EVIDENCE FOR CYCLODEXTRIN DIOXIRANES

alpha-Cyclodextrin, beta-cyclodextrin, 2,6-di-O-methyl-beta-cyclodextr in, methyl-beta-cyclodextrin and sucrose have been oxidised by aqueous bromine solution at neutral pH. Both ketone and carboxylic acid conta ining materials are among the products of the oxidations. For alpha-cy clodextrin there is clear C-13 NMR evidence for the presence of a keto ne group and its hydrate form. This together with the continued abilit y of the product to complex p-nitrophenol indicates that the ketone is present at the secondary rim of an intact cyclodextrin ring. A pH dep endence for the reaction of bromine with cyclodextrin shows that the m aximum rate of bromine loss roughly coincides with the maximum concent ration of hypobromous acid, HOBr, indicating that this is the reactive species in these oxidations. The results are consistent with a mechan ism involving attack by one of the secondary hydroxyls of cyclodextrin on HOBr, with Br- leaving to yield an intermediate dehydroxy hydroper oxy cyclodextrin that subsequently decomposes to a keto-cyclodextrin v ia a Kornblum-De La Mare-type reaction. An alternative pathway prevail s when the reaction is carried out under alkaline conditions, where ca rboxylic acids are the principle products. The keto derivatives produc ed by bromine oxidation at neutral pH are capable of catalysing the ox idation of p-nitrophenol and aryl-alkyl sulfoxides by peroxomono-sulfa te in an analogous way to cyclohexanone, which is known to form a diox irane upon reaction with peroxomonosulfate. It is likely that dioxiran e formation is responsible for the observed catalysis in the present c ase also. (C) 1998 Elsevier Science Ltd. All rights reserved.