A (Z)-ETHYLIDENECYCLOPENTANE ANNULATION METHOD - TOTAL SYNTHESES OF (+/-)-ANHYDROOPLOPANONE, (+/-)-OPLOPANONE, AND (+/-)-8-EPI-OPLOPANONE

Transmetalation of (Z)-5-chloro-3-trimethylsiannyl-2-pentene (8) with MeLi in tetrahydrofuran (THF) at -78¼C provides the novel bifunctional reagent (E)-5-chloro-3-lithio-2-pentene (11), which can be transformed readily into the Grignard reagent 12. Copper(I)-catalyzed conjugate addition of 12 to cyclic a,3-unsaturated ketones (e.g., 23-29), followed by intramolecular alkylation of the resultant chloro ketones (e.g., 30-36), affords the corresponding (Z)-ethylidenecyclopentane annulation products (e.g., 37-43). This new annulation method played a key role in total syntheses of the sesquiterpenoids (±)-anhydrooplopanone (51), (±)-oplopanone (50), and (±)-8-ppi-oplopane (70). © 1990, American Chemical Society. All rights reserved.