Total Syntheses of (±)-Acetoxymarsupellone and (±)-Marsupellins A, B, and D Enabled by a Reductive Cyclization and Semipinacol Rearrangement Strategy

We report herein the total syntheses of three marsupellin-family
sesquiterpenoids, (±)-acetoxymarsupellone and (±)-marsupellins
B and D, in 14–19 steps from our known precursor, making marsupellin
A also accessible from marsupellin B through a known procedure. The
critical tricyclic framework bearing the challenging C7 bridgehead
all-carbon quaternary center is strategically constructed through
a Ti-mediated reductive cyclization and semipinacol rearrangement
sequence. This study provides a general approach to the syntheses
of (ent-)longipinane-type molecules.