Enantioselective Synthesis of 4H-Pyran via Amine-Catalyzed Formal (3 + 3) Annulation of δ-Acetoxy Allenoate.

The formal (3 + 3) annulations of δ-acetoxy allenoates and 1C,3O-bisnucleophiles are reported with the use of 6'-deoxy-6'-perfluorobenzamido-quinine (4g) as a catalyst, which provide rapid access to 4H-pyrans with excellent enantioselectivity. The reaction features a wide reaction scope and mild reaction conditions. The crucial roles of amide NH of 4g as a H-bond donor have also been elucidated, which not only activates allenoate to facilitate formation of cationic intermediate A but also enhances the electrophilicity of its δ-position for nucleophilic 1,6-addition.