On the reactions of superoxide with keto enols, aci-reductones and ascorbic acid derivatives

The superoxide-mediated base catalyzed autoxidation of alpha-oxo enols is initiated by the deprotonation of the labile hydroxyl group. Thus, the reaction of O2-. (generated from KO2/crown ether in aprotic media) with 3-hydroxycoumarin (1), followed by a CH3I-workup, generates products 2-4 via a deprotonation-oxidation sequence complicated by a competing saponification of the lactone linkage. The related coumarin reductone (alpha-oxo enediol) 8 is rapidly oxidized by O2-., HO- and t-butoxide to the corresponding triketone, which in turn undergoes further oxidation and rearrangement ultimately yielding (upon methyl iodide workup) products 9-14. When the O2-. mediated oxidation is carried out under argon in completely degassed solutions, large amounts (greater than 20%) of monodeprotonation product (detected as 9) accumulate. These results are discussed in light of the differing mechanisms proposed by Sawyer and Afanas'ev for the interaction of O2-. with the reductone ascorbic acid.