Dielectrophilic Allenic Ketone-Enabled [4 + 2] Annulation with 3,3’-Bisoxindoles: Enantioselective Creation of Two Contiguous Quaternary Stereogenic Centers

We introduced a type of allenic ketone
as a dielectrophilic C4
synthon in phosphine-mediated reactions. The high electrophilicity
of the advanced intermediates created upon phosphine activation empowered
the utilization of 3,3′-bis-oxindoles as a two-carbon reaction
partner in a highly enantioselective [4 + 2] annulation, allowing
for facile creation of spirocyclic bisindoline structures containing
two contiguous quaternary stereogenic centers. Synthetic manipulations
of the [4 + 2] annulation product led to concise total synthesis of
(−)-folicanthine.