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

Article abstract of DOI:10.1021/jo01313a003

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 1-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 separatedand 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 cylopropanation-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 37percent overall yield by this methodology. 13C NMR data for several tricyclo<6.3.0.0^2,6>undecane compounds are also provided.