Organocatalytic Enantioselective α-Alkylation of Aldehydes

Iridium-Catalyzed Asymmetric Allylic Alkylation of Boron Enolates to Construct Acyclic All-Carbon Quaternary Stereocenters

Enolates are ubiquitous intermediates in organic synthesis. Among them, boron enolates exhibit distinctive reactivity patterns and selectivities due to the presence of a boron atom, making their synthesis highly attractive. Although methods for accessing ketone- or ester-derived boron enolates are well-developed, much less progress has been made in the development of aldehyde-derived boron enolates due to aldehydes' high tendency toward self-condensation. Therefore, the practical applications of aldehyde-derived boron enolates are significantly hindered. We present herein an efficient method for the preparation of aldehyde-derived boron enolates via the 1,2-hydroboration of ketenes with boranes, avoiding the use of acidic R2BCl/R2BOTf and bases and leading to improved functional group tolerance. Utilizing this convenient protocol, we developed an Ir-catalyzed asymmetric allylic alkylation of boron enolates, yielding a wide array of chiral aldehydes bearing acyclic all-carbon quaternary centers with high chemo-, regio-, and enantioselectivity, which are prevalent in various natural products and bioactive molecules. The synthetic utility and practicality of this method are demonstrated through gram-scale reactions and asymmetric syntheses of the ent-5HT1 A antagonist as well as biological activity studies in inhibiting the growth of plant pathogens.

The biochemistry of the carcinogenic alcohol metabolite acetaldehyde

Acetaldehyde (AcH) is the first metabolite of ethanol and is proposed to be responsible for the genotoxic effects of alcohol consumption. As an electrophilic aldehyde, AcH can form multiple adducts with DNA and other biomolecules, leading to function-altering and potentially toxic and carcinogenic effects. In this review, we describe sources of AcH in humans, including AcH biosynthesis mechanisms, and outline the structures, properties and functions of AcH-derived adducts with biomolecules. We also describe human AcH detoxification mechanisms and discuss ongoing challenges in the field.

Enantioselective synthesis, characterization, molecular docking simulation and ADMET profiling of α-alkylated carbonyl compounds as antimicrobial agents

All living organisms produce only one enantiomer, so we found that all natural compounds are presented in enantiomerically pure form. Asymmetric synthesis is highly spread in medicinal chemistry because enantiomerically pure drugs are highly applicable. This study initially demonstrated the feasibility of a good idea for the asymmetric synthesis of α-alkylated carbonyl compounds with high enantiomeric purity ranging from 91 to 94% using different quinazolinone derivatives. The structure of all compounds was confirmed via elemental analysis and different spectroscopic data and the enantioselectivity was determined via HPLC using silica gel column. The synthesized compounds' mode of action was investigated using molecular docking against the outer membrane protein A (OMPA) and exo-1,3-beta-glucanase, with interpreting their pharmacokinetics aspects. The results of the antimicrobial effectiveness of these compounds revealed that compound 6a has a broad biocidal activity and this in-vitro study was in line with the in-silico results. Overall, the formulated compound 6a can be employed as antimicrobial agent without any toxicity with high bioavailability in medical applications.

Catalytic Enantioselective Alkylation of Aldehydes with 3-Bromooxindoles

An asymmetric conjugate addition of aldehydes with o-azaxylylene intermediates (indol-2-ones) from 3-bromooxindoles has been developed. The use of a novel spiro-pyrrolidine (SPD)-derived bifunctional N-sulfonylated amide catalyst is essential for a highly diastereo- and enantioselective transformation to provide a wide array of enantioenriched C3 quaternary oxindoles with structurally diverse β-aldehyde appendages. Further application of this synthetic methodology enables the construction of the tricyclic cores of akuammiline-type alkaloids.

Enantioselective catalytic radical decarbonylative azidation and cyanation of aldehydes

Empowered by the ubiquity of carbonyl functional groups in organic compounds, decarbonylative functionalization was prevalent in the construction of complex molecules. Under this context, asymmetric decarbonylative functionalization has emerged as an efficient pathway to accessing chiral motifs. However, ablation of enantiomeric control in a conventional 2e transition metal-catalyzed process was notable because of harsh conditions (high temperatures, etc.) that are usually required. To address this challenge and use readily accessible aldehyde directly, we report the asymmetric radical decarbonylative azidation and cyanation. Diverse aldehydes were directly used as alkyl radical precursor, engaging in the subsequent inner-sphere or outer-sphere ligand transfer where functional motifs (CN and N3) could be incorporated in excellent site- and enantioselectivity. Mild conditions, broad scope, excellent regioselectivity (driven by polarity-matching strategy), and enantioselectivity were shown for both transformations. This radical decarbonylative strategy using aldehydes as alkyl radical precursor has offered a powerful reaction manifold in asymmetric radical transformations to construct functional motifs regio- and stereoselectively.

Merger of Visible Light-Driven Chiral Organocatalysis and Continuous Flow Chemistry: An Accelerated and Scalable Access into Enantioselective α-Alkylation of Aldehydes

The electron donor-acceptor complex-enabled asymmetric photochemical alkylation strategy holds potential to attain elusive chiral α-alkylated aldehydes without an external photoredox catalyst. The photosensitizer-free conditions are beneficial concerning process costs and sustainability. However, lengthy organocatalyst preparation steps as well as limited productivity and difficult scalability render the current approaches unsuitable for synthesis on enlarged scales. Inspired by these limitations, a protocol was developed for the enantioselective α-alkylation of aldehydes based on the synergistic combination of visible light-driven asymmetric organocatalysis and a controlled continuous flow reaction environment. With the aim to reduce process costs, a commercially available chiral catalyst has been exploited to achieve photosensitizer-free enantioselective α-alkylations using phenacyl bromide derivates as alkylating agents. As a result of elaborate optimization and process development, the present flow strategy furnishes an accelerated and inherently scalable entry into enantioenriched α-alkylated aldehydes including a chiral key intermediate of the antirheumatic esonarimod.

BINOL-phosphoric acid and metalloporphyrin derived chiral covalent organic framework for enantioselective α-benzylation of aldehydes

The catalytic asymmetric α-benzylation of aldehydes represents a highly valuable reaction for organic synthesis. For example, the generated α-heteroarylmethyl aldehydes, such as (R)-2-methyl-3-(pyridin-4-yl)propanal ((R)-MPP), are an important class of synthons...

Catalytic Enantioselective Alkylation of Prochiral Enolates

The asymmetric alkylation of enolates is a particularly versatile method for the construction of α-stereogenic carbonyl motifs, which are ubiquitous in synthetic chemistry. Over the past several decades, the focus has shifted to the development of new catalytic methods that depart from classical stoichiometric stereoinduction strategies (e.g., chiral auxiliaries, chiral alkali metal amide bases, chiral electrophiles, etc.). In this way, the enantioselective alkylation of prochiral enolates greatly improves the step- and redox-economy of this process, in addition to enhancing the scope and selectivity of these reactions. In this review, we summarize the origin and advancement of catalytic enantioselective enolate alkylation methods, with a directed emphasis on the union of prochiral nucleophiles with carbon-centered electrophiles for the construction of α-stereogenic carbonyl derivatives. Hence, the transformative developments for each distinct class of nucleophile (e.g., ketone enolates, ester enolates, amide enolates, etc.) are presented in a modular format to highlight the state-of-the-art methods and current limitations in each area.

Probing α-Amino Aldehydes as Weakly Acidic Pronucleophiles: Direct Access to Quaternary α-Amino Aldehydes by an Enantioselective Michael Addition Catalyzed by Brønsted Bases

The high tendency of α-amino aldehydes to undergo 1,2-additions and their relatively low stability under basic conditions have largely prevented their use as pronucleophiles in the realm of asymmetric catalysis, particularly for the production of quaternary α-amino aldehydes. Herein, it is demonstrated that the chemistry of α-amino aldehydes may be expanded beyond these limits by documenting the first direct α-alkylation of α-branched α-amino aldehydes with nitroolefins. The reaction produces densely functionalized products bearing up to two, quaternary and tertiary, vicinal stereocenters with high diastereo- and enantioselectivity. DFT modeling leads to the proposal that intramolecular hydrogen bonding between the NH group and the carbonyl oxygen atom in the starting α-amino aldehyde is key for reaction stereocontrol.

Rhodium-Catalyzed Branched and Enantioselective Direct α-Allylic Alkylation of Simple Ketones with Alkynes

Herein, we report the first direct branched-selective α-allylic alkylation of simple ketones with alkynes under rhodium and secondary amine cooperative catalysis. Through a rhodium-hydride-catalyzed allylic substitution pathway, a series of valuable γ,δ-unsaturated ketones are obtained with excellent regioselectivity in an atom-economic and byproduct-free manner. With a chiral BIPHEP ligand, high enantioselectivity has been achieved for this transformation.

Construction of Quaternary Carbon Center by Catalytic Asymmetric Alkylation of 3-Arylpiperidin-2-ones Under Phase-Transfer Conditions

A highly enantioselective synthesis of δ-lactams having a chiral quaternary carbon center at the α-position has been developed through an asymmetric alkylation of 3-arylpiperidin-2-ones under phase-transfer conditions. In this transformation, a 2,2-diarylvinyl group on the δ-lactam nitrogen atom plays a crucial role as a novel protecting group and an achiral auxiliary for improving both yield and enantioselectivity of the reaction.

Asymmetric α-alkylation of cyclic β-keto esters and β-keto amides by phase-transfer catalysis

A facile and efficient asymmetric α-alkylation of β-keto esters and β-keto amides has been achieved by phase-transfer catalysis.

Direct Asymmetric Alkylation of Ketones: Still Unconquered

The alkylation of ketones is taught at basic undergraduate level. In many cases this transformation leads to the formation of a new stereogenic center. However, the apparent simplicity of the transformation is belied by a number of problems. So much so, that a general method for the direct asymmetric alkylation of ketones remains an unmet target. Despite the advancement of organocatalysis and transition-metal catalysis, neither field has provided an adequate solution. Indeed, even use of an efficient and general stoichiometric chiral reagent has yet to be reported. Herein we describe the state-of-the-art in terms of direct alkylation reactions of some carbonyl groups. We outline the limited progress that has been made with ketones, and potential routes towards ultimately achieving a widely applicable methodology for the asymmetric alkylation of ketones.

Enantioselective α-Alkylation of Aldehydes by Photoredox Organocatalysis: Rapid Access to Pharmacophore Fragments from β-Cyanoaldehydes

The combination of photoredox catalysis and enamine catalysis has enabled the development of an enantioselective α-cyanoalkylation of aldehydes. This synergistic catalysis protocol allows for the coupling of two highly versatile yet orthogonal functionalities, allowing rapid diversification of the oxonitrile products to a wide array of medicinally relevant derivatives and heterocycles. This methodology has also been applied to the total synthesis of the lignan natural product (-)-bursehernin.

A general Pd-catalyzed α- and γ-benzylation of aldehydes for the formation of quaternary centers

A palladium-catalyzed benzylation of α-branched aldehydes has been developed using benzyl methyl carbonates.

A catalytic reactor for the organocatalyzed enantioselective continuous flow alkylation of aldehydes

The use of immobilized metal-free catalysts offers the unique possibility to develop sustainable processes in flow mode. The challenging intermolecular organocatalyzed enantioselective alkylation of aldehydes was performed for the first time under continuous flow conditions. By using a packed-bed reactor filled with readily available supported enantiopure imidazolidinone, different aldehydes were treated with three distinct cationic electrophiles. In the organocatalyzed α-alkylation of aldehydes with 1,3-benzodithiolylium tetrafluoroborate, excellent enantioselectivities, in some cases even better than those obtained in the flask process (up to 95% ee at 25 °C), and high productivity (more than 3800 h(-1) ) were obtained, which thus shows that a catalytic reactor may continuously produce enantiomerically enriched compounds. Treatment of the alkylated products with Raney-nickel furnished enantiomerically enriched α-methyl derivatives, key intermediates for active pharmaceutical ingredients and natural products.