Regioselective oxidation of hydroxyl groups of sugar and its derivatives using silver catalysts mediated by TEMPO and peroxodisulfate in water

Primary hydroxyl groups were oxidized regioselectively to carboxylic acid u sing organic nitrosonium salts generated on supported silver catalysts, whi ch promote disproportionation of 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMP O) in aqueous solution. The oxidation reactions were performed at pH 9.5 in a batch reactor at room temperature using heterogeneous silver catalysts a nd peroxides as primary co-oxidants; e.g., 99 mol% selectivity to methyl-al pha-D-glucopyrasiduronic acid was obtained at 90% conversion of the pyranos ide using a silver carbonate-celite catalyst. The efficiency of the system was increased by adding carbonates to the silver catalyst. This result is e xplained by the increase of the electron charge deficiency on silver in the presence of carbonate, which accelerates the nucleophilic attack of hydrox yls and/or TEMPO. In the case of the Ag-Al2O3 catalyst, this result was pro ved by temperature-programmed desorption measurements using ammonia. With p rimary/secondary polyols, the selectivity for the primary hydroxyl groups i s high. In addition, primary hydroxyl groups, as in the case of pyranosides , were oxidized more selectively than those of the furanosides. The observe d regioselectivity is due to the sterical hindrance caused by the four meth yl groups in TEMPO. (C) 2000 Academic Press.