Singlet Oxygen and Electron Transfer Induced Oxygenation Pathways in Reactions Photosensitized by Dicyanoanthracene and Hydroxy-Anthraquinones

Abstract

Some time ago, Schenck and Gollnick (1958) observed that ascaridole formation by hydroxy-anthraquinone-photosensitized oxygenation of α - terpinene, now recognized as a (4+2)-cycloaddition of singlet oxygen to the cyclic 1,3-diene system, was dependent on the number as well as on the positions of the OH groups in the anthraquinone moiety. Since naturally occurring quinones carrying hydroxy groups such as hypericin (for a recent review, see Duran and Song, 1986), cercosporin and dohistromin (Youngman and Elstner, 1984) are of importance in many biochemical processes, for example in photodynamic actions, it appears to be interesting to study the capability of model compounds such as anthraquinone and a series of its hydroxy derivatives as sensitizers of Type II (singlet oxygen) and Type I ((1) H-atom induced, and (2) electron transfer induced) photooxygenation reactions. Since the methods available for distinguishing between singlet oxygen and electron transfer induced oxygenation reactions have their severe drawbacks (Davidson et al., 1987), our approach is to apply simple, chemically well-defined systems which may be able to give a clear-cut answer with regard to the mechanisms involved. These results may be useful to photobiologists for their studies on phtooxygenations that proceed in the rather complicated biological systems. Although our only recently commenced studies on Type I and Type II photooxygenations sensitized by hydroxy-anthraquinones are far from being complete, first conclusions may be drawn from the following results.