We initiated our studies by screening a panel of hemoproteins for the model reaction of benzyl aziridine-1-carboxylate 1 with ethyl diazoacetate (EDA) as a carbene precursor to provide enantioenriched azetidine 2 (Table ) in suspensions of Escherichia coli (E. coli) whole cells. We were delighted to find that a variant of P411BM3-CIS with the additional mutations P248T, I263G, and L437F ("Parent F2"), provided the product with 3.7% yield, 73 total turnover numbers (TTNs), and 90:10 er favoring the (S)-enantiomer (Entry 1). Parent F2 is derived from hemoproteins originally engineered for the cyclopropanation of heteroatomsubstituted olefins and is 17 mutations away from its wild-type progenitor, cytochrome P450BM3 from Bacillus megaterium, which natively catalyzes the oxidation of longchain fatty acids. Control experiments revealed that hemin is unable to catalyze this reaction (see SI for details). Further control reactions indicated that the observed formation of the ring-opened hydrolysis product of 1 is not an enzyme-dependent process. No other aziridine-derived byproducts (e.g., cheletropic extrusion products , carbene insertion into the benzylic C-H 21c , or α-N-H bonds of the substrate 21e ) were identified, including a second ring expansion to form the corresponding pyrrolidine. Further experiments demonstrated that neither 2 nor the unsubstituted benzyl azetidine-1-carboxylate underwent ring expansion under the disclosed conditions. Chemoselectivity for aziridine ring expansion over azetidine ring expansion in this system can be attributed to the increased pyramidalization at nitrogen observed for acylaziridines and related compounds, which increases their N-nucleophilicity relative to less strained amides. 26