Colouring matters of the aphididae. Part XXX. Coupling reactions involving the quinone A derived by reduction of protoaphin-fb

Abstract

When warmed in aqueous borate or shaken in air with platinum, the quinone A, obtained by reduction of protoaphin-fb, undergoes oxidative coupling to form a dimeric quinone analogous to skyrin. Alternatively, under anaerobic conditions, it is converted into the blue extended quinone, xylaphin, a derivative of peri-xanthenoxanthene. The latter process involves, stoicheiometrically, the loss of two molecules of water. These reactions are closely related to one another and are thought to involve addition to quinone A of an electron donor, e.g., the corresponding quinol. The ease with which they proceed constrasts with the lack of reactivity of similarly substituted anthraquinones under the same conditions. This is associated, at least in part, with the lower oxidation–reduction potentials of anthraquinones relative to naphthaquinones.