Cyclopentene Annulation via Intramolecular Addition of Diazo Ketones to 1,3-Dienes. Applications to the Synthesis of Cyclopentanoid Terpenes

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]octenones. Several l-diazo-5,7-octadien-2-ones substituted at the 1-, 7-, or 8-position were decomposed in the presence of Cu(acac)₂ to yield stereospecifically the corresponding vinylcyclopropanes 13,16,21,24, and 27. The thermal as well as the rhodium-promoted modes of rearrangement to the appropriate cyclopentenes 14,17, 22a,b, and 28a,b were studied. Where necessary, diastereomers were separated and structurally assigned by relying on ¹³C NMR spectroscopy. ¹³C NMR data are provided for all new compounds in the bicyclooctane series to serve as an aid in assignments of cyclopentanoid terpenes synthesized by this methodology. The intramolecular cyclopropanation-rearrangement sequence of dienic diazo ketones has been shown to provide facile access to bicyclo[3.3.0]octanes of the type 14, 17, 22a,b, and 25a,b which are of value as terpene synthons. Enhanced stereoselectivity was observed in the rhodium-promoted cyclopentene rearrangements in favor of the less concave diastereomers (22a, 25a, and 28a). Finally, the sesquiterpene hirsutene (31) was synthesized in 37% overall yield by this methodology. ¹³C NMR data for several tricyclo[6.0.0.0^2,6] undecane compounds are also provided.