One-pot synthesis of benzo[b]fluorenones via a cobalt-catalyzed MHP-directed [3+2] annulation/ring-opening/dehydration sequence.

We have developed a one-pot synthesis of benzo[b]fluorenones via a cobalt-catalyzed [3+2] annulation of oxabicyclic alkenes followed by a ring-opening/dehydration sequence in good to excellent yields. With the use of 2-(1-methylhydrazinyl)pyridine (MHP), first explored by our group as the bidentate directing group in C-H functionalization of benzoic hydrazides, the new reaction sequence proceeded smoothly and tolerated a wide range of functional groups, featuring facile scalability. Notably, oxygen served as the green oxidant in the reaction system. Careful tuning of the reaction parameters assured the direct synthesis of benzo[b]fluorenones through aromatization of the dihydroepoxybenzofluorenone intermediate.