Mechanisms for hydroperoxide degradation of disaccharides and related compounds

Abstract

The reactions of disaccharides with alkaline hydrogen peroxide were studied under diverse conditions. Treatment of cellobiose, lactose, and maltose with aqueous sodium peroxide afforded, in each instance, the corresponding aldobionic acid, the next lower aldobionic acid 2- O- d-glucopyranosyl- d-erythronic acid, and formic acid. On the other hand, melibiose and gentiobiose afforded the corresponding aldobionic acid, the next lower aldobionic acid, and a 2- O- d-glycopyranosylglycolic acid. The yields of products varied widely with the experimental conditions, especially with the proportions of alkali peroxide and hydrogen peroxide. The reactions with alkali peroxide were slow, but rapid in the presence of hydrogen peroxide with the gradual addition of alkali. The results indicate that degradation of carbohydrates by alkaline hydrogen peroxide takes place by five reaction paths. These are designated the alpha-hydroxy hydroperoxide cleavage-mechanism, the Baeyer-Villiger mechanism, the ester mechanism, the dihydroxy-epoxide mechanism, and a newly proposed peroxy-radical mechanism. The last-named mechanism is more rapid than the others. With an excess of hydrogen peroxide and slow addition of alkali, it results in rapid, stepwise conversion of both reducing and nonreducing saccharides into formic acid. The process begins with formation of a hydroperoxide adduct of the carbohydrate. Reaction of the adduct with hydrogen peroxide affords a peroxy radical and a hydroxyl radical. The peroxy radical decomposes, affording formic acid, the next lower aldose and hydroxyl radical. Hydroxyl radical produced in a chain reaction oxidizes alditols and aldonic acids by reactions analogous to those of the Fenton reagent.