Aliphatic Friedel-Crafts reactions. Part VIII. Preparation of unsaturated ketones by the acylation of 1-alkylcyclopentenes

Abstract

Reaction of 1-ethylcyclopentene with zinc chloride–acetic anhydride affords 5-acetyl-1-ethylcyclopentene (39%) and 1-acetyl-2-ethylidenecyclopentane (13%). Similar products are obtained in the acetylation of other n-alkylcyclopentenes, with the exception of 1-methylcyclopentene which yields a mixture of 5-acetyl-1-methylcyclopentene and 1-acetyl-2-methylcyclopentene in a ratio of ca. 3 : 2. Treatment of the initial acylation products with base in all cases afforded the corresponding 1-acyl-2-alkylcyclopentene.