Total Syntheses of Vellosimine, N-Methylvellosimine, and 10-Methoxyvellosimine and Formal Synthesis of 16-Epinormacusine B through a [5+2] Cycloaddition

To date, more than 100 members of the sarpagine alkaloid family have been isolated. Their structural variations originate from oxidative transformations of the carboskeleton and the presence of both absolute configurations at the C-16 atom, which is established in the course of their biosynthesis. More than 40 sarpagine alkaloids belong to the either the 16-regular or 16-epi subgroups, depending on the stereochemistry at C-16. Herein, we report the formal synthesis of 16-epinormacusine B, a member of the 16-epi group, by using our well-established generalized strategy for the total synthesis of these alkaloids. Furthermore, we provide the synthetic details and pitfalls of the asymmetric total syntheses of vellosimine, N-methylvellosimine, and 10-methoxyvellosimine, all members of the 16-regular group.