Enantioselective Hydrogenation of α-Aryloxy and α-Alkoxy α,β-Unsaturated Carboxylic Acids Catalyzed by Chiral Spiro Iridium/Phosphino-Oxazoline Complexes

A New Antidiabetic Agent Showing Short- and Long-Term Effects Due to Peroxisome Proliferator-Activated Receptor Alpha/Gamma Dual Agonism and Mitochondrial Pyruvate Carrier Inhibition

A new series of analogues or derivatives of the previously reported PPARα/γ dual agonist LT175 allowed the identification of ligand 10, which was able to potently activate both PPARα and -γ subtypes as full and partial agonists, respectively. Docking studies were performed to provide a molecular explanation for this different behavior on the two different targets. In vivo experiments showed that this compound induced a significant reduction in blood glucose and lipid levels in an STZ-induced diabetic mouse model displaying no toxic effects on bone, kidney, and liver. By examining in depth the antihyperglycemic activity of 10, we found out that it produced a slight but significant inhibition of the mitochondrial pyruvate carrier, acting also through insulin-independent mechanisms. This is the first example of a PPARα/γ dual agonist reported to show this inhibitory effect representing, therefore, the potential lead of a new class of drugs for treatment of dyslipidemic type 2 diabetes.

Highly enantioselective Ni-catalyzed asymmetric hydrogenation of β,β-disubstituted acrylic acids

A highly enantioselective Ni-catalyzed hydrogenation of β,β-disubstituted acrylic acids was first realized using Ph-BPE, providing straightforward access to chiral carboxylic acids in high yields with excellent enantioselectivities, up to 99% ee.

Highly Efficient Ruthenium-Catalyzed Asymmetric Hydrogenation of N-Heteroaryl Vinyl Ethers

Highly efficient N-heteroaryl-directed Ru-catalyzed asymmetric hydrogenation of α,α-disubstituted vinyl ethers is successfully achieved by using commercially available ligand (R/S)-RuCl[(p-cymene)(BINAP)]Cl. Hundred-gram scale-up of 6b has been successfully completed with consistent ee...

Catalytic asymmetric synthesis of α-stereogenic carboxylic acids: recent advances

Chiral carboxylic acids bearing an α-stereogenic center constitute the backbone of many natural products and therapeutic reagents as well as privileged chiral ligands and catalysts. Hence, it is not surprising that a large number of elegant catalytic asymmetric strategies have been developed toward the efficient synthesis of α-chiral carboxylic acids, such as α-hydroxy acids and α-amino acids. In this review, the recent advances in asymmetric synthesis of α-stereogenic free carboxylic acids via organocatalysis and transition metal catalysis are summarized (mainly from 2010 to 2020). The content is organized by the reaction type of the carboxyl source involved, including asymmetric functionalization of substituted carboxylic acids, cyclic anhydrides, α-keto acids, substituted α,β-unsaturated acids and so on. We hope that this review will motivate further interest in catalytic asymmetric synthesis of chiral α-substituted carboxylic acids.

Reshaping the active pocket of esterase Est816 for resolution of economically important racemates

Eight 2-arylpropionic acids with high E values were generated by engineered Est816, which overcomes the contradiction between the wide substrate scope and high enantioselectivity of esterases.

Design of oxa-spirocyclic PHOX ligands for the asymmetric synthesis of lorcaserin via iridium-catalyzed asymmetric hydrogenation

Phosphine-oxazoline (PHOX) ligands are a very important class of privileged ligands in asymmetric catalysis. A series of highly rigid oxa-spiro phosphine-oxazoline (O-SIPHOX) ligands based on O-SPINOL was synthesized efficiently, and their iridium complexes were synthesized by coordination of the O-SIPHOX ligands to [Ir(cod)Cl]₂ in the presence of sodium tetrakis-3,5-bis(trifluoromethyl)phenylborate (NaBArF). The cationic iridium complexes showed high reactivity and excellent enantioselectivity in the asymmetric hydrogenation of 1-methylene-tetrahydro-benzo[d]azepin-2-ones (up to 99% yield and up to 99% ee). A key intermediate of the anti-obesity drug lorcaserin could be efficiently synthesized using this protocol.

Cobalt-catalyzed highly enantioselective hydrogenation of α,β-unsaturated carboxylic acids

Asymmetric hydrogenation of α,β-unsaturated acids catalyzed by noble metals has been well established, whereas, the asymmetric hydrogenation with earth-abundant-metal was rarely reported. Here, we describe a cobalt-catalyzed asymmetric hydrogenation of α,β-unsaturated carboxylic acids. By using chiral cobalt catalyst bearing electron-donating diphosphine ligand, high activity (up to 1860 TON) and excellent enantioselectivity (up to >99% ee) are observed. Furthermore, the cobalt-catalyzed asymmetric hydrogenation is successfully applied to a broad spectrum of α,β-unsaturated carboxylic acids, such as various α-aryl and α-alkyl cinnamic acid derivatives, α-oxy-functionalized α,β-unsaturated acids, α-substituted acrylic acids and heterocyclic α,β-unsaturated acids (30 examples). The synthetic utility of the protocol is highlighted by the synthesis of key intermediates for chiral drugs (6 cases). Preliminary mechanistic studies reveal that the carboxy group may be involved in the control of the reactivity and enantioselectivity through an interaction with the metal centre.

A large number of marketed drugs contains a chiral carboxylic acid scaffold. Here, the authors report the asymmetric hydrogenation of α,β-unsaturated carboxylic acids to α-chiral carboxylic acids using a cobalt catalyst bearing an electron-donating chiral diphosphine ligand.

Access to Optically Enriched α-Aryloxycarboxylic Esters via Carbene-Catalyzed Dynamic Kinetic Resolution and Transesterification

Optically active α-aryloxycarboxylic acids and their derivatives are important functional molecules. Disclosed here is a carbene-catalyzed dynamic kinetic resolution and transesterification reaction for access to this class of molecules with up to 99% yields and 99:1 er values. Addition of a chiral carbene catalyst to the ester substrate leads to two diastereomeric azolium ester intermediates that can quickly epimerize to each other and thus allows for effective dynamic kinetic resolution to be realized. The optically enriched ester products from our reaction can be quickly transformed to chiral herbicides and other bioactive molecules.

Two new secondary metabolites, saccharochlorines A and B, from a marine bacterium Saccharomonospora sp. KCTC-19160

Two new chlorinated secondary metabolites, saccharochlorines A and B (1 and 2), were isolated from the saline cultivation of a marine-derived bacterium Saccharomonospora sp. (KCTC-19160). The chemical structures of the saccharochlorines were elucidated by 2D NMR and MS spectroscopic data. Saccharochlorines A and B (1 and 2) exhibit weak inhibition of β-secretase (BACE1) in biochemical inhibitory assay, but they induced the release of Aβ (1-40) and Aβ (1-42) in H4-APP neuroglial cells. This discrepancy might be derived from the differences between the cellular and sub-cellular environments or the epigenetic stimulation of BACE1 expression.

A dienamine-mediated deconjugative addition/cyclization cascade of γ,γ-disubstituted enals with carboxylic acid-activated enones: a rapid access to highly functionalized γ-lactones

An aminocatalytic deconjugative addition/cyclization cascade of γ,γ-disubstituted enals with carboxylic acid-activated enones was realized, giving rise to highly functionalized γ-lactones with excellent enantioselectivities.

A practical strategy to access chiral α-aryloxy carboxylic acids through ion-pairing directed asymmetric hydrogenation

A series of optically active α-aryloxy functionalized carboxylic acids were obtained via non-covalent interaction assisted highly efficient asymmetric hydrogenation.

Synthesis of enantiomerically pure 2-aryloxy carboxylic acids and their derivatives

Recent publications on the key preparation methods of the enantiomers of 2-aryloxy carboxylic acids are summarized and comparative analysis of the methods is given. The information is arranged according to the type of the starting compound, being classified into syntheses from enantiomerically pure chiral precursors and syntheses from prochiral precursors, which imply generation of an asymmetric centre in the substrate molecule. Data on the chemical resolution of racemic mixtures of the title compounds are addressed in a separate Section. Attention is focused on the preparation of practically valuable 2-aryloxy acids. Examples of biologically active derivatives of 2-aryloxy carboxylic acids are given.

The bibliography includes 121 references.

Spiro Indane-Based Phosphine-Oxazolines as Highly Efficient P,N Ligands for Enantioselective Pd-Catalyzed Allylic Alkylation of Indoles and Allylic Etherification

A series of indane-based phosphine-oxazoline ligands with a spirocarbon stereogenic center were examined for palladium-catalyzed asymmetric allylic alkylation of indoles. Under optimized conditions, high yields (up to 98%) and enantioselectivities (up to 98% ee) were obtained with a broad scope of indole derivatives. The ligand was determined to be the most efficient P,N-ligand for this reaction. Moreover, the ligand was also efficient for Pd-catalyzed asymmetric allylic etherification with hard aliphatic alcohols as nucleophiles.

Catalytic enantioselective oxidative coupling of saturated ethers with carboxylic acid derivatives

Catalytic enantioselective C–C bond forming process through cross-dehydrogenative coupling represents a promising synthetic strategy, but it remains a long-standing challenge in chemistry. Here, we report a formal catalytic enantioselective cross-dehydrogenative coupling of saturated ethers with diverse carboxylic acid derivatives involving an initial oxidative acetal formation, followed by nickel(II)-catalyzed asymmetric alkylation. The one-pot, general, and modular method exhibits wide compatibility of a broad range of saturated ethers not only including prevalent tetrahydrofuran and tetrahydropyran, but also including medium- and large-sized cyclic moieties and acyclic ones with excellent enantioselectivity and functional group tolerance. The application in the rapid preparation of biologically active molecules that are difficult to access with existing methods is also demonstrated.

Cross-dehydrogenative coupling (CDC) is a powerful method for C-C bond formation however the enantioselective variant is underdeveloped. Here, the authors show a formal enantioselective CDC method involving unactivated ethers and carboxylic acid derivatives allowing for the rapid preparation of biologically active molecules.

An Alkaloid from Scorpion Venom: Chemical Structure and Synthesis

While most scorpion venom components identified in the past are peptidic or proteinic in nature, we report here a new alkaloid isolated from the venom of the Mexican scorpion Megacormus gertschi. Nuclear magnetic resonance and mass spectrometric investigations elucidate the structure of the alkaloid as ( Z)- N-(2-(1 H-imidazol-4-yl)ethyl)-3-(4-hydroxy-3-methoxyphenyl)-2-methoxyacrylamide (1). A chemical method of synthesizing this alkaloid is also described. Although abundant in venom, the above alkaloid was not found to have insecticidal activity. Structural analysis suggests that this venom alkaloid might be of potential interest for evaluating its medicinal effect.

Reversible C-C bond formation at a triply cyclometallated platinum(iv) centre

The oxidation of the tribenzylphosphine derivative of the doubly cylcometallated platinum(ii) complex of diphenylpyridine, 1, with PhICl2 led, as a first step, to the formation of a highly electrophilic metal centre which attacked the benzyl phosphine to give a triply cyclometallated species as the arenium ion. The highly acidic arenium ion protonated unreacted starting 1, a reaction that could be supressed by the addition of water, and gave the neutral species 2(t). Octahedral complex 2(t) was induced to reductively couple, with two five-membered rings coupling to give square planar complex 5 containing a nine-membered ring. The crystal structure of 5 showed the nine-membered ring to span trans across the square planar metal accompanied by considerable distortion: the P-Pt-N bond angle is 155.48(5)°. Oxidation of 5 with PhICl2 resulted in the addition of two chlorides and a change of the nine-membered ring ligand coordination to cis at an octahedral centre, still with considerable distortions: the P-Pt-N bond angle in the crystal structure of 6 is 99.46(5)°. Treatment of 2(t) with AgBF4 also induced a coupling to give a nine-membered ring, and the fluxional three coordinate complex 7. A mono-methylated version of 1, Me-1, was prepared and similar reactions were observed. The presence of the methyl group allowed us to observe selectivity in the coupling reaction to give the nine-membered ring, with two products (a-Me-7 and b-Me7) being initially formed in the ratio 7 : 1. The concentrations of two products changed with time giving a final ratio of 1 : 8 at room temperature (half-life 48 hours), the equilibration being made possible by a reversible C-C bond forming reaction. Reaction of complexes 7 with CO or hydrogen left the nine-membered ring intact, though oxidative degradation resulted in decomplexation of the phosphine donor, accompanied by formation of a P[double bond, length as m-dash]O group.

Iridium-Catalyzed Asymmetric Hydrogenation of Unsaturated Carboxylic Acids

Chiral carboxylic acid moieties are widely found in pharmaceuticals, agrochemicals, flavors, fragrances, and health supplements. Although they can be synthesized straightforwardly by transition-metal-catalyzed enantioselective hydrogenation of unsaturated carboxylic acids, because the existing chiral catalysts have various disadvantages, the development of new chiral catalysts with high activity and enantioselectivity is an important, long-standing challenge. Ruthenium complexes with chiral diphosphine ligands and rhodium complexes with chiral monodentate or bidentate phosphorus ligands have been the predominant catalysts for asymmetric hydrogenation of unsaturated acids. However, the efficiency of these catalysts is highly substrate-dependent, and most of the reported catalysts require a high loading, high hydrogen pressure, or long reaction time for satisfactory results. Our recent studies have revealed that chiral iridium complexes with chiral spiro-phosphine-oxazoline ligands and chiral spiro-phosphine-benzylamine ligands exhibit excellent activity and enantioselectivity in the hydrogenation of α,β-unsaturated carboxylic acids, including α,β-disubstituted acrylic acids, trisubstituted acrylic acids, α-substituted acrylic acids, and heterocyclic α,β-unsaturated acids. On the basis of an understanding of the role of the carboxy group in iridium-catalyzed asymmetric hydrogenation reactions, we developed a carboxy-group-directed strategy for asymmetric hydrogenation of olefins. Using this strategy, we hydrogenated several challenging olefin substrates, such as β,γ-unsaturated carboxylic acids, 1,1-diarylethenes, 1,1-dialkylethenes, and 1-alkyl styrenes in high yield and with excellent enantioselectivity. All these iridium-catalyzed asymmetric hydrogenation reactions feature high turnover numbers (up to 10000) and turnover frequencies (up to 6000 h^-1), excellent enantioselectivities (greater than 95% ee with few exceptions), low hydrogen pressure (<12 atm), and operational simplicity. These features make chiral iridium catalysts superior or comparable to well-established chiral ruthenium and rhodium catalysts for asymmetric hydrogenation of unsaturated carboxylic acids. A number of chiral natural products and pharmaceuticals have been prepared by concise routes involving an iridium-catalyzed asymmetric hydrogenation of an unsaturated carboxylic acid as a key step. As part of a mechanistic study of iridium-catalyzed asymmetric hydrogenation of unsaturated acids, we isolated, for the first time, the migratory insertion intermediate in the iridium-catalyzed asymmetric hydrogenation of olefins, and this result strongly supports the involvement of an Ir(III)/Ir(V) catalytic cycle. The rigid, bulky scaffold of the chiral spiro-P,N-ligands of the catalysts not only prevents them from undergoing deactivating aggregation under the hydrogenation conditions but also is responsible for the efficient chiral induction. The carboxy group of the substrate acts as an anchor to ensure coordination of the substrate to the iridium center of the catalyst during the reaction and makes the hydrogenation proceed smoothly.

Neutral iridium catalysts with chiral phosphine-carboxy ligands for asymmetric hydrogenation of unsaturated carboxylic acids

We developed neutral iridium catalysts with chiral spiro phosphine-carboxy ligands (SpiroCAP) for asymmetric hydrogenation of unsaturated carboxylic acids. Different from the cationic Crabtree-type catalysts, the iridium catalysts with chiral spiro phosphine-carboxy ligands are neutral and do not require the use of a tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BAr_F^-) counterion, which is necessary for stabilizing cationic Crabtree-type catalysts. Another advantage of the neutral iridium catalysts is that they have high stability and have a long lifetime in air. The new iridium catalysts with chiral spiro phosphine-carboxy ligands exhibit unprecedented high enantioselectivity (up to 99.4% ee) in the asymmetric hydrogenations of various unsaturated carboxylic acids, particularly for 3-alkyl-3-methylenepropionic acids, which are challenging substrates for other chiral catalysts.

Iridium catalysts with modular axial-unfixed biphenyl phosphine–oxazoline ligands: asymmetric hydrogenation of α,β-unsaturated carboxylic acids

Axial-unfixed biphenyl phosphine–oxazoline ligands were successfully applied to Ir-catalyzed asymmetric hydrogenation of α,β-unsaturated carboxylic acids.

Synthesis of 2-aryloxy butenoates by copper-catalysed allylic C–H carboxylation of allyl aryl ethers with carbon dioxide

2-Aryloxy butenoates were efficiently synthesized by deprotonative alumination of ally aryl ethers and subsequent Cu-catalysed carboxylation with CO₂.

Asymmetric Hydrogenation of β-Aryloxy/Alkoxy Cinnamic Nitriles and Esters

A highly efficient and enantioselective hydrogenation of β-aryloxy/alkoxy cinnamic nitriles and esters under mild conditions has been realized by using a rhodium catalyst with a chiral f-spiroPhos ligand. The method provides efficient access to the asymmetric synthesis of a variety of chiral β-oxy-functionalized nitriles and esters with excellent enantioselectivities (up to 99.9% ee) and high turnover numbers (TON of up to 50000). This methodology has also been successfully applied to the concise and practical synthesis of the chiral pharmaceutical nisoxetine.

Palladium-catalyzed C(sp3)–H arylation of lactic acid: efficient synthesis of chiral β-aryl-α-hydroxy acids

A palladium-catalyzed oxidative arylation of lactic acid is described.

Highly enantioselective hydrogenation of α-oxy functionalized α,β-unsaturated acids catalyzed by a ChenPhos–Rh complex in CF3CH2OH

A Chenphos–Rh complex is demonstrated to be a highly efficient catalyst for asymmetric hydrogenation of α-oxy functionalized α,β-unsaturated carboxylic acidsviaan ionic interaction between the ligand and the substrate.

Remote Meta-C-H Activation Using a Pyridine-Based Template: Achieving Site-Selectivity via the Recognition of Distance and Geometry

The pyridyl group has been extensively employed to direct transition-metal-catalyzed C-H activation reactions in the past half-century. The typical cyclic transition states involved in these cyclometalation processes have only enabled the activation of ortho-C-H bonds. Here, we report that pyridine is adapted to direct meta-C-H activation of benzyl and phenyl ethyl alcohols through engineering the distance and geometry of a directing template. This template takes advantage of a stronger σ-coordinating pyridine to recruit Pd catalysts to the desired site for functionalization. The U-shaped structure accommodates the otherwise highly strained cyclophane-like transition state. This development illustrates the potential of achieving site selectivity in C-H activation via the recognition of distal and geometric relationship between existing functional groups and multiple C-H bonds in organic molecules.

Synthesis of erythrocentaurin derivatives as a new class of hepatitis B virus inhibitors

Twenty-four derivatives of erythrocentaurin (ET) were synthesized and evaluated for their anti-HBV activities on HepG 2.2.15 cell line in vitro. Eight compounds 1, 2, 5, 8, 9, 1e, 1k, and 1m increased activity against HBV DNA replication with the SI values higher than 11. In particular, derivatives 1e and 1k exhibited the most potent inhibition on HBV DNA replication with the IC50 values of 0.026 mM (SI>70.8) and 0.045 mM (SI>36.0), respectively. The primary structure-activity relationships (SARs) of ET derivatives were summarized for exploring potent anti-HBV agents.