Total Syntheses of Novel Cannabinoids and Lundurines A-C with a Golden Touch

Research in our group is focused primarily upon the development of new transformations and methodologies employing gold catalysis and on their application to the total synthesis of natural products. Thus, the previously developed gold(I)-catalyzed intramolecular hydroheteroarylation of alkynes was considered a promising strategy for the synthesis of lundurines A–C. The elaboration of an efficient method to generate the required C20 stereocenter resulted in the development of a tandem condensation / lactamization / Claisen rearrangement. The good level of enantioselectivity for this transformation was achieved by optimization of the chiral auxiliary. We took advantage of a highly 8-endo-dig selective hydroheteroarylation using gold(I) as the catalyst in order to build the polyhydroazocine ring. A new intramolecular cyclopropanation of indoles by formation of a pyrazoline and subsequent C–C bond formation through loss of dinitrogen and toluenesulfinic acid has been discovered. In the course of our investigations, a remarkably facile olefin migration through a vinylcyclopropane retro-ene / ene rearrangement was found and applied to the synthesis of this family of alkaloids. A unified has been developed and successfully applied to the racemic and enantioselective total syntheses of lundurine A–C. The absolute configuration of lundurine C has been assigned as (5aR,5bS,6aR,12aR) by the X-ray crystal structure of its N-Me ammonium salt.

The fully diastereoselective gold(I)-catalyzed cycloisomerization of simple 1,5-enynes was applied as the key step in the total synthesis of cannabimovone and anhydrocannabimovone. We also revised the stereochemistry of the ring fusion of anhydrocannabimovone by X-ray crystallography and DFT calculations.