A Homo-Mannich Reaction Strategy Enables Collective Access to Ibophyllidine, Aspidosperma, Kopsia, and Melodinus Alkaloids.

We report here a homo-Mannich reaction of cyclopropanol with iminium ion generated by an asymmetric allylic dearomatization of indole to construct a tricyclic hydrocarbazole core, which is shared by a variety of monoterpenoid indole alkaloids across families. Through this skeleton-constructing process, an all-carbon quaternary stereogenic center as well as an allyl and a ketone groups were installed. Using this functionalized hydrocarbazole as the structural platform, D ring and E rings of different sizes (i.e., five-, six-, and seven-membered) were successively or simultaneously assembled, leading to a collective asymmetric synthesis of seven alkaloids belonging to the ibophyllidine, Aspidosperma, Kopsia, and Melodinus alkaloid families.