Synergistic Rhodium/Phosphoric Acid Catalysis for the Enantioselective Addition of Oxonium Ylides to ortho-Quinone Methides

We report herein a powerful and highly stereoselective protocol for the domino‐type reaction of diazoesters with ortho‐quinone methides generated in situ to furnish densely functionalized chromans with three contiguous stereogenic centers. A transition‐metal and a Brønsted acid catalyst were shown to act synergistically to produce a transient oxonium ylide and ortho‐quinone methide, respectively, in two distinct cycles. These intermediates underwent subsequent coupling in a conjugate‐addition–hemiacetalization event in generally good yield with excellent diastereo‐ and enantioselectivity. Hand in hand: A rhodium‐catalyzed diazo ester decomposition with subsequent oxonium ylide formation and a phosphoric acid‐catalyzed generation of an ortho‐quinone methide from an ortho‐hydroxy benzhydryl alcohol were efficiently coupled to furnish highly substituted and densely functionalized chromans with three contiguous chiral centers in one synthetic step and with generally good yields and excellent diastereo‐ and enantioselectivities (PA=phosphoric acid).