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.