Monoketal radicals

The voltammetric response of curcumin and carthamin must, in principle, be dominated by the oxidation of the phenol and/or methoxyphenol groups (see Scheme 2.2). The electrochemistry of methoxyphenols has claimed considerable attention because of their applications in organic synthesis [159-163]. As studied by Quideau et al., in aprotic media, 2-methoxyphenols are oxidized in two successive steps into cyclohexadienone derivatives [163], whereas a-(2)- and a-(4-methoxyphenoxy) alkanoic acids undergo electrochemically induced spirolac-tonization to develop synthetically useful orthoquinone bis- and monoketals. In the presence of methanol, the electrochemical pathway involves an initial one-electron loss, followed by proton loss, to form a monoketal radical. This undergoes a subsequent electron and proton loss coupled with the addition of alcohol to form an orthoquinone monoketal. The formal electrode potential for the second electron transfer... 

Cation radical cyclization can be performed (and accelerated significantly) under sonication. Thus, the reaction of methyl vanillate with phenyliodonium bis(acetate) is initiated by ultrasonic irradiation and gives rise to the corresponding oquinone monoketal, which is trapped by a series of furanes. Monoadducts are formed reactions can be conducted at room temperature and are essentially complete within 15-50 min (Avalos et al. 2000). 

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