Initial model studies investigating the utility of a new intramolecular cyclopentene annulation procedure are described as pertaining to the preparation of bicyclo [3.3. 0]~ nones. Several l- b5, 7-octadien-2-ones substituted at the I-, 7-, or &position were decomposed in the presence of C u (a~ ac)~ to yield stereospecifically the corresponding vinylcyclopropanes 13, 16, 21, 24, and 27. The thermal as well as the rhodium-promoted modes of ...