A surprising mechanistic "switch" in Lewis acid activation: a bifunctional, asymmetric approach to alpha-hydroxy acid derivatives

Catalytic Metal Ion-Substrate Coordination during Nonenzymatic RNA Primer Extension

Nonenzymatic template-directed RNA copying requires catalysis by divalent metal ions. The primer extension reaction involves the attack of the primer 3'-hydroxyl on the adjacent phosphate of a 5'-5'-imidazolium-bridged dinucleotide substrate. However, the nature of the interaction of the catalytic metal ion with the reaction center remains unclear. To explore the coordination of the catalytic metal ion with the imidazolium-bridged dinucleotide substrate, we examined catalysis by oxophilic and thiophilic metal ions with both diastereomers of phosphorothioate-modified substrates. We show that Mg2+ and Cd2+ exhibit opposite preferences for the two phosphorothioate substrate diastereomers, indicating a stereospecific interaction of the divalent cation with one of the nonbridging phosphorus substituents. High-resolution X-ray crystal structures of the products of primer extension with phosphorothioate substrates reveal the absolute stereochemistry of this interaction and indicate that catalysis by Mg2+ involves inner-sphere coordination with the nonbridging phosphate oxygen in the pro-SP position, while thiophilic cadmium ions interact with sulfur in the same position, as in one of the two phosphorothioate substrates. These results collectively suggest that during nonenzymatic RNA primer extension with a 5'-5'-imidazolium-bridged dinucleotide substrate the interaction of the catalytic Mg2+ ion with the pro-SP oxygen of the reactive phosphate plays a crucial role in the metal-catalyzed SN2(P) reaction.

Enantioselective α-heterofunctionalization reactions of catalytically generated C1-Lewis base enolates

Chiral Lewis base (LB) organocatalysis has emerged as a powerful covalent catalysis concept which allows for highly selective asymmetric C-C and C-heteroatom bond formations. Considering significant recent progress in the development of strategies to access α-heterofunctionalized carboxylic acid derivatives under chiral LB catalysis, we wish to summarize the most significant concepts and advances in this field within this mini review now.

Biocatalytic Synthesis of Homochiral 2-Hydroxy-4-butyrolactone Derivatives by Tandem Aldol Addition and Carbonyl Reduction

Chiral 2-hydroxy acids and 2-hydroxy-4-butyrolactone derivatives are structural motifs often found in fine and commodity chemicals. Here, we report a tandem biocatalytic stereodivergent route for the preparation of these compounds using three stereoselective aldolases and two stereocomplementary ketoreductases using simple and achiral starting materials. The strategy comprises (i) aldol addition reaction of 2-oxoacids to aldehydes using two aldolases from E. coli, 3-methyl-2-oxobutanoate hydroxymethyltransferase (KPHMT _Ecoli ), 2-keto-3-deoxy-l-rhamnonate aldolase (YfaU _Ecoli ), and trans-o-hydroxybenzylidene pyruvate hydratase-aldolase from Pseudomonas putida (HBPA _Pputida ) and (ii) subsequent 2-oxogroup reduction of the aldol adduct by ketopantoate reductase from E. coli (KPR _Ecoli ) and a Δ¹-piperidine-2-carboxylate/Δ¹-pyrroline-2-carboxylate reductase from Pseudomonas syringae pv. tomato DSM 50315 (DpkA _Psyrin ) with uncovered promiscuous ketoreductase activity. A total of 29 structurally diverse compounds were prepared: both enantiomers of 2-hydroxy-4-butyrolactone (>99% ee), 21 2-hydroxy-3-substituted-4-butyrolactones with the (2R,3S), (2S,3S), (2R,3R), or (2S,3R) configuration (from 60:40 to 98:2 dr), and 6 2-hydroxy-4-substituted-4-butyrolactones with the (2S,4R) configuration (from 87:13 to 98:2 dr). Conversions of aldol adducts varied from 32 to 98%, while quantitative conversions were achieved by both ketoreductases, with global isolated yields between 20 and 45% for most of the examples. One-pot one-step cascade reactions were successfully conducted achieving isolated yields from 30 to 57%.

Ruthenium-Catalyzed Asymmetric Transfer Hydrogenation of β-Substituted α-Oxobutyrolactones

A concise and effective ruthenium-catalyzed asymmetric transfer hydrogenation of β-substituted α-oxobutyrolactones has been developed, delivering a series of cis-β-substituted α-hydroxybutyrolactone derivatives with excellent yields, enantioselectivities, and diastereoselectivities. Two consecutive stereogenic centers were constructed in one step through dynamic kinetic resolution under basic conditions. The reaction could be conducted on a gram scale without loss of activity and enantioselectivity. The reductive products could be easily transformed into useful building blocks.

Generation and Reactivity of C(1)-Ammonium Enolates by Using Isothiourea Catalysis

C(1)-Ammonium enolates are powerful, catalytically generated synthetic intermediates applied in the enantioselective α-functionalisation of carboxylic acid derivatives. This minireview describes the recent developments in the generation and application of C(1)-ammonium enolates from various precursors (carboxylic acids, anhydrides, acyl imidazoles, aryl esters, α-diazoketones, alkyl halides) using isothiourea Lewis base organocatalysts. Their synthetic utility in intra- and intermolecular enantioselective C-C and C-X bond forming processes on reaction with various electrophiles will be showcased utilising two distinct catalyst turnover approaches.

Tertiary Amine Lewis Base Catalysis in Combination with Transition Metal Catalysis

The cooperation between two orthogonal catalytic events during the formation of carbon-carbon and carbon-heteroatom bonds has emerged as an effective strategy for enantioselective chemical synthesis. In recent years, a number of pioneering investigations have described useful chemical synthesis methods whereby the reactivity or nucleophile-electrophile combinations can be fine-tuned or extended far beyond the effect and influence of a single catalyst. The recognition of this has had profound implications for the development cooperative catalysis as a field and has provided a foundation for the development of broadly useful chemical synthesis methods. This chapter focuses on the combination of tertiary amine Lewis base and transition metal catalysts, which the authors hope will simulate further developments and advances.

Addition-Rearrangement of Ketenes with Lithium N- tert-Butanesulfinamides: Enantioselective Synthesis of α,α-Disubstituted α-Hydroxycarboxylic Acid Derivatives

Addition of the lithium salts of chiral N-substituted tert-butanesulfinamides to ketenes and subsequent silylation initiates stereoselective [2,3]-rearrangement, which affords enantioenriched α,α-disubstituted α-sulfenyloxy carboxamides through a reaction that faithfully transfers the absolute stereochemistry of the lithiated sulfinylamides to the α-carbon of the amide products. This addition-rearrangement can be performed together with ketene formation from acyl chloride in a single flask, providing a new and practical synthetic route to α-hydroxycarboxylic acid derivatives.

Synthesis of α-Fluorinated Imides via Direct Fluorohydroxylation of Ynamides

A practical synthesis of α-fluorinated imides via the catalyst-free fluorohydroxylation of ynamides is developed. The reaction employs commercially available Selectfluor (F-TEDA-BF₄) and H₂O as the fluorine and hydroxyl sources, respectively. A broad range of aryl- or alkyl-substituted ynamides were well applicable to the reaction with good functional group tolerance under simple and mild reaction conditions. The synthetic utility of the α-fluoroimide products was demonstrated by several value-added transformations. Preliminary mechanistic studies were conducted.

Fluorofunctionalization of C═C Bonds with Selectfluor: Synthesis of β-Fluoropiperazines through a Substrate-Guided Reactivity Switch

The halofunctionalization of alkene substrates remains an essential tool for synthetic chemists. Herein, we report regioselective ammoniofluorination of unactivated alkenes through photochemical means. A one-pot transformation of the ammonium fluoride products into pharmaceutically relevant β-fluoropiperazines is highlighted. Furthermore, a substrate-guided reactivity switch is observed: certain alkenes are shown to react with the same fluorinating reagent to instead give the less-substituted fluoride. We hope that the ammoniofluorination reaction will be of utility in the area of medicinal chemistry, where nitrogen and fluorine are among the most important heteroatoms.

Traversing Steric Limitations by Cooperative Lewis Base/Palladium Catalysis: An Enantioselective Synthesis of α-Branched Esters Using 2-Substituted Allyl Electrophiles

Cooperative catalysis enables the direct enantioselective α-allylation of linear prochiral esters with 2-substituted allyl electrophiles. Critical to the successful development of the method was the recognition that metal-centered reactivity and the source of enantiocontrol are independent. This feature is unique to simultaneous catalysis events and permits logical tuning of the supporting ligands without compromising enantioselectivity.

Asymmetric Reaction of p-Quinone Diimide: Organocatalyzed Michael Addition of α-Cyanoacetates

Hitherto unknown catalytic enantioselective transformation of p-quinone diimides is achieved using chiral bifunctional organic molecules. Bifunctional thiourea compounds catalyze the Michael addition of cyanoacetates with excellent yields and enantioselectivities. The initially formed Michael adducts undergo cyclization to yield functionally rich, fused cyclic imidines bearing a quaternary benzylic chiral center. Density functional theory calculations of the competing transition states (TSs) were carried out to explain the observed stereochemical outcome.

New Benzenoid Derivatives and Other Constituents from Lawsonia inermis with Inhibitory Activity against NO Production

Three new benzenoid derivatives, lawsoinermone (1), inermidioic acid (2), and inermic acid (3) have been isolated from the aerial part of Lawsonia inermis, together with 11 known compounds (4-14). The structures of three new compounds were determined through spectroscopic and MS analyses. Compounds 1, 4-6, 13 and 14 were evaluated for inhibition of nitric oxide production in LPS-stimulated product of nitrite in RAW 264.7 cells with IC₅₀ values of 6.12, 16.43, 18.98, 9.30, 9.30 and 14.90 μg/mL, respectively.

Catalytic Asymmetric Inverse-Electron-Demand Hetero-Diels-Alder Reactions

In this review, the recent developments in catalytic asymmetric inverse-electron-demand hetero-Diels-Alder reaction, which is recognized as one of the most powerful routes to construct highly functionalized and enantioenriched six-membered heterocycles, are described. The article is organized on the basis of different kinds of electron-deficient heterodienes, including α,β-unsaturated ketones/aldehydes, o-benzoquinones, α,β-unsaturated imines, N-aryl imines, o-benzoqinone imides, and other aza-olefins.

A Quantitative Scale of Oxophilicity and Thiophilicity

Oxophilicity and thiophilicity are widely used concepts with no quantitative definition. In this paper, a simple, generic scale is developed that solves issues with reference states and system dependencies and captures empirically known tendencies toward oxygen. This enables a detailed analysis of the fundamental causes of oxophilicity. Notably, the notion that oxophilicity relates to Lewis acid hardness is invalid. Rather, oxophilicity correlates only modestly and inversely with absolute hardness and more strongly with electronegativity and effective nuclear charge. Since oxygen is highly electronegative, ionic bonding is stronger to metals of low electronegativity. Left-side d-block elements with low effective nuclear charges and electronegativities are thus highly oxophilic, and the f-block elements, not because of their hardness, which is normal, but as a result of the small ionization energies of their outermost valence electrons, can easily transfer electrons to fulfill the electron demands of oxygen. Consistent with empirical experience, the most oxophilic elements are found in the left part of the d block, the lanthanides, and the actinides. The d-block elements differ substantially in oxophilicity, quantifying their different uses in a wide range of chemical reactions; thus, the use of mixed oxo- and thiophilic (i.e., "mesophilic") surfaces and catalysts as a design principle can explain the success of many recent applications. The proposed scale may therefore help to rationalize and improve chemical reactions more effectively than current qualitative considerations of oxophilicity.

Lewis acid-promoted [2 + 2] cycloadditions of alkenes with aryl ketenes

A method for the [2 + 2] cycloaddition of aryl ketenes and alkenes is presented. The process involves the in situ generation of a ketene in the presence of a Lewis acid. The utility of products is demonstrated towards the synthesis of a common scaffold found in several natural product families.

Platinum functionalized multiwall carbon nanotube composites as recyclable catalyst for highly efficient asymmetric hydrogenation of methyl pyruvate

A highly efficient platinum nanoparticle functionalized carbon nanocomposite is demonstrated to be a recyclable heterogeneous catalyst for asymmetric hydrogenation of methyl pyruvate.

From bifunctional to trifunctional (tricomponent nucleophile-transition metal-lewis acid) catalysis: the catalytic, enantioselective α-fluorination of acid chlorides

We report in full detail our studies on the catalytic, asymmetric α-fluorination of acid chlorides, a practical method that produces an array of α-fluorocarboxylic acid derivatives in which improved yield and virtually complete enantioselectivity are controlled through electrophilic fluorination of a ketene enolate intermediate. We discovered, for the first time, that a third catalyst, a Lewis acidic lithium salt, could be introduced into a dually activated system to amplify yields of aliphatic products, primarily through activation of the fluorinating agent. Through our mechanistic studies (based on kinetic data, isotopic labeling, spectroscopic measurements, and theoretical calculations) we were able to utilize our understanding of this "trifunctional" reaction to optimize the conditions and obtain new products in good yield and excellent enantioselectivity.

Catalytic, asymmetric alpha-fluorination of acid chlorides: dual metal-ketene enolate activation

In this Communication, we disclose a catalytic, highly enantioselective (up to >99% ee) alpha-fluorination of acid chlorides to produce a variety of optically active carboxylic acid derivatives from readily accessible and commercially available starting materials. The reaction depends on dually activated ketene enolates generated from two discrete catalysts--a chiral nucleophile and an achiral transition metal complex working in tandem. The active, putative alpha-fluorobis(sulfonimide) intermediates readily transacylate in situ under mild conditions upon addition of a wide variety of nucleophiles, including complex natural products. As a consequence, the power of this method is witnessed by the broad range of alpha-fluorinated products that can be accessed efficiently depending on the work up conditions.