Medium-ring systems .6. Synthesis and isomerizations of medium-ring 3-methylenecycloalkanones and 3-methylcycloalkenones

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 beta-methyl shifts these equilibria toward the Delta(2) isomer in each ring relative to the unsubstituted compounds and to compounds containing electron-withdrawing groups at the beta position. In the 9- and 10-membered rings, the Delta(2) isomers were isomerized to the Delta(3) isomers only with difficulty, and the reverse process could not be accomplished under the reaction conditions utilized. The ease of interconversion of the Delta(2) and Delta(3) isomers is directly related to the thermodynamic stability of planar conjugated dienes in medium ring carbocycles.