Highly stereoselective and enantiodivergent synthesis of cyclopropylphosphonates with engineered carbene transferases

Mechanistic investigation of sustainable heme-inspired biocatalytic synthesis of cyclopropanes for challenging substrates

Engineered heme proteins exhibit excellent sustainable catalytic carbene transfer reactivities toward olefins for value-added cyclopropanes. However, unactivated and electron-deficient olefins remain challenging in such reactions. To help design efficient heme-inspired biocatalysts for these difficult situations, a systematic quantum chemical mechanistic study was performed to investigate effects of olefin substituents, non-native amino acid axial ligands, and natural and non-natural macrocycles with the widely used ethyl diazoacetate. Results show that electron-deficient substrate ethyl acrylate has a much higher barrier than the electron-rich styrene. For styrene, the predicted barrier trend is consistent with experimentally used heme analogue cofactors, which can significantly reduce barriers. For ethyl acrylate, while the best non-native axial ligand only marginally improves the reactivity versus the native histidine model, a couple of computationally studied macrocycles can dramatically reduce barriers to the level comparable to styrene. These results will facilitate the development of better biocatalysts in this area.

Hybrid Metal Catalysts as Valuable Tools in Organic Synthesis: An Overview of the Recent Advances in Asymmetric C─C Bond Formation Reactions

Carbon-carbon bond formation represents a key reaction in organic synthesis, resulting in paramount importance for constructing the carbon backbone of organic molecules. However, traditional metal-based catalysis, despite its advantages, often struggles with issues related to efficiency, selectivity, and sustainability. On the other hand, while biocatalysis offers superior selectivity due to an extraordinary recognition process of the substrate, the scope of its applicable reactions remains somewhat limited. In this context, Artificial Metalloenzymes (ArMs) and Metallo Peptides (MPs) offer a promising and not fully explored solution, merging the two fields of transition metal catalysis and biotransformations, by inserting a catalytically active metal cofactor into a customizable protein scaffold or coordinating the metal ion directly to a short and tunable amino acid (Aa) sequence, respectively. As a result, these hybrid catalysts have gained attention as valuable tools for challenging catalytic transformations, providing systems with new-to-nature properties in organic synthesis. This review offers an overview of recent advances in the development of ArMs and MPs, focusing on their application in the asymmetric carbon-carbon bond-forming reactions, such as carbene insertion, Michael additions, Friedel-Crafts and cross-coupling reactions, and cyclopropanation, underscoring the versatility of these systems in synthesizing biologically relevant compounds.

Biocatalytic Strategy for the Highly Stereoselective Synthesis of Fluorinated Cyclopropanes

Fluorinated cyclopropanes are highly desired pharmacophores in drug discovery owing to the rigid nature of the cyclopropane ring and the beneficial effects of C-F bonds on the pharmacokinetic properties, cell permeability, and metabolic stability of drug molecules. Herein a biocatalytic strategy for the stereoselective synthesis of mono-fluorinated and gem-difluoro cyclopropanes is reported though the use of engineered myoglobin-based catalysts. In particular, this system allows for a broad range of gem-difluoro alkenes to be cyclopropanated in the presence of diazoacetonitrile with excellent diastereo and enantiocontrol (up to 99 : 1 d.r. and 99 % e.e.), thereby enabling a transformation not currently accessible with chemocatalytic methods. The synthetic utility of the present approach is further exemplified through the gram-scale synthesis of a key gem-difluorinated cyclopropane intermediate useful for the preparation of fluorinated bioactive molecules.

Stereodivergent Synthesis of Pyridyl Cyclopropanes via Enzymatic Activation of Pyridotriazoles

Hemoproteins have recently emerged as powerful biocatalysts for new-to-nature carbene transfer reactions. Despite this progress, these strategies have remained largely limited to diazo-based carbene precursor reagents. Here, we report the development of a biocatalytic strategy for the stereoselective construction of pyridine-functionalized cyclopropanes via the hemoprotein-mediated activation of pyridotriazoles (PyTz) as stable and readily accessible carbene sources. This method enables the asymmetric cyclopropanation of a variety of olefins, including electron-rich and electrodeficient ones, with high activity, high stereoselectivity, and enantiodivergent selectivity, providing access to mono- and diarylcyclopropanes that incorporate a pyridine moiety and thus two structural motifs of high value in medicinal chemistry. Mechanistic studies reveal a multifaceted role of 7-halogen substitution in the pyridotriazole reagent toward favoring multiple catalytic steps in the transformation. This work provides the first example of asymmetric olefin cyclopropanation with pyridotriazoles, paving the way to the exploitation of these attractive and versatile reagents for enzyme-catalyzed carbene-mediated reactions.

Chemoenzymatic synthesis of optically active α-cyclopropyl-pyruvates and cyclobutenoates via enzyme-catalyzed carbene transfer with diazopyruvate

Cyclopropanes are recurrent structural motifs in natural products and bioactive molecules. Recently, biocatalytic cyclopropanations have emerged as a powerful approach to access enantioenriched cyclopropanes, complementing chemocatalytic approaches developed over the last several decades. Here, we report the development of a first biocatalytic strategy for cyclopropanation using ethyl α-diazopyruvate as a novel enzyme-compatible carbene precursor. Using myoglobin variant Mb(H64V,V68G) as the biocatalyst, this method afforded the efficient synthesis of α-cyclopropylpyruvates in high diastereomeric ratios and enantiomeric excess (up to 99% ee). The ketoester moiety in the cyclopropane products can be used to synthesize diverse optically pure cyclopropane derivatives. Furthermore, the enzymatically obtained α-cyclopropylpyruvate products could be converted into enantiopure cyclobutenoates via a metal-free photochemical ring expansion without loss of optical activity.

Application of Vitreoscilla Hemoglobin as a Green and Efficient Biocatalyst for the Synthesis of Benzoxazoles in Water

We report an efficient and eco-friendly method for the Vitreoscilla hemoglobin (VHb)-catalyzed synthesis of benzoxazoles in water at room temperature. tert-Butyl hydroperoxide and 2,2,6,6-tetramethyl-1-piperidinyloxy were used as oxidant and radical scavenger, respectively. A total of 27 functionally diverse benzoxazoles were prepared in moderate to high yields (62 %-94 %) by the annulation reaction of phenols with amines in the presence of VHb in 1 h. Thus, this method is highly viable for practical applications. This work broadens the application of hemoglobin to organic synthesis.

Mechanistic manifold in a hemoprotein-catalyzed cyclopropanation reaction with diazoketone

Hemoproteins have recently emerged as promising biocatalysts for new-to-nature carbene transfer reactions. However, mechanistic understanding of the interplay between productive and unproductive pathways in these processes is limited. Using spectroscopic, structural, and computational methods, we investigate the mechanism of a myoglobin-catalyzed cyclopropanation reaction with diazoketones. These studies shed light on the nature and kinetics of key catalytic steps in this reaction, including the formation of an early heme-bound diazo complex intermediate, the rate-determining nature of carbene formation, and the cyclopropanation mechanism. Our analyses further reveal the existence of a complex mechanistic manifold for this reaction that includes a competing pathway resulting in the formation of an N-bound carbene adduct of the heme cofactor, which was isolated and characterized by X-ray crystallography, UV-Vis, and Mössbauer spectroscopy. This species can regenerate the active biocatalyst, constituting a non-productive, yet non-destructive detour from the main catalytic cycle. These findings offer a valuable framework for both mechanistic analysis and design of hemoprotein-catalyzed carbene transfer reactions.

Recent Synthetic Advances on the Use of Diazo Compounds Catalyzed by Metalloporphyrins

Diazo compounds are organic substances that are often used as precursors in organic synthesis like cyclization reactions, olefinations, cyclopropanations, cyclopropenations, rearrangements, and carbene or metallocarbene insertions into C-H, N-H, O-H, S-H, and Si-H bonds. Typically, reactions from diazo compounds are catalyzed by transition metals with various ligands that modulate the capacity and selectivity of the catalyst. These ligands can modify and enhance chemoselectivity in the substrate, regioselectivity and enantioselectivity by reflecting these preferences in the products. Porphyrins have been used as catalysts in several important reactions for organic synthesis and also in several medicinal applications. In the chemistry of diazo compounds, porphyrins are very efficient as catalysts when complexed with low-cost metals (e.g., Fe and Co) and, therefore, in recent years, this has been the subject of significant research. This review will summarize the advances in the studies involving the field of diazo compounds catalyzed by metalloporphyrins (M-Porph, M = Fe, Ru, Os, Co, Rh, Ir) in the last five years to provide a clear overview and possible opportunities for future applications. Also, at the end of this review, the properties of artificial metalloenzymes and hemoproteins as biocatalysts for a broad range of applications, namely those concerning carbene-transfer reactions, will be considered.

Reversing the Enantioselectivity of Enzymatic Carbene N-H Insertion Through Mechanism-Guided Protein Engineering

We report a computationally driven approach to access enantiodivergent enzymatic carbene N-H insertions catalyzed by P411 enzymes. Computational modeling was employed to rationally guide engineering efforts to control the accessible conformations of a key lactone-carbene (LAC) intermediate in the enzyme active site by installing a new H-bond anchoring point. This H-bonding interaction controls the relative orientation of the reactive carbene intermediate, orienting it for an enantioselective N-nucleophilic attack by the amine substrate. By combining MD simulations and site-saturation mutagenesis and screening targeted to only two key residues, we were able to reverse the stereoselectivity of previously engineered S-selective P411 enzymes. The resulting variant, L5_FL-B3, accepts a broad scope of amine substrates for N-H insertion with excellent yields (up to >99 %), high efficiency (up to 12 300 TTN), and good enantiocontrol (up to 7 : 93 er).

Structure-guided semi-rational design of an imine reductase for enantio-complementary synthesis of pyrrolidinamine

In this study, engineered imine reductases (IREDs) of IRED M5, originally from Actinoalloteichus hymeniacidonis, were obtained through structure-guided semi-rational design. By focusing on mutagenesis of the residues that directly interact with the ketone donor moiety, we identified two residues W234 and F260, playing essential roles in enhancing and reversing the stereoselectivity, respectively. Moreover, two completely enantio-complementary variants S241L/F260N (R-selectivity up to 99%) and I149D/W234I (S-selectivity up to 99%) were achieved. Both variants showed excellent stereoselectivity toward the tested substrates, offering valuable biocatalysts for synthesizing pyrrolidinamines. Its application was demonstrated in a short synthesis of the key intermediates of potential drug molecules leniolisib and JAK1 inhibitor 4, from cheap and commercially available pro-chiral N-Boc-piperidone 1 (2 and 3 steps, respectively).

Regiodivergent and Enantioselective Hydroxylation of C-H bonds by Synergistic Use of Protein Engineering and Exogenous Dual-Functional Small Molecules

It is a great challenge to optionally access diverse hydroxylation products from a given substrate bearing multiple reaction sites of sp³ and sp² C-H bonds. Herein, we report the highly selective divergent hydroxylation of alkylbenzenes by an engineered P450 peroxygenase driven by a dual-functional small molecule (DFSM). Using combinations of various P450BM3 variants with DFSMs enabled access to more than half of all possible hydroxylated products from each substrate with excellent regioselectivity (up to >99 %), enantioselectivity (up to >99 % ee), and high total turnover numbers (up to 80963). Crystal structure analysis, molecular dynamic simulations, and theoretical calculations revealed that synergistic effects between exogenous DFSMs and the protein environment controlled regio- and enantioselectivity. This work has implications for exogenous-molecule-modulated enzymatic regiodivergent and enantioselective hydroxylation with potential applications in synthetic chemistry.

Hot off the Press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as hyjapone A from Hypericum japonicum.