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

Abstract

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.