Enzymatic Stereodivergent Synthesis of Azaspiro[2.y]alkanes

Azaspiro[2.y]alkanes are increasingly valuable scaffolds in pharmaceutical drug discovery; however, an asymmetric catalytic method for their synthesis remains unknown. Here, we present a stereodivergent carbene transferase platform for the cyclopropanation of unsaturated exocyclic N-heterocycles to provide structurally diverse and pharmaceutically relevant azaspiro[2.y]alkanes in high yield (21->99% yield) and excellent diastereoselectivity and enantioselectivity (diastereomeric ratio (dr) values from 52.5:47.5 to >99.5:0.5; enantiomeric ratio (er) values from 51:49 to >99.5:0.5). These engineered protoglobin-based enzymes operate on gram scale, with no organic cosolvent, at substrate concentrations up to 150 mM (25 g/L) using lyophilized E. coli lysate as the catalyst. This platform represents a practical, scalable, and stereodivergent biocatalytic synthesis of azaspiro[2.y]alkanes, using low-cost engineered protoglobins and their native iron-heme cofactors.