Interestingly, the direct use of unprotected hydroxylamines as nitrene equivalents is unprecedented and might be applicable in other types of reactions involving nitrene precursors. A number of sterically and electronically distinct N-alkyl and N-aryl hydroxylamines were found to be compatible in the reaction when 2a was used as substrate, leading to a variety of unsubstituted β-lactams (6a-6h). Moreover, an N-hydroxy-amino acid derivative could be employed, effectively leading to an N-capped phenylalanine building block in similar yield (6i). Importantly, the reaction was found to be particularly efficient when performed from chiral 2substituted cyclopropanone precursors, leading to high yields of the corresponding 4-substituted β-lactams, either using N-alkylor N-aryl hydroxylamines (6j-6m). Sterically hindered cyclopropanone equivalent 5j was also found to be compatible in the transformation, providing access to congested β-lactam 6n in moderate yield. Bicyclic cyclopropanone precursor 5l led to formation of ring-fused lactam 6o, albeit in lower yield likely due to competitive ring-opening of the hemiaminal intermediate to a hydroxamic acid, similar to what is observed in the Favorskii rearrangement. In an effort to increase the sustainability and cost-efficiency of this transformation, we sought to evaluate the viability of a catalytic version. Although direct application of our previous conditions with a substoichiometric amount of Lewis acid did not yield any of the βlactam product, 11 it was found that similar efficiency could be realized when the reaction was performed in a two-step manner using only 1 mol% of Al(OTf)3, where the hemiaminal intermediate was used crude after aqueous workup (Scheme 5). Considering the high nucleophilicity of sulfinate anions, we believe that the leaving group liberated in the first step is capable of poisoning the Lewis acid catalyst, thus forming various aluminum species of type Al(PhSO2)x(OR)3-x too electron-rich to catalyze the subsequent rearrangement. Therefore, performing an aqueous workup after the first step eliminates all sulfinate salts initially formed and allows for the use of a catalytic amount of Lewis acid in the subsequent step.