Stereoselective synthesis of (±)-tacamonine

We report a stereocontrolled approach to the pentacyclic indole alkaloid tacamonine by modifying an earlier route using norbornadiene to supply the nontryptamine portion. By maintaining a bridged system the reduction step of the Bischler–Napieralski reaction proceeded to deliver a bridged diol in which three methine hydrogen atoms are in an all-cis configuration. All 19 skeletal carbon atoms are fully incorporated, therefore, the only remaining steps involved cleavage of the vic-diol subunit in the seven-membered ring and further oxidation and reduction of the resulting lactam aldehyde.