Synthesis of the ABC Substructure of Brevenal by Sequential <i>exo</i>‑Mode Oxacyclizations of Acyclic Polyene Precursors

Exploratory studies on the sequential exo-mode oxacyclizations of acyclic polyene precursors have provided a substantial substructure of brevenal, including the fused tricyclic polyether with stereochemical patterns consistent with the AB and BC ring fusions. The synthesis of acyclic substrates featured two variations of Cr­(II)/Ni­(II) couplings for preparing 1,1-disubstituted allylic alcohols. A sequence of iodine-promoted cycloetherification, base-promoted intramolecular conjugate addition, and mercury-promoted cycloetherification produced the tricyclic substructure.