Facial selectivity in addition reactions to the highly strained enone in tricyclo[5.2.1.02,6]deca-2(6),8-dienone and tricyclo[5.2.1.02,6]dec-2(6)-enone

Tricyclo[5.2.1.02,6]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 tricyclo[5.2.1.02,6]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 tricyclo[5.2.1.02,6]deca-2(6),8-dien-3-one proving significant electronic participation of the C8-C9 double bond in reactions with this substrate.