Nitroxide supported on nanometric metal oxides as new hybrid catalysts for selective sugar oxidation.

A new series of supported organocatalysts, prepared by a simple method, were used for selective sugar oxidation. This approach is based on the immobilization of a nitroxide derivative through a carboxylic function on nanometric metal oxides (TiO2, Al2O3 and CeO2), allowing the recovery of the catalyst. These hybrid materials were carefully characterized by Diffuse Reflectance FT-IR spectroscopy (DRIFT), ThermoGravimetric Analysis (TGA), X-Ray Diffraction (XRD), Brunauer-Emmet-Teller surface area measurements (B.E.T.), elemental and electrochemical analyses, showing different characteristics and behaviors depending on the nature of the metal oxide used. The activity of the supported nitroxide catalyst was evaluated on methyl α-d-glucoside oxidation, used as model reaction. In all cases, high catalytic activity was highlighted, with up to 25 times less nitroxyl radical required for complete conversion than under homogeneous conditions. The influence of several experimental conditions such as the use of phosphate buffer and recyclability of the catalyst were also investigated.

Influence of chemically modified alginate esters on the preparation of microparticles by transacylation with protein in W/O emulsions.

Microencapsulation using the transacylation reaction in a W/O emulsion is based on the creation of amide bonds between the protein's amine functions and the ester groups of a polysaccharide in the aqueous phase after alkalization. Commercial propylene glycol alginate (PGA) has been the only modified polysaccharide involved in the process up to now. In the present work, we describe the effect of substituting the commercial PGA by other chemically modified alginates in the formation of microparticles. Alkyl and hydroxyalkyl alginate esters, were synthesized and tested in the encapsulation process with human serum albumin (HSA). It was found that the hydroxyalkyl alginates were suitable polysaccharide substitutes for PGA in the transacylation reaction, whereas the alkyl alginates did not lead to microparticle formation in the same process. Hydroxyalkyl alginates with high esterification degree (DE) (>50) led to microparticles when involved in the preparation procedure. However with lower DE (<30), no microparticles could be obtained from 2% ester solution concentrations. This difference in reactivity was explained by the formation of hydrophobic microdomains with the alkyl esters that hindered the transacylation reaction, as opposed to hydroxyalkyl esters that bore hydrophilic ester groups.

Catalytic Synthesis of a New Series of Alkyl Uronates and Evaluation of Their Physicochemical Properties.

Large quantities (>3 g) of a new series of alkyl uronates were synthesized in two steps from commercial methyl hexopyranosides. Firstly, several tens of grams of free methyl α-d-glucopyranoside were selectively and quantitatively oxidized into corresponding sodium uronate using 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO)-catalyzed oxidation. Hydrophobic chains of different length were then introduced by acid-mediated esterification with fatty alcohols (ethyl to lauryl alcohol) leading to the desired alkyl glucuronates with moderate to good yields (49%-72%). The methodology was successfully applied to methyl α-d-mannopyranoside and methyl ß-d-galactopyranoside. Physicochemical properties, such as critical micelle concentration (CMC), equilibrium surface tension at CMC (γcmc), solubility, and Krafft temperature were measured, and the effect of structural modifications on surface active properties and micelle formation was discussed.