Synthetic Utility of (Propargyl)dicobalt Hexacarbonyl Cations. A New Cyclopentenone Annelation

The regiospecific alkylation of cyclic ketones and TMS enol ethers by the cobalt-stabilized propargyl cations 1 followed by demetalation and regiospecific acetylene hydration proceeds in good overall yield to produce 1,4-diketones. The latter are efficiently cyclized in the presence of base to annelated cyclopentenones.