An Enantiospecific Approach to Tricyclic Sesquiterpenes Mayurone and Thujopsenes¹

An enantiospecific approach to mayurone and thujopsenes, sesquiterpenes containing three contiguous quaternary carbon atoms, starting from (<i>R</i>)-carvone (<b>8</b>), is described. (<i>S</i>)-3,4,4-Trimethylcarvone (<b>7</b>), obtained from (<i>R</i>)-carvone, was transformed into the bicyclo[2.2.2]octanone <b>13</b> via regioselective intramolecular alkylation of the allyl bromide <b>11</b>. Regioselective ozonolysis and Criegee fragmentation of the bicyclic ketone <b>13</b> furnished the keto ester <b>14</b>. Reductive deoxygenation followed by one-carbon homologation transformed the keto ester <b>19</b> into the ester <b>6</b>. Intramolecular cyclopropanation of the diazo ketone <b>25</b>, derived from the acid <b>5</b>, furnished (−)-dihydromayurone (<b>4</b>), thus constituting a formal enantiospecific synthesis of mayurone and thujopsenes.