Intramolecular Cobalt-Mediated Cycloaddition of Linear Enediynes. A Useful Synthetic Entry into Cobalt-Protected Tricyclic Dienes and Their Synthetic Elaboration

Article abstract of DOI:10.1021/jo00183a017

CpCo(CO)2 undergoes reaction with the linear α,δ,ω-enediynes 4, 10, 18, 22 and 25 to give the CpCo-complexed tricyclic dienes 5, 11, 19, 20, 23, 24, 26, and possibly 27.The free ligands may be obtained in good yield by oxidative demetalation.Treatment of the dienylsilane 12 with bromine gave the desilylated aromatic 13, whereas reaction with m-chloroperbenzoic acid furnished the dienol 14 and α-trimethylsilyl β,γ-enone 15.The latter rearranged to the desilylated α,β-enone 17 with acid.Some mechanistic discussion is presented concerned with the course of the cobalt-mediated cyclization reaction.Hydride abstraction from 11 resulted in the cation 28 which underwent unexpected nucleophilic addition to both ligands, in addition to deprotonation to benzene complex 32 and the free aromatic ligand 31.