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

Abstract

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.

Pd-Catalyzed Intramolecular Dearomative [4 + 2] Cycloaddition of Naphthalenes with Arylalkynes

“…Typically, the use of olefins as starting materials enables numerous variations of [ n + 2] cycloaddition reactions . Recently, dearomative cycloaddition reaction employing planar aromatic molecules as nonclassic CC π-systems has been proved to be an efficient and straightforward avenue toward complex 3D-polyalicyclic compounds by breaking the aromaticity. Significant progress has been achieved in the transition-metal-catalyzed dearomatizing cycloaddition reactions, while most of the documented reports employ five-membered-ring heteroarenes with relatively lower resonance energies as substrates, such as indoles, benzofurans, and pyrroles .…”