Total Synthesis of Dendrochrysanene through a Frame Rearrangement.

The first total synthesis of dendrochrysanene (1) was achieved. The key reaction for the construction of dendrochrysanene was an oxidative frame rearrangement reaction from a phenanthrene dimer to a spiro-lactone skeleton, which we serendipitously identified. Owing to the steric hindrance of the substituent on the peri position of the phenanthrene dimer, high-temperature conditions were required for the rearrangement reaction; however, at such temperatures, the substrate decomposed. To address this issue, we added phenylethylamine or benzylamine to the reaction system. We assumed that the amine trapped generated hydrochloric acid and acted as a ligand for iron, helping to maintain an appropriate redox potential. The total synthesis of dendrochrysanene, involving this rearrangement reaction, is an important sequence interlinking phenanthrene derivatives, phenanthrene dimers, and spiro-lactone compounds, which are frequently isolated from plants of Orchidaceae.