Two new anthraquinone photoreactions

Two new photochemical reactions of 1-allyloxy-9,10-anthraquinones have been discovered. Both reactions involve initial photoinduced hydrogen abstraction and subsequent intramolecular electron transfers. The reactions were discovered as part of a project investigating the utility of intramolecular abstraction/SET processes for the release of biologically active aldehydes. The expected photochemistry for these compounds has been used to prepare trans-4-hydroxy-2-nonenal in 91% yield. However, under certain conditions, a number of unexpected reaction pathways became important. The first reaction included a remarkable CC bond formation; mechanistic possibilities are suggested. The second reaction involved a 6-endo cyclization of a photochemically generated phenolic radical on a tethered alkene, followed by an unprecedented electron transfer from a secondary carbon-centered radical to a semiquinone radical. The resulting carbocation was then trapped by solvent. In one molecule, both reactions were observed, along with a third photoinduced Claisen rearrangment.