Ruthenium(0) Catalyzed Endiyne-α-Ketol [4+2] Cycloaddition: Convergent Assembly of Type II Polyketide Substructures via C-C Bond Forming Transfer Hydrogenation

Upon exposure of 3,4-benzannulated 1,5-diynes (benzo-endiynes) to alpha-ketols (alpha-hydroxyketones) in the presence of Ru(0) catalysts derived from Ru-3(CO)(12) and RuPhos or CyJohnPhos, successive redox-triggered C-C coupling occurs to generate products of [4 + 2] cycloaddition. The proposed catalytic mechanism involves consecutive alkyne-carbonyl oxidative couplings to form transient oxaruthanacycles that suffer alpha-ketol mediated transfer hydrogenolysis. This process provides a new, convergent means of assembling Type II polyketide substructures.