Divergent synthesis of multi-substituted phenanthrenes via an internal redox reaction/ring expansion sequence

We report an effective synthetic route to multi-substituted phenanthrenes via an internal redox reaction/ring expansion sequence. The interesting feature of the present system is that it allows for the divergent synthesis of the target skeleton depending on the selected Lewis acid catalyst. When benzylidene malonates with a cyclic structure at the ortho-position were treated with BF₃·OEt₂, three sequential processes (internal redox reaction/elimination of the alkoxy group/ring expansion) proceeded to give phenanthrene derivatives in which the alkoxycarbonyl (CO₂R) group and the alkyl (R) group were in close proximity to each other, in good chemical yields. In sharp contrast, treatment with Bi(OTf)₃ exclusively led to the formation of another type of phenanthrene, whose R group was positioned distal to the CO₂R group.