Tetrahedral Intermediate Research Articles

Catalytic mechanism of DcsB: Arginase framework used for hydrolyzing its inhibitor.

DcsB, an enzyme produced from the d-cycloserine biosynthetic gene cluster, displays moderate similarity to arginase in the sequence and three-dimensional structure. Arginase is a ubiquitous enzyme hydrolyzing l-arginine to generate l-ornithine and urea, whereas DcsB hydrolyzes Nω -hydroxy-l-arginine (l-NOHA), an arginase inhibitor, to generate l-ornithine and hydroxyurea. We determined the crystal structure of DcsB associated with l-ornithine and that with the tetrahedral derivative of 2(S)-amino-6-boronohexanoic acid, whose boron atom forms a covalent bond with an oxygen atom bridging two manganese ions at the active center. The substrate-binding pocket of DcsB is narrower than that of arginase, suggesting that DcsB is unsuitable for the binding of l-NOHA in an inhibitory manner. The transition state-like structure demonstrated that Asp210 and Glu241 have a role to trap a positively charged ion near the dimanganese cluster. Kinetic analysis using the mutated DcsB showed that the enzyme employs different catalytic mechanisms under the neutral and alkaline pH conditions. Glu241 in DcsB is likely involved in the recognition of the hydroxyguanidino group of l-NOHA, whereas Asp210, in cooperation with Glu241, seems to contribute to the reactivity toward the protonated l-NOHA, which is a preferable species under the neutral pH conditions. After entering of the protonated l-NOHA to the substrate-binding pocket of DcsB, a hydronium ion may be trapped at the positive ion-binding site. Then, the ion serves as a specific acid catalyst to facilitate the collapse of the tetrahedral intermediate of l-NOHA.

Quantum Mechanics/Molecular Mechanics Studies on the Catalytic Mechanism of a Novel Esterase (FmtA) of Staphylococcus aureus.

FmtA is a novel esterase that shares the penicillin-binding protein (PBP) core structural folding but found to hydrolyze the removal of d-Ala from teichoic acids. Molecular docking, dynamics, and MM-GBSA of FmtA and its variants S127A, K130A, Y211A, D213A, and K130AY211A, in the presence or absence of wall teichoic acid (WTA), suggest that active site residues S127, K130, Y211, D213, N343, and G344 play a role in substrate binding. Quantum mechanics (QM)/molecular mechanics (MM) calculations reveal that during WTA catalysis, K130 deprotonates S127, and the nucleophilic S127 attacks the carbonyl carbon of d-Ala bound to WTA. The tetrahedral intermediate (TI) complex is stabilized by hydrogen bonding to the oxyanion holes. The TI complex displays a high energy gap and collapses to an energetically favorable acyl-enzyme complex.

Complexities of Regioselective Ring-Opening vs Transcarbonylation-Driven Structural Metamorphosis during Organocatalytic Polymerizations of Five-Membered Cyclic Carbonate Glucose Monomers.

Rigorous investigations of the organobase-catalyzed ring-opening polymerizations (ROPs) of a series of five-membered cyclic carbonate monomers derived from glucose revealed that competing transcarbonylation reactions scrambled the regiochemistries of the polycarbonate backbones. Regioirregular poly(2,3-α-d-glucose carbonate) backbone connectivities were afforded by 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)-catalyzed ROPs of three monomers having different cyclic acetal protecting groups through the 4- and 6-positions. Small molecule studies conducted upon isolated unimers and dimers indicated a preference for Cx–O2 vs Cx–O3 bond cleavage from tetrahedral intermediates along the pathways of addition–elimination mechanisms when the reactions were performed at room temperature. Furthermore, treatment of isolated 3-unimer or 2-unimer, having the carbonate linkage in the 3- or 2-position as obtained from either Cx–O2 or Cx–O3 bond cleavage, respectively, gave the same 74:26 (3-unimer:2-unimer) ratio, confirming the occurrence of transcarbonylation reactions with a preference for 3-unimer vs. 2-unimer formation in the presence of organobase catalyst at room temperature. In contrast, unimer preparation at −78 °C favored Cx–O3 bond cleavage to afford a majority of 2-unimer, presumably due to a lack of transcarbonylation side reactions. Computational studies supported the experimental findings, enhancing fundamental understanding of the regiochemistry resulting from the ring-opening and subsequent transcarbonylation reactions during ROP of glucose carbonates. These findings are expected to guide the development of advanced carbohydrate-derived polymer materials by an initial monomer design via side chain acetal protecting groups, with the ability to evolve the properties further through later-stage structural metamorphosis.

Detection of the tetrahedral reaction intermediate of the reaction of acetyl chloride with ethanol in microdroplets via laser desorption/ionization tandem mass spectrometry.

Understanding the fundamental mechanisms of chemical reactions is of great interest to scientists working in many fields as it enables the rationalization, prediction, and design of reactions. Many chemical processes involve the formation of short-lived reaction intermediates, most of which cannot be isolated and are challenging to detect. One such intermediate is the tetrahedral intermediate often proposed to be generated upon the reactions of acetyl chlorides with simple alcohols via an addition/elimination mechanism. However, the formation of this tetrahedral intermediate is a subject of controversy as it has not been detected. Furthermore, some kinetic evidence suggests the SN2 mechanism for this reaction. In the present investigation, a 266 nm pulsed Nd:YAG laser was used to evaporate and ionize reactants, reaction intermediates, and products in microdroplets of acetyl chloride and ethanol. A linear quadrupole ion trap mass spectrometer was used to detect the ions and collision-activated dissociation (CAD) experiments were employed for their structural characterization. The results demonstrate the formation of the protonated tetrahedral intermediate of the addition/elimination reaction. The protonated reaction intermediate was isolated and subjected to CAD, which resulted in the loss of water and ethylene, thus confirming its structure. These results demonstrate that the ethanolysis of acetyl chloride proceeds via an addition/elimination mechanism involving a tetrahedral reaction intermediate. However, the parallel occurrence of the SN2 mechanism cannot be ruled out.

Transamidation of thioamides with nucleophilic amines: thioamide N-C(S) activation by ground-state-destabilization.

Thioamides are 'single-atom' isosteres of amide bonds that have found broad applications in organic synthesis, biochemistry and drug discovery. In this New Talent themed issue, we present a general strategy for activation of N-C(S) thioamide bonds by ground-state-destabilization. This concept is outlined in the context of a full study on transamidation of thioamides with nucleophilic amines, and relies on (1) site-selective N-activation of the thioamide bond to decrease resonance and (2) highly chemoselective nucleophilic acyl addition to the thioamide CS bond. The follow-up collapse of the tetrahedral intermediate is favored by the electronic properties of the amine leaving group. The ground-state-destabilization concept of thioamides enables weakening of the N-C(S) bond and rationally modifies the properties of valuable thioamide isosteres for the development of new methods in organic synthesis. We fully expect that in analogy to the burgeoning field of destabilized amides introduced by our group in 2015, the thioamide bond ground-state-destabilization activation concept will find broad applications in various facets of chemical science, including metal-free, metal-catalyzed and metal-promoted reaction pathways.

A CO2-Catalyzed Transamidation Reaction.

Transamidation reactions are often mediated by reactive substrates in the presence of overstoichiometric activating reagents and/or transition metal catalysts. Here we report the use of CO2 as a traceless catalyst: in the presence of catalytic amounts of CO2, transamidation reactions were accelerated with primary, secondary, and tertiary amide donors. Various amine nucleophiles including amino acid derivatives were tolerated, showcasing the utility of transamidation in peptide modification and polymer degradation (e.g., Nylon-6,6). In particular, N,O-dimethylhydroxyl amides (Weinreb amides) displayed a distinct reactivity in the CO2-catalyzed transamidation versus a N2 atmosphere. Comparative Hammett studies and kinetic analysis were conducted to elucidate the catalytic activation mechanism of molecular CO2, which was supported by DFT calculations. We attributed the positive effect of CO2 in the transamidation reaction to the stabilization of tetrahedral intermediates by covalent binding to the electrophilic CO2.

Amino Acid Based Antimicrobial Agents - Synthesis and Properties.

Structures of several dozen of known antibacterial, antifungal or antiprotozoal agents are based on the amino acid scaffold. In most of them, the amino acid skeleton is of a crucial importance for their antimicrobial activity, since very often they are structural analogs of amino acid intermediates of different microbial biosynthetic pathways. Particularly, some aminophosphonate or aminoboronate analogs of protein amino acids are effective enzyme inhibitors, as structural mimics of tetrahedral transition state intermediates. Synthesis of amino acid antimicrobials is a particular challenge, especially in terms of the need for enantioselective methods, including the asymmetric synthesis. All these issues are addressed in this review, summing up the current state‐of‐the‐art and presenting perspectives fur further progress.

Reaction Mechanism of MHETase, a PET Degrading Enzyme

In 2016, one of the two enzymes involved in the polyethylene terephthalate (PET) degradation pathway of Ideonella sakaiensis 201-F6, MHETase, was found to exhibit a strong ability to degrade the PET monomer mono-(2-hydroxyethyl)terephthalate (MHET) at room temperature, converting it back into the precursors used in PET production. MHETase engineering to improve efficiency is an active field that suffers from an incomplete characterization of its reaction mechanism. In this paper, we analyze the reaction mechanism of MHETase using umbrella sampling molecular dynamics simulations at the B3LYP/MM level of theory. The combination of a high theoretical level and extensive sampling generated a very robust computational prediction. We found that MHETase catalyzed the conversion of MHET in two steps, with a rate-limiting step activation barrier of ΔG⧧ = 19.35 ± 0.15 kcal·mol–1 (from the weighted-histogram analysis). Our calculations are in line with the hypothesis that a transient tetrahedral intermediate mediates the reaction mechanism in each step, which is quite common in the serine hydrolase class. The energy of the first tetrahedral intermediate was similar to that of the reactant state, while the tetrahedral intermediate of the deacylation step was observed to lie closer to the rate-limiting transition state. Nevertheless, both determined tetrahedral states were found to be transient, with activation barriers close to ∼2.0 kcal·mol–1 relative to the product state of the acylation and deacylation steps, corresponding to a half-life of about 3 ps at 303.15 K.

Hydrogen Bonds and n → π* Interactions in the Acetylation of Propranolol Catalyzed by Candida antarctica Lipase B: A QTAIM Study.

Enzyme–substrate interactions play a crucial role in enzymatic catalysis. Quantum theory of atoms in molecules (QTAIM) calculations are extremely useful in computational studies of these interactions because they provide very detailed information about the strengths and types of molecular interactions. QTAIM also provides information about the intramolecular changes that occur in the catalytic reaction. Here, we analyze the enzyme–substrate interactions and the topological properties of the electron density in the enantioselective step of the acylation of (R,S)-propranolol, an aminoalcohol with therapeutic applications, catalyzed by Candida antarctica lipase B. Eight reaction paths (four for each enantiomer) are investigated and the energies, atomic charges, hydrogen bonds, and n → π* interactions of propranolol, the catalytic triad (composed of D187, H224, and S105), and the oxyanion hole are analyzed. It is found that D187 acts as an electron density reservoir for H224, and H224 acts as an electron density reservoir for the active site of the protein. It releases electron density when the tetrahedral intermediate is formed from the Michaelis complex and receives it when the enzyme–product complex is formed. Hydrogen bonds can be grouped into noncovalent and covalent hydrogen bonds. The latter are stronger and more important for the reaction than the former. We also found weak n → π* interactions, which are characterized by QTAIM and the natural bond orbital (NBO) analysis.

Stability comparison of four lipases and catalytic mechanism during the synthesis of 1,3-di-oleic-2-medium chain triacylglycerols in a trace water-in-oil system: Experimental analyses and computational simulations.

In the present study, a kind of structured lipids, namely 1,3-di-oleic-2-medium chain (OMO) triacylglycerols, were synthesized through lipase-catalyzed reactions using coconut oil and rapeseed acid as materials in a trace water-in-oil system. Experimental analysis and computational simulations were undertaken to compare the stability of four lipases including Lipozyme RMIM, Lipozyme TLIM, Novozym 435, and Aspergillus oryzae immobilized lipase (AOIM), and illustrate catalytic mechanism of Novozym 435 during the synthesis of OMO. Fourier transform infrared and molecular dynamics simulation results demonstrated that a decrease in ordered structure (α-helix and β-sheet) led to a reduction in enzyme activity. Compared with Lipozyme RMIM and Novozym 435, Lipozyme TLIM and AOIM exhibited better stability due to a short-chain lid in TLIM, which covers activity sites, and hydrogen bonds formed between activity center of AOIM and water. Among four lipases, AOIM exhibited best catalytic performance: a OMO yield of 30.7% at 3hr and a good stability of long term (48hr). Density functional theory results demonstrated that specifically, during the synthesis of OMO triacylglycerol, the addition of Novozym 435 (derived from Candida antarctica lipase B, CALB) substantially lowered reaction barriers (64.4KJ/mol with CALB vs. 332.7KJ/mol with no lipase), aiding in the generation of OMO because of the formations of transitional tetrahedral intermediates. A trace water-in-oil system was a green and efficient alternative for lipase-catalyzed production of OMO, and this study provided crucial insights into the stability/instability and catalytic mechanisms of lipase in the synthesis of structured lipids. PRACTICAL APPLICATIONS: We compared the stability of Lipozyme RMIM, Lipozyme 435, Lipozyme TLIM, and AOIM during the synthesis of the OMO triacylglycerols in a trace water-in-oil system, where exhibited a high catalytic activity of lipase in water-oil interface. AOIM had the highest stability and showed the best catalytic performance due to the formation of hydrogen bonds. Besides, for the first time, the transition tetrahedral structure was revealed in the enzymatic synthesis of medium- and long-chain triacylglycerols. This study provides a rational approach to compare lipase stability and a possible hint to choose appropriate enzyme in a specific catalytic condition.

Acyl Transfer Catalytic Activity in De Novo Designed Protein with N-Terminus of α-Helix As Oxyanion-Binding Site.

The design of catalytic proteins with functional sites capable of specific chemistry is gaining momentum and a number of artificial enzymes have recently been reported, including hydrolases, oxidoreductases, retro-aldolases, and others. Our goal is to develop a peptide ligase for robust catalysis of amide bond formation that possesses no stringent restrictions to the amino acid composition at the ligation junction. We report here the successful completion of the first step in this long-term project by building a completely de novo protein with predefined acyl transfer catalytic activity. We applied a minimalist approach to rationally design an oxyanion hole within a small cavity that contains an adjacent thiol nucleophile. The N-terminus of the α-helix with unpaired hydrogen-bond donors was exploited as a structural motif to stabilize negatively charged tetrahedral intermediates in nucleophilic addition-elimination reactions at the acyl group. Cysteine acting as a principal catalytic residue was introduced at the second residue position of the α-helix N-terminus in a designed three-α-helix protein based on structural informatics prediction. We showed that this minimal set of functional elements is sufficient for the emergence of catalytic activity in a de novo protein. Using peptide-αthioesters as acyl-donors, we demonstrated their catalyzed amidation concomitant with hydrolysis and proved that the environment at the catalytic site critically influences the reaction outcome. These results represent a promising starting point for the development of efficient catalysts for protein labeling, conjugation, and peptide ligation.

Lipase-Catalyzed Fully Aliphatic Copolyesters Based on Renewable Isohexide Isomers

Isohexides are versatile carbohydrate-based building blocks for designing biodegradable polymers with tunable biodegradability and enhanced material properties owing to their unique high structural rigidity and hydrophilicity. However, the limited reactivity and thermal stability of isohexides, especially the isomers with endo-hydroxyl groups, hamper their practical applications. In this work, fully aliphatic copolyesters based on two isohexide stereoisomers, isosorbide (IS) and isomannide (IM), were comparatively synthesized via a mild lipase-catalyzed polymerization [enzymatic polymerization (EP)] technique. The products were obtained with fairly high molecular weights (Mn values: 15,300–31,500 g·mol–1), negligible degree of discoloration, and 20–40 °C higher thermal stabilities (Td,5%: 335–360 °C) compared to those of their counterparts obtained by melt polymerization (MP). Molecular dynamics (MD) simulation revealed that the endo-OH is preferred to the exo-OH under the EP process having a high hydrogen-bonding frequency with the catalytic site of CALB (lipase immobilized from Candida antarctica, CALB), and it also requires considerably low energy (70–100 kJ·mol–1) to form the second tetrahedral transition-state intermediates. The wide-angle X-ray diffraction (WAXD) study further elucidates the interesting influence of the EP process on inducing specific β-type crystalline structures.

X-ray Crystallographic Snapshots of Substrate Binding in the Active Site of Histone Deacetylase 10.

Histone deacetylase 10 (HDAC10) is a zinc-dependent polyamine deacetylase enriched in the cytosol of eukaryotic cells. The active site of HDAC10 contains catalytic residues conserved in other HDAC isozymes that function as lysine deacetylases: Y307 assists the zinc ion in polarizing the substrate carbonyl for nucleophilic attack, and the H136-H137 dyad serves general base-general acid functions. As an inducer of autophagy, HDAC10 is an attractive target for the design of selective inhibitors that may be useful in cancer chemotherapy. Because detailed structural information regarding the catalytic mechanism of HDAC10 may inform new approaches to inhibitor design, we now report X-ray crystal structures of HDAC10 in which reaction intermediates with substrates N8-acetylspermidine and N-acetylputrescine are trapped in the active site. The Y307F substitution prevents activation of the substrate carbonyl for nucleophilic attack by the zinc-bound water molecule, thereby enabling crystallographic isolation of intact enzyme-substrate complexes. The H137A substitution removes the catalytically obligatory general acid, thereby enabling crystallographic isolation of oxyanionic tetrahedral intermediates. Finally, the acetate complex with the wild-type enzyme represents a product complex after dissociation of the polyamine coproduct. Taken together, these structures provide snapshots of the reaction coordinate of acetylpolyamine hydrolysis and are consistent with a mechanism in which tandem histidine residues H136 and H137 serve as general base and general acid catalysts, respectively. The function of the histidine dyad in the HDAC10 mechanism appears to be similar to that in HDAC6, but not HDAC8 in which both functions are served by the second histidine of the tandem pair.

Unique, yet Typical Oxyanion Holes in Aspartic Proteases

From studies on pepsin nearly 200 years ago to the modern pursuit of drugs targeting renin, HIV protease, and β-secretases, aspartic proteases continue to shape the understanding of proteins. Though their structure and reactivity are of great interest for the design of therapeutics, mechanistic details remain elusive. We reveal within a stereoelectronic link between “the most obscure of all the proteases” and the oxyanion hole of serine proteases—the vivid illustration of nature’s catalytic strategy of transition state stabilization. Specifically, rate-limiting breakdown of the tetrahedral intermediate is facilitated by n → π* donation from the forming C-terminus into an active-site glycine. Cooperative H-bonds strengthen this rare, if not unreported, mode of enzyme catalysis in a fashion resembling that found in serine proteases. Exploiting this interaction provides a strategy for the design of next-generation inhibitors.

The specific features of the liquid-phase oxidation of saturated esters. Kinetics, reactivity and mechanisms of formation of destruction products

Data on kinetics, reactivity, and mechanisms of formation of destruction products of the liquid-phase oxidation of saturated esters by molecular oxygen have been systematized and discussed. The ester group has both an activating and deactivating effects on the reactivity of the CH bonds of the alkoxyl (α′-, β′-, γ′-, and δ′-) and acyl (α-, β-, and γ-) groups. The activating effect in esters is weaker than in alcohols, ketones, and ethers and extends to the α- and α′-CH bonds. The reactivity of the β-, β′-, and γ′-CH bonds is significantly lower than that of the secondary CH bonds of hydrocarbons. Oxidation of the β-CH bonds of the acyl moieties of esters leads to HOO· radical, α,β-epoxy-, α,β-dihydroxy-, and α,β-unsaturated esters, but not to hydroperoxides. Upon the oxidation of esters, the highly active α-carboxy-alkylperoxy-, α-acyloxyperoxy-, and HOO· -radicals are formed. The recombination reactions involving these radicals occur both with and without chain termination. The specificity of the decomposition of ester hydroperoxides are due to intra- and intermolecular interactions of the hydroperoxide groups with the ester groups. Labile peroxide intermediates (α-hydroxyperoxy-esters, peroxyesters, and peroxylactones) are formed through the corresponding tetrahedral intermediates and decay predominantly by the non-radical pathway. The recombination of the peroxyl radicals and the decay of labile peroxides result in the parallel accumulation of peroxide and non-peroxide products during the oxidation of esters.

One- and Two-Proton Transfer Mechanisms Coexist in One Active Site.

Acibenzolar-S-methyl (ASM) is one of the most successfully commercialized plant activators of the systemic acquired resistance (SAR). However, its activation (hydrolysis) mechanism catalyzed by the salicylic acid binding protein 2 (SABP2) remains elusive. The fundamental catalytic mechanism of the SABP2-catalyzed hydrolysis of the ASM had been investigated by extensive computational and experimental studies, including QM/MM simulations, charge transfer analysis, small-molecule synthesis, and biochemical assays. Here we report that the promiscuous SABP2 shows different catalytic mechanisms toward different substrates. To catalyze the ASM hydrolysis, the SABP2 uses a two-proton transfer mechanism, and the key intermediate is stabilized by the charge transfer effect; to catalyze the ethyl 1,2,3-benzothiadiazole-7-carboxylate (BTM, an ASM analogue) hydrolysis, the SABP2 applies the one-proton transfer mechanism, and the classic tetrahedral intermediate is stabilized by the electrostatic effect. The HPLC analyses of the SABP2 esterase activities toward the ASM and the BTM show comparable results with our computaional results, suggesting that the obtained computational mechanism insights are reasonable. The obtained mechanism is not only an important supplement to the theory of enzymes' catalytic promiscuity, but it also contributes a possible strategy for the design of next generation plant SAR activators.

Kinetic Studies of the Effect of pH on the Trypsin-Catalyzed Hydrolysis of N-α-benzyloxycarbonyl-l-lysine-p-nitroanilide: Mechanism of Trypsin Catalysis.

The pH dependence of the trypsin-catalyzed hydrolysis of N-α-benzyloxycarbonyl-l-lysine p-nitroanilide has been studied at 25 °C. kcat/KM was maximal at alkaline pH values but decreased with decreasing pH. kcat/KM was dependent on free enzyme pKa values of 6.75 ± 0.09 and 4.10 ± 0.13, which were assigned to the ionization of the active site histidine-57 and aspartate-189, respectively. Protonation of either group abolished catalytic activity. kcat is shown to equal the acylation rate constant k2 over the pH range studied. k2 decreased on the protonation of two groups with pKa values of 4.81 ± 0.15 and 4.23 ± 0.19. We assign the pKa of 4.23 to the ionization of the aspartate-189 residue and the pKa of 4.81 to the oxyanion of the tetrahedral intermediate formed during acylation. We conclude that during acylation, breakdown of the catalytic tetrahedral intermediate is rate-limiting and that there is a strong interaction between the imidazolium ion of histidine-57 and the oxyanion of the catalytic tetrahedral intermediate, which perturbs their pKa values. From the pH dependence of k3, we conclude that deacylation depends on a pKa of 6.41 ± 0.22 and that the ionization of the carboxylate group of aspartate-189 does not have a significant effect on the rate of deacylation (k3). A catalytic mechanism is proposed to explain the pH dependence of catalysis.

MurD inhibitors as antibacterial agents: a review

Peptidoglycan is an essential component of the bacterial cell wall and is required for the survival of bacteria. ATP-dependent MurC–F ligases are responsible for the early stages of peptidoglycan biosynthesis. Second in the series, MurD catalyzes the addition of d-glutamic acid to UDP-MurNAc-l-Ala which involves acyl-phosphate and tetrahedral intermediates. Due to its high specificity for d-glutamic acid substrate and its absence in mammalian cells, MurD is considered as an attractive target for the design of novel antibacterial agents. Several MurD inhibitors have been designed and tested for their activity. These include phosphinates, N-acetylmuramic acids, pulvinones, phosphinodipeptide, cyclic nonapeptides, macrocyclic compounds, benzene-1,3-dicarboxylic acid, naphthalene sulphonamides, 2-thioxothiazolidin-4-ones, benzylidene-2,4-thiazolidindiones, etc. In the present review, an updated status of MurD enzyme inhibitors is presented which will serve as a useful source of structural information and may be utilized for the design of potent inhibitors against this enzyme.

Kinetically Controlled, Highly Chemoselective Acylation of Functionalized Grignard Reagents with Amides by N-C Cleavage.

The direct transition-metal-free acylation of amides with functionalized Grignard reagents by highly chemoselective N-C cleavage under kinetic control has been accomplished. The method offers rapid and convergent access to functionalized biaryl ketones through transient tetrahedral intermediates. The direct access to functionalized Grignard reagents by in situ halogen-magnesium exchange promoted by the versatile turbo-Grignard reagent (iPrMgCl⋅LiCl) permits excellent substrate scope with respect to both the amide and Grignard coupling partners. These reactions enable facile, operationally simple and chemoselective access to tetrahedral intermediates from amides under significantly milder conditions than chelation-controlled intermediates. This novel direct two-component coupling sets the stage for using amides as acylating reagents in an alternative paradigm to the metal-chelated approach, acyl metals and Weinreb amides.

Transition-Metal-Free Activation of Amides by N-C Bond Cleavage.

The amide bond N-C activation represents a powerful strategy in organic synthesis to functionalize the historically inert amide linkage. This personal account highlights recent remarkable advances in transition-metal-free activation of amides by N-C bond cleavage, focusing on both (1) mechanistic aspects of ground-state-destabilization of the amide bond enabling formation of tetrahedral intermediates directly from amides with unprecedented selectivity, and (2) synthetic utility of the developed transformations. Direct nucleophilic addition to amides enables a myriad of powerful methods for the formation of C-C, C-N, C-O and C-S bonds, providing a straightforward and more synthetically useful alternative to acyl-metals.