Braca

Abstract

Here, we present an innovative approach to achieve Minisci-type alkylation of cyclic N-ketimines, enabling access to a diverse range of valuable alkylated sulfonyl ketimines via iron hydride hydrogen atom transfer. More than 35 examples are shown with a wide range of substrates. Additionally, this system is easy to operate and can be efficiently scaled up. Mechanistic investigations revealed that the alkyl radical process involved and the air acts as the sole oxidant in this system.

An effective catalytic synthesis method comprising dual photoredox and cobalt catalysis is established for heterocoupling two similar benzyl radicals from abundant methylarenes and benzyl NHPI esters. The results showed successful 1°–1°, 1°–2°, 1°–3°, 2°–2°, 2°–3° and 3°–3° benzyl couplings, benzylation of drug structure-containing molecules, scale-up experiments, continuous photochemical flow synthesis, and a one-pot procedure.

Abstract

Transition-metal-regulated radical cross coupling enables the selective bonding of two distinct transient radicals, whereas the catalytic method for sorting two almost identical transient radicals, especially similar benzyl radicals, is still rare. Herein, we show that leveraging dual photoredox/cobalt catalysis can selectively couple two similar benzyl radicals. Using easily accessible methylarenes and phenylacetates (benzyl N-hydroxyphthalimide (NHPI) esters) as benzyl radical sources, a range of unsymmetrical 1,2-diarylethane classes via the 1°–1°, 1°–2°, 1°–3°, 2°–2°, 2°–3° and 3°–3° couplings were obtained with broad functional group tolerance. Besides the photochemical continuous flow synthesis, the one-pot procedure that directly uses phenylacetic acids and NHPI as the starting materials to avoid the pre-preparation of benzyl NHPI esters for the gram-scale synthesis is also feasible and affords good yields, showcasing the synthetic utility of our protocol.

Nature Chemical Biology, Published online: 24 December 2024; doi:10.1038/s41589-024-01798-9

Cyclic peptides offer access to therapeutics with properties that sit between biologics and small molecules. A 2009 study reported the design, construction and selection of a phage-encoded bicyclic peptide library that used a chemical linker for cyclization. This and other advanced display technologies have accelerated the de novo discovery of functional cyclic peptides.

Hydrogen atom transfer (HAT) constitutes a powerful mechanism exploited in biology and chemistry alike to functionalize otherwise inert C(sp3)–H bonds in organic molecules. Despite its synthetic potential, achieving stereocontrol in chemical HAT-mediated C–H functionalization transformations remains challenging. By merging the radical reactivity of thiamine (ThDP)-dependent enzymes with chemical hydrogen atom transfer, we report here a photobiocatalytic strategy for the enantioselective C(sp3)–H acylation of an organic substrate, a transformation not found in nature, nor currently attainable by chemical means. This method enables the direct functionalization of benzylic C(sp3)–H sites in a broad range of substrates to furnish valuable enantioenriched ketone motifs with good to high enantioselectivity (up to 96% e.e.). Mechanistic and reactivity studies support the involvement of radical species derived from the Breslow intermediate and C–H substrate, along with the critical role of the photocatalyst and hydrogen atom abstraction reagent for productive catalysis. This study illustrates the productive integration of ThDP-mediated biocatalysis with chemical HAT, expanding the range of asymmetric C(sp3)–H functionalization transformations accessible through biocatalysis.

Nature Catalysis, Published online: 20 December 2024; doi:10.1038/s41929-024-01263-9

Photoredox catalysis is merged with metalloenzymatic catalysis to enable asymmetric decarboxylative azidation and thiocyanation. These transformations are achieved by coupling the photoredox activation of N-hydroxyphthalimide esters using a synthetic photocatalyst with enantioselective radical capture by Fe(iii) intermediates of non-haem iron enzymes.

Science, Volume 386, Issue 6728, Page 1421-1427, December 2024.

Nature Communications, Published online: 18 December 2024; doi:10.1038/s41467-024-55212-y

Affinity chromatography allows for the separation of biomolecules such as proteins, based on a change in the chemical solvent composition and the resulting impacts on ligand binding. Here, authors introduce a physical principle by exploiting the light-dependent interaction between the Azo-tag and an α- CD chromatography matrix.

Genetic code expansion strategies tend to incorporate non-canonical amino acids (ncAAs) with singular, specialised functions into proteins. In this study, a dual-purpose ncAA that can undergo inverse-electron-demand Diels–Alder (IEEDA) reactions and coordinate luminogenic metal complexes was designed, synthesised, and incorporated into a protein. Significantly, luminescence studies revealed emission shifts upon dual functionalisation.

Abstract

A rationally designed dual-purpose non-canonical amino acid (Trz) has been synthesised and successfully incorporated into a protein scaffold by genetic code expansion. Trz contains a 5-pyridyl-1,2,4-triazine system, which allows for inverse-electron-demand Diels–Alder (IEDDA) reactions to occur on the triazine ring and for metal ions to be chelated both before and after the click reaction. Trz was successfully incorporated into a protein scaffold and the IEDDA utility of Trz demonstrated through the site-specific labelling of the purified protein with a bicyclononyne. Additionally, Trz was shown to successfully coordinate a cyclometallated iridium(III) centre, providing access to a bioorthogonal luminogenic probe. The luminescent properties of the Ir(III)-bound protein blue-shift upon IEDDA click reaction with bicyclononyne, providing a unique method for monitoring the extent and location of the labelling reaction. In summary, Trz is a new dual-purpose non-canonical amino acid with great potential for myriad bioapplications where metal-based functionality is required, for example in imaging, catalysis, and photo-dynamic therapy, in conjunction with a bioorthogonal reactive handle to impart additional functionalities, such as dual-modality imaging or therapeutic payloads.

Bipyridyl-l-alanine (BpyAla) is a highly sought, metal-chelating non-canonical amino acid. However, its high cost has hindered many applications. Here, we develop a chemoenzymatic approach to efficiently construct BpyAla. This strategy is general to many types of metal-chelating amino acid, and we show that a newly available BpyAla analog can be incorporated into proteins using existing amber suppression technology.

Abstract

Metal-chelating noncanonical amino acids (ncAAs) are uniquely functional building blocks for proteins, peptide catalysts, and small molecule sensors. However, catalytic asymmetric approaches to synthesizing these molecules are hindered by their functional group variability and intrinsic propensity to ligate metals. In particular, bipyridyl-l-alanine (BpyAla) is a highly sought ncAA, but its complex, inefficient syntheses have limited utility. Here, we develop a chemoenzymatic approach to efficiently construct BpyAla. Three enzymes that can be produced in high titer together react to convert Gly and an aldehyde into the corresponding β-hydroxy ncAA, which is subsequently deoxygenated. We explore approaches to synthesizing biaryl aldehydes and show how the three-enzymatic cascade can access a range of α-amino acids with bulky side chains, including a variety of metal-chelating amino acids. We show that newly accessible BpyAla analogues are compatible with existing amber suppression technology, which will enable future merging of traditional synthetic and biosynthetic approaches to tuning metal reactivity.

We disclosed that activation of alkyl boronic esters by MeOLi is highly efficient for the generation of alkyl radicals under photocatalysis conditions. This strategy was successfully applied to nickel/photoredox dual-catalyzed C(sp²)−C(sp³) coupling of aryl halides with alkyl boronic esters. Mechanistic experiments such as radical trapping experiments, stoichiometric Ar−Ni(II)-X mediated or Ar−Ni(II)-X, Ni(I)-X or Ni(0) catalyzed reactions, fluorescence quenching experiments etc. were performed. DFT calculations indicated that the cross-coupling likely proceeds via a Ni(I)-catalyzed pathway. In addition, the method can also be extended to alkyl boronic acids.

Abstract

Nickel/photoredox dual catalyzed cross-coupling of aryl halides with alkylboron compounds is one of the effective methodologies for the construction of C(sp²)−C(sp³) bonds. Although elegant results have been achieved by using alkyl trifluoroborates as alkyl radical precursors, the generation of alkyl radicals from readily available alkyl boronic esters is still limited due to their high oxidation potential. We disclosed here that activation of alkyl boronic esters by MeOLi is highly efficient for the generation of alkyl radicals under photocatalysis conditions. The reaction featured with a wide substrate scope, high functional group tolerance, and late-stage modification of bioactive substances. In addition, the method was also successfully extended to alkyl boronic acids. Experimental and computational mechanistic studies indicated that the cross-coupling likely proceeds via a Ni(I)-catalyzed pathway.

Enzymes are attractive catalysts due to their high chemo-, regio-, and enantio-selectivity. In recent years the application of enzymes in organic synthesis has expanded dramatically, especially for the synthesis of chiral alcohols and amines, two very important functional groups found in many active pharmaceutical ingredients (APIs). Indeed, many elegant routes employing such compounds have been described by industry. Yet, for the synthesis chiral thiols and thioethers, likewise found in APIs albeit less ubiquitous, only very few biocatalytic syntheses have been reported, and stereo-control has proved challenging. Here we apply ene-reductases (EREDs), whose ability to initiate and control chemically challenging radical chemistries has recently emerged, to the synthesis of chiral thioethers from α-bromoacetophenones and pro-chiral vinyl sulfides, without requiring light. Depending on the choice of ERED either enantiomer of the product could be accessed. Highest conversion and selectivity were achieved with GluER T36A using fluorinated substrates, reaching up to 82% conversion and >99.5% ee. With α-bromoacetophenone and α-(methylthio)styrene, the reaction could be performed on a 100 mg scale, affording the product in 46% isolated yield with 93% ee. Finally, mechanistic studies were carried out using stopped-flow spectroscopy and protein mass-spectrometry, providing insight into the preference of the enzyme for the inter-molecular reaction. This work paves the way for new routes for the synthesis of thioether-containing compounds.

We report an efficient halogen bond-catalyzed Pictet-Spengler reaction using diaryliodonium salts as catalysts as a metal-free alternative to traditional acid catalysis. Through systematic optimization, exceptional catalytic activity was achieved with only 0.5 mol-% of a simple dibenzoiodolium with a perfluorinated borate counterion. The protocol demonstrates a broad substrate scope, converting various N-protected tryptamines and diverse carbonyl compounds (aromatic, heteroaromatic, and aliphatic aldehydes) to the corresponding tetrahydro-β-carbolines (THβCs) in up to 98% yield. The reaction versatility was further demonstrated by a successful oxa-variant using tryptophol. Control experiments revealed the crucial role of halogen bonding in promoting both the initial condensation and the final cyclization steps.

Asymmetric allylation reaction between unsymmetrical allyl carbonates and α-hydroxy ketones synergistically catalyzed by a palladium/borinic acid catalytic platform has been developed. The reaction proceeds through nucleophilic activation of α-hydroxy ketone by an achiral borinic acid to form a tetracoordinated enediol boronate complex and its subsequent interception of electrophilic π-allyl palladium complex generated from a chiral Pd catalyst and an allyl component. A broad scope of substrates was suitable in this reaction, providing a series of allyl-substituted α-hydroxy ketones bearing vicinal stereocenters with excellent enantioselectivities (up to 99% ee) and diastereoselectivities (up to 24:1 dr). The methodology also achieved effective kinetic resolution of 1,3-disubstituted allylic carbonates, with the unreacted substrates being recovered with high ee values. A plausible mechanism and model for chiral induction in this reaction are proposed. The utility of this method was demonstrated by a gram scale reaction and subsequent elaboration of the allylation products.

Mn(salen)-based artificial metalloenzymes (ArMs) were constructed by embedding biotinylated Mn(salen) complexes into streptavidin. Their activities were improved by genetic optimization of protein scaffold and the ArM variants catalyzed the aziridination of styrene and oxidation of benzylic C−H bonds with up to 19 and 146 turnover numbers.

Abstract

The development of artificial metalloenzymes (ArMs) offers a potent approach to incorporate non-natural chemical reactions into biocatalysis. Here we report the assembly of Mn(salen)-based ArMs by embedding biotinylated Mn(salen) complexes into streptavidin (Sav) variants. Using commercially available nitrene and oxo transfer reagents, these biohybrid catalysts catalyzed the aziridination of alkenes and oxidation of benzylic C−H bonds with up to 19 and 146 turnover numbers.

Nature Catalysis, Published online: 11 December 2024; doi:10.1038/s41929-024-01259-5

Compatibility issues often limit chemoenzymatic systems. Now it is shown that the proximity between catalytic polymers grafted from the membrane of microorganisms and intracellular heterologous enzymes enhances the reaction rates of a photoenzymatic system, while the coating increases the stability.

The generic incorporation of a thioxanthone-containing amino acid into a protein scaffold is described. The resulting artificial photoenzyme was engineered to catalyze a dearomative [2+2] cycloaddition reaction in high yields with excellent enantioselectivity.

Abstract

Genetically encodable photosensitizers allow the design of artificial photoenzymes to expand the scope of abiological reactions. Herein, we report the genetic incorporation of a thioxanthone-containing amino acid into a protein scaffold via an engineered pyrrolysyl-tRNA/pyrrolysyl-tRNA synthetase pair. The designer enzyme was engineered to catalyze a dearomative [2+2] cycloaddition reaction in high yields (up to>99 % yield) with excellent enantioselectivity (up to 98 : 2 e.r.). This work provides a robust and facile method for photoenzyme design and lays the foundation for the development of further photoenzymatic reactions.

This study presents the biochemical and structural characterization of CyanoPOX, a heme-containing peroxidase from Cyanobacterium sp. TDX16. Expressed in E. coli, the enzyme exhibits properties similar to bovine lactoperoxidase despite low sequence identity. CyanoPOX demonstrates optimal activity in slightly acidic conditions and efficiently oxidizes various substrates. Structural analysis reveals a non-covalently bound b-type heme cofactor and a conserved active site pocket.

Abstract

Peroxidases belong to a group of enzymes that are widely found in animals, plants and microorganisms. These enzymes are effective biocatalysts for a wide range of oxidations on various substrates. This work presents a biochemical and structural characterization of a novel heme-containing peroxidase from Cyanobacterium sp. TDX16, CyanoPOX. This cyanobacterial enzyme was successfully overexpressed in Escherichia coli as a soluble, heme-containing monomeric enzyme. Although CyanoPOX shares relatively low sequence identity (37 %) with bovine lactoperoxidase, it displays comparable biochemical properties. CyanoPOX is most stable and active in slightly acidic conditions (pH 6–6.5) and moderately thermostable (melting temperature around 48 °C). Several compounds that are typical substrates for mammalian lactoperoxidases were tested to establish the catalytic potential of CyanoPOX. Potassium iodide showed the highest catalytic efficiency (126 mM⁻¹ s⁻¹), while various aromatic compounds were also readily converted. Structural elucidation of CyanoPOX confirmed the presence of a non-covalently bound b-type heme cofactor that is situated in the central core of the protein. Except for a highly similar overall structure, CyanoPOX also has a conserved active site pocket when compared with mammalian lactoperoxidases. Due to its catalytic properties and high expression in a bacterial host, this newly discovered peroxidase shows promise for applications.

Oxazolidinones are important heterocycles widely utilized in medicinal chemistry for the synthesis of antifungals, antibacterials, and other bioactive compounds and in organic chemistry as chiral auxiliaries for asymmetric synthesis. Herein, we report a biocatalytic strategy for the synthesis of enantioenriched oxazolidinones through the intramolecular C(sp3)‑H amination of carbamate derivatives using engineered myoglobin-based catalysts. This method is applicable to a diverse range of substrates, with high functional group tolerance, providing enantioenriched oxazolidinones in good yields and with high enantioselectivity. The synthetic utility of this methodology is further highlighted by the development of enantiodivergent biocatalysts for this transformation and through the preparative-scale synthesis of key oxazolidinone intermediates for the production of the cholesterol-lowering drugs Ezetimibe and CJ-15-161. An outer sphere mutation, Y146F, was found to be beneficial to favor the productive C–H amination reaction over an unproductive reductive pathway commonly observed in hemeprotein-catalyzed nitrene transfer reactions. This study demonstrates a biocatalytic, enantiodivergent synthesis of oxazolidinones via C-H amination of carbamate derivatives, offering an attractive strategy for the synthesis of these valuable intermediates for applications in medicinal chemistry, target-directed synthesis, and asymmetric synthesis.