Short Approach toward the Nonracemic A,B,E Tricyclic Core of Calyciphylline B‑Type Alkaloids

A suitably
functionalized tricyclic adduct containing the common A,B,E rings
found in calyciphylline B-type alkaloids was obtained in nine linear
steps. The key transformation features an efficient one-pot sequence
of intramolecular Vilsmeier–Haack cyclization and azomethine
ylide 1,3-dipolar cycloaddition in which three cycles, three new carbon–carbon
bonds, and three stereocenters are formed, one being fully substituted.
This work also demonstrates the first use of a chiral, nonracemic
cyclic enol ether as an internal carbon nucleophile.