Stereoelectronic Effects Dictate Mechanistic Dichotomy between Cu(II)-Catalyzed and Enzyme-Catalyzed Reactions of Malonic Acid Half Thioesters

Electrochemical decarboxylative alkylation of β-ketoacids with phenol derivatives

An electrochemical method for the decarboxylative alkylation of β-ketoacids with phenol derivatives has been developed. The protocol was carried out in readily available unseparated cells at room temperature in the absence of catalysts and oxidants. The corresponding aryl ketones were obtained in satisfactory yields without additional electrolytes, and were easy to produce in gram-scale synthesis. Based on control experiments and cyclic voltammetry, a plausible reaction mechanism was proposed.

Stereoelectronic Effect in the Reaction of α-Methylene Lactones with Tertiary Phosphines and Its Application in Organocatalysis

The kinetic data indicate that the addition of tertiary phosphines to α-methylene lactones in acetic acid is strongly accelerated in comparison to the reactions of related open-chain esters. Six-membered α-methylene-δ-valerolactone exhibited a more pronounced rate increase than five-membered α-methylene-γ-butyrolactone. The use of α-methylene-γ-butyrolactam as a nitrogen analogue of α-methylene-γ-butyrolactone resulted in a total loss of the reaction acceleration. The observed reactivities were rationalized by DFT calculations at the RwB97XD/6-31+G(d,p) level of theory, showing that the intramolecular interaction between phosphonium and enolate oxygen centers provided by the locked s-cis-geometry of the heterocycles plays an important role in the stabilization of intermediate zwitterions. The reactivity is also controlled by the conformational flexibility of the heterocycle. The geometries of five-membered and, especially, six-membered lactone cycles are slightly changed upon the nucleophilic attack of phosphine, leading to the stabilizing stereoelectronic effect by the Ρ···Ο interaction. The addition of phosphine to α-methylene-γ-butyrolactam significantly distorts the initial geometry of the heterocycle, making the nucleophilic attack unfavorable. The application of the stereoelectronic effect to enhance the efficiency of the phosphine-catalyzed Michael and Pudovik reactions of α-methylene lactones was demonstrated.

A catalytic enantioselective stereodivergent aldol reaction

The aldol reaction is among the most powerful and strategically important carbon-carbon bond-forming transformations in organic chemistry. The importance of the aldol reaction in constructing chiral building blocks for complex small-molecule synthesis has spurred continuous efforts toward the development of direct catalytic variants. The realization of a general catalytic aldol reaction with control over both the relative and absolute configurations of the newly formed stereogenic centers has been a longstanding goal in the field. Here, we report a decarboxylative aldol reaction that provides access to all four possible stereoisomers of the aldol product in one step from identical reactants. The mild reaction can be carried out on a large scale in an open flask, and generates CO₂ as the only by-product. The method tolerates a broad substrate scope and generates chiral β-hydroxy thioester products with substantial downstream utility.

Asymmetric synthesis of δ-substituted-β-keto esters and β-substituted ketones via carboxyl-assisted site- and enantio-selective addition reactions

Highly site- and enantio-selective additions of polyfunctional nucleophiles to imines or carbonyls were achieved via a carboxyl-assisted strategy, affording δ-substituted-β-keto esters and β-substituted ketones with good yield and enantioselectivity.

Zn/Cu Complexes Constructed through Selective in Situ Br−Cl Exchange: Synthesis, Structures, Properties, and DFT Insights into the Cu-Catalyzed Mechanism

Two coordination polymers (CPs) [Cu2(L1)(µ3-OH)(H2O)]n (1) and {[Zn4(L)2(4,4′-bipy)3]•4H2O}n (2) were synthesized under solvothermal conditions. It is unexpectedly found the transformation of aryl−Br to aryl−Cl (Br-Cl exchange) in the formation of...

Bronsted acidic surfactants: efficient organocatalysts for diverse organic transformations

Organic transformations using efficient, atom-economical, cost-effective and environmentally benign strategies for the construction of diversified molecules have attracted synthetic chemists worldwide in recent years. These processes often minimize the waste production and avoid the use of hazardous flammable organic solvents. Among various green protocols, the procedures using surfactant-based catalytic systems have received a considerable attention in organic synthesis. In this context, Bronsted acidic surfactants have emerged as efficient catalysts for various C–C, C–O, C–N and C–S bond forming reactions. Many of these reactions occur in water, as Bronsted acidic surfactants have a unique ability of creating hydrophobic pocket through micelle formation in aqueous medium and the substrate molecules react efficiently to afford the targeted products in good yields. In the past, Bronsted acidic surfactant combined catalysts successfully displayed their potential to accelerate the reaction rates of diverse organic transformations. This chapter presents a complete overview on Bronsted acidic surfactants catalyzed organic reactions to construct a variety of aromatic and heteroaromatic molecular frameworks.

β-Keto acids in asymmetric metal catalysis and organocatalysis

β-Keto acids, ideal surrogates of inactive ketones, play an important role in organic synthesis. The asymmetric decarboxylative reaction using β-keto acids is the one which is being studied the most. Herein we present a comprehensive review on this research topic, which is generally classified according to different catalytic systems and chiral induction modes. Additionally, some extended utilities of these methodologies for synthesizing bioactive compounds were also summarized. This review will facilitate the synthetic community to understand the role of β-keto acids in asymmetric reactions, providing many new opportunities for further exploration in this field.

Copper Catalyzed Decarboxylative Functionalization of Ketoacids

Selective copper catalyzed activation of ketoacids and notably bio-sourced 1,3-acetonedicarboxylic acid, represents an attractive strategy to solve key synthetic challenges. Condensation with aldehydes under exceedingly mild conditions can create more rapidly known natural products scaffolds such as 1,3 polyols. In this account, the recent progress in this field, notably through multicatalytic combination with organocatalysis is described. In addition to the rapid preparation of natural product fragments, cascade incorporation of fluorine also provided new type of synthetic analogues of improved properties in a broad range of applications.

Asymmetric Construction of α-Substituted β-Hydroxy Lactones via Ni Catalyzed Decarboxylative Addition Reaction

We described a Ni-bidentate oxazoline catalyzed highly enantio- and diastereoselective decarboxylative aldol reaction of 2-oxotetrahydrofuran-3-carboxylic acid/2-oxochromane-3-carboxylic acid derivatives with different kinds of carbonyls. Under optimal reaction conditions, α-substituted β-hydroxy butyrolactones and dihydrocoumarins with an all-carbon quaternary stereocenter have been generated with high levels of functional-group compatibility. Furthermore, proficient transformations of products were also described, in which an aliphatic tertiary alcohol and a multi-substituted 1,4-diol were smoothly constructed through hydrogenation and ring-opening reaction, respectively.

Exploiting the role of stereoelectronic effects to design the antagonists of the human complement C3a receptor

Stereoelectronic effects are crucial in governing the conformational behaviour of small molecules bearing heterocyclic rings adjacent to amides.

Stereo-electronic control of reaction selectivity in short-chain dehydrogenases: Decarboxylation, epimerization, and dehydration

Sugar nucleotide-modifying enzymes of the short-chain dehydrogenase/reductase type use transient oxidation-reduction by a tightly bound nicotinamide cofactor as a common strategy of catalysis to promote a diverse set of reactions, including decarboxylation, single- or double-site epimerization, and dehydration. Although the basic mechanistic principles have been worked out decades ago, the finely tuned control of reactivity and selectivity in several of these enzymes remains enigmatic. Recent evidence on uridine 5'-diphosphate (UDP)-glucuronic acid decarboxylases (UDP-xylose synthase, UDP-apiose/UDP-xylose synthase) and UDP-glucuronic acid-4-epimerase suggests that stereo-electronic constraints established at the enzyme's active site control the selectivity, and the timing of the catalytic reaction steps, in the conversion of the common substrate toward different products. The mechanistic idea of stereo-electronic control is extended to epimerases and dehydratases that deprotonate the Cα of the transient keto-hexose intermediate. The human guanosine 5'-diphosphate (GDP)-mannose 4,6-dehydratase was recently shown to use a minimal catalytic machinery, exactly as predicted earlier from theoretical considerations, for the β-elimination of water from the keto-hexose species.

Mechanistic characterization of UDP-glucuronic acid 4-epimerase

UDP-glucuronic acid (UDP-GlcA) is a central precursor in sugar nucleotide biosynthesis and common substrate for C4-epimerases and decarboxylases releasing UDP-galacturonic acid (UDP-GalA) and UDP-pentose products, respectively. Despite the different reactions catalyzed, the enzymes are believed to share mechanistic analogy rooted in their joint membership to the short-chain dehydrogenase/reductase (SDR) protein superfamily: Oxidation at the substrate C4 by enzyme-bound NAD⁺ initiates the catalytic pathway. Here, we present mechanistic characterization of the C4-epimerization of UDP-GlcA, which in comparison with the corresponding decarboxylation has been largely unexplored. The UDP-GlcA 4-epimerase from Bacillus cereus functions as a homodimer and contains one NAD⁺ /subunit (k_cat  = 0.25 ± 0.01 s^-1 ). The epimerization of UDP-GlcA proceeds via hydride transfer from and to the substrate's C4 while retaining the enzyme-bound cofactor in its oxidized form (≥ 97%) at steady state and without trace of decarboxylation. The k_cat for UDP-GlcA conversion shows a kinetic isotope effect of 2.0 (±0.1) derived from substrate deuteration at C4. The proposed enzymatic mechanism involves a transient UDP-4-keto-hexose-uronic acid intermediate whose formation is rate-limiting overall, and is governed by a conformational step before hydride abstraction from UDP-GlcA. Precise positioning of the substrate in a kinetically slow binding step may be important for the epimerase to establish stereo-electronic constraints in which decarboxylation of the labile β-keto acid species is prevented effectively. Mutagenesis and pH studies implicate the conserved Tyr149 as the catalytic base for substrate oxidation and show its involvement in the substrate positioning step. Collectively, this study suggests that based on overall mechanistic analogy, stereo-electronic control may be a distinguishing feature of catalysis by SDR-type epimerases and decarboxylases active on UDP-GlcA.

Development of copper-catalyzed enantioselective decarboxylative aldolization for the preparation of perfluorinated 1,3,5-triols featuring supramolecular recognition properties

Fluorine is able to confer unique properties to organic molecules but the scarcity of natural organofluorine sources renders the development of new synthetic methods highly desirable. Using a chiral BOX/Cu combination, enantioselective decarboxylative aldolization of perfluorinated aldehydes has been developed. Most notably, the reaction occurring under mild conditions and with high enantiocontrol can create ketodiols in one single synthetic operation, which are precursors of crucial perfluorinated 1,3,5-triols. In addition, the reaction performed with chloral, validates the proposed transition state model based on steric interactions and provides the first enantioselective synthesis of hexachlorinated ketodiol of great synthetic utility. The ability of perfluorinated 1,3,5-triols to form a central hydrogen-bonding framework allows strong coordination of anions and the chirality obtained through the catalyst-controlled synthetic sequence demonstrates the selective chiral anion recognition ability of polyols.

Catalytic strategies towards 1,3-polyol synthesis by enantioselective cascades creating multiple alcohol functions

This review highlights the different enantioselective catalyst-controlled cascades creating multiple alcohol functions through the formation of several carbon-carbon bonds. Through subsequent simple derivatization, these strategies ensure the rapid preparation of 1,3-polyols. Thanks to the use of efficient metal- or organo-catalysts, these cascades enable the selective assembly of multiple substrates considerably limiting operations and waste generation. For this purpose, several mono- or bi-directional approaches have been devised allowing successive C-C bond-forming events. The considerable synthetic economies these cascades enable have been demonstrated in the preparation of a wide variety of complex bioactive natural products, notably polyketides.

Ni(II)-Catalyzed Enantioselective Synthesis of β-Hydroxy Esters with Carboxylate Assistance

A Ni-oxazoline complex-catalyzed asymmetric decarboxylative aldol reaction between malonic acid half-oxyesters and various carbonyls with carboxylate assistance was developed, affording structurally diverse β-hydroxy esters with good yields and enantioselectivities under mild conditions. Importantly, the broad substrate scope of this methodology enabled rapid accesses to several natural products and their analogues as exemplified by phenylpropanoid, phaitanthrin B, and phthalide.

Copper(I)-catalyzed asymmetric decarboxylative Mannich reaction enabled by acidic activation of 2H-azirines

Chiral aziridines are structure units found in many biologically active compounds and are important building blocks in organic synthesis. Herein, by merging nucleophilic generation through copper(I)-catalyzed decarboxylation and activation of poorly electrophilic 2H-azirines through protonation with carboxylic acids, an asymmetric decarboxylative Mannich reaction between α,α-disubstituted cyanoacetic acids and 2H-azirines is uncovered, which leads to generation of chiral aziridines containing vicinal tetrasubstituted and acyclic quaternary stereogenic carbon centers in good to excellent diastereo- and enantioselectivities. At last, transformations of the produced chiral aziridine are successfully carried out to deliver synthetically useful compounds.

Due to their poor electrophilicity, 2H-azirines do not easily react with nucleophiles. Here, the authors show an acidic activation of 2H-azirines by cyanoacetic acid coupling partners affording chiral aziridines containing vicinal tetrasubstituted and acyclic quaternary stereogenic carbon centers.

Mild Homologation of Esters through Continuous Flow Chloroacetate Claisen Reactions

The selective chloromethylenation of functionalized esters using chloroacetic acid (CA) and LiHMDS (HMDS=hexamethyldisilazide) in a continuous-flow setup is reported. This Claisen homologation is for the first time extended to bis-chloromethylenation using dichloroacetic acid (DCA), thus giving access to under-explored α,α'-bis-chloroketones. The use of flow conditions enables efficient generation and reaction of the unstable chloroacetate dianion intermediates, leading to unprecedented mild and scalable reaction conditions at an economical reagent stoichiometry (-10 °C, <1 min, 1.0-2.4 equiv dianion). The clean reaction profiles allow subsequent use of the unpurified crude products, which is demonstrated in the synthesis of various heterocycles of broad interest. Furthermore, we report a novel, catalyst-free substitution of the obtained monochloro ketone products with (hetero)aryl zinc enolates to give valuable 1,4-diketones.

Nickel/Copper Dual Catalysis for Sequential Nazarov Cyclization/Decarboxylative Aldol Reaction

A nickel/copper cocatalyzed sequential Nazarov cyclization/decarboxylative aldol reaction is reported. Various β-hydroxycyclopentenones containing three contiguous stereocenters were prepared in good yields with high diastereoselectivities. A gram-scale reaction and further derivatizations were easily achieved to highlight the synthetic utility of this transformation. Preliminary attempts for sequential asymmetric transformation were also conducted.

Fluorescent Small Molecules Are BIG Enough To Sense Biomacromolecule: Synthesis of Aromatic Thioesters and Understanding Their Interactions with ctDNA

The visible fluorescent chromophoric moiety present in the water-soluble photoactive yellow protein (PYP) of Ectothiorhodospira halophila is p-hydroxycinnamic acid linked to the cysteine residue (Cys-69) by a thioester bond and it controls the key photoinduced biological processes of the host organism. In the present work, we have synthesized and characterized three structurally different thiophenyl esters [viz., p-hydroxycinnamic-thiophenyl ester (1), p-N,N-dimethylaminocinnamic-thiophenyl ester (2), and S-phenyl-3-(4-chlorophenyl)-3-(phenylthio)propanethioate (3)] in addition to a novel (to the best of our knowledge) stilbene-type olefinic compound, N¹,N¹,N²,N²-tetramethyl-1,2-bis(phenylthio)ethene-1,2-diamine (4), under the same reaction condition. All of these four compounds showed characteristic and distinguishable chromophoric/fluorophoric behavior in ethanol and also at pH 7.4. However, we have observed that the intrinsic chromophoric/fluorophoric activities of (1) and (2) were greatly influenced during their interactions with calf-thymus DNA, studied by a range of spectroscopic and physicochemical measurements. We have also applied density functional theory [B3LYP, 6-311G⁺(d,p)]-based method to get optimized structures of (1) and (2), which were explored further for molecular docking studies to understand their mode of interaction with DNA. The present study opens up their possible applications as fluorescence probes for biomacromolecules like DNA in future.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Enantioselective [3+2] cycloaddition of azomethine ylides and aldehydes via Ni/bis(oxazoline)-catalyzed ring opening of N-tosylaziridines through a chirality transfer approach

An enantioselective [3+2] cycloaddition of N-tosylaziridines and aldehydes catalyzed by a Ni(ii)-bisoxazoline complex has been accomplished through a chirality transfer approach.

Direct carbon-carbon bond formation via reductive soft enolization: a syn-selective Mannich addition of α-iodo thioesters

The β-amino carboxylic acid moiety is a key feature of numerous important biologically active compounds. We describe a syn-selective direct Mannich addition reaction that uses α-iodo thioesters and sulfonyl imines and produces β-amino thioesters. Enolate formation is achieved by reductive soft enolization. The products of the reaction provide straightforward access to biologically important β-lactams through a variety of known reactions.

Tandem Rh-catalysis: decarboxylative β-keto acid and alkyne cross-coupling

Herein, we describe a regioselective Rh-catalyzed decarboxylative cross-coupling of β-keto acids and alkynes to access branched γ,δ-unsaturated ketones. Rh-hydride catalysis enables the isomerization of an alkyne to generate a metal-allyl species that can undergo carbon-carbon bond formation. Ketones are generated under mild conditions, without the need for base or activated electrophiles.