Total Synthesis of (±)-Baphicacanthcusine A Enabled by Sequential Ring Contractions.

Reported herein is the first total synthesis of the poly-pseudoindoxyl natural product baphicacanthcusine A. The synthesis leverages the oxidative rearrangement of indoles to pseudoindoxyls to install vicinal pseudoindoxyl heterocycles in a diastereoselective manner. Key steps include an acid-mediated cyclization/indole transposition, two diastereoselective oxidative ring contractions, and a site-selective C-H oxygenation. The synthesis of the oxidation precursors was guided by recognition of an element of hidden symmetry. This work provides a foundation for the chemical synthesis of other poly-pseudoindoxyl alkaloids.