Enantioselective Synthesis of Spirocyclic Oxindole‐β‐lactones via N‐Heterocyclic Carbene‐Catalyzed Cycloaddition of Ketenes and Isatins

Chiral N‐heterocyclic carbenes were found to be efficient catalysts for the formal [2+2] cycloaddition reaction of disubstituted ketenes and isatins to give the corresponding spirocyclic oxindole‐β‐lactones in good yields with good diastereoselectivities and excellent enantioselectivities. Ring opening with Grignard reagents or decarboxylation of the oxindole spirocyclic‐β‐lactone gave the corresponding 3‐hydroxy‐ or 3‐alkylenyloxindoles in good yields.