The rearrangement of strained ketone derivatives, typically yielding ring-expanded or ring-opened products, constitutes a key strategy for the elaboration of complex molecules relevant to the pharmaceutical industry. While myriad transformations have been developed with small cyclic ketones such as cyclobutanones, 2 the analogous use of cyclopropanone derivatives as substrates has seriously lagged behind due to the kinetic instability often associated with these compounds, as a result of their extreme strain and multiple decomposition pathways. Indeed, while cyclopropane and cyclobutane derivatives generally possess similar strain energies, the corresponding cycloalkanones differ in this regard by ca. 20 kcal/mol (Scheme 1a). Although cyclobutanone derivatives are known to be relatively stable and in some cases are commercially available, cyclopropanones often decompose instantly at room temperature, as they are kinetically unstable to heat, light and moisture. The most common decomposition pathways involve either polymerization (heat-and moisture-induced), decarbonylation to alkenes (lightinduced), or nucleophilic ring-opening as in the Favorskii rearrangement. Nevertheless, cyclopropanone itself has been previously prepared by reaction of diazomethane with ketene at -78 °C followed by distillation at the same temperature, 6 affording an ethereal solution that must be used immediately at low temperature. A more common and practical approach popularized by Wasserman in the 1960's involves the use of alcohol adducts of cyclopropanone derivatives, such as cyclopropanone hemiketal 1, that can be transformed in situ to the corresponding strained ketone via base-or acid-induced α-elimination (Scheme 1b). Due in part to the poor leaving group ability of alkoxides, these unstable precursors require harsh conditions to equilibrate to the corresponding cyclopropanones, which is problematic considering the kinetic instability of these highly strained species. This paradox is the critical reason why these hemiketal precursors to cyclopropanone are rarely suitable substrates and often lead to low yields of desired rearranged prod-ucts. The absence of a general class of well-behaved cyclopropanone precursors has thus largely defined these highly strained species as chemical curiosities rather than useful building blocks in organic synthesis. By analogy to the use of sulfinic acid adducts as base-labile protecting groups for unstable aldehydes and imines in nucleophilic addition chemistry, Chen reported that the phenylsulfinic acid adduct of unsubstituted cyclopropanone 2a, a white crystalline solid, is more reactive and surprisingly wellbehaved as compared with classical hemiketals, with equilibration to cyclopropanone taking place in mildly basic conditions at room temperature or below (Scheme 1c). We envisioned that such stability and increased reactivity of 1-sulfonylcyclopropanols could be key to unlocking the tremendous potential of cyclopropanone derivatives as synthetic intermediates, allowing reactions to proceed under mild conditions and thus suppressing undesired decomposition pathways. Moreover, the steric and electronic tunability of the sulfinic acid leaving group in these compounds could prove beneficial to establishing a general class of cyclopropanone precursors. (d) Herein, we report the first enantioselective synthesis of 1sulfonylcyclopropanols via an unprecedented α-hydroxylation of readily accessible cyclopropylsulfones using a bis(silyl) peroxide reagent as oxidant, and their application as substrates in a novel stereospecific formal [3+1] cycloaddition with simple hydroxylamines, leading to enantioenriched β-lactams (Scheme 1d). Kinetic studies using a pyrazole trapping reaction reveal the crucial influence of the sulfinic acid leaving group's steric and electronic properties, where hindered and/or electron-poor derivatives led to considerably faster equilibration to cyclopropanone, highlighting the modular reactivity of these substrates. Considering the breadth of reported transformations using strained ketones in organic synthesis, 1 our studies should find widespread application in the elaboration of complex molecules using previously inaccessible cyclopropanone-based rearrangements.