Evidence for cyclodextrin dioxiranes

α-Cyclodextrin, β-cyclodextrin, 2,6-di-O-methyl-β-cyclodextrin, methyl-β-cyclodextrin and sucrose have been oxidised by aqueous bromine solution at neutral pH. Both ketone and carboxylic acid containing materials are among the products of the oxidations. For α-cyclodextrin there is clear 13C NMR evidence for the presence of a ketone group and its hydrate form. This together with the continued ability of the product to complex p-nitrophenol indicates that the ketone is present at the secondary rim of an intact cyclodextrin ring. A pH dependence for the reaction of bromine with cyclodextrin shows that the maximum rate of bromine loss roughly coincides with the maximum concentration of hypobromous acid, HOBr, indicating that this is the reactive species in these oxidations. The results are consistent with a mechanism involving attack by one of the secondary hydroxyls of cyclodextrin on HOBr, with Br - leaving to yield an intermediate dehydroxy hydroperoxy cyclodextrin that subsequently decomposes to a keto-cyclodextrin via a Kornblum-De La Mare-type reaction. An alternative pathway prevails when the reaction is carried out under alkaline conditions, where carboxylic acids are the principle products. The keto derivatives produced by bromine oxidation at neutral pH are capable of catalysing the oxidation of p-nitrophenol and aryl-alkyl sulfoxides by peroxomono-sulfate in an analogous way to cyclohexanone, which is known to form a dioxirane upon reaction with peroxomonosulfate. It is likely that dioxirane formation is responsible for the observed catalysis in the present case also.