Stereocontrolled synthesis of beta-D-mannuronic acid esters: synthesis of an alginate trisaccharide

A facile synthesis route toward beta-linked mannuronic acid oligomers using the corresponding 1-thiomannuronic acid esters in combination with the Ph2SO/Tf2O or NIS/TMSOTf reagent combinations is presented. The presence of the remotely attached carboxylic ester sufficiently influences the electronic environment to allow good to excellent beta-selectivities.     

Energy levels in YPO4:Ce3+,Sm3+ studied by thermally and optically stimulated luminescence

Energy-resolved optically stimulated luminescence (OSL) spectra and thermoluminescence (TL) glow curves of a powder sample of YPO₄:Ce³⁺,Sm³⁺ were measured to investigate the nature of the trapping centre and to locate its energy level relative to the valence and conduction bands of the YPO₄ host. The high-temperature glow peak could unequivocally be assigned to Sm²⁺ (thus Sm³⁺ acts as an electron trap). The trap depth of this centre, as derived from the OSL excitation spectra, is in good agreement with the Dorenbos model prediction. The OSL excitation spectra also reveal excited states of Sm²⁺ well below the conduction band. These excited states produce a broadening of the high-temperature TL glow peak and also cause the activation energy determined by the Hoogenstraten method to underestimate the trap depth.     

Tensor product Gauss-Lobatto points are Fekete points for the cube

Tensor products of Gauss-Lobatto quadrature points are frequently used as collocation points in spectral element methods. Unfortunately, it is not known if Gauss-Lobatto points exist in non-tensor-product domains like the simplex. In this work, we show that the -dimensional tensor-product of Gauss-Lobatto quadrature points are also Fekete points. This suggests a way to generalize spectral methods based on Gauss-Lobatto points to non-tensor-product domains, since Fekete points are known to exist and have been computed in the triangle and tetrahedron. In one dimension this result was proved by Fejér in 1932, but the extension to higher dimensions in non-trivial.

     

On the Lebesgue constant of barycentric rational interpolation at equidistant nodes

Recent results reveal that the family of barycentric rational interpolants introduced by Floater and Hormann is very well-suited for the approximation of functions as well as their derivatives, integrals and primitives. Especially in the case of equidistant interpolation nodes, these infinitely smooth interpolants offer a much better choice than their polynomial analogue. A natural and important question concerns the condition of this rational approximation method. In this paper we extend a recent study of the Lebesgue function and constant associated with Berrut’s rational interpolant at equidistant nodes to the family of Floater–Hormann interpolants, which includes the former as a special case.     

Search for leptoquark pairs decaying into nunu+jets in pp collisions at square root[s] = 1.8 TeV

We present the results of a search for leptoquark (LQ) pairs in (85.2+/-3.7) pb(-1) of pp* collider data collected by the D0 experiment at the Fermilab Tevatron. We observe no evidence for leptoquark production and set a limit on sigma(pp*-->LQLQ-->nunu+jets) as a function of the mass of the leptoquark (m(LQ)). Assuming the decay LQ-->nuq, we exclude scalar leptoquarks for m(LQ) < 98 GeV/c(2), and vector leptoquarks for m(LQ) < 200 GeV/c(2) and coupling which produces the minimum cross section, at a 95% confidence level.     

Photolysis of cellulose and related compounds by 253.7 nm light. II. The role of defects in hydrogen production

Cellobiose and myoinositol, both as received and in physically modified form, were photolyzed at 60.0°C under vacuum with 253.7 nm light. The rates of hydrogen production were compared with the H₂ rate obtained from analogous photolyses of hydrocellulose. The similar physical state and optical properties of cellobiose and hydrocellulose allowed a direct comparison of H₂ production rates, from which it is clear that neither the glycosidic linkage, nor the number of primary or secondary hydroxyl groups, is the basis for observed differences in rates. The relative rates of H₂ production from myoinositol and levoglucosan indicate that strain in the pyranose ring has only a slight effect on the photolysis rate, and further, since published carbohydrate structures show that the local skeletal environment about each hydroxyl in a “defectless” unstrained carbohydrate structure is very similar in all the relevant compounds thus far determined, differences in H₂ rates must therefore arise as a result of possible differences in the hydrogen-bonding environment of the hydroxyl. From an extrapolation of optical properties of simple alcohols in hydrogen-bond-free environments, together with a consideration of possible cage effects during photolysis, it is concluded that the precursors of the H₂ are hydrogen-bond-free hydroxyl groups of the carbohydrates. Confirmation of this hypothesis has been obtained from the H₂ rates observed on photolyzing cellobiose and myoinositol after these compounds had been freeze-dried or recrystallized, which treatment results in a concentration of “defective” hydroxyls differing from the original solid, but is without effect on the overall numbers of “ordinary” hydroxyls.     

Thioglycuronides: synthesis and application in the assembly of acidic oligosaccharides

[reaction: see text] Partially protected thioglycuronic acids are prepared efficiently by chemo- and regioselective oxidation of the corresponding thioglycosides using the TEMPO/BAIB reagent combination. After esterification, the thioglycuronic acids proved to be useful as both donor and acceptor in sulfonium-mediated condensations toward acidic di- and trisaccharides.