Concise Synthesis of (-)-GA 18 Methyl Ester.

Gibberellins (GAs) are important plant hormones, but some of their family members are in extremely limited natural supply including GA 18 . Herein, we report a concise synthesis of (-)-GA 18 methyl ester, a member of the C 20 gibberellins, from commercially available and cheap andrographolide. Our synthesis features an intramolecular ene reaction to form the C ring, an oxidative cleavage followed by aldol condensation to realize a ring contraction and form the challenging trans -hydrindane (AB ring), and a photochemical [2+2] cycloaddition accompanied by a subsequent SmI 2 -mediated skeletal rearrangement to construct the methylenebicyclo[3.2.1]octanol moiety (CD ring).