Efficient Chemoenzymatic Synthesis of α-Aryl Aldehydes as Intermediates in C–C Bond Forming Biocatalytic Cascades
Anthony Meza, Meghan E. Campbell, Anna Zmich, Sierra A. Thein, Abbigail M. Grieger, Matthew J. McGill, Patrick H. Willoughby, Andrew R. Buller
Abstract
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 α-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 β-hydroxy-α-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 α-aryl aldehydes was further established by intercepting them with a recently engineered decarboxylative aldolase to yield γ-hydroxy nsAAs. Together, these data establish a versatile method of producing α-aryl aldehydes in simple, whole-cell conditions and show that these intermediates are useful synthons in C–C bond forming cascades.