Positioning-Group-Enabled Biocatalytic Oxidative Dearomatization

Abstract

Biocatalysts have the potential to perform reactions with exceptional selectivity and high catalytic efficiency while utilizing safe and sustainable reagents. Despite these positive attributes, the utility of a biocatalyst can be limited by the breadth of substrates that can be accommodated in the active site in a reactive pose. Proven strategies exist for optimizing the performance of a biocatalyst toward unnatural substrates, including protein engineering; however, these methods can be time intensive and require specialized equipment that renders these approaches inaccessible to synthetic chemists. Strategies accessible to chemists for the expansion of a natural enzyme’s substrate scope, while maintaining high levels of site- and stereoselectivity, remain elusive. Here, we employ a computationally guided substrate engineering strategy to expand the synthetic utility of a flavin-dependent monooxygenase. Specifically, experimental observations and computational modeling led to the identification of a critical interaction between the substrate and protein which is responsible for orienting the substrate in a pose productive for catalysis. The fundamental hypothesis for this positioning group strategy is supported by binding and kinetic assays as well as computational studies with a panel of compounds. Further, incorporation of this positioning group into substrates through a cleavable ester linkage transformed compounds not oxidized by the biocatalyst SorbC into substrates efficiently oxidatively dearomatized by the wild-type enzyme with the highest levels of site- and stereoselectivity known for this transformation.

Stereodivergent, Chemoenzymatic Synthesis of Azaphilone Natural Products

“…Beyond the importance of Tyr position in the active site, it is clear that other mechanisms for stereocontrol have evolved in this class of enzymes. For example, catalysts AzaH and SorbC break from this Tyr control mechanism, and, in the case of SorbC, we have proposed an alternative mechanism for control of substrate position in the active site …”

“…For example, catalysts AzaH and SorbC break from this Tyr control mechanism, and, in the case of SorbC, we have proposed an alternative mechanism for control of substrate position in the active site. 47 Ultimately, this survey of sequence space surrounding known enzymes provides a greater understanding of the sequence features that can predict site-and stereoselectivity and has increased the number of biocatalysts vetted for this transformation. With catalysts capable of delivering enantiomeric products in hand, we chose to pursue an enantiodivergent synthetic strategy with AzaH and AfoD based on the robust expression and reactivity of these enzymes, in addition to the excellent stereoselectivity of each catalyst.…”

ACS Central Science Virtual Issue on Bioinspired Catalysis

“…Importantly, the critical crotyl ester positioning group can readily be removed with [Pd(PPh 3 ) 4 ] and morpholine. 19…”

“…This positioning strategy is beautifully illustrated by an oxidative phenol dearomatization to afford highly enantioenriched quinol products using wild-type SorbC. Importantly, the critical crotyl ester positioning group can readily be removed with [Pd­(PPh 3 ) 4 ] and morpholine …”

Photocatalytic Oxidative Dearomatization of Orcinaldehyde Derivatives

“… These reactive intermediates can be readily transformed through a breadth of carbon–carbon and carbon–heteroatom bond-forming events and have been leveraged by Nature and chemists alike in the synthesis of complex molecules . The use of lead­(IV) tetraacetate (LTA) for oxidative dearomatization was first reported over half a century ago and continues to be widely used today, despite the generation of stoichiometric quantities of lead waste and lack of site-selectivity. , More recently, hypervalent iodide reagents have been employed to perform this transformation, such as PIDA, PIFA, and IBX . which result in stoichiometric iodide byproducts that can be challenging to separate from the desired quinol.…”