Preparation and thermal decarbonylation of some sterically compressed 5,6-endo-disubstituted norborn-2-en-7-ones.

Reaction of 1,2,3,4-tetrachloro-5,5-dimethoxycyclopentadiene with norbornadiene and benzo-norbornadiene gave adducts having the endo-exo-1,4,4a,5,8,8a-hexahydro-1,4:5,8-dimethano-naphthalene and endo-exo-1,4,4a,9,9a,l0-hexahydro-1,4:9,l0-dimethanoanthracene skeletons, respectively. These adducts were dechlorinated to give acetals, which were hydrolysed to give the corresponding ketones. The initial adducts, and the derived acetals and ketones, are all sterically compressed as a result of crowding of the C2-C3 etheno bridge against a methano hydrogen atom syn to that bridge. This steric compression manifests itself in a marked deshielding of the relevant methano proton in the N.M.R. spectra of these compounds, and in an increased rate of thermal decarbonylation of the ketones. The location of the deuterium atom in benzonorbornadiene[7-anti-D] (12) has been established chemically by observing an intramolecular deuterium transfer during photochemical half-cage formation from the adduct of (12) and 1,2,3,4-tetrachloro-5,5- dimethoxycyclopentadiene.