Medium-Ring Systems. 6.1 Synthesis and Isomerizations of Medium-Ring 3-Methylenecycloalkanones and 3-Methylcycloalkenones

Abstract

A series of 3-methylenecycloalkanones with ring sizes 7−10 has been prepared and subjected to base-catalyzed isomerization. Equilibrium between these exocyclic isomers and the corresponding 3-methyl-2-cycloalkenones and 3-methyl-3-cycloalkenones was reached in the 7- and 8-membered rings. The presence of the β-methyl shifts these equilibria toward the Δ2 isomer in each ring relative to the unsubstituted compounds and to compounds containing electron-withdrawing groups at the β position. In the 9- and 10-membered rings, the Δ2 isomers were isomerized to the Δ3 isomers only with difficulty, and the reverse process could not be accomplished under the reaction conditions utilized. The ease of interconversion of the Δ2 and Δ3 isomers is directly related to the thermodynamic stability of planar conjugated dienes in medium ring carbocycles.