Asymmetric assembly of congested contiguous stereocenters via photobiocatalytic three-component radical coupling

Despite rapid advances in new-to-nature biocatalysis, trimolecular enzymatic reactions that generate previously inaccessible molecular entities, particularly those bearing well-defined, congested stereochemical dyads, remain exceedingly rare. Here, we report a novel diastereo- and enantioselective photobiocatalytic three-component radical coupling unknown in both organic chemistry and enzymology, enabled by cooperative catalysis employing an evolved pyridoxal biocatalyst and a transition-metal photosensitizer. Directed evolution using high-throughput photobiocatalysis furnished two threonine aldolase variants, enabling the stereoselective assembly of contiguous tri- and tetrasubstituted stereocenters as well as vicinal tetrasubstituted stereocenters, which were long regarded as a challenging goal for stereoselective radical chemistry and asymmetric catalysis. Furthermore, a variety of radical precursors, including alpha-iodoesters, alpha-iodoamides, alpha-iodoketones, and alpha-iodonitriles, could be transformed into structurally diverse, densely substituted non-canonical amino acid derivatives via C-H functionalization with excellent stereocontrol. Leveraging the broad substrate compatibility of engineered pyridoxal biocatalysts, combinatorial biocatalysis was achieved by simultaneously varying all three coupling partners, delivering products with a 98% success rate. Collectively, this diversity-generating multicomponent photobiocatalytic radical coupling afforded a powerful strategy for addressing long-standing challenges in asymmetric catalysis while enabling access to valuable stereochemically complex small molecules with applications in medicinal chemistry.