Lewis acid-catalyzed successive skeletal rearrangement of cyclobutene-fused diphenylhomoquinone

The tricyclic enedione, which was synthesized in the [2+2] photocycloaddition of homoquinone with alkyne, underwent a novel Lewis acid-mediated skeletal rearrangement to provide bi-, tri- and tetracyclic cage compounds, depending on the identity of the Lewis acid used. The prolonged reaction and the independent treatment of the intermediary products by other Lewis acids revealed that the reaction proceeds through successive, multi-step transformations involving a cyclobutene ring-cleavage, an intramolecular Friedel–Crafts addition, and a cyclopropane ring-opening as well as an intramolecular cyclization. The product distributions were governed by the identity of the Lewis acid and the coordinated carbonyl site.