Asymmetric Formal Synthesis of Cortistatins via a Gold-Catalyzed Semi-Pinacol Rearrangement Strategy

Over the past decade, Gold complexes have emerged as efficient and mild catalysts for the transformation of substrates possessing alkyne functionality into a range of useful scaffolds.These powerful methods have enabled the development of novel approaches for the total synthesis of biologically active natural products by gold catalysis.In this case, we found that the intramolecular nucleophilic addition of a hydroxyl group to a carbon-carbon triple bond, which activated by a gold catalyst, followed by further useful transformation has proven to be an excellent method for rapid construction of structural diversity of molecular scaffolds.The cortistatins are a family of 11 steroidal alkaloids which exhibit significant biological activities.The intriguing biological properties and their low natural abundance have elevated cortistatins to be a typical target for both partial and total synthesis.Up to now, more than a dozen research groups have published approaches directed toward the synthesis of cortistatins, including one semi-synthesis, five total syntheses and five formal syntheses, as well as a number of synthetic studies about the pentacyclic core and some illuminating model studies.One of the biggest challenges for the synthesis of cortistatins is how to construct the unprecedented oxabicyclo[3.2.1]octane ring system which lies within a complex tetracarbocyclic skeleton.In our previous work, we have developed a gold-catalyzed semi-pinacol rearrangement strategy to diastereoselective synthesis of the oxabicyclo[3.2.1]octane ring system.The wide substrate scope as well as the high diastereoselectivity have made us to apply this method into the asymmetric formal synthesis of Cortistatins.Herein, full details about our efforts towards the formal synthesis of cortistatins were described by employing our developed gold-catalyzed cascade reaction to oxabicyclo[3.2.1]octane ring systems.This route is featured with a novel gold-catalyzed cascade reaction involving intramolecular nucleophilic addition of hydroxyl group to the carbon-carbon triple bond, followed by an oxonium ion initiated semi-pinacol-type 1,2-migration to construct the key oxabicyclo[3.2.1]octane skeleton.