Facial selectivity in addition reactions to the highly strained enone in tricyclo[5.2.1.0]deca-2(6),8-dienone and tricyclo[5.2.1.0]dec-2(6)-enone

Tricyclo5.2.1.0^2,6]deca-2(6),8-dien-3-one contains a highly strained central double bond due to geometrical constraints imposed by the tricyclic skeleton which does not allow optimal sp² hybridization at the C₂ and C₆ bridgehead positions. Michael addition of various nucleophiles (alkoxides, cyanide, and malonate) under protic conditions resulted in an exclusive exo-facial selectivity. This preference can be explained by steric and electronic factors. Michael additions using lithium dialkylcuprates resulted in predominant formation of endo-products, but also some exo-products were obtained. These exo-products arising from endo-approach may be the result of coordination of the cuprate with both the enone moiety and the olefinic C₈-C₉ bond. Michael additions to tricyclo5.2.1.0^2,6]dec-2(6)-en-3-one, which lacks this C₈-C₉ double bond showed exclusive exo-facial selectivity to give exo-products. Besides these additions were all considerably slower than those to tricyclo5.2.1.0^2,6]deca-2(6),8-dien-3-one proving significant electronic participation of the C₈-C₉ double bond in reactions with this substrate.