Static to inducibly dynamic stereocontrol: The convergent use of racemic ß-substituted ketones.

The synthesis of stereochemically complex molecules in the pharmaceutical and agrochemical industries requires precise control over each distinct stereocenter, a feat that can be challenging and time consuming using traditional asymmetric synthesis. Although stereoconvergent processes have the potential to streamline and simplify synthetic routes, they are currently limited by a narrow scope of inducibly dynamic stereocenters that can be readily epimerized. Here, we report the use of photoredox catalysis to enable the racemization of traditionally static, unreactive stereocenters through the intermediacy of prochiral radical species. This technology was applied in conjunction with biocatalysts such as ketoreductases and aminotransferases to realize stereoconvergent syntheses of stereodefined γ-substituted alcohols and amines from ß-substituted ketones.

Photoenzymatic Hydrogenation of Heteroaromatic Olefins Using 'Ene'-Reductases with Photoredox Catalysts.

Flavin-dependent 'ene'-reductases (EREDs) are highly selective catalysts for the asymmetric reduction of activated alkenes. This function is, however, limited to enones, enoates, and nitroalkenes using the native hydride transfer mechanism. Here we demonstrate that EREDs can reduce vinyl pyridines when irradiated with visible light in the presence of a photoredox catalyst. Experimental evidence suggests the reaction proceeds via a radical mechanism where the vinyl pyridine is reduced to the corresponding neutral benzylic radical in solution. DFT calculations reveal this radical to be "dynamically stable", suggesting it is sufficiently long-lived to diffuse into the enzyme active site for stereoselective hydrogen atom transfer. This reduction mechanism is distinct from the native one, highlighting the opportunity to expand the synthetic capabilities of existing enzyme platforms by exploiting new mechanistic models.

Asymmetric redox-neutral radical cyclization catalysed by flavin-dependent 'ene'-reductases.

Flavin-dependent 'ene'-reductases (EREDs) are exquisite catalysts for effecting stereoselective reductions. Although these reactions typically proceed through a hydride transfer mechanism, we recently found that EREDs can also catalyse reductive dehalogenations and cyclizations via single electron transfer mechanisms. Here, we demonstrate that these enzymes can catalyse redox-neutral radical cyclizations to produce enantioenriched oxindoles from α-haloamides. This transformation is a C-C bond-forming reaction currently unknown in nature and one for which there are no catalytic asymmetric examples. Mechanistic studies indicate the reaction proceeds via the flavin semiquinone/quinone redox couple, where ground-state flavin semiquinone provides the electron for substrate reduction and flavin quinone oxidizes the vinylogous α-amido radical formed after cyclization. This mechanistic manifold was previously unknown for this enzyme family, highlighting the versatility of EREDs in asymmetric synthesis.

Preparation of dihydroxy polycyclic aromatic hydrocarbons and activities of two dioxygenases in the phenanthrene degradative pathway.

Dihydroxy phenanthrene, fluoranthene, and pyrene derivatives are intermediates in the bacterial catabolism of the corresponding parent polycyclic aromatic hydrocarbon (PAH). Ring-opening of the dihydroxy species followed by a series of enzyme-catalyzed reactions generates metabolites that funnel into the Krebs Cycle with the eventual production of carbon dioxide and water. One complication in delineating these pathways and harnessing them for useful purposes is that the initial enzymatic processing produces multiple dihydroxy PAHs with multiple ring opening possibilities and products. As part of a systematic effort to address this issue, eight dihydroxy species were synthesized and characterized as the dimethoxy or diacetate derivatives. Several dihydroxy compounds were examined with two dioxygenases in the phenanthrene degradative pathway in Mycobacterium vanbaalenii PYR-1. One, 3,4-dihydroxyphenanthrene, was processed by PhdF with a kcat/Km of 6.0 × 106 M-1s-1, a value that is consistent with the annotated function of PhdF in the pathway. PhdI processed 1-hydroxy-2-naphthoate with a kcat/Km of 3.1 × 105 M-1s-1, which is also consistent with the proposed role in the pathway. The observations provide the first biochemical evidence for these two reactions in M. vanbaalenii PYR-1 and, to the best of our knowledge, the first biochemical evidence for the reaction of PhdF with 3,4-dihydroxyphenanthrene. Although PhdF is upregulated in the presence of pyrene, it did not process two dihydroxypyrenes. Methodology was developed for product analysis of the extradiol dioxygenases.

Enantioselective Hydrogen Atom Transfer: Discovery of Catalytic Promiscuity in Flavin-Dependent 'Ene'-Reductases.

Flavin has long been known to function as a single electron reductant in biological settings, but this reactivity has rarely been observed with flavoproteins used in organic synthesis. Here we describe the discovery of an enantioselective radical dehalogenation pathway for α-bromoesters using flavin-dependent 'ene'-reductases. Mechanistic experiments support the role of flavin hydroquinone as a single electron reductant, flavin semiquinone as the hydrogen atom source, and the enzyme as the source of chirality.