Catalytic Enantioselective Formal (3 + 3) Cycloaddition of 3-Halooxindoles with Pyridinium 1,4-Zwitterionic Thiolates

The first catalytic enantioselective formal (3 + 3) cycloaddition of pyridinium 1,4-zwitterionic thiolates with indol-2-ones, generated in situ from 3-halooxindoles and serving as C-C-O three-atom reaction components, was successfully developed. A series of optically pure indolenine-fused 2H-1,4-oxathiines were obtained in moderate to good yields with high enantioselectivities using a catalyst system comprising a chiral tridentate Pybox ligand and Ni(acac)₂. The products could be further converted to chiral 3-sulfenyl-3,3'-disubstituted oxindoles with stereoretention. This work also represents the first report on a catalytic asymmetric cycloaddition reaction involving pyridinium 1,4-zwitterionic thiolates.