Synthesis of the Benzo[b]fluorene Core of the Kinamycins by Arylalkyne-Allene and Arylalkyne-Alkyne Cycloadditions

Arylalkyne-allene and arylalkyne-alkyne cycloadditions yields benzo[a]fluorenones, which are related to the tetracyclic core of the kinamycins. In the arylalkyne-alkyne cycloadditions, we found a rearrangement that produces benzo[a]fluorenones, in addition to the expected benzo[b]fluorenones. This rearrangement could be suppressed in the presence of phenol, which allowed the synthesis of 4,9-dimethoxy-2-methyl-11H-benzo[b]fluoren-11-one in excellent yield.