Enantioselective total syntheses of the novel tricyclic sesquiterpene hydrocarbons (+)- and (−)-kelsoene. Absolute configuration of the natural product

Simple and preparatively efficacious lipase-catalysed kinetic resolution of endo,endo–cis-bicyclo[3.3.0]octane-2,6-diol rac-3 has provided ready access to bicyclo[3.3.0]octane-2,6-diones (−)-2 and (+)-2 of high enantiomeric purity. These C2-symmetric diones have been further elaborated to the sesquiterpene hydrocarbon (+)-kelsoene 1 and ent-kelsoene (−)-1, respectively, thereby establishing the absolute configuration of the natural product. In the light of these results, the absolute configuration assigned earlier to (+)-kelsoene needs to be revised as (+)-1.