Herein is reported the formal (3 3) annulations of delta-acetoxy allenoates with 1C,30-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 BN, but enhance the electrophilicity of delta(C) of B-N for nucleophilic attack. (C) 2017 Elsevier Ltd. All rights reserved.
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