G. Descotes

Esterification of unprotected sucrose with acid chlorides in aqueous medium: kinetic reactivity versus acyl- or alkyloxycarbonyl-group migrations

Abstract The study of the competition between formation, migration and hydrolysis of sucrose mono-esters and -carbonates confirmed that the pre-eminent reactivity of some secondary positions persists in aqueous medium, and in particular, OH-2, even though fast subsequent migrations finally enrich the mixture in compounds substituted at the primary positions. The reaction was also shown to depend strongly on the starting sucrose concentration providing higher substituted derivatives in the case of hydrophobic acylating agents, in contrast to smaller chains.

Sucrose tricarboxylate by sonocatalysed TEMPO-mediated oxidation.

Oxidation of sucrose by the NaOCl/TEMPO system provided sucrose tricarboxylate without the addition of sodium bromide as co-catalyst when high-frequency (500 kHz) ultrasound was applied, in contrast to very limited conversion without sonication. In the presence of sodium bromide, sonication also caused acceleration of the oxidation. The rate increase due to sonication of the oxidant system prior to sucrose addition suggests that ultrasound acts at the level of the formation of the nitrosonium ion, the active oxidising species in the catalytic cycle.

Synthesis of glycosyl isocyanides

Abstract Substitution of benzylated or methylated glycopyranosyl and furanosyl bromides with silver cyanide gives rise to glycosylisocyanides as main products. The isocyanide structure is ascertained by physical (13C NMR, IR) and by chemical methods (hydrolysis, isomerisation to nitriles).

Radical Anomefhc Chemistry

Abstract The main synthetic reactions o f carbohydrates a t the anomeric center involve ionic processes. commonly cationic processes for glycosylations and anionic processes for C-branching reactions. The use of homolytic or radical reactions has emerged in the last decade for the synthesis of complex molecules. These developments are of great interest for synthetic purposes and for the study of stereoelectronic effects at the anomeric center of carbohydrates. This short review summarizes the principal results of radical transformations at C-1 of aldoses.

Proteinase N-catalysed transesterifications in DMSO–water and DMF–water: preparation of sucrose monomethacrylate

Abstract The selective acylation of sucrose was catalysed by proteinase N (from Bacillus subtilis ) in DMF suspensions, providing 1′-monoesters, and notably, methacryloyl esters. Presence of water up to 7% (vol.) led to faster reactions. Alternatively, a slurry of the enzyme with a minimum volume of DMSO could also be used, despite its ability to denature enzymes.

Synthesis in water of amphiphilic sucrose hydroxyalkyl ethers

Abstract We describe the preparation of amphiphilic hydroxyalkylsucrose ethers from unprotected sucrose in water and we show that among the monosubstituted products, the 2- and 1′- regiosiomers account for 60% of the mixture. The high reactivity of these positions is thus confirmed in water as it is dipolar aprotic solvents. A careful analysis also show evidence of the formation of oligomerisation products as by-products.

Hydrogen peroxide oxidation of palatinose and trehalulose: direct preparation of carboxymethyl α-d-glucopyranoside

Abstract The direct oxidation of palatinose and trehalulose by hydrogen peroxide in acidic medium provided carboxymethyl α- d -glucopyranoside (α-CMG) in moderate to fair yields in a single step. The effect of catalytic sodium tungstate on the reaction was studied. α-CMG is a versatile synthon able to supply a glucosyl moiety through the acylation of alcohols or amines, via the opening of the δ-lactone obtained by acetylation.

Study of the competition between homogeneous and interfacial reactions during the synthesis of surfactant sucrose hydroxyalkyl ethers in water

Abstract The study of the heterogeneous reaction of sucrose with 1,2-epoxydodecane in water showed how the emulsification can change both the reaction rates, the yields and the distribution of the products, namely sucrose monoethers, sucrose diethers, polysubstituted and oligomerisation products. The yield of sucrose ethers can thus be increased to the detriment of hydrolysis and oligomerisation reactions. From the results of the kinetic monitoring, we suggested a model explaining the competition between the different reactions depending on their location in the microheterogeneous medium. The reaction is submitted to both a physicochemical autocatalysis and a chemical autocatalysis. Macroscopic progressions of the mixtures, from emulsion stages to liquid crystals stages, are also described.

Sonocatalysis of the TEMPO-mediated oxidation of glucosides

Abstract Ultrasound is shown to increase the rate of the TEMPO-mediated oxidation of methyl α- d -glucopyranoside or sucrose in the presence of stoichiometric amounts of sodium hypochlorite in basic aqueous medium. The reaction can then occur without the usually necessary sodium bromide, showing that ultrasound acts at the level of the formation of the nitrosonium ion, the active oxidizing species in the catalytic cycle.

Ultrasound in carbohydrate chemistry: sonophysical glucose oligomerisation and sonocatalysed sucrose oxidation.

Both aspects of the interest in using ultrasound are illustrated by our results in the field of carbohydrate chemistry. The course of the heterogeneous reaction of glucose with hydrophobic alcohols in acidic medium is directed towards the oligomerisation of glucose because of the wetting of the glucose suspension due to the efficiency of the sonophysical mixing. On the other hand, a sonocatalysis is observed during the course of the oxidation of primary hydroxyl groups in homogeneous aqueous medium by the NaOCl/TEMPO system.