Remarkable Formation of Indene Derivatives Upon Friedel–Crafts Acylation Reactions

Alkylbenzene derivatives react with two equivalents of pivaloyl chloride (PivCl) in the presence of aluminum trichloride under formation of indene derivatives with up to 92% yield. Monosubstituted benzenes undergo this cyclopentannulation reaction with quantitative regioselectivity, which was proved by X‐ray single crystal structure analysis. A mechanistic proposal is based on the in situ‐formation of 2,2,5‐trimethyl‐4‐hexene‐3‐one from the acid chloride. This enone forms the indene derivatives in a sequence of two Friedel–Crafts alkylation reactions. Indeed, the 2,2,5‐trimethyl‐4‐hexene‐3‐one can be prepared from PivCl and AlCl 3 in 68% yield. It can then be directly converted to the indene derivatives with yields up to 99%.