Synthesis and conformational analysis of new troponyl aromatic amino acid

Synthetic peptides are in huge demand in expansion of potential peptide mimics, which may have improved or comparable function as natural one. With these concerns, phenyl bearing aromatic amino acids and peptides has extensively explored, because phenyl residue has high probability in forming stable secondary structure, owing to the presence of an extra stabilizing factor as π–π non-covalent interactions. Apart from phenyl bearing benzenoid aromatic amino acids, a few non-benzenoid aromatic derivatives such as tropolone and related compounds are also occurred in nature, but troponyl containing amino acids and peptides are very poorly understood. Tropolonyl derivatives also contain carbonyl functional group, which may play an important role to provide stable conformation in peptide. Herein we report the synthesis, and conformational analysis of rationally designed new unnatural δ-amino acid, troponyl aminoethylglycine (Tr-aeg), which contains troponyl residue as side chain in flexible aminoethylglycine (aeg) amino acid backbone. We also demonstrate the role of troponyl carbonyl of Tr-aeg residue in hydrogen bonding with adjacent amide NH of their hybrid di/tri-peptides with NMR methods and DFT calculations. In future, Tr-aeg amino acid would be a potential building block in development of promisable peptide mimics.     

Total synthesis of (+)-anamarine

An enantiospecific total synthesis of polyhydroxy δ-pyrone natural product (+)-anamarine is accomplished. The main features of the synthesis include the stereoselective reduction of the ketone obtained by the desymmetrization of the bis-dimethyl amide of tartaric acid and further elaboration involving asymmetric Brown's allylation and ring closing metathesis.     

Co掺杂对YBa2 Cu3 O7-δ电阻率与塞贝克系数的影响

用固态反应法制备了YBa2Cu3-xCoxO7-δ(x=0.0,0.1,0.2,0.5)样品,研究了Co掺杂对YBa2Cu3O7-δ高温区的电阻率和塞贝克系数的影响。随着Co含量的增加,样品的电阻率和塞贝克系数逐渐增大,从金属性导电转变为p型半导体导电。x=0.2和0.5样品的电导活化能在500K处发生突变,高温区的活化能大于低温区的活化能。通过塞贝克系数与温度的关系,计算出x=0.2和0.5样品的费米能级分别为0.02和0.12 eV。当温度高于650-700K时,氧脱附显著影响样品的电输运性质,导致电阻率和塞贝克系数随温度增加而增大。     

Double perovskite oxides Sr_2Mg_(1-x)Fe_xMoO(6-δ) for catalytic oxidation of methane

The double perovskite oxides Sr2Mg1-xF exMoO6-δ were investigated as catalysts for the methane oxidation.The structural properties of catalysts were characterized in detail by X-ray diffraction,X-ray photoelectron spectroscopy and X-ray absorption spectroscopy.The catalytic property was strongly influenced by the Fe substitution.The relation between catalytic performance and the degree of Fe substitution was examined with regard to the structure and surface characteristics of the mixed oxides.The Fe-containing catalysts exhibited higher activity attributable to the possible(Fe2+,Mo6+) and (Fe3+,Mo5+)valency pairs,and the highest activity was observed for Sr2Mg0.2Fe0.8MoO6-δ.The enhancement of the catalytic activity may be correlated with the Fe-relating surface lattice oxygen species and was discussed in view of the presence of oxygen vacancies.     

A new approach to the synthesis of lactams of muramic,isomuramic and normuramic acids via intramolecular O-alkylation: Stereochemical features of the intramolecular nucleophilic substitution

The title compounds were synthesized from the selectively protected N-acylated d-glucosamine derivatives, containing α-halo carboxylic acid moieties, via intramolecular 3-O-alkylation. It was found that if the starting compound contains asymmetric electrophilic center, isomuramic acid derivatives were mainly formed, regardless of the configuration of the electrophilic carbon atom. An explanation for the observed stereochemical results was proposed on the basis of the analysis of steric interactions in the molecules of the starting compounds, as well as using the concept of anchimeric assistance. It was shown that N-acetylation of the obtained lactam derivatives and subsequent methanolysis under mild conditions led to the selective cleavage of δ-lactam ring resulting in the formation of the corresponding ester derivatives of N-acetylmuramic acid or its analogues in high yields.     

Pd-catalyzed intramolecular direct arylations at high temperature

Pd-catalyzed cyclodehydrohalogenation involving oxidative addition of aromatic C-Br or activated azaheteroaromatic C-Cl bonds and C(sp²)-H activation have been investigated at high reaction temperatures (180-200 °C). This allowed the fast (10-30 min) synthesis of a variety of azaheteroaromatic ring systems (dibenzo[f,h]phthalazine, dibenzo[f,h]cinnoline, benzofuro[2,3-d]pyridazine, 5H-pyridazino[4,5-b]indole, 7H-indolo[2,3-c]quinoline and 5H-δ-carboline) in moderate to good yields.     

Stereoselective total synthesis of (+)-anamarine via cross-metathesis protocol

A convergent stereoselective total synthesis of (+)-anamarine via cross-metathesis (CM) protocol starting from 2-butyn-1,4-diol and vinyl lactone is reported. Other key features of the strategy include the use of Sharpless asymmetric epoxidation, Sharpless dihydroxylation, and Red-Al reduction.     

Luminescence enhancement of Eu, Ce co-doped Ba3Si5O13−δNδ phosphors

Host lattice Ba₃Si₅O_13−δN_δ oxonitridosilicates have been synthesized by the traditional solid state reaction method. The lattice structure is based on layers of vertex-linked SiO₄ tetrahedrons and Ba²⁺ ions, where each Ba²⁺ ion is coordinated by eight oxygen atoms forming distorted square antiprisms. Under an excitation wavelength of 365 nm, Ba₃Si₅O_13−δN_δ:Eu²⁺ and Ba₃Si₅O_13−δN_δ:Eu²⁺,Ce³⁺ show broad emission bands from about 400-620 nm, with maxima at about 480 nm and half-peak width of around 130 nm. The emission intensity is strongly enhanced by co-doping Ce³⁺ ions into the Ba₃Si₅O_13−δN_δ:Eu²⁺ phosphor, which could be explained by energy transfer. The excitation band from the near UV to the blue light region confirms the possibility that Ba₃Si₅O_13−δN_δ:Eu²⁺, Ce³⁺ could be used as a phosphor for white LEDs.     

生物组织的δ-P1近似漫反射光学模型

本文研究了生物组织的改进的δ-P₁近似漫反射光学模型,推导了含有等效光源一阶矩的双点源近似空间分辨漫反射解R_δ－P1(ρ).研究表明,考虑等效光源一阶矩的光学模型,较好地描述了具有强的前向散射特性和较大吸收系数的生物组织散射特性;与漫射近似下的漫反射率R_SDA(ρ)相比,新的光学模型能较好地描述光源附近的漫辐射强度分布,并且由于解析表达式中含有散射相函数的二阶参量γ,这对 关键词： 组织光学 1近似')" href="#">δ-P₁近似 等效光源 微区漫反射     

Oxygen nonstoichiometry, defect structure and electrical properties of LaFe0.7Ni0.3O3 − δ

Oxygen nonstoichiometry (δ), total conductivity (σ) and thermoelectric power (S) of the LaFe_0.7Ni_0.3O_3 − δ sample have been studied as functions of temperature and oxygen partial pressure. Based on the results of the direct reduction of the sample in hydrogen flow at 1100 °C the absolute oxygen content (3 − δ) has been found to vary from 2.999 to 2.974 in the range of 1273-1373 K and 10^− 3-0.21 atm. The point defect equilibrium models have been proposed and fitted to the set of experimental data in the form of log p(O₂) = f(δ)_T dependences. The values of standard thermodynamic quantities of defect formation reactions have been assessed. The joint analysis of oxygen nonstoichiometry, total conductivity and thermoelectric power has been performed using a small-polaron approach. The values of partial conductivity, partial thermopower and mobilities of electronic charge carriers have been calculated. The p-type semiconducting behavior of LaFe_0.7Ni_0.3O_3 − δ has been explained by the higher mobility values of electron holes than those of electrons in the whole range of thermodynamic parameters studied.