Synthetic studies towards bruceantin. Part 2. The synthesis of a pentacyclic intermediate

In a synthetic approach to the quassinoid bruceantin (2), the key intermediate 8 obtained via alkylation of a dianion has been transformed into the pentacyclic intermediate 33 via an ABDC ring forming strategy. The key steps involved in this route are as follows: a unique acid catalyzed cyclization, 19 → 20; an intramolecular Michael reaction, 24 → 28; and an allyl sulfoxide [2,3]-sigmatropic rearrangement to introduce the axial C12 alcohol, 31 → 33. Key words: bruceantin, quassinoids, cyclization, sulfoxides, sigmatropic rearrangement.