The bicyclobutylidenes 2 and 5 have been rearranged to the bicyclooctenes 3 and 6, respectively, and further transformed to the tricyclic ketones 16 and 21, respectively, as known precursors of (±)-ceratopicanol (4) and (±)-hirsutene (7). For the cyclopentane annelation, a [3+2]cycloaddition with cyclopropane-1,1-dicarboxylic acid ester, followed by an alkaline hydrolysis and an oxidative decarboxylation