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%.
Bantel et al. (Thu,) studied this question.