The γ-carbonyl cations generated from propargyl ether-Co₂(CO)₆ complexes undergo intramolecular Nicholas reactions to give dehydrobenzoxacin-3-one-Co₂(CO)₆ complexes in good yields. Reductive decomplexation and subsequent manipulation allows the synthesis of (±)-heliannuol K methyl ether and the formal syntheses of (±)-heliannuol K, (±)-heliannuol A, and (−)-heliannuol L.