Construction of the [6,5,7,5] Tetracyclic Core of Calyciphylline A Type Alkaloids via a Tandem Semipinacol Rearrangement/Nicholas Reaction

A novel and efficient
approach toward the assembly of the synthetically
challenging tetracyclic [6,5,7,5] core structure of calyciphylline
A-type alkaloids is developed. The synthetic route features a tandem
semipinacol rearrangement/Nicholas reaction that has been devised
strategically to construct the spirocyclic A/B ring and the sterically
congested vicinal all-carbon quaternary carbon centers in high diastereoselectivity.
Late-stage installation of the hydropyrrole ring and the strained
7-membered ring via a double-reductive amination and ring-closing
metathesis, respectively, has also been realized.