Dicobaltoctacarbonyl-Mediated Synthesis of Tricyclic 5,6-Diydropyran-2-one Derivatives via Tandem Cycloaddition Reaction between cis-Epoxyalkynes, a Tethered Olefin, and Carbon Monoxide

Cobalt carbonyl complex Co2(CO)8 implemented an intramolecular carbonylation of cis-epoxyalkynes to generate Co2(CO)6-stabilized γ-lactonyl allene species. For 1,1,2-trisubstituted epoxyalkynes, this Co2(CO)6-allene species reacted with a tethered olefin to give [2 + 2]-cycloadducts, and with CO and a tethered olefin to produce [2 + 2 + 1]-cycloadducts. These resulting cycloadducts have a 5,6-diydropyran-2-one core fused with a cyclobutane and a cyclopentanone ring, respectively. For 1,2-disubstituted cis-epoxyalkyne and 1,1,2-trisubstituted cis-epoxyalkynes bearing a heteroatom constituent, cyclization of the corresponding epoxyalkyne with a tethered alkene is invariably accompanied by incorporation of CO to produce a [2 + 2 + 1]-cycloadduct, even in the absence of CO. We have prepared various 1,1,2-trisubstituted and 1,2-disubstituted cis-epoxyalkynes to generalize such cycloaddition pathways. Attempt to use an organic promoter to perform these tandem cycloadditions was unsuccessful because of a competing Pauson−Khand reaction. Cyclization of a 1,2-disubstituted epoxyalkyne with a tethered diene was achieved successfully in one case, but the yield was low (25%).