Total Synthesis of (+)-Arborisidine

The first total synthesis of arborisidine, a unique Kopsia indole alkaloid possessing a fully substituted cyclohexanone ring system with two quaternary carbons, has been achieved in seven steps in racemic format from tryptamine and in nine steps in asymmetric format from D-tryptophan methyl ester. Key elements of the design include a carefully orchestrated decyanation protocol to finalize the asymmetric formation of an aza-quaternary center that is challenging to access in optically active format via direct Pictet-Spengler cyclizations with tryptamine, a metal-promoted 6-endo-dig cyclization of an enyne to establish the second core quaternary center, and regiospecific functionalizations of the resultant complex diene to finalize the target structure. The distinct and efficient nature of the developed solution is highlighted by several unsuccessful approaches and unexpected rear-rangements.