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The Synthesis of ent-Cleistanthane Diterpenoid Spruceanol: Construction of Aromatic C Ring via Lewis Acid-Controlled Regioselective Diels-Alder Cycloaddition.

The first synthesis of ent-cleistanthane-type diterpenoid spruceanol with significant anti-cancer activity is described. A chiral pool approach was employed with a linear sequence of 13 steps beginning from readily available and inexpensive andrographolide. The approach features the construction of the aromatic ring with hydroxyl and methyl groups at C-12 and C-13 of the target compound respectively via Lewis acid-controlled regioselective Diels-Alder cycloaddition and the regioselective removal of the primary hydroxyl group of Diels-Alder adduct.