Enantiocomplementary Total Biosynthesis of Cyclic Imino Acids Enabled by an SN2 Reaction Catalyzed by a Class I Aldolase and a Transaminase

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Cyclic noncanonical imino acids are highly valuable in drug design due to their rigid conformational constraints. However, their structural complexity has limited the development of robust synthetic methods. Herein, we report a concise strategy for the enantiocomplementary total synthesis of these compounds. This route is driven by two key biocatalytic steps: a non-natural SN2 reaction catalyzed by a class I pyruvate aldolase, followed by an enantiocomplementary reductive amination catalyzed by amino acid transaminases. We successfully implement this synthesis in E. coli whole-cell catalysts for streamlined production. From simple precursors, we synthesize 42 cyclic imino acids, including the natural product proferrorosamine A, in yields of up to 98% and with enantioselectivity up to >99% ee. This work underscores the potential of leveraging non-natural activities of class I aldolases to create concise enzymatic routes for synthesizing complex chiral heterocycles beyond imino acids.