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 pr epared and subjected to base-catalyzed isomerization. Equilibrium betw een these exocyclic isomers and the corresponding 3-methyl-2-cycloalke nones and 3-methyl-3-cycloalkenones was reached in the 7- and 8-member ed rings. The presence of the beta-methyl shifts these equilibria towa rd the Delta(2) isomer in each ring relative to the unsubstituted comp ounds and to compounds containing electron-withdrawing groups at the b eta position. In the 9- and 10-membered rings, the Delta(2) isomers we re isomerized to the Delta(3) isomers only with difficulty, and the re verse process could not be accomplished under the reaction conditions utilized. The ease of interconversion of the Delta(2) and Delta(3) iso mers is directly related to the thermodynamic stability of planar conj ugated dienes in medium ring carbocycles.