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

The superoxide-mediated base catalyzed autoxidation of αoxo enols is initiated by the dcprotonation of the labile hydoxyl group. Thus, the reaction of O^-₂ (generated from KO ₂/ crown ether in aprotic media) with 3-hydroxycoumarin (I). followed by a CH,I-workup, generates products 24 via a deprotonation-oxidation sequence complicated by a competing saponification of the lactone linkage. The related coumarin reductone (αoxo enediol) 8 is rapidly oxidized by O^-₂, HO^- and t-butoxide to the corresponding triketone, which in turn undergoes further oxidation and rearrangenlent ultimately yielding (upon methyl iodide workup) products 9-14. Whcn the O^-₂ mediated oxidation is carried out under argon in completely degassed solutions, large amounts (≥ 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 O^-₂ with the reductone ascorbic acid.