A ring-closing metathesis route to 7-membered aza-heteroannulated sugars

Azepane rings have been constructed diastereoselectively upon a carbohydrate derivative utilising reductive amination and RCM. The stereochemistry of the ring junctions was confirmed by X-ray crystallography and NMR. Diastereoselective dihydroxylation has also been employed to afford a tetrahydroxylated azepane carbohydrate derivative with potential biological activity.     

Multiparameter Polynomial Adaptive Stabilizers

In 1983 Morse proved, for unknown scalar one-dimensional linearsystems, the nonexistence of rational or polynomial universalstabilizers (UAS). In 1983, Nussbaum gave an example of an analyticUAS. In our paper, it is shown that there exist time-invariantpolynomial UAS's with multidimensional gain adaptation. Thedesign procedure is developed for linear, minimum-phase systemsof relative degree one. Convergence of the closed-loop systemis proved. Some numerical simulations are provided.     

The effect of metal oxides on the oxidation of polybutadiene

Carboxy-terminated polybutadiene (CTPB) was oxidised as a coating on various metal oxides by heating in air on a thermobalance. The oxidation was found to be free from diffusion control and was catalysed by certain of the oxides. The catalysis was shown to be independent of semiconductor type and to be associated with those oxides for which the width of the forbidden zone between the valence and conduction bands (U) is less than 1.9 eV. In general, low values of U result in high catalytic activity. It is proposed that catalysis occurs by a redox mechanism, viz. RO₂H + p → RO₂^. + H⁺, RO₂H + e → RO^. + OH^?.     

The thermal degradation and oxidation of polybutadiene

Detailed studies have been made of the successive stages in the thermal degradation in air and nitrogen of carboxy-terminated polybutadiene (CTPB). During oxidation at high temperatures, a protective surface film is first formed; this film ruptures at temperatures where pyrolysis leads to the formation of volatile products in the bulk of the polymer. Thermogravimetric curves for the degradation of CTPB in nitrogen are complex in shape; it appears that the free-radical crosslinking and cyclisation reactions cause an increase in the thermal stability of the polymer during degradation.     

Phosphoryl and Thiophosphoryl Transfer Reactions: Stereochemical Imperatives for Metaphosphate and Thiometaphosphate

Abstract

The stereochemical courses of phosphoryl transfer reactions in aprotic solvents and thiophosphoryl transfer reactions in protic solvent have been determined. The extensive racemisation observed in both instances is discussed in terms of metaphosphate and thiometaphosphate intermediates of significant life-times.     

Asymmetric synthesis of pedamide using I_2-induced heterocyclization to construct the skeleton

An alternative approach to synthesize pedamide,a key building block of pederin was described.Iodine-induced asymmetric heterocyclization was used as the key step to construct the skeleton,a tetrahydropyran ring with three chiral centers.Brown's asymmetric allylation and Lewis acid-mediated allylation were investigated to introduce chains and chiral alcohols.Sharpless dihydroxylation decorated the side chain.And high optically pure target was obtained by removing the epimers formed in these reactions on c...     

Photostabilisation of wood using aromatic vinyl esters

Thin wood veneers were esterified to different weight gains with vinyl benzoate, vinyl cinnamate or vinyl-4-T-butylbenzoate to graft aromatic groups to wood's molecular components. We hypothesised that such modification would increase the resistance of wood to photodegradation. There was a linear relationship between the level of esterification of wood with vinyl benzoate and the photostability of the modified veneers exposed to natural weathering. Vinyl benzoate protected lignin and cellulose in wood from photodegradation at high weight gains (>30%) whereas modification of wood with vinyl-4-T-butylbenzoate provided no such protection and vinyl cinnamate increased the photodegradation of wood. We provide an explanation for why these effects occurred and discuss the implications of our findings for the development of weather-resistant wood materials.     

Microwave-assisted one-pot synthesis of 3-substituted-3,4-dihydrocoumarins via tendem Konevenagel and Hantzsch reactions

A convenient route for the synthesis of 3,4-dihydrocoumarin derivates from salicylaldehyde derivates and 1,3-dicarbonyl compounds under solvent-free microwave irradiation conditions was described. In this way, a range of compounds was obtained in moderate to good yields in a short reaction time.     

As-rich reconstruction stability observed by high temperature scanning tunnelling microscopy

Atomic resolution scanning tunnelling microscopy (STM) has been used to study in-situ the As-terminated reconstructions formed on GaAs(0 0 1) surfaces in the presence of an As₄ flux. The reconstructions c(4×4), (2×4) and (3×1) are long established for GaAs(0 0 1) between 400 and 600 °C for varying Ga and As flux, however the stoichiometry of incommensurate transient reconstructions is still uncertain. By performing high temperature STM on an initial (2×4) surface between 250 and 450 °C in the absence of an As flux, small domains with varying reconstruction are observed in a similar manner to the InAs/GaAs(0 0 1) wetting layer. The local storage of excess Ga in Ga-rich domains could provide insight into sub ML homo- and hetero-epitaxial growth.     

Self-injection length in La0.7Ca0.3MnO3-YBa2Cu3O7-δ ferromagnet-superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La_0.7Ca_0.3MnO₃ (LCMO) and YBa₂Cu₃O_7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO₃ (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 μm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I _c, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J _c of ∼ 2 × 10⁶ A/cm², the LCMO side showed about half the value for the same thickness (1800 ?). The difference in J _c indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of J _c at two different thicknesses (1800 ? and 1500 ?) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 ?. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO₃ deposited by PLD on YBCO did not show any evidence of self-injection.