Tetrahedron
Tetrahedron
5 years ago

Tertiary amine-catalyzed (3+3) annulations of δ-acetoxy allenoates: Substrate scope, synthetic application and mechanistic insight

Tertiary amine-catalyzed (3+3) annulations of δ-acetoxy allenoates: Substrate scope, synthetic application and mechanistic insight
Herein is reported the formal (3 + 3) annulations of δ-acetoxy allenoates with 1C,3O-bisnucleophiles by using 6’-deoxy-6’-perfluorobenzamido-quinine (5g) as the catalyst, providing rapid access to 4H-pyrans with excellent enantioselectivity. The reaction features wide substrate scope and mild reaction conditions. The utility of this method is demonstrated in highly stereoselective synthesis of the core of calyxin I. Mechanistic experiments suggest the involvement of 3-ammonium-2,4-dienoate intermediate B N . This type of cationic intermediate arises from an addition-elimination process between allenoate substrate and amine catalyst, and is stable enough for isolation and characterization. Mechanistic studies also disclose the crucial role of amide NH of 5g, which is able to not only activate allenoate to facilitate the formation of B N , but enhance the electrophilicity of δ C of B N for nucleophilic attack.

Publisher URL: www.sciencedirect.com/science

DOI: S0040402017304490

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