A short stereoselective synthesis of the protected uracil 3′-epi-polyoxin C

A short synthetic approach to the protected uracil 3′-epi-polyoxin C 20 has been developed. The stereoselective [3,3]-sigmatropic rearrangement of the corresponding 7-thiocyanato-α-d-xylo-hept-5-enfuranose 6 was employed as the key step to construct the C-5 stereocentre in 5-isothiocyanato-α-d-gluco-hept-6-enfuranose 8 and the formal synthesis of uracil 3′-epi-polyoxin C has been accomplished for the first time. This synthesis provides a facile method for multigram scale preparation and thus is useful for the research into the polyoxins’ structure-activity relationship and to search for more potent and effective anticandidal agents.     

Sensitivity of Xanthoria parietina to UV-A: role of metabolic modulators

Effects of methyl jasmonate (MeJA), salicylic acid (SA) or 2-aminoindane-2-phosphonic acid (AIP) pre-treatments on the sensitivity of Xanthoria parietina exposed to UV-A were studied. UV decreased chlorophylls and stimulated increase in hydrogen peroxide and superoxide level. Accumulation of soluble phenols and flavonoids increased in response to UV treatment. Metabolic modulators had negligible impact on these UV-induced changes. Within free amino acids, AIP (-UV variant) and SA and MeJA (+UV variants) altered their accumulation. AIP had no effect on the amount of phenylalanine. Benzoic and cinnamic acids were elevated by UV and mainly MeJA influenced their accumulations. Among lichen specific metabolites, vulpinic acid and ergosterol increased while usnic acid and atranorin decreased after exposure to UV; accumulation of parietin was not affected. Applied modulators showed a different effect on these lichen metabolites but biosynthetic pathway-specific trend of alteration was visible. Overall, MeJA showed the most pronounced effect among studied parameters. Accumulation of selected phenolics in response to UV-A seems to be an important feature of Xanthoria tolerance. Present finding in the context of phenolic metabolism in non-vascular plants and with respect to limited data about effect of studied modulators on non-vascular plants are discussed.     

3‐Butyl‐1‐(5‐nitro­benzo­[c][1,2]­thia­zol‐3‐yl)‐3‐phenyl­triazene

The mol­ecule of the title compound, C₁₇H₁₇N₅O₂S, consists of three π systems, viz. two aromatic rings and the triazene moiety, which are mutually deconjugated although coplanar. The n‐butyl chain is roughly perpendicular to the molecular plane, with the terminal methyl­ene and methyl groups disordered between two equally populated positions. The mol­ecules in the crystal associate in an antiparallel fashion, forming dimers across the centre of symmetry, the principal intradimer interaction being stacking of the π‐electron portions of the mol­ecules.     

Study of D_s~±→μ~±V_μ at Belle

We present a preliminary measurement of the B(D_s→μν_μ)with the Belle experiment at the KEKB collider.We select D~s→μV_μdecays with a method that provides a high-purity of the selected sample and an absolute measurement of the branching fraction.The results are based on a data sample of 550 fb~(-1)and are compared to similar measurements by other experiments as well as to the predictions of LQCD.We conclude with short prospects for improvements in the accuracy of the measurement.     

Preparation and structural properties of MgO films grown on GaAs substrate

Epitaxial MgO thin films have been grown on semiinsulating GaAs (0 0 1) substrates using electron beam (e-beam) evaporation. X-ray diffraction indicates c-axis oriented MgO with (0 0 2) reflection only and rocking curve widths ∼2.2-3°. Transmission electron microscopy (TEM) analyses confirm an epitaxial growth of the MgO films. We study the microstructure and the defects at the interface between the MgO film and the GaAs substrate. Auger electron Spectroscopy (AES) concentration depth profiles reveal no contamination of the MgO films by As and Ga at different temperatures of the deposition process.     

A free boratriptycene-type Lewis superacid

Bicyclic pyrazabole-bridged ferrocenes with BH groups at their bridgehead positions were prepared from [Li(thf)]₂[1,1′-fc(BH₃)₂] and pyrazole or 3,5-dimethylpyrazole in the presence of Me₃SiCl (1 or 1Me, respectively; 1,1′-fc = 1,1′-ferrocenylene); Me₃SiH and H₂ are released as byproducts. Treatment of 1 or 1Me with 1 eq. of the hydride scavenger [Ph₃C][B(C₆F₅)₄] afforded the borenium salts [2][B(C₆F₅)₄] (72%) and [2Me][B(C₆F₅)₄] (77%). According to X-ray crystallography, [2Me]⁺ contains one trigonal-planar borenium cation, the cyclopentadienyl (Cp) rings of the 1,1′-fc fragment remain parallel to each other, but the Cp–B bond vector is bent out of the Cp plane by an unprecedentedly large dip angle α* of 40.6°. The Fe⋯B(sp²) distance is very short (2.365(4) Å) and the ¹¹B NMR signal of the cationic B(sp²) center is remarkably upfield shifted (23.4 ppm), suggesting a direct Fe(3d) → B(2p) donor–acceptor interaction. Although this interpretation is confirmed by quantum-chemical calculations, the coupling between the associated orbitals corresponds to an energy of only 12 kJ mol⁻¹. Accordingly, both the experimental (e.g., Gutmann–Beckett acceptor number AN = 111) and theoretical assessment (e.g., Et₃PO and F⁻-ion affinities) of the Lewis acidity proves that [2]⁺ is among the strongest boron-based Lewis acids available to date.

An exceptionally strong ferrocene-containing, cationic boratriptycene-type Lewis acid is stabilized by a weak Fe⋯B through-space interaction.     

Cooperative motion in amorphous polymers; application to the study of free-radical migration in solids

A Monte Carlo method has been used for studying the effect of the motion of some submolecular structures on the migration of radical centres and on the free-radical decay and its dependence on density. Motions of crank, crankshaft, kink, and double kink type are considered. A cooperative type of motions is also taken into account. The results show that cooperative motions support diffusion of radical centres and thus also the free-radical decay but, at higher densities, the cooperation of motions is restricted. The density of the system, where the decay of radicals is followed, affects the rate of the decay, its increase causes radical decay retardation. This is in line with the high pressure effect, which also retards the radical decay rate.     

New materials with high π conjugation by reaction of 2-oxazoline containing phenols with polyamidines and inorganic base

The non-covalent interactions between 2-oxazoline containing phenols and an aliphatic polyamidines as well as an inorganic base were studied. The reaction of a weak acid with a strong base results in the formation of the deprotonated species and subsequently in the formation of a new electronic structure. A bathochromic shift of the wavelength of the absorption maxima of the chromophores bounded to polyamidine was observed. Depending on the structure of the chromophore, the shift of the absorption maxima is 40-100 nm. The changes in photochemical behavior can be explained by the higher portion of quinoid structures in the conjugated π-system. The degree of deprotonation is dependent on the molar ratio of the chromophore and the polymeric base. Analogous results were obtained with an inorganic base.     

Correlation between glass transition temperature and chain structure for randomly crosslinked high polymers

The empirical form for the dependence, T_g(n) ≅ T_g(∞)·(1 + α/n), of the glass transition temperature T_g on the average number n of repeat units between crosslinks, is generalized for randomly crosslinked high polymers. The new form, T_g(n) ≅ T_g(∞) · [1 + c/(n·N_rot)], is based on a correlation study of data for 77 samples of 10 different sets of resins. The fitting parameter α is resolved into composition-dependent N_rot and composition-independent c terms. N_rot summarizes the average number of rotational degrees of freedom per repeat unit, and is estimated in a straightforward manner from the structure and mol fraction of each repeat unit. The value of c is found from data analysis to be 5 ± 2. The results of this work are consistent with expectations based on the entropy theory of glasses, and provide improved understanding and predictive ability for the properties of crosslinked polymers. © 1996 John Wiley & Sons, Inc.     

Calculation of rotational barriers in amorphous polymers

The Monte Carlo method was used for generation of amorphous polyethylene configurations on a diamond lattice. Chain building was performed on the tetrahedral lattice of edges 36, 62 and 43 Å with periodic boundary conditions imposed.32 chains were generated, each with a length of 100 CH₂-groups (resulting density = 0.81 g·cm⁻³). Small spherical volumes with a radius of 10 Å were chosen at random from the total volume for the calculation of rotational barriers. The rotating bond was chosen to be close to the center of this sphere. We employed the method of molecular mechanics in order to calculate the rotational barriers. The calculation was made for 578 rotating bonds and the obtained distribution of rotational barriers is approximated by the corresponding Γ-distribution.