Enantioselective Trapping of Pd-Containing 1,5-Dipoles by Photogenerated Ketenes: Access to 7-Membered Lactones Bearing Chiral Quaternary Stereocenters.

An enantioselective [5+2] cycloaddition of vinylethylene carbonates and α-diazoketones was achieved for the first time by merging photoactivation and asymmetric Pd catalysis. The key to the success of this method is the enantioselective trapping of Pd-containing, 1,5-dipolar intermediates by ketenes, a class of reactive C2 synthons, which were generated in an in situ and traceless manner under visible light irradiation. Through this trapping, a variety of 7-membered lactones bearing challenging chiral quaternary stereocenters can be accessed in a facile manner with good efficiency and high enantioselectivity (up to 99% yield and 96:4 er).