calcium and magnesium sensitivity. Parathyroid hormone (PTH) secretion could be suppressed by rising calcium and magnesium concentrations. The secretory behaviour of the cells was not altered by cultivation or cryopreservation. The morphological integrity of the individual cells after mechanical and enzymatic preparation was confirmed by light and electron microscopy.
Label-Free Pyrophosphate Recognition with Functionalized Asymmetric Nanopores
2016
[EN] The label¿free detection of pyrophosphate (PPi) anions with a nanofluidic sensing device based on asymmetric nanopores is demonstrated. The pore surface is functionalized with zinc complexes based on two di(2¿picolyl)amine [bis(DPA)] moieties using carbodiimide coupling chemistry. The complexation of zinc (Zn2+) ion is achieved by exposing the modified pore to a solution of zinc chloride to form bis(Zn2+¿DPA) complexes. The chemical functionalization is demonstrated by recording the changes in the observed current¿voltage (I¿V) curves before and after pore modification. The bis(Zn2+¿DPA) complexes on the pore walls serve as recognition sites for pyrophosphate anion. The experimental re…
Physicochemical characterization of passive films on niobium by admittance and electrochemical impedance spectroscopy studies
2005
An analysis of the electronic properties of amorphous semiconductor-electrolyte junction is reported for thin (D ox < 20 nm) passive film grown on Nb in acidic electrolyte. It will be shown that the theory of amorphous semiconductor-electrolyte junction (a-SC/EI) both in the low band-bending and high band-bending regime is able to explain the admittance data of a-Nb 2 O 5 /El interface in a large range (10 Hz-10 kHz) of frequency and electrode potential values. A modelling of experimental EIS data at different potentials and in the frequency range of 0.1 Hz-100 kHz is presented based on the theory of amorphous semiconductor and compared with the results of the fitting of the admittance data…
On the use of the electromechanical impedance technique for the assessment of dental implant stability: Modeling and experimentation
2014
We propose the electromechanical impedance technique to monitor the stability of dental implants. The technique consists of bonding one wafer-type piezoelectric transducers to the implant system. When subjected to an electric field, the transducer induces structural excitations which, in turn, affect the transducer’s electrical admittance. The hypothesis is that the health of the bone surrounding the implant affects the sensor’s admittance. A three-dimensional finite element model of a transducer bonded to the abutment of a dental implant placed in a host bone site was created to simulate the progress of the tissue healing that occurs after surgery. The healing was modeled by changing the …