The bicyclobutylidene to bicyclooctene rearrangement:: Formal syntheses of (±)-ceratopicanol and (±)-hirsutene

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-l,1-dicarboxylic acid ester, followed by an alkaline hydrolysis and an oxidative decarboxylation of the diacids formed was used (3-8-14-16 6-18-20-21). The structures of the cycloadduct 9, the diacid 15 and the ketone 17 followed from an X ray analysis of the ditosylate 13, itself obtained by reduction and tosylation (9-12-13). The new syntheses of 16 and 21 represent short entries to 4 and 7. (C) 1998 Elsevier Science Ltd. All rights reserved.