Organotellurides as a source of organometallics: application in the synthesis of (+/−)-frontalin

A masked lithium homoenolate, generated by tellurium/lithium exchange, was reacted with epoxides. The lithium compound was also converted into other organometallics such as Grignard, and cuprates and the reactivity of those organometallics with epoxides was evaluated. The same building block was employed in the synthesis of (+/−)-frontalin.     

Synthesis of optically active 6-substituted 2-(aminomethyl)chromans

Three novel, optically active, 6-substituted 2-(aminomethyl)chromans were synthesized from readily available chroman 2-carboxylic acid precursors. These chroman-containing primary amines are useful building blocks for the synthesis of chroman-derived pharmaceutical agents.     

Electron spin lifetimes in Hg0.78Cd0.22 Te and InSb

We have made direct pump–probe measurements of spin lifetimes in long wavelength narrow-gap semiconductors at wavelengths between 4 and 10 μm and from 4 to 300 K. In particular, we measure remarkably long spin lifetimes, τ_s300 ps, even at 300 K for epilayers of degenerate n-type InSb. In this material the mobility is approximately constant between 77 and 300 K, and we find that τ_s is approximately constant in this temperature range. In order to determine the dominant spin relaxation mechanism we have investigated the temperature dependence of τ_s in non-degenerate lightly n-type Hg_0.78Cd_0.22Te of approximately the same band gap as InSb, and find that τ_s varies from 356 ps at 150 K to 24 ps at 300 K. Our results, both in magnitude and temperature dependence of τ_s, imply that the Elliott–Yafet model dominates in these materials.     

Some examples for the Poincaré and Painlevé problems

In 1891, Poincaré started a series of three papers in which he tried to answer the following question (cf. [21-23]): “Is it possible to decide if an algebraic differential equation in two variables is algebraically integrable?” (in the sense that it has a rational first integral). More or less at the same time P. Painlevé asked the following question: “Is it possible to recognize the genus of the general solution of an algebraic differential equation in two variables which has a rational first integral?”. In this paper we give examples of one-parameter families which show that both problems have a negative answer. With some of the families we can also answer a question posed by M. Brunella in [5].     

Measurement of the neutron electric form factor via recoil polarimetry

The ratio G _e ⁿ /G _m ⁿ of the electric to the magnetic form factor of the neutron has been measured by analyzing the polarization of the recoiling neutron in quasi-elastic scattering of longitudinally polarized electrons from deuterium at the Q ² values of 0.45, 1.15, and 1.47 (GeV/c)². The experiment has been performed in Hall C of the Thomas Jefferson National Accelerator Facility. With G _m ⁿ being known G _e ⁿ can be deduced. The preliminary results show that the lowest Q ² points follow the Galster parametrization and that the 1.47 (GeV/c)² point rises above this parametrization.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.40.Gp Electromagnetic form factors - 14.20.Dh Protons and neutrons - 24.70. + s Polarization phenomena in reactions     

Electrochemical impedance spectroscopy and linear polarization applied to evaluation of porosity of phosphate conversion coatings on electrogalvanized steels

In this work, electrochemical impedance spectroscopy and linear polarization are used in determining porosity of zinc phosphates, and of nickel and manganese modified zinc phosphates on electrogalvanized steel. The porosity of the phosphate layers ranges from 0.1% for the manganese-modified hopeite to 8% for hopeite, using the linear polarization and the electrochemical impedance spectroscopy techniques. The porosity values measured using the two techniques were in agreement. All impedance spectra of coated samples showed two steps, with two capacitive arcs and an inductive looping.     

Kinetic and structural characterization of human manganese superoxide dismutase containing 3-fluorotyrosines

Incorporation of 3-fluorotyrosine and site-specific mutagenesis have been used with stopped-flow spectrophotometry and pulse radiolysis to investigate the catalytic properties of human manganese superoxide dismutase (MnSOD). All of the nine tyrosine residues in each of the four subunits of the homotetramer of human MnSOD were replaced with 3-fluorotyrosine. Previous studies showed that the crystal structures of the unfluorinated and fluorinated human MnSOD are nearly superimposable with the root-mean-square deviation for 198 -carbon atoms at 0.3 Å. However, the catalytic activity k_cat/K_m of the fluorinated MnSOD at 30 μM⁻¹ s⁻¹ was less than unfluorinated wild type at 800 μM⁻¹ s⁻¹. Comparison of the values of k_cat/K_m for fluorinated and unfluorinated wild-type andY34F MnSOD showed that this decrease for the fluorinated enzyme was in significant part due to 3-fluorotyrosine residues distant (>7 Å) from the active-site metal, not to 3-fluorotyrosine at position 34 close (5 Å) to the metal. Although many rate constants for the catalysis are decreased by this fluorination, the rate of dissociation of the product-inhibited complex appears unchanged by the presence of fluorinated tyrosines. These results suggest that Tyr34 is not a proton donor in the release of the product-inhibited complex, which involves protonation of a peroxo complex of the metal with release of hydrogen peroxide.     

The Synthesis of 3,5-Diamino-1,2,4-oxadiazoles. 1st Communication

Reaction of the 1-substituted-3-cyano-isothioureas 6 with hydroxylamine gave mixtures of the 5-amino-3-substituted-amino-1,2,4-oxadiazoles 1 and the isomeric 3-amino-5-substituted-amino-1,2,4-oxadiazoles 8 in which 1 usually predominated. The structural assignment of these products is discussed. In a second method, the 2-hydroxy-1-methyl-1-phenyl-guanidine 15 was converted to the corresponding 3-disubstituted-amino-5-trichloromethyl-1,2,4-oxadiazole 16 , a precursor to the 5-amino derivatives 17 by nucleophilic displacement of the trichloromethyl group.     

Increased structural complexity leads to higher activity: peptides as efficient and versatile catalysts for asymmetric aldol reactions

[reaction: see text] Peptides containing a secondary amine and a carboxylic acid in a specific orientation to each other are presented as highly efficient catalysts for asymmetric aldol reactions: (1) their activity is considerably higher compared to that of proline, and (2) the enantioselectivity of the peptidic catalysts can be changed from (R)- to (S)-selectivity by simple modifications of the secondary structure.