Asymmetric Total Synthesis of Propindilactone G, Part 3: The Final Phase and Completion of the Synthesis.

Two independent synthetic approaches were evaluated for the final phase of the asymmetric total synthesis of propindilactone G (1). The key steps that led to the completion of the asymmetric total synthesis included: 1) an intermolecular oxidative heterocoupling reaction of enolsilanes to link the core structure to the side chain; 2) an intermolecular Wittig reaction for the formation of the α,β,γ,δ-unsaturated ester; and 3) a regio- and stereoselective OsO4 -catalyzed dihydroxylation of an α,β,γ,δ-unsaturated enone, followed by an intramolecular lactonization reaction to afford the final product. These reactions enabled the synthesis of (+)-propindilactone G in only 20 steps. As a consequence of our synthetic studies, the structure of (+)-propindilactone G has been revised. Furthermore, the direct oxidative coupling strategy for ligation of the core of propindilactone G with its side chain may find application in the syntheses of other natural products and complex molecules.