Cobalt-Mediated [2+2+2] Cycloadditions of Alkynes to Benzo[b]furans and Benzo[b]thiophenes: A Potential Route toward Morphanoids

Exploratory studies of the CpCo-mediated [2+2+2] cycloaddition of alkynes to the 2,3-double bond of benzo[b]furans (and some benzo[b]thiophenes) are presented, with the general aim to access morphinan substructures. The basic feasibility of constructing Co-complexed tetrahydrophenanthro[4,5-bcd]furans (and -thiophenes) in moderate to good yields is demonstrated, with complete to extensive diastereoselectivity. Limitations are the apparent necessity for bulky (silylated) monoalkynes, the lack of regioselectivity in the cocyclization with unsymmetrical alkynes, and the sensitivity of the ligands, both complexed and uncomplexed, with respect to ring opening and rearrangement.