Enantioselective Copper(I)-Catalyzed Alkynylation of Oxocarbenium Ions to Set Diaryl Tetrasubstituted Stereocenters

Enantioselective Synthesis of α-Quaternary Isochromanes by Oxidative Aminocatalysis and Gold Catalysis

A novel strategy that combines oxidative aminocatalysis and gold catalysis allows the preparation of chiral α-quaternary isochromanes, a motif that is prevalent in natural products and synthetic bioactive compounds. In the first step, α-branched aldehydes and propargylic alcohols are transformed into α-quaternary ethers with excellent optical purities (>90 % ee) via oxidative umpolung with DDQ and an amino acid-derived primary amine catalyst. Subsequent gold(I)-catalyzed intramolecular hydroarylation affords the isochromane products with retention of the quaternary stereocenter. A second approach explores the use of allylic alcohols as reaction partners for the oxidative coupling to furnish α-quaternary ethers with generally lower enantiopurities. Stereoretentive cyclization to isochromane products is achieved via intramolecular Friedel-Crafts type alkylation with allylic acetates as a reactive handle. A number of synthetic elaborations and a biological study on these α-quaternary isochromanes highlight the potential applicability of the presented method.

Dehydrative alkynylation of 3-hydroxyisoindolinones with terminal alkynes for the synthesis of 3-alkynylated 3,3-disubstituted isoindolinones

A brand-new procedure for the synthesis of 3-alkynylated 3,3-disubstituted isoindolinones has been disclosed via a HOTf or Fe(OTf)3-catalyzed dehydrative alkynylation of 3-hydroxyisoindolinones with terminal alkynes. Aryl, alkenyl and alkyl terminal alkynes are suitable to couple with a broad range of 3-hydroxyisoindolinones to afford the desired products in moderate to good yields. This protocol features the use of an inexpensive catalyst, mild reaction conditions, broad substrate scope and easy elaboration of the products.

Catalytic Enantioselective [4+2] Cycloadditions of Salicylaldehyde Acetals with Enol Ethers

A readily accessible conjugate-base-stabilized carboxylic acid (CBSCA) catalyst facilitates highly enantioselective [4+2] cycloaddition reactions of salicylaldehyde-derived acetals and cyclic enol ethers, resulting in the formation of polycyclic chromanes with oxygenation in the 2- and 4-positions. Stereochemically more complex products can be obtained from racemic enol ethers. Spirocyclic products are also accessible.

Copper-Catalyzed Benzylic Functionalization of Lignin-Derived Monomers

Within the field of lignin biorefining, significant research effort has been dedicated to the advancement of catalytic methods for lignocellulose depolymerization. However, another key challenge in lignin valorization is the conversion of the obtained monomers into higher value-added products. To address this challenge, new catalytic methods that can fully embrace the inherent complexity of their target substrates are needed. Here, we describe copper-catalyzed reactions for benzylic functionalization of lignin-derived phenolics via intermediate formation of hexafluoroisopropoxy-masked para-quinone methides (p-QMs). By controlling the rates of copper catalyst turnover and p-QM release, we have developed copper-catalyzed allylation and alkynylation reactions of lignin-derived monomers to install various unsaturated fragments amenable to further synthetic applications.

Transition Metal-Catalyzed Enantioselective Synthesis of Chiral Five- and Six-Membered Benzo O-heterocycles

Enantiomerically enriched five- and six-membered benzo oxygen heterocycles are privileged architectures in functional organic molecules. Over the last several years, many effective methods have been established to access these compounds. However, comprehensive documents cover updated methodologies still in highly demand. In this review, recent transition metal catalyzed transformations lead to chiral five- and six-membered benzo oxygen heterocycles are presented. The mechanism and chirality transfer or control processes are also discussed in details.

Highly Enantioselective Catalytic Alkynylation of Quinolones: Substrate Scope, Mechanistic Studies, and Applications in the Syntheses of Chiral N-Heterocyclic Alkaloids and Diamines

The alkynylation of 4-siloxyquinolinium triflates has been achieved under the influence of copper bis(oxazoline) catalysis. The identification of the optimal bis(oxazoline) ligand was informed through a computational approach that enabled the dihydroquinoline products to be produced with up to 96% enantiomeric excess. The conversions of the dihydroquinoline products to biologically relevant and diverse targets are reported.

Mechanism of a Dually Catalyzed Enantioselective Oxa-Pictet-Spengler Reaction and the Development of a Stereodivergent Variant

Enantioselective oxa-Pictet-Spengler reactions of tryptophol with aldehydes proceed under weakly acidic conditions utilizing a combination of two catalysts, an indoline HCl salt and a bisthiourea compound. Mechanistic investigations revealed the roles of both catalysts and confirmed the involvement of oxocarbenium ion intermediates, ruling out alternative scenarios. A stereochemical model was derived from density functional theory calculations, which provided the basis for the development of a highly enantioselective stereodivergent variant with racemic tryptophol derivatives.

Diastereoselective Generation of C2-Azlactonized 2H-Chromenes via Brønsted Acid-Catalyzed Oxo-Cyclization of Propargyl Alcohols

A new Brønsted acid-catalyzed oxo-cyclization of propargyl alcohols with azlactones to synthesize C₂-azlactonized 2H-chromenes has been established that uses 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (BiNPO₄H) as the catalyst and gives excellent diastereoselectivities (≥19:1 dr) in most cases. This protocol has a high compatibility with various substituents of substrates, offering a catalytic and useful entry to the fabrication of the synthetically important C₂-functionalized 2H-chromene scaffold.

Autotandem Catalysis: Inexpensive and Green Access to Functionalized Ketones by Intermolecular Iron-Catalyzed Amidoalkynylation/Hydration Cascade Reaction via N-Acyliminium Ion Chemistry

Iron-based catalysts were applied in cascade-type reactions for the synthesis of different carbonyl compounds. The reactions proceeded by a new iron-catalyzed cascade of alkynylation/hydration by using both the σ- and π-Lewis acid properties of iron salts. The alkynylation reactions of several endo and exocyclic acetoxylactams were achieved with three different catalysts including FeCl 3 .6H 2 O, FeCl 3 , and Fe(OTf) 3 showing the efficiency of σ-Lewis acidity of iron (III) salts in catalyzing the alkynylation reaction. We also demonstrated that the reaction sequence could be shortened by the direct use of hydroxylactams, leading to an environmentally friendly protocol, avoiding the need to perform unnecessary lengthy steps. A combination of the hard/soft iron Lewis acid properties was then used to implement an unprecedented tandem intermolecular alkynylation/intramolecular hydration sequence allowing expedient access to a new carbonyl structures from trivial materials.

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Enantioselective Alkynylation of Unstabilized Cyclic Iminium Ions

An enantioselective copper-catalyzed alkynylation of unstabilized cyclic iminium ions has been developed. Whereas such alkynylations typically utilize pyridinium, quinolinium and isoquinolinium intermediates, this method enables use of cyclic iminium ions unstabilized by resonance. With the use of a Lewis acid and copper catalyst, these iminium ions are generated in situ from readily available hemiaminal methyl ethers and transformed into highly enantioenriched α-alkynylated cyclic amines. A variety of terminal alkynes can be incorporated in high yields and enantiomeric excesses.

Modular Design of Chiral Conjugate-Base-Stabilized Carboxylic Acids: Catalytic Enantioselective [4 + 2] Cycloadditions of Acetals

Readily available 1,2-amino alcohols provide the framework for a new generation of chiral carboxylic acid catalysts that rival the acidity of the widely used chiral phosphoric acid catalyst (S)-TRIP. Covalently linked thiourea sites stabilize the carboxylate conjugate bases of these catalysts via anion-binding, an interaction that is largely responsible for the low pK_a values. The utility of the new catalysts is illustrated in the context of challenging [4 + 2] cycloadditions of salicylaldehyde-derived acetals with homoallylic and bishomoallylic alcohols, providing polycyclic chromanes in a highly enantioselective fashion.

Catalytic enantioselective alkylation of 2-alkoxy-tetrahydrofurans

A nickel(ii)-catalyzed asymmetric alkylation of non-resonance-stabilized oxocarbenium ions with carboxylic acid derivatives on a large scale has been disclosed.

Catalyst-Free Synthesis of α-Functionalized 2H-Chromenes in Water: A Tandem Self-Promoted pseudo-Substitution and Decarboxylation Process

A catalyst-free decarboxylative reaction between β-keto acids and 2H-chromene acetals in water was developed. This reaction featured a broad substrate scope and easily obtainable starting materials to afford α-functionalized 2H-chromenes in high yields. The synthetic value of this protocol was also demonstrated by the scale-up synthesis and versatile conversions of the title product into other useful compounds. In addition, control experiments indicated that water was essential for the reactivity. Mechanistic studies further revealed that the reaction proceeded through a self-promoted tandem pseudo-substitution and decarboxylation process.

A Facile Enantioselective Alkynylation of Chromones

The first catalytic enantioselective alkynylation of chromones is reported. In this process, chromones are silylated to form silyloxybenzopyrylium ions that lead to silyl enol ethers after Cu-catalyzed alkyne addition using StackPhos as a ligand. The outcome of the reaction is impacted by distal ligand substituents with differing electronic character and it was found that successful reactions could be achieved with different ligand congeners by using different solvents. This sequence enables access to different products by protonation or further functionalization, thus increasing complexity in a divergent manner. The transformation is high yielding over a broad scope to provide a variety of useful chromanones in high enantioselectivity.

Copper-catalysed enantioselective intramolecular etherification of propargylic esters: synthetic approach to chiral isochromans

The first example for direct synthesis of chiral terminal alkynyl isochromans.

Catalytic enantioselective cross-dehydrogenative coupling of 3,6-dihydro-2H-pyrans with aldehydes

The first catalytic asymmetric cross-dehydrogenative coupling of 3,6-dihydro-2H-pyrans and aldehydes with excellent enantioselectivity is described.

HClO4 catalysed aldol-type reaction of fluorinated silyl enol ethers with acetals or ketals toward fluoroalkyl ethers

A highly efficient metal-free catalytic aldol-type reaction of fluorinated enol silyl ethers and acetals/ketals toward functionalized fluoroalkyl ethers is developed by using less than 1.0 mol% HClO₄ (70 wt%, aq.).

Asymmetric construction of polycyclic indole derivatives with different ring connectivities by an organocatalysis triggered two-step sequence

Organocatalyzed reaction sequences between hemiacetals and indole-containing nitroolefins were developed for the preparation of indole derivatives with different ring connectivities.

Bimolecular oxidative C–H alkynylation of α-substituted isochromans

The first bimolecular oxidative C–H functionalization of secondary benzylic ethers for tertiary ether synthesis has been established in high efficiency.

Organocatalytic Asymmetric Synthesis of Spiro-Bridged and Spiro-Fused Heterocyclic Compounds Containing Chromane, Indole, and Oxindole Moieties

Following the reactivity inversion strategy, two different two-step sequences were designed and successfully applied to the asymmetric synthesis of spiro-bridged and spiro-fused heterocyclic compounds, which combined chromane, indole, and oxindole, three potential pharmacophores, in one molecule. The power of these two organocatalytic pathways is underscored by mild reaction conditions and high efficiency in the production of synthetically challenging, but biologically important heterocyclic products, which could be transformed into more biologically interesting heterocyclic structures.

Can a Ketone Be More Reactive than an Aldehyde? Catalytic Asymmetric Synthesis of Substituted Tetrahydrofurans

O-heterocycles bearing tetrasubstituted stereogenic centers are prepared via catalytic chemo- and enantioselective nucleophilic additions to ketoaldehydes, in which the ketone reacts preferentially over the aldehyde. Five- and six-membered rings with both aromatic and aliphatic substituents, as well as an alkynyl substituent, are obtained. Moreover, 2,2,5-trisubstituted and 2,2,5,5-tetrasubstituted tetrahydrofurans are synthesized with excellent stereoselectivities. Additionally, the synthetic utility of the described method is demonstrated with a three-step synthesis of the side chain of anhydroharringtonine.

Redox deracemization of 1,3,4,9-tetrahydropyrano[3,4-b]indoles

The existing asymmetric synthesis of enantiopure α-substituted cyclic ethers predominantly relies on the enantioselective C-C bond formation involving a prochiral oxocarbenium ion intermediate. In such a strategy, enantioselectivity and efficiency are typically susceptible to the electronic and substituent effects of either nucleophile or electrophile partners. Here, we describe a strategically different redox deracemization of α-substituted 1,3,4,9-tetrahydropyrano[3,4-b]indoles. This method exhibits good compatibility with the regional variation of the electronic or substituent effect of substrates, thus providing a practical and efficient supplement to the traditional strategy.

Photoinduced Copper-Catalyzed Coupling of Terminal Alkynes and Alkyl Iodides

We have developed a photoinduced copper-catalyzed alkylation of terminal alkynes with primary, secondary, or tertiary alkyl iodides as electrophiles. The reaction has a broad substrate scope and can be successfully performed in the presence of ester, nitrile, aryl halide, ketone, sulfonamide, epoxide, alcohol, and amide functional groups. The alkylation is promoted by blue light (λ≈450 nm) and proceeds at room temperature in the absence of any additional metal catalysts. The use of a terpyridine ligand is essential for the success of the reaction and is shown to prevent photoinduced copper-catalyzed polymerization of the starting materials.

Asymmetric Induction via a Helically Chiral Anion: Enantioselective Pentacarboxycyclopentadiene Brønsted Acid-Catalyzed Inverse-Electron-Demand Diels-Alder Cycloaddition of Oxocarbenium Ions

An enantioselective catalytic inverse-electron-demand Diels-Alder reaction of salicylaldehyde acetal-derived oxocarbenium ions and vinyl ethers to generate 2,4-dioxychromanes is described. Chiral pentacarboxycyclopentadiene (PCCP) acids are found to be effective for a variety of substrates. Computational and X-ray crystallographic analyses support the unique hypothesis that an anion with point-chirality-induced helical chirality dictates the absolute sense of stereochemistry in this reaction.

Different hybridized oxygen atoms controlled chemoselective formation of oxocarbenium ions: synthesis of chiral heterocyclic compounds

The chemoselective formation of oxocarbenium ions was realized by the chemoselective protonation of different hybridized oxygen atoms.

Efficient access to chiral benzo[c]chromenes via asymmetric transfer hydrogenation of ketals

A highly enantioselective method to chiral α-substituted 6H-benzo[c]chromenes through chiral imidodiphosphoric acid-catalyzed asymmetric transfer hydrogenation of ketals has been established.