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 α-ketols (α-hydroxyketones) in the presence of Ru(0)
catalysts derived from Ru3(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 α-ketol mediated
transfer hydrogenolysis. This process provides a new, convergent means
of assembling Type II polyketide substructures.