Radical reactions of carbohydrates. Part 6. An electron spin resonance. Study of the reaction of the hydroxyl radical with some lactones derived from sugars and with ascorbic acid

For a series of lactones derived from sugars it is shown that reaction of ˙OH (from the TiIII–H2O2 couple) takes place preferentially at C(2) to give the appropriate carboxy-conjugated radical. Radicals formed by abstraction of C(3)–H in the γ-lactones are shown to react readily both by ring-opening [via fission of the C(β)–O(acyloxy) bond] and by rapid oxidation with H2O2, evidently to give enediols related to ascorbic acid. Oxidation of the latter with ˙OH is suggested to proceed both via direct electron transfer (at pH >5) and, at lower pH, addition to the enolic double bond followed by either acid-or base-catalysed elimination of water.