Evidence for cyclodextrin dioxiranes. Part 2. Catalytic and enantioselective properties of cyclodextrin dioxiranes formed from keto-derivatised hydroxypropyl–cyclodextrins

Following our recent study of the bromine oxidation, at neutral pH, of α-cyclodextrin, β-cyclodextrin, O-methylated β-cyclodextrins and sucrose, which yield ketone and carboxylic acid-containing materials in the oxidation products (M.E. Deary, D.M. Davies, Carbohydr. Res., 309 (1998) 17), we have extended the work to hydroxypropyl-α-cyclodextrin and hydroxypropyl-β-cyclodextrin. 13C NMR analysis confirms the presence of ketone groups (δ 207) in the oxidation products of both of these compounds. The continued ability of the products of these oxidations to complex p-nitrophenol demonstrates that ring integrity is maintained. The ketone-containing products are capable of catalysing the peroxomonosulfate (PMS) oxidation of a range of substrates including aryl alkyl sulfoxides, pyridine, 4-bromopyridine, aniline, 4-aminobenzoate, 4-bromoaniline and several amino acids, most probably by the formation of a more reactive cyclodextrin–dioxirane intermediate. A small degree of enantioselectivity is observed in the oxidation of (R)-(+)- and (S)-(−)-methyl p-tolyl sulfoxide by PMS in the presence of the keto derivative of hydroxypropyl-α-cyclodextrin, though not for the β analogue.