Benzene reacts with a cyclic anhydride X in the presence of anhydrous AlCl ₃ to form a keto-acid Y. Compound Y undergoes Clemmensen reduction to form Z. Heating Z with polyphosphoric acid yields benzosuberone (6,7,8,9-tetrahydro-5H-benzo[a]cyclohepten-5-one), which contains a seven-membered cyclic ketone fused to the benzene ring. Identify the starting cyclic anhydride X.

Step-by-step solution

The final product, benzosuberone, consists of a benzene ring fused to a seven-membered cyclic ketone. The seven-membered ring shares two carbon atoms with the benzene ring, meaning the side chain that cyclizes must contain five carbon atoms. Working backwards (retrosynthetically): The intramolecular acylation of Z to form a seven-membered ring requires Z to be 5-phenylpentanoic acid. Z is formed by the Clemmensen reduction of the keto-acid Y. Therefore, Y must be 5-oxo-5-phenylpentanoic acid. Y is produced by the F