Total Synthesis of Shearinines D and G: A Convergent Approach to Indole Diterpenoids

Abstract

The first total syntheses of the indole diterpenoids (+)-shearinine G and D are disclosed. The successful routes rely on late-stage coupling of two complex fragments. Formation of the challenging trans-hydrindane motif was accomplished by diastereoselective, intramolecular cyclopropanation. A one-pot sequence consisting of Sharpless dihydroxylation/Achmatowicz reaction was developed to install the dioxabicyclo[3.2.1]octane motif. The indenone subunit was accessed by Prins cyclization. Tuning the electronic nature of the substituents on the parent arylcarboxaldehyde allowed access to divergent products that were further transformed into shearinines G and D. Riley-type oxidation of a bicyclic enone yielded a surprising stereochemical outcome.

Keywords: alkaloids; natural products; ring synthesis; terpenoids; total synthesis.