A New Co2(CO)8-Mediated Tandem [5 + 1]/[2 + 2 + 1]-Cycloaddition Reaction: A One-Pot Synthesis of Tricyclic δ-Lactones from cis-Epoxy Ene-ynes

Abstract
In the presence of Co2(CO)8 and CO, cis-epoxyalkynes bearing a tether olefin undergo a tandem [5 + 1]/[2 + 2 + 1]-cycloaddition to give tricyclic δ-lactones efficiently in a one-pot operation. The reaction mechanism is proposed to involve a cobalt-coordinated cyclic allene species.