Formal Synthesis of Sarain A: Intramolecular Cycloaddition of an Eight-Membered Cyclic Nitrone to Construct the 2-Azabicyclo[3.3.1]nonane Framework.

An enantioselective route to the tetracyclic skeleton of sarain A has been developed. Asymmetric reduction of an ynone introduced a chiral center which was transferred to the contiguous tertiary stereogenic centers through an Ireland-Claisen rearrangement. The 2-azabicyclo[3.3.1]nonane framework was constructed by an unprecedented intramolecular cycloaddition of an eight-membered cyclic nitrone. Using the steric bias of the bicyclic system, the quaternary carbon atom was constructed by a stereoselective aldol reaction. Further ring formations were performed by ring-closing metathesis for the 13-membered ring and an iodoamidation reaction for the pyrrolidine ring. The present synthesis has successfully provided an alternative route to the late-stage intermediate of Overman's synthesis.