13C magnetic resonance studies. 119. Tricyclo[3.3.0.0] and [3.2.1.0]octanones from substituted norbornenones via cyclopropanation and homoketonization

Simmons–Smith cyclopropanation of the trimethylsilyi enoi ethers of 7-isopropylidene- and two 7-spirocyclopropyl-norbomenones leads directly to the 8-substituted 2-trimethylsilyl ethers of homoquadricyclene. Homoketonization of the unsaturated ether proceeds regiospecifically to the corresponding tricyclo[3.3.0.02.8]octan-4-one while the others give the tricyclo[3.2.1.02.6]octan-4-ones exclusively, all with high stereoselectivity favoring inversion of configuration. This difference in the regiochemistry of homoketonization can be attributed to a directive effect of β-functionalization overcoming thermodynamic control of cleavage.