NHC-Mediated [2+2] Cycloaddition Route to Nitro-β-thiolactams

Significance

Reported is the synthesis of nitro-β-thiolactams by the N-heterocyclic carbene (NHC)-mediated [2+2] cycloaddition of trans-­nitroolefins with isocyanates. The reaction did not proceed in the absence of the NHC. Through analogy with the well-known reaction of ketenes with imines, it was suggested that the most likely mechanism involves NHC attack on the isocyanate, followed by subsequent Michael-type addition onto the nitro alkene, then ring closure to give the final compound with regeneration of the catalyst.