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