Asymmetric Synthesis of Oxa-Bridged Oxazocines through a Catalytic RhII /ZnII Relay [4+3] Cycloaddition Reaction.

Oxa-bridged oxazocines bearing three chiral carbon centers were synthesized efficiently through a bimetallic catalytic asymmetric tandem reaction of β,γ-unsaturated α-ketoesters with diazoimides. The process contained a rhodium-promoted in situ generation of isomünchnone from diazoimide decomposition, and a [4+3]-cycloaddition of β,γ-unsaturated α-ketoester catalyzed by a chiral N,N'-dioxide-ZnII complex. Ligand-accelerated catalysis was found, and a possible transition-state model was proposed to explain the origin of stereoselectivity.