Acid-catalyzed reactions of a strained ring nazarov substrate

The synthesis and acid-catalyzed rearrangements of cross-conjugated cyclobutylidene ketone 1 are described. With strong Bronsted acids it gave a mixture of 5,7-dimethyltetralin 10 and 2-cyclohexenyl-1-methyl-3-phenylbenzene 11, the former by an initial retroaldol reaction and the latter by a series of tautomerizations and electrocyclic reactions following cation induced four-membered ring cleavage. Iodotrimethylsilane converted 1 to 11 in excellent yield. Tin (IV) chloride converted 1 to a kinetically controlled mixture of Nazarov rearrangement products 2, 3 and 4. This mixture was stable under the reaction conditions, but treatment with toluene sulfonic acid converted 2 completely to 3. Even under more severe conditions, however, 4 could not be induced to isomerize.