Mechanism of Novel Consecutive Rearrangements of Cyclobutene-Fused Diphenylhomobenzoquinones Catalyzed by Lewis Acids

Lewis acid catalyzed rearrangements of highly strained [2 + 2] photoadducts 1ad of diphenylhomobenzoquinone with various acetylenes were investigated under the influence of AlCl3, SnCl4, BF3, and TiCl4. With the relief of steric strain, these tricyclo[5.2.0.03,5]non-8-ene-2,6-diones underwent the three steps of consecutive skeletal transformations. The first step was the two-way cyclobutene ring-cleavage reaction with a Wagner−Meerwein vinyl migration to either Lewis acid activated carbonyl function. This process virtually occurred under the anchimeric assistance of the endo-phenyl ring to give, after proton transfer, the phenylene-bridged tetracyclic keto alcohols 2 and 3, respectively. The next step was the acid-induced cyclopropane ring cleavage of only 3 to lead to bicyclic diones 4 via a following stereoselective proton transfer. The last one involved a Michael-type intramolecular cyclization of 4 accompanied by a proton transfer to afford thermodynamically less stable tricyclic diones 5α which epimerized to 5β only by TiCl4. The factors that control the selectivity and the reactivity of these tandem reactions were addressed on the basis of the X-ray crystal analyses as well as the PM3 calculations. It was found the present Lewis acid-catalyzed rearrangements were very dependent on the substituents of 1ad and the nature of the Lewis acids.