Syntheses of a prenylbisabolane diterpene, a natural insecticide from Croton linearis, and of the bisabolane sesquiterpenes (−)-delobanone and (−)-epi-delobanone

An enantioselective first total synthesis of a constituent of Croton linearis , the (−)-7-hydroxy-3,10-prenylbisaboladien-2-one 1 , is described as well as the syntheses of the 7-hydroxy-3,10-bisaboladien-2-ones (−)- epi -delobanone ( 14a ) and (−)-delobanone ( 14b ). The model compounds, 7-hydroxy-11-nor-methyl-3-bisabolen-2-one ( 8a ), and 11,15-nor-dimethyl-7-hydroxy-3-bisabolen-2-one ( 8b ), were successfully prepared by opening of the protected carvone epoxide derivative 6 with the appropriate organocuprates. An alternative approach was used for compounds 1 and 14 . Thus, these were obtained from homogeranyllithium or homoprenyl Grignard reagent, which reacted successfully with a masked nor-carvone, ketone 11 , prepared in four steps from ( R )-carvone. Graphic