Synthesis of the ABC Ring of Calyciphylline A‑Type Alkaloids by a Stereocontrolled Aldol Cyclization: Formal Synthesis of (±)-Himalensine A

A synthetic approach to a functionalized ABC-tricyclic
framework
of calyciphilline A-type alkaloids, a building block toward this class
of alkaloids, is reported. The key synthetic steps involve a radical
cyclization to form the hydroindole system and piperidine ring closure
through a stereocontrolled aldol cyclization. The resulting alcohol
allows the methyl group to be installed in the bowl-shaped azatricyclic
structure; this crucial reaction takes place with configuration retention.
The synthesis of azatricyclic compound I constitutes
a formal synthesis of himalensine A.