The Palladium-Catalyzed Enyne Cycloisomerization Reaction in a General Approach to the Asymmetric Syntheses of the Picrotoxane Sesquiterpenes. Part I. First-Generation Total Synthesis of Corianin and Formal Syntheses of Picrotoxinin and Picrotin

The palladium-catalyzed enyne cycloisomerization is used as a key ring-forming process for the obtention of the cis-fused hydrindane carbon skeleton characteristic of the picrotoxanes sesquiterpenes. The enyne cycloisomerization product is suitably functionalized so that each carbon of the hydrindane core can be modified to permit access to many members of the picrotoxane family, not only to picrotoxinin itself. Part I of this report describes our first-generation approach to the picrotoxane sesquiterpenes as illustrated by the asymmetric synthesis of corianin and the asymmetric formal syntheses of picrotoxinin and picrotin. A new catalyst system to effect the cycloisomerization emerged from this study. Subsequent work proved the generality of this catalyst system.