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At least 775 records · Page 43Linked to original sources
PubMed2000

Total synthesis of 17-isoLevuglandin E4 and the structure of C22-PGF4alpha.

The isolevuglandin 17-isoLGE4 (10-acetyl-11-formyl-14-hydroxynonadeca-4(Z),7(Z),12(E),16(Z)-tetr aenoic acid) is a levulinaldehyde derivative that is expected to be generated during the free radical-induced oxidation of docosahexaenoic acid. A total synthesis was executed to facilitate detection and identification of 17-isoLGE4 in biological samples. Conjugate addition of a higher order vinyl cyanocuprate to a gamma-alkoxy enone achieved the final carbon-carbon bond formation to complete a convergent elaboration of the 17-isoLGE4 carbon skeleton. Attempted construction of the requisite vinyl nucleophile synthon using hydrostannylation of an alkyne was foiled by tri-n-butylstannyl radical-promoted isomerization of a cis to a trans double bond. Derivatization of 17-isoLGE4 with methoxylamine under anhydrous or wet conditions generated bismethoximes of 17-isoLGE4 or the isomerized delta11-17-isoLGE4 respectively. Analysis of the mass spectrum of a bismethoxime-pentafluorobenzyl ester-trimethylsilyl ether derivative of 17-isoLGE4 provided presumptive evidence that an incorrect structure was proposed earlier for C22-PGF4alpha, the only F4-isoprostane which is produced enzymatically. We conclude that the 22-carbon analogue of PGF2alpha, produced from docosahexaenoic acid by a cyclooxygenase from trout gill, does not have the same side chains as 17-isoLGE4. Furthermore, we now propose that mass spectral data reported for "C22-PGF4alpha" support a 14-PGF4alpha structure rather than the 17-PGF4alpha structure suggested previously.

Hydrolysis↗
PubMed2002

Synthesis of symmetrical carotenoids by a two-fold Stille reaction.

beta-Carotene 1 and (3R,3'R)-zeaxanthin 2 have been stereoselectively prepared in a highly convergent fashion by a 2-fold Stille cross-coupling reaction. The C(12)-pentaenylbis-stannane 8 is the central "lynchpin" that connects two units of the terminal C(14)-iodides 9 and 17 to afford 1 and 2, respectively.

Carotenoids↗
PubMed2004

Syntheses and reactions of optically active alpha-aminoallenylstannanes.

Lithiation/stannylation of optically active N-propargyloxazolidinones produced optically active alpha-oxazolidinonylallenylstannanes. Reaction of these with aldehydes in the presence of BF(3).OEt(2) produced beta-hydroxypropargylamines with high syn diastereoselectivity and ee. These were converted to gamma-hydroxy-beta-amino acids by oxidative cleavage of the alkyne.

Magnetic Resonance Spectroscopy↗
PubMed2005

Enantio- and diastereoselective additions to aldehydes using the bifunctional reagent 2-(chloromethyl)-3-(tributylstannyl)propene: application to a synthesis of the C16-C27 segment of bryostatin 1.

[reaction: see text] Reactions of the bifunctional allylstannane 2-(chloromethyl)-3-(tributylstannyl)propene with aldehydes have been examined. These generally occur in high yields using Lewis acid promoters and the products can be isolated and purified without incident. Good yields and high enantioselectivities are also realized in catalytic asymmetric allylations (CAA reactions) using the previously described BITIP catalyst system. Protection of the free hydroxyl can be accomplished without cyclization to the derived tetrahydrofuran, although this transformation is also facile. The utility of the incorporated allyl chloride functionality allows for the obvious use of such products in reactions with nucleophiles. Use of these products in a less obvious connective strategy is demonstrated in the synthesis of the C12-C27 segment of bryostatin 1 where a connective, or "lynchpin", double-allylation process was employed. The beta-hydroxy allyl chloride obtained from an initial chelation-controlled allylation of aldehyde 16 was converted to allylstannane 19 and applied in a second allylation reaction, thus allowing for a highly convergent synthesis of the bryostatin C ring backbone in a stereoselective fashion.

Aldehydes↗
PubMed2004

Novel Kumada coupling reaction to access cyclic (2-azaallyl)stannanes. Cycloadditions of cyclic nonstabilized 2-azaallyllithium species derived from cyclic (2-azaallyl)stannanes.

A Kumada cross-coupling reaction involving organomagnesium reagents and (3-methylthio-2-azaallyl)stannanes with a Ni(0) catalyst provided cyclic nonstabilized (2-azaallyl)stannanes in moderate to good yields. Primary alkyl, aryl, and allylic organomagnesium reagents can be used as the cross-coupling partner. In general, NiCl(2)dppp in toluene at room temperature provided the shortest reaction times and most consistent yields. The azomethine ylides and 2-azaallyllithium species derived from these stannanes were shown to undergo efficient [3 + 2] cycloaddition reactions to provide azabicyclo[n.2.1]alkanes as the endo cycloadducts. These cycloadducts were found to be useful as starting materials for further elaboration into aza-bridged bicyclic natural and unnatural products of biological interest. Although cyclic 2-azaallyllithium species have been generated previously, this work reports the first generation and cycloaddition of entirely nonstabilized 2-azaallyllithium species. In addition a novel extension of the Kumada coupling was developed to allow for the preparation of the cyclic (2-azaallyl)stannanes, which are precursors to the nonstabilized 2-azaallyllithium species.

Lithium↗
PubMed2006

Synthesis of N,N-bis(3-butenyl)amines from 2-azaallyl dication synthetic equivalents and conversion to 2,3,6,7-tetrahydroazepines by ring-closing metathesis.

N,N-Bis(3-butenyl)amines can be prepared by the double allylation of either (2-azaallyl)stannanes or (2-azaallyl)nitriles, both of which thereby act as synthetic equivalents to amine alpha,alpha'-dications (2-azaallyl dications). Allylmagnesium bromide is the reagent of choice for the double allylation of both substrates, although allyllithium also effects the double allylation of (2-azaallyl)nitriles. Ring-closing metathesis can be performed on the N-protected amines, or with in situ protonation, on the free amines to provide 2,3,6,7-tetrahydroazepines. (2-Azaallyl)nitriles can also be monoallylated to provide N-(3-butenyl)aminonitriles, whereas the double allylation of (2-azaallyl)stannanes cannot be stopped at monoallylation. (2-Azaallyl)silanes undergo monoallylation to give N-(3-butenyl)aminosilanes but do not undergo double allylation.

Amines↗
PubMed2005

First-principles studies of SnS2 nanotubes: a potential semiconductor nanowire.

First principles calculations are used to predict the stability and electronic structures of SnS(2) nanotubes. Optimization of several structures and their corresponding strain energies confirm the stability of SnS(2) nanotube structures. Band structure calculations show that SnS(2) nanotubes could have moderate band gaps regardless of their chirality. It suggests that SnS(2) nanotubes would be well-suited to use as semiconductor wires in nanoelectronic devices if they are synthesized. Adsorption of NH(3) onto SnS(2) is also investigated and discussed with regard to potential sensor application.

Adsorption↗
PubMed2005

Silver-nanoparticle-attached indium tin oxide surfaces fabricated by a seed-mediated growth approach.

By applying a seed-mediated growth method that had been reported for the chemical synthesis of Ag nanorods and nanowires in aqueous solution, we successfully attached Ag nanosphere and nanorod particles to indium tin oxide (ITO) surfaces. In this method, it is characteristic that the attachment can be performed without using bridging reagents, such as 3-mercaptopropyltrimethoxysilane, but rather through a two-step immersion into the seed solution first and then into the growth solution containing AgNO(3), cetyltrimethylammonium bromide, and ascorbic acid. It was found that the formed nanostructures were very sensitive to the amount of ascorbic acid in the growth solution. Whereas Ag nanoparticles grew on the ITO surface with a moderate dispersion when the concentration of ascorbic acid in the growth solution was 0.64 mM, the formation of nanorods and nanowires was observed when the ascorbic acid concentration was increased to 0.86 mM. The attachment of Ag nanoparticles onto the ITO surfaces was strong enough for further use, e.g., as a working electrode. From electrochemical measurements, it was confirmed that the outer spheres of the Ag nanoparticles involved in the redox reaction show the typical oxidation and reduction waves of Ag. In addition, the redox behavior of [Fe(CN)(6)](3-)/[Fe(CN)(6)](4-) was improved on the Ag-nanoparticle-attached ITO (AgNP/ITO) electrode, reflecting the low electron-transfer resistivity, which is a remarkable advantage of the present fabrication without using bridging reagents. This result indicated that the Ag nanoparticles promote the electron-transfer reactions by being present on the conducting ITO surface. The AgNP/ITO electrode was examined for the reduction of the methyl viologen dication in order to discuss some features of the present fabrication.

Electrochemistry↗
PubMed2005

Combined X-ray diffraction and diffuse reflectance analysis of nanocrystalline mixed Sn(II) and Sn(IV) oxide powders.

Nanocrystalline mixtures of Sn(II) and Sn(IV) oxide powders, potential gas sensor materials, are synthesized via a simple precipitation route using SnCl(2) as the precursor. Materials are characterized by powder X-ray diffraction, thermogravimetric analysis, UV-visible diffuse reflectance spectroscopy (DRS), and Fourier transform infrared spectroscopy. The ratio of Sn(II)/Sn(IV) in powders precipitated at room temperature, as well as the identity of the primary Sn(II) product (SnO or Sn(6)O(4)(OH)(4)), can be varied by adjusting aging time and washing procedures. The identity of the initial Sn(II) product influences the subsequent phase composition and degree of disorder in the tetragonal SnO(2) phase obtained following sintering in air. Analysis of the DRS absorption edge and long-wavelength (Urbach) absorption tail is used to determine the SnO(2) optical band gap and extent of disorder. SnO(2) obtained by heating the SnO/SnO(2) mixture at 600 or 800 degrees C has a smaller optical band gap and a broader Urbach tail than the analogous sample obtained from heating Sn(6)O(4)(OH)(4), indicating a more disordered material.

Crystallization↗
PubMed2005

Charge transport versus recombination in dye-sensitized solar cells employing nanocrystalline TiO2 and SnO2 films.

We report a comparison of charge transport and recombination dynamics in dye-sensitized solar cells (DSSCs) employing nanocrystalline TiO(2) and SnO(2) films and address the impact of these dynamics upon photovoltaic device efficiency. Transient photovoltage studies of electron transport in the metal oxide film are correlated with transient absorption studies of electron recombination with both oxidized sensitizer dyes and the redox couple. For all three processes, the dynamics are observed to be 2-3 orders of magnitude faster for the SnO(2) electrode. The origins of these faster dynamics are addressed by studies correlating the electron recombination dynamics to dye cations with chronoamperometric studies of film electron density. These studies indicate that the faster recombination dynamics for the SnO(2) electrodes result both from a 100-fold higher electron diffusion constant at matched electron densities, consistent with a lower trap density for this metal oxide relative to TiO(2), and from a 300 mV positive shift of the SnO(2) conduction band/trap states density of states relative to TiO(2). The faster recombination to the redox couple results in an increased dark current for DSSCs employing SnO(2) films, limiting the device open-circuit voltage. The faster recombination dynamics to the dye cation result in a significant reduction in the efficiency of regeneration of the dye ground state by the redox couple, as confirmed by transient absorption studies of this reaction, and in a loss of device short-circuit current and fill factor. The importance of this loss pathway was confirmed by nonideal diode equation analyses of device current-voltage data. The addition of MgO blocking layers is shown to be effective at reducing recombination losses to the redox electrolyte but is found to be unable to retard recombination dynamics to the dye cation sufficiently to allow efficient dye regeneration without resulting in concomitant losses of electron injection efficiency. We conclude that such a large acceleration of electron dynamics within the metal oxide films of DSSCs may in general be detrimental to device efficiency due to the limited rate of dye regeneration by the redox couple and discuss the implications of this conclusion for strategies to optimize device performance.

Electrons↗
PubMed2005

Effects of the polarizability and packing density of transparent oxide films on water vapor permeation.

The tin oxide and silicon oxide films have been deposited on polycarbonate substrates as gas barrier films, using a thermal evaporation and ion beam assisted deposition process. The oxide films deposited by ion beam assisted deposition show a much lower water vapor transmission rate than those by thermal evaporation. The tin oxide films show a similar water vapor transmission rate to the silicon oxide films in thermal evaporation but a lower water vapor transmission rate in IBAD. These results are related to the fact that the permeation of water vapor with a large dipole moment is affected by the chemistry of oxides and the packing density of the oxide films. The permeation mechanism of water vapor through the oxide films is discussed in terms of the chemical interaction with water vapor and the microstructure of the oxide films. The chemical interaction of water vapor with oxide films has been investigated by the refractive index from ellipsometry and the OH group peak from X-ray photoelectron spectroscopy, and the microstructure of the composite oxide films was characterized using atomic force microscopy and a transmission electron microscope. The activation energy for water vapor permeation through the oxide films has also been measured in relation to the permeation mechanism of water vapor. The diffusivity of water vapor for the tin oxide films has been calculated from the time lag plot, and its implications are discussed.

Algorithms↗
PubMed2005

Influence of the anion on lone pair formation in Sn(II) monochalcogenides: a DFT study.

The electronic structure of SnO, SnS, SnSe, and SnTe in the rocksalt, litharge, and herzenbergite structures has been calculated using density functional theory. Comparison of the distorted and undistorted structures allows for an explanation of the unusual experimentally observed structural transitions seen along the Sn(II) monochalcogenides. Analysis of the electronic structure shows a strong anion dependence of the Sn(II) lone pair, with the Sn(5s) and Sn(5p) states too far apart to couple directly. However, the interaction of Sn(5s) with anion states of appropriate energy produce a filled antibonding Sn(5s)-anion p combination which allows coupling of Sn(5s) and Sn(5p) to occur, resulting in a sterically active asymmetric density on Sn. While the interaction between Sn(5s) and O(2p) is strong, interactions of Sn with S, Se, and Te become gradually weaker, resulting in less high energy 5s states and hence weaker lone pairs. The stability of the distorted structures relative to the symmetric structures of higher coordination is thereby reduced, which induces the change from highly distorted litharge SnO to highly symmetric rocksalt SnTe seen along the series.

Anions↗
PubMed2006

Derivation of force field parameters for SnO2-H2O surface systems from plane-wave density functional theory calculations.

Plane-wave density functional theory (DFT-PW) calculations were performed on bulk SnO2 (cassiterite) and the (100), (110), (001), and (101) surfaces with and without H2O present. A classical interatomic force field has been developed to describe bulk SnO2 and SnO2-H2O surface interactions. Periodic density functional theory calculations using the program VASP (Kresse et al., 1996) and molecular cluster calculations using Gaussian 03 (Frisch et al., 2003) were used to derive the parametrization of the force field. The program GULP (Gale, 1997) was used to optimize parameters to reproduce experimental and ab initio results. The experimental crystal structure and elastic constants of SnO2 are reproduced reasonably well with the force field. Furthermore, surface atom relaxations and structures of adsorbed H2O molecules agree well between the ab initio and force field predictions. H2O addition above that required to form a monolayer results in consistent structures between the DFT-PW and classical force field results as well.

Elasticity↗