A stereoselective route to (±)-pentalenene and (±)-9-epi-pentalenene

The (±)-pentalenene 1 and (±)-epi-pentalenene 12 have been synthesised by a novel spiro-annulation strategy that allowed the subsequent formation of two of the five-membered rings by aldol condensations; the order of reduction of two double bonds in a bicyclic intermediate 8 led, in a Stereoselective manner, to one C-9 epimer or the other.