Preparation and reductive lithiation of 2-deoxy-D-glucopyranosyl phenylsulfones: A highly stereoselective route to C-glycosides

2-Deoxy-D-glucopyranosyl phenylsulfones 3a–c have been synthesized in 75–83% yield from commercial tri-O-acetyl-D-glucal 1. Their reductive desulfonylation by lithium naphthalenide and reaction of the intermediate glycosyl-lithium 5 with aldehydes leads to α-D-C-glucopyranosides with high stereoselectivity.     

Stabilization of flows through porous media

In this article, we study the motion of an incompressible homogeneous Newtonian fluid in a rigid porous medium of infinite extent. The fluid is bounded below by a fixed layer having an external source (with an injection rate b), and above by a free surface moving under the influence of gravity. The flow is governed by Darcy’s law. If b(c) = 0 for some c > 0 then the system admits (u, f) ≡ (c, c) as an equilibrium solution. We shall prove that the stability properties of this equilibrium are determined by the slope of b in c : The equilibrium is unstable if b′(c) < 0, whereas b′(c) > 0 implies exponential stability. Zhaoyong Feng: He is grateful to the DFG for financial support through the Graduiertenkolleg 615 “Interaction of Modeling, Computation Methods and Software Concepts for Scientific-Technological Problems”.     

Global existence for fully parabolic boundary value problems

We present some results on the global existence of classical solutions for quasilinear parabolic equations with nonlinear dynamic boundary conditions in bounded domains with a smooth boundary.     

Prediction of rheological behavior of MWCNTs–SiO2/EG–water non-Newtonian hybrid nanofluid by designing new correlations and optimal artificial neural networks

In this paper, at the first, new correlations were proposed to predict the rheological behavior of MWCNTs–SiO₂/EG–water non-Newtonian hybrid nanofluid using different sets of experimental data for the viscosity, consistency and power law indices. Then, based on minimum prediction errors, two optimal artificial neural network models (ANNs) were considered to forecast the rheological behavior of the non-Newtonian hybrid nanofluid. One hundred and ninety-eight experimental data were employed for predicting viscosity (Model I). Two sets of forty-two experimental data also were considered to predict the consistency and power law indices (Model II). The data sets were divided to training and test sets which contained respectively 80 and 20% of data points. Comparisons between the correlations and ANN models showed that ANN models were much more accurate than proposed correlations. Moreover, it was found that the neural network is a powerful instrument in establishing the relationship between a large numbers of experimental data. Thus, this paper confirmed that the neural network is a reliable method for predicting the rheological behavior of non-Newtonian nanofluids in different models.

     

Application of benzyl protecting groups in the synthesis of prenylated aromatic compounds

Benzyl ethers have proven to be useful protecting groups for synthesis of phenols bearing isoprenoid chains because the benzyl groups can tolerate the conditions of halogen-metal exchange used to introduce the side chains yet are cleaved in good yields upon treatment with sodium s-butanol.     

Synthesis and inhibition properties of conformational probes for the mutase-catalyzed UDP-galactopyranose/furanose interconversion

UDP-galactose mutase is a flavoenzyme that catalyzes the isomerization of UDP-galactopyranose into UDP-galactofuranose, a key step in the biosynthesis of important bacterial oligosaccharides. Several mechanisms for this unique ring-contraction have been proposed, one of them involving a putative 1,4-anhydrogalactopyranose as an intermediate in the reaction. The purpose of this study was to probe the mutase binding site with conformationally restricted analogues of its substrate. Thus, we describe the straightforward synthesis of two C-glycosidic UDP-galactose derivatives: analogue 1, presenting a galactose moiety locked in a bicyclic (1,4)B boat conformation, and UDP-C-Galf 2, where the galactose residue is locked in the conformation of the mutase substrate. The two molecules were found to be inhibitors of UDP-galactose mutase at levels depending on the redox state of the enzyme. Strong inhibition of the native enzyme, but a low one of the reduced mutase, were observed with UDP-C-Galf 2, whereas 1 displayed intermediate inhibition levels under both native and reducing conditions. These data provide evidence of a significant conformational difference of the mutase binding pocket in the reduced enzyme and in the native one, the enzyme switching from a low Galf-affinity state (reduced enzyme) to a very strong one (native enzyme). It is remarkable that the mutase binds the boat-locked analogue 1 with similar affinities in both its conformational states. These results support a mechanism involving the formation of 1,4-anhydrogalactopyranose as a low-energy intermediate. An alternative explanation would be that the distortion of the galactose moiety during the cycle contraction transiently brings the carbohydrate into a conformation close to a (1,4)B boat.     

Solid-phase Synthesis of C-Terminal Peptide Amino Alcohols

The solid-phase syntheses of enkephalin and somatostatin analogues with C-terminal OH functions instead of the normal carboxylates are described. The OH function of the N-terminal amino alcohol was acylated with succinic acid and esterified to the solid support. Normal Boc-TFA solid-phase strategy can be applied to build up and cleave these peptides. The succinic ‘handle’ can be removed by mild basic hydrolysis after cleavage.     

L-o-Carboranylalanine,a Boron Analogue of Phenylalanine

The synthesis of a new, boron-containing analogue of phenylalanine is described. Carboranylalanine (Car) carries a 1,2-carborane cage in place of the benzene ring of phenylalanine. It is obtained by the reaction of derivatives of propargylglycine with decaborane. Possibilities of practical application derive from the facile neutron activation of boron nuclei with mass number 10.     

Synthese des substances de groupe sanguin—IV : Synthese du 2-acetamido-2-desoxy-4-O-[2-O-(α-l-fucopyranosyl)-β-d-galactopyranosyl]-d-glucopyranose,porteur de la specificite H

The synthesis of a H blood group specific trisaccharide was performed by using benzyl ethers as temporary blocking groups for hydroxylic functions. Benzyl 2-acetamido-3,6-di-O-benzyl-2-deoxy-α-d- glucopyranoside was glycosylated by 3,4,6-tri-O-benzyl-1,2-O-benzyl-1,2-O-(tert-butoxyethylidene)-α-d-galactopyranose; after O-deacetylation, glycosylation by 2,3,4-tri-O-benzyl-α-l-fucopyranosyl bromide, and hydrogenolysis, 2-acetamido-2-deoxy-4-O-[2-O-(α-l-fucopyranosyl)-β-d-galactopyranosyl]-d-glucopyranose is obtained.     

A Low-Temperature Broadband NMR Probe for Multinuclear Cross-Polarization

Dissolution dynamic nuclear polarization (D-DNP) probes are usually designed for one or at most two specific nuclei. Investigation of multiple nuclei usually requires manufacturing a number of costly probes. In addition, changing the probe is a time-consuming process since a system that works at low temperature (usually between 1.2 and 4.2 K) must be warmed up, thus increasing the risks of contamination. Here, an efficient apparatus is described for D-DNP designed not only for microwave-enhanced direct observation of a wide range of nuclei S such as ¹H, ¹³C, ²H, ²³Na, and ¹⁷O, but also for cross-polarization (CP) from I=¹H to such S nuclei. Unlike most conventional designs, the tuning and matching circuits are partly immersed in superfluid helium at temperatures down to 1.2 K. Intense radio-frequency (RF) fields with amplitudes on the order of 50 kHz or better can be applied simultaneously to both nuclei I and S using RF amplifiers with powers on the order of 90 and 80 W, respectively, without significant losses of liquid helium. The system can operate at temperatures over a wide range between 1.2 and 300 K.