Chemoenzymatic Synthesis of Fluorinated Mycocyclosin Enabled by the Engineered Cytochrome P450-Catalyzed Biaryl Coupling Reaction.

Installing fluorine atoms onto natural products holds great promise for the generation of fluorinated molecules with improved or novel pharmacological properties. The enzymatic oxidative carbon-carbon coupling reaction represents a straightforward strategy for synthesizing biaryl architectures, but the exploration of this method for producing fluorine-substituted derivatives of natural products remains elusive. Here, in this study, we report the protein engineering of cytochrome P450 from Mycobacterium tuberculosis (MtCYP121) for the construction of a series of new-to-nature fluorine-substituted Mycocyclosin derivatives. This protocol takes advantage of a "hybrid" chemoenzymatic procedure consisting of tyrosine phenol lyase-catalyzed fluorotyrosine preparation from commercially available fluorophenols, intermolecular chemical condensation to give cyclodityrosines, and an engineered MtCYP121-catalyzed intramolecular biphenol coupling reaction to complete the strained macrocyclic structure. Computational mechanistic studies reveal that MtCYP121 employs Cpd I to abstract a hydrogen atom from the proximal phenolic hydroxyl group of the substrate to trigger the reaction. Then, conformational change makes the two phenolic hydroxyl groups close enough to undergo intramolecular hydrogen atom transfer with the assistance of a pocket water molecule. The final diradical coupling process completes the intramolecular C-C bond formation. The efficiency of the biaryl coupling reaction was found to be influenced by various fluorine substitutions, primarily due to the presence of distinct binding conformations.

Computational insights into the binding modes, keto-enol tautomerization and stereo-electronically controlled decarboxylation of oxaloacetate in the active site of macrophomate synthase.

Oxaloacetic acid (OAA) is a ß-ketocarboxylic acid, which plays an important role as an intermediate in some metabolic pathways, including the tricarboxylic acid cycle, gluconeogenesis and fatty acid biosynthesis. Animal studies have indicated that supplementing oxaloacetic acid shows an increase of lifespan and other substantial health benefits including mitochondrial DNA protection, and protection of retinal, neural and pancreatic tissues. Most of the chemical transformations of OAA in the metabolic pathways have been extensively studied; however, the understanding of decarboxylation of OAA at the atomic level is relatively lacking. Here, we carried out MD simulations and combined quantum mechanical/molecular mechanical (QM/MM) calculations as an example to systematically elucidate the binding modes, keto-enol tautomerization and decarboxylation of OAA in the active site of macrophomate synthase (MPS), which is a Mg(II)-dependent bifunctional enzyme that catalyzes both the decarboxylation of OAA and [4+2] cycloaddition of 2-pyrone with the decarboxylated intermediate of OAA (pyruvate enolate). On the basis of our calculations, it was found that the Mg2+-coordinated oxaloacetate may exist in enol forms and keto forms. The four keto forms can be transformed into each other by simply rotating the C2-C3 single bond, nevertheless, the keto-enol tautomerization strictly requires the assistance of pocket water molecules. In addition, the decarboxylation is stereo-electronically controlled, i.e., it is the relative orientation of the terminal carboxyl anion that determines the rate of decarboxylation. As such, the chemistry of oxaloacetate in the active site of MPS is complex. On one hand, the most stable binding mode (K-I) may undergo enol-keto tautomerization to isomerize to the enol form, which may further react with the second substrate; on the other hand, K-I may isomerize to another binding mode K-II to proceed decarboxylation to generate pyruvate enolate and CO2. Starting from K-I, the enol-keto tautomerization corresponds to a barrier of 16.2 kcal mol-1, whereas the decarboxylation is associated with an overall barrier of 19.7 kcal mol-1. These findings may provide useful information for understanding the chemistry of OAA and the catalysis of related enzymes, and they are basically in agreement with the available experimental kinetic data.

Photocatalyzed Cascade Hydrogen Atom Transfers for Assembly of Multi-Substituted α-SCF3 and α-SCF2H Cyclopentanones.

A photocatalyzed formal (3+2) cycloaddition has been developed to construct original polysubstituted α-SCF3 cyclopentanones in a regio- and diastereoselective manner. This building block approach leverages trifluoromethylthio alkynes and branched/linear aldehydes, as readily available reaction partners, in consecutive hydrogen atom transfers and C-C bond formations. Difluoromethylthio alkynes are also compatible substrates. Furthermore, the potential for telescoped reaction starting from alcohols instead of aldehydes was demonstrated, as well as process automatization and scale-up under continuous microflow conditions. This prompted density functional theory (DFT) calculations to support a radical-mediated cascade process.

Selective Chiral Recognition between Amino Acids and Growing Gypsum Crystals.

In nature, selective chiral interactions between biomolecules and minerals provide insight into the mysterious origin of homochirality. Here, we show growing gypsum crystals in a nonequilibrium state can recognize chiral enantiomers of amino acids. The chiral selection for amino acids with different functional groups by growing minerals are distinct. For 11 amino acids, the d-isomer slows dynamic gypsum growth more than the l-isomer, whereas for another 7 amino acids, the opposite was observed. These differences in chiral recognition are attributed to the different stereochemical matching between the chiral amino acids and the dynamic steps of growing gypsum. These stereoselective interactions between amino acid enantiomers and dynamic growing crystals can be applied toward the fabrication of gypsum cements to regulate their structure and mechanical properties. These findings provide insight into understanding the mechanism of the origin of homochirality in nature and suggest a pathway for constructing advanced functional materials.

Regioselective [3 + 2] cycloaddition of di/trifluoromethylated hydrazonoyl chlorides with fluorinated nitroalkenes: a facile access to 3-di/trifluoroalkyl-5-fluoropyrazoles.

Herein we describe the base-mediated [3 + 2] cycloaddition reaction of di/trifluoromethylated hydrazonoyl chlorides with fluorinated nitroalkenes. The reaction protocol provides a direct and facile strategy for the dual incorporation of a fluorine atom and fluoroalkyl group into pyrazole cores, thus allowing rapid access to a wide variety of densely functionalized 3-di/trifluoroalkyl-5-fluoropyrazoles in generally high yields with excellent regioselectivities. Furthermore, several drug-like 3-di/trifluoroalkyl-5-fluoropyrazoles have been synthesized, demonstrating potent inhibitory activities against cyclooxygenase 2 (COX-2).

Triazines: Syntheses and Inverse Electron-demand Diels-Alder Reactions.

Triazines are an important class of six-membered aromatic heterocycles possessing three nitrogen atoms, resulting in three types of regio-isomers: 1,2,4-triazines (a-triazines), 1,2,3-triazines (v-triazines), and 1,3,5-triazines (s-triazines). Notably, the application of triazines as cyclic aza-dienes in inverse electron-demand Diels-Alder (IEDDA) cycloaddition reactions has been established as a unique and powerful method in N-heterocycle synthesis, natural product preparation, and bioorthogonal chemistry. In this review, we comprehensively summarize the advances in the construction of these triazines via annulation and ring-expansion reactions, especially emphasizing recent developments and challenges. The synthetic transformations of triazines are focused on IEDDA cycloaddition reactions, which have allowed access to a wide scope of heterocycles, including pyridines, carbolines, azepines, pyridazines, pyrazines, and pyrimidines. The utilization of triazine IEDDA reactions as key steps in natural product synthesis is also discussed. More importantly, a particular attention is paid on the bioorthogonal application of triazines in fast click ligation with various strained alkenes and alkynes, which opens a new opportunity for studying biomolecules in chemical biology.

Constructing Photoactivatable Protein with Genetically Encoded Photocaged Glutamic Acid.

Genetically replacing an essential residue with the corresponding photocaged analogues via genetic code expansion (GCE) constitutes a useful and unique strategy to directly and effectively generate photoactivatable proteins. However, the application of this strategy is severely hampered by the limited number of encoded photocaged proteinogenic amino acids. Herein, we report the genetic incorporation of photocaged glutamic acid analogues in E. coli and mammalian cells and demonstrate their use in constructing photoactivatable variants of various fluorescent proteins and SpyCatcher. We believe genetically encoded photocaged Glu would significantly promote the design and application of photoactivatable proteins in many areas.

Tetrazole Diversification of Amino Acids and Peptides via Silver-Catalyzed Intermolecular Cycloaddition with Aryldiazonium Salts.

Selective structural modification of amino acids and peptides is a central strategy in organic chemistry, chemical biology but also in pharmacology and material science. In this context, the formation of tetrazole rings, known to possess significant therapeutic properties, would expand the chemical space of unnatural amino acids but has received less attention. In this study, we demonstrated that the classic unimolecular Wolff rearrangement of α-amino acid-derived diazoketones could be replaced by a faster intermolecular cycloaddition reaction with aryldiazonium salts under identical practical conditions. This strategy provides an efficient synthetic platform that could transform proteinogenic α-amino acids into a plethora of unprecedented tetrazole-decorated amino acid derivatives with preservation of the stereocenters. Density functional theory studies shed some light on the reaction mechanism and provided information regarding the origins of the chemo- and regioselectivity. Furthermore, this diazo-cycloaddition protocol was applied to construct tetrazole-modified peptidomimetics and drug-like amino acid derivatives.

Highly Enantio- and Diastereoselective Hydrogenation of Cyclic Tetra-Substituted ß-Enamido Phosphorus Derivatives.

Catalytic asymmetric hydrogenation of enamido phosphorus derivatives is one of the most efficient methods for the construction of chiral amino phosphorus products, among which the congested tetra-substituted substrates remains an unaddressed challenge. In this study, we utilize a commercially available Rh-Josiphos system for the efficient and stereoselective hydrogenation of a wide set of tetra-substituted cyclic ß-enamido phosphonates/phosphine oxides, thus enabling access to chiral ß-amino phosphorus compounds featuring two vicinal stereocenters. This protocol was broadly applicable to different ring systems possessing various phosphonate/phosphine oxide groups and further applied in the preparation of amino-phosphine ligands. DFT mechanistic explorations indicate that the C=C migratory insertion into RhIII -H bond could be the rate- and stereo-determining step. The origins of stereoselectivity are revealed through distortion/interaction analysis, which is primarily regulated by distinguished dispersion interactions and steric repulsions.

Difluoromethylation of Unactivated Alkenes Using Freon-22 through Tertiary Amine-Borane-Triggered Halogen Atom Transfer.

The application of abundant and inexpensive fluorine feedstock sources to synthesize fluorinated compounds is an appealing yet underexplored strategy. Here, we report a photocatalytic radical hydrodifluoromethylation of unactivated alkenes with an inexpensive industrial chemical, chlorodifluoromethane (ClCF2H, Freon-22). This protocol is realized by merging tertiary amine-ligated boryl radical-induced halogen atom transfer (XAT) with organophotoredox catalysis under blue light irradiation. A broad scope of readily accessible alkenes featuring a variety of functional groups and drug and natural product moieties could be selectively difluoromethylated with good efficiency in a metal-free manner. Combined experimental and computational studies suggest that the key XAT process of ClCF2H is both thermodynamically and kinetically favored over the hydrogen atom transfer pathway owing to the formation of a strong boron-chlorine (B-Cl) bond and the low-lying antibonding orbital of the carbon-chlorine (C-Cl) bond.

Expanding the Scope of Genetically Encoded Lysine Post-Translational Modifications with Lactylation, ß-Hydroxybutyrylation and Lipoylation.

Post-translational modifications (PTMs) occurring on lysine residues, especially diverse forms of acylations, have seen rapid growth over the past two decades. Among them, lactylation and ß-hydroxybutyrylation of lysine side-chains are newly identified histone marks and their implications in physiology and diseases have aroused broad research interest. Meanwhile, lysine lipoylation is highly conserved in diverse organisms and well known for its pivotal role in central metabolic pathways. Recent findings in the proteomic profiling of protein lipoylation have nonetheless suggested a pressing need for an extensive investigation. For both basic and applied research, it is necessary to prepare PTM-bearing proteins particularly in a site-specific manner. Herein, we use genetic code expansion to site-specifically generate these lysine PTMs, including lactylation, ß-hydroxybutyrylation and lipoylation in proteins in E. coli and mammalian cells. Notably, using strategies including activity-based selection, screening and rational design, unique pyrrolysyl-tRNA synthetase variants were successfully evolved for each of the three non-canonical amino acids, which enabled efficient production of recombinant proteins. Through encoding these ncAAs, we examined the deacylase activities of mammalian sirtuins to these modifications, and importantly we unfold the lipoamidase activity of several sirtuins.

Amphiphilic Polyfluorinated Amino Ethers from Cyclic Sulfamidates.

Regioselective ring opening of cyclic sulfamidates was achieved by means of nucleophilic polyfluorinated alkoxides to access achiral and chiral ß- and γ-ORF amines and α-amino esters. Subsequent transformations provide free amines ready for incorporation into bioactive substances through amide bond formation or nucleophilic aromatic substitution.

Radical 1,5-Chloropentafluorosulfanylation of Unactivated Vinylcyclopropanes and Transformation into α-SF5 Ketones.

The radical 1,5-chloropentafluorosulfanylation of vinyl cyclopropanes (VCPs) initiated by Et3B/O2 affords allylic pentafluorosulfanyl/homoallylic chloride products through the ring-strain release of the cyclopropane. The VCP substitution pattern was investigated. The utility of this reaction was illustrated in post-transformation of the C═C bond by ozonolysis, giving access to valuable α-SF5 carbonyl compounds.

Regioselective [3 + 2] Cycloaddition Reaction of 3-Alkynoates with Seyferth-Gilbert Reagent.

A Et3N-triggered regioselective [3 + 2] cycloaddition reaction of 3-alkynoates with Seyferth-Gilbert reagent has been developed to furnish a series of trisubstituted pyrazole-3-phosphonates. A one-pot cycloaddition/alkylation sequence further offered access to the corresponding fully substituted pyrazoles.

Remote Fluorination and Fluoroalkyl(thiol)ation Reactions.

Remote functionalization reactions have the power to transform a C-H (or C-C) bond at a distant position from a functional group. This Review summarizes recent advances and key breakthroughs in remote fluorination, trifluoromethylation, difluoromethylation, trifluoromethylthiolation, and fluoroalkenylation reactions. Several powerful strategies have emerged to control the reactivity and distal selectivity such as the undirected radical approach, the 1,5-hydrogen atom transfer, the metal migration, the use of distant directing groups, and the ring-opening reactions. These unconventional and predictable C-H (and C-C) functionalization transformations should allow for the preparation of a wide range of otherwise-difficult-to-access alkyl, aromatic, heteroaromatic, and structurally complex fluorides.

Direct Enamido C(sp2 )-H Diphosphorylation Enabled by a PCET-Triggered Double Radical Relay: Access to gem-Bisphosphonates.

Herein we report a novel and straightforward protocol for the construction of valuable gem-BPs by means of proton-coupled electron-transfer (PCET)-triggered enamido C(sp2 )-H diphosphorylation. This reaction represents a rare example of realizing the challenging double C-P bond formation at a single carbon atom, thus providing facile access to a broad variety of structurally diverse bisphosphonates from simple enamides under silver-mediated conditions. Initial mechanistic studies demonstrated that the diphosphorylation involves two rounds of PCET-initiated radical relay process.

Catalytic Asymmetric Access to Noncanonical Chiral α-Amino Acids from Cyclic Iminoglyoxylates and Enamides.

Here we describe an enantioselective Mannich reaction of cyclic iminoglyoxylates with enamides by virtue of chiral phosphoric acid catalysis in a one-pot manner. The wide substrate scope, mild reaction conditions, and constantly excellent enantioselectivities (>95% ee in most cases) render this protocol highly practical for the rapid construction of valuable noncanonical chiral α-amino-acid building blocks.

General Synthesis of Tri-Carbo-Substituted N2-Aryl-1,2,3-triazoles via Cu-Catalyzed Annulation of Azirines with Aryldiazonium Salts.

The general synthesis of fully substituted N2-aryl-1,2,3-triazoles is hitherto challenging compared with that of the N1-aryl counterparts. Herein, we describe a Cu-catalyzed annulation reaction of azirines and aryldiazonium salts. This regiospecific method allows access to a broad spectrum of tri-carbo N2-aryl-1,2,3-triazoles substituted with diverse aryl and alkyl moieties. Its utility is highlighted by the synthesis of several triazole precursors applicable in drug discovery, as well as novel chiral binaphthyl ligands bearing triazole moieties.

Catalytic alkylation of unactivated C(sp3)-H bonds for C(sp3)-C(sp3) bond formation.

The construction of carbon-carbon bonds is a central tenet of modern synthetic chemistry. Metal-catalyzed C-H functionalization can serve as a particularly powerful tool for achieving carbon-carbon bond formation. This review summarizes the early adventures and recent advancements in catalytic transformation of unactivated C(sp3)-H bonds into C(sp3)-C(sp3) bonds. To date, three main strategies have emerged to enable this transformation, namely, metallocarbene-triggered C(sp3)-H alkylation, auxiliary-directed C(sp3)-H alkylation, and photo-induced C(sp3)-H alkylation. Compared with traditional cross-coupling reactions having both coupling partners activated with functional groups or base-promoted enolate chemistry, catalytic alkylation of unactivated C(sp3)-H bonds for C(sp3)-C(sp3) bond formation not only offers novel disconnections in retrosynthetic analysis, but also represents the future trend in green and atom-economic chemistry.

Asymmetric synthesis of CF2-functionalized aziridines by combined strong Brønsted acid catalysis.

A diastereo- and enantioselective approach to access chiral CF2-functionalized aziridines from difluorodiazoethyl phenyl sulfone (PhSO2CF2CHN2) and in situ-formed aldimines is described. This multicomponent reaction is enabled by a combined strong Brønsted acid catalytic platform consisting of a chiral disulfonimide and 2-carboxyphenylboronic acid. The optical purity of the obtained CF2-substituted aziridines could be further improved by a practical dissolution-filtration procedure.