Efficient Chemoenzymatic Synthesis of Α-Aryl Aldehydes As Intermediates in C–C Bond Forming Biocatalytic Cascades

Multi-enzyme biocatalytic cascades are emerging as practical routes for the synthesis of complex bioactive molecules. However, the relative sparsity of water-stable carbon electrophiles limits the synthetic complexity of molecules made from such cascades. Here, we develop a chemoenzymatic platform that leverages styrene oxide isomerase (SOI) to convert readily accessible aryl epoxides into alpha-aryl aldehydes through Meinwald rearrangement. These unstable aldehyde intermediates are then intercepted with a C-C bond forming enzyme, ObiH, that catalyzes a transaldolase reaction with L-threonine to yield synthetically challenging beta-hydroxy-alpha-amino acids. Co-expression of both enzymes in E. coli yields a whole-cell biocatalyst capable of synthesizing a variety of stereopure non-standard amino acids (nsAA) and can be produced on a gram scale. We used isotopically labeled substrates to probe the mechanism of SOI, which we show to catalyze a concerted isomerization featuring a stereospecific 1,2-hydride shift. The viability of in situ generated alpha-aryl aldehydes was further established by intercepting them with a recently engineered decarboxylative aldolase to yield gamma-hydroxy nsAAs. Together, these data establish a versatile method of producing alpha-aryl aldehydes in simple, whole -cell conditions and show that these intermediates are useful synthons in C-C bond forming cascades.