Reaction of Superoxide with Aci-Reductones

Three reductones, 2,3-dihydroxy-4,4-diphenyl-2,5-cyclohexadien-1-one (11), 3,4-dihydroxycoumarin (35), and 3,4-dihydroxyspiro[5.5]undecan-3-en-4-one (64), were prepared and subsequently reacted with superoxide anion radical (O₂^●-), generated from KO₂ and 18-crown-6 polyether. The reactions were carried out in aprotic media and quenched with methyl iodide which facilitates the trapping of the various oxyanions formed. While a plethora of products were formed in each case [2-hydroxy-2-methyl-4,4-diphenyl-5-cyclohexene-1,3-dione (17), dimethyl 4,4-diphenylglutaconate (18), methyl 4,4-diphenyl-3-butenoate (19), phenylcinnamaldehyde (20), methyl 3-phenylcinnamate (21), and benzophenone (22) from 11; 3-hydroxy-2-methoxycoumarin (39), 2-carbomethoxy-2-hydroxy-3-coumaranone (40), 2-hydroxy-2-methyl-3-coumaranone (41), methyl o-hydroxyphenylglyoxylate (42), methyl salicylate (43), and catechol (44) from 35; and 2,4-dihydroxyspiro[5.5]undeca-1,4-dien-3- one (66), 2-hydroxyspiro[4.5]dec-1-en-3-one (70), dimethyl 1,1-cyclohexanediacetate (73), and dimethyl α-keto-1-[(methoxycarbonyl)methyl]cyclohexane-1-propionate) (75) from 64], an overall analysis of the product distribution indicates that the basic elements of the reaction sequence are the same. The first step involves facile deprotonation and the concomitant generation of the reductone monoanion, a process which lends support to the suggestion of Afanas’ev and co-workers (Afanas’ev, I. B.; Grabovetskii, V. V.; Kuprianova, N. S. J. Chem. Soc. Perkin Trans. 2 1987, 281-285). Oxidation of this monoanion yields the corresponding triketone. Of the various options available to this polyketone, superoxide attack at the most electrophilic central carbonyl followed by oxidative cleavage and/or benzylic acid rearrangement are clearly the most prominent. These are followed by a variety of base catalyzed autoxidative processes which are highly dependent on the nature of the substrate.