Non-native Intramolecular Radical Cyclization Catalyzed by a B₁₂ -Dependent Enzyme

Despite the unique reactivity of vitamin B₁₂ and its derivatives, B₁₂ -dependent enzymes remain underutilized in biocatalysis. In this study, we repurposed the B₁₂ -dependent transcription factor CarH to enable non-native radical cyclization reactions. An engineered variant of this enzyme, CarH*, catalyzes the formation γ- and δ-lactams through either redox-neutral or reductive ring closure with marked enhancement of reactivity and selectivity relative to the free B₁₂ cofactor. CarH* also catalyzes an unusual spirocyclization by dearomatization of pendant arenes to produce bicyclic 1,3-diene products instead of 1,4-dienes provided by existing methods. These results and associated mechanistic studies highlight the importance of protein scaffolds for controlling the reactivity of B₁₂ and expanding the synthetic utility of B₁₂ -dependent enzymes.