73%, >99% ee (>99) 35%, 94% ee (94) 84%, 97% ee (96) 91%, 90% ee (93) 56%, 96% ee ( ) The general reactivity trend observed revealed that electron-poor substrates afford higher isolated yields in this transformation (e.g. 5c and 5d), with most reactions proceeding with complete retention of stereochemical information. Aliphatic substrates 4f-4l were synthesized in a single step from phenyl methyl sulfone and the corresponding epoxides, building on a one-pot ringopening/activation/substitution sequence first reported by Tanaka and co-workers (Method B). 15c To our delight, this procedure was found to be fully stereospecific to form the methyl-substituted sulfonylcyclopropane 4f when starting from an enantioenriched epoxide, subsequently allowing us to access enantiopure cyclopropanone precursor 5f in good yield using our hydroxylation procedure. Importantly, trifluoromethyl-substituted cyclopropanone equivalent 5g could also be accessed by this protocol, in addition to 2,3-disubstituted derivatives 5h and 5i. A more sterically congested 2,2-dimethyl-substituted derivative was also found to be compatible (5j), as well as achiral ring-fused analogs 5k and 5l. Interestingly, these bicyclic derivatives constitute precursors of cyclopropanones commonly encountered as high energy intermediates in the Favorskii rearrangement, eventually affording ring-opened carboxylic acid derivatives in the presence of protic nucleophiles. Synthesis of β-lactams by formal [3+1] cycloaddition. With general access to a wide variety of highly reactive cyclopropanone precursors, we sought to apply these substrates in strain-releasing rearrangements. Considering the biological relevance of β-lactam derivatives, a particularly attractive transformation we were prompted to study is the formal [3+1] cycloaddition of cyclopropanones with nitrene equivalents to afford β-lactams. An analogous Schmidt-type rearrangement had previously been reported by Aubé and co-workers using (1-ethoxycyclopropoxy)trimethylsilane in the presence of organoazides and BF3•OEt2, although only unsubstituted achiral β-lactam derivatives could be obtained in low to moderate yield. In that work, the relatively harsh conditions required for equilibration to cyclopropanone likely led to multiple decomposition pathways. Moreover, we sought to avoid organoazides in order to improve the applicability of the method on a larger scale. After evaluating various other nitrene equivalents, 11 it was found that simple hydroxylamines or their corresponding ethers could play the same role when the reaction was run in presence of a Lewis acid (Scheme 4). Indeed, treatment of a chosen hydroxylamine with the 1-sulfonylcyclopropanol substrate leads to smooth formation of a stable hemiaminal intermediate under mild basic conditions, which then rearranges in situ to the corresponding β-lactam following addition of Al(OTf)3. Scheme 4. Scope of accessible β-lactams by formal [3+1] cycloaddition of cyclopropanone equivalents with hydroxylamines a a Isolated yields on 1 mmol scale. Yield obtained from the corresponding O-benzyl-protected hydroxylamine. Yield obtained from the corresponding hydroxylamine hydrochloride salt, using 2.2 equiv Et3N.