Copper(II)-Catalyzed Reaction of 2,3-Allenoic Acids, Sulfur Dioxide, and Aryldiazonium Tetrafluoroborates: Route to 4-Sulfonylated Furan-2(5 H)-ones.

A copper(II)-catalyzed three-component reaction of 2,3-allenoic acids, sulfur dioxide, and aryldiazonium tetrafluoroborates under mild conditions is developed, leading to 4-sulfonylated furan-2(5 H)-ones in good yields. Not only sodium metabisulfite but also 1,4-diazabicyclo[2.2.2]octane-sulfur dioxide (DABCO·(SO2)2) is workable under the conditions. This transformation proceeds through a radical process initiated by the addition of arylsulfonyl radical to the C-central position of 2,3-allenoic acid. A broad substrate scope is demonstrated, and many sensitive functional groups are tolerated.