Total Synthesis of (−)-Verrucarol¹

We have achieved the total synthesis of (−)-verrucarol, a trichothecene sesquiterpenoid obtained as a hydrolysis product of the naturally occurring verrucarin A. Our total synthesis began with the previously reported enantiomerically pure bicyclic α-methylated γ-lactone, which was prepared from d-glucose. The key steps for the total synthesis were (1) aldol-like carbon−carbon bond formation applied to the starting lactone using a four-carbon aldehyde as an electrophile for introduction of the quaternary stereogenic carbon sharing the B and C-rings of the trichothecene skeleton, (2) Dieckmann cyclization of the derived ε-ester lactone for construction of the C-ring equivalent, (3) Barton's decarboxylative oxygenation for conversion of a carboxylic acid functionality in the Dieckmann cyclization product into a hydroxyl group, (4) skeletal enlargement strategy for the crucial trichothecene skeleton construction, and (5) the final stereoselective formation of the exo-epoxy ring at the methylene carbon in the bridge constituting the B and C-rings.