A Key Intermediate for the Chiral Synthesis of Elemanoids, Synthesis of (+)-ß-Elemenone

(1R,5S)-3-Phenylsulfenyl-6,6-dimethylbicyclo[3.1.1]heptanone obtained from (+)-nopinone was transformed into (1R,4S,5S)-4-methyl-4-vinylbicyclo[3.1.1]heptan-2-one, whose cyclobutane ring was cleaved with BF3·EtO2–Zn(OAc)2in acetic anhydride to provide (4S,5S)-1-acetoxy-4-isopropenyl-5-methyl-5-vinyl-1-cyclohexene (5), the key intermediate, in a highly regio- and stereoselective manner. Regioselective introduction of a three-carbon unit to 5with acetone followed by dehydration yielded (+)-ß-elemenone.