Aldol Reaction Research Articles

Stereoselective synthesis of the northern hemisphere of the proposed structure of neaumycin B.

The stereoselective synthesis of the northern hemisphere (C20-C41) of the purported structure of the extremely potent anticancer natural product neaumycin B has been accomplished. Twelve out of nineteen asymmetry centers have been installed chemically. The key highlights of this synthesis include the Krische iridium catalyzed anti-diastereoselective carbonyl crotylation, the Crimmins aldol reaction, HWE olefination, CBS reduction, vanadium promoted stereoselective epoxidation, Evans methylation and spiroketalization.

Advancements and Expansions in the Proline-Catalyzed Asymmetric Aldol Reaction

Advancements and Expansions in the Proline-Catalyzed Asymmetric Aldol Reaction

Enantioselective vinylogous aldol reaction between β,γ-unsaturated amides and isatins

A highly enantioselective vinylogous aldol reaction between β,γ-unsaturated amides and isatins has been developed to provide chiral multifunctional 3-hydroxy-2-oxindoles.

Harboring organocatalysts in a star-shaped block copolymer for micellar catalysis and emulsion catalysis

Conducting an asymmetric aldol reaction in both micelles and emulsions with high interfacial reactivity and stereoselectivity is achieved by incorporating an organocatalyst in the inherently hydrophobic center of a star-shaped polymer.

Installation of superacidic carbon acid moieties into polymer materials via post-polymerization modification.

In the fields of polymer and material chemistries, strong acid units have mainly included sulfonic acids, which has limited the extension of related material chemistries. Here, a unique carbon acid functionality, namely the bis[(trifluoromethyl)sulfonyl]methyl group, was integrated with polymers via a simple postpolymerization modification with the outstandingly electrophilic 1,1-bis[(trifluoromethyl)sulfonyl]ethylene. The proposed synthesis protocol was verified as an efficient process even for solid-state reactions. The synthesis afforded an organic material with a surface decorated with bis[(trifluoromethyl)sulfonyl]methyl units. The fabricated membranes featuring surface bis[(trifluoromethyl)sulfonyl]methyl units functioned as efficient organocatalysts with high catalytic activity for the Mukaiyama aldol reaction. This study provides a simple method for installing superacidic carbon acid moieties onto the surfaces of materials without tedious chemical treatments.

Screening of novel carbohydrate-derived thioureas for antibacterial activity

Compounds synthesized from carbohydrate substrates have yielded promising lead molecules for pharmaceutical developments, as many are approved for the clinical management of a wide range of diseases. Among them, thiourea moieties are the gold mines of antibacterial molecules. In view of the lack and need for new antibiotics, a series of 16 D-fructose-based thiourea compounds were synthesized from asymmetric aldol reaction and isopropylidenation. The new compounds were established from nuclear magnetic resonance, Fourier-transform infrared, and electrospray ionization mass spectrometry. Using disk diffusion assay for antibiotic susceptibility, four of the novel compounds were found to have concentration-dependent antibacterial activities. The addition of trifluoromethyl groups at the meta-position on the phenyl ring of the thioureas appeared to promote bacterial inhibitory action. Thiourea 2a exhibited the most potent activity against the Gram-negative species, Klebsiella pneumoniae, Escherichia coli, and Salmonella typhi, as well as the Gram-positive species, Micrococcus luteus; but not against all bacteria tested. Thiourea 2b and 3a had lower efficacy but showed better broad-spectrum activity against most bacteria tested. Thiourea 3b displayed species-specific activity only against K. pneumoniae and S. typhi. Our findings show the benefits and prospects of synthesizing new carbohydrate compounds for therapeutic medication.

Synthesis of fluorinated amino acids by low-specificity, promiscuous aldolases coupled to in situ fluorodonor generation.

Fluorine (F) is an important element in the synthesis of molecules broadly used in medicine, agriculture, and materials. F addition to organic structures represents a unique strategy for tuning molecular properties, yet this atom is rarely found in Nature and approaches to produce fluorometabolites (such as fluorinated amino acids, key building blocks for synthesis) are relatively scarce. This chapter discusses the use of L-threonine aldolase enzymes (LTAs), a class of enzymes that catalyze reversible aldol addition to the α-carbon of glycine. The C-C bond formation ability of LTAs, together with their known substrate promiscuity, make them ideal for in vitro F biocatalysis. Here, we describe protocols to harness the activity of the low-specificity LTAs isolated from Escherichia coli and Pseudomonas putida on 2-fluoroacetaldehyde to efficiently synthesize 4-fluoro-L-threonine in vitro. This chapter also provides a comprehensive account of experimental protocols to implement these activities in vivo. These methods are illustrative and can be adapted to produce other fluorometabolites of interest.

Chiral auxiliary-induced asymmetric synthesis of (R)- and (S)-Garner’s aldehydes

This article describes the synthesis of optically pure four N-Boc and N-Cbz-protected (R)- and (S)-isomers of Garner’s aldehyde. The key step involves TiCl4-catalyzed Evans' aldol reaction of the uncommon chiral synthon “glycine enolate equivalent” of N-(N-Boc/Cbz-glycinyl) oxazolidinone with gaseous formaldehyde. The chromatographically separable Evans and non-Evans adduct formation takes place through the installation of hydroxymethyl functionality on chiral glycine enolate in good yields. The orthogonality of the reaction was maintained via the “reverse addition” in the aldol reaction on Evans chiral auxiliary. This method provides a promising opportunity for the synthesis of expensive (R)-Garner’s aldehyde evading the use of unnatural d-serine. Moreover, asymmetric alkylation of Li-enolate of N-(N-Boc-glycinyl) oxazolidinone is a promising opportunity for the formation of an exclusive single isomer of (R)-Garner’s aldehyde.

Enabling High Throughput Kinetic Experimentation by Using Flow as a Differential Kinetic Technique.

Kinetic data is most commonly collected through the generation of time-series data under either batch or flow conditions. Existing methods to generate kinetic data in flow collect integral data (concentration over time) only. Here, we report a method for the rapid and direct collection of differential kinetic data (direct measurement of rate) in flow by performing a series of instantaneous rate measurements on sequential small-scale reactions. This technique decouples the time required to generate a full kinetic profile from the time required for a reaction to reach completion, enabling high throughput kinetic experimentation. In addition, comparison of kinetic profiles constructed at different residence times allows the robustness, or stability, of homogeneously catalysed reactions to be interrogated. This approach makes use of a segmented flow platform which was shown to quantitatively reproduce batch kinetic data. The proline mediated aldol reaction was chosen as a model reaction to perform a high throughput kinetic screen of 216 kinetic profiles in 90 hours, one every 25 minutes, which would have taken an estimated continuous 3500 hours in batch, an almost 40-fold increase in experimental throughput matched by a corresponding reduction in material consumption.

Enabling High Throughput Kinetic Experimentation by Using Flow as a Differential Kinetic Technique**

AbstractKinetic data is most commonly collected through the generation of time‐series data under either batch or flow conditions. Existing methods to generate kinetic data in flow collect integral data (concentration over time) only. Here, we report a method for the rapid and direct collection of differential kinetic data (direct measurement of rate) in flow by performing a series of instantaneous rate measurements on sequential small‐scale reactions. This technique decouples the time required to generate a full kinetic profile from the time required for a reaction to reach completion, enabling high throughput kinetic experimentation. In addition, comparison of kinetic profiles constructed at different residence times allows the robustness, or stability, of homogeneously catalysed reactions to be interrogated. This approach makes use of a segmented flow platform which was shown to quantitatively reproduce batch kinetic data. The proline mediated aldol reaction was chosen as a model reaction to perform a high throughput kinetic screen of 216 kinetic profiles in 90 hours, one every 25 minutes, which would have taken an estimated continuous 3500 hours in batch, an almost 40‐fold increase in experimental throughput matched by a corresponding reduction in material consumption.

A proposed total synthesis of a clerodane-type diterpenoid Scaparin C

A theoretically feasible synthetic route for Scaparin C, a clerodane-type diterpenoid new-ly discovered in nature is depicted. A Diels-Alder reaction followed by oxidative cleavage of the olefin generates a 1,3,6-tricarbonyl compound as the substrate. Based on the rela-tive reactivities of the three carbonyl groups, the primary aldehyde group is protected as the acetal. It is expected that the right aldol and alkylation reactions take place for the remaining carbonyl groups, collaboratively giving a bi-fused ring system which is the most important constituent of Scaparin C. After deprotection, the primary aldehyde group is connected with the 3-bromofuran through a Friedel-Crafts reaction. Finally, reactions including sequential reduction of the carbonyl groups, selective protection, acylation and oxidation of the alcohols can preserve the diol and the ketone groups needed, intramolec-ularly forming the cyclic acetal and epoxidation of alkene with the m-CPBA completing the synthesis.

Enantioselective Catalytic Aldol Reactions in the Presence of Knoevenagel Nucleophiles: A Chemoselective Switch Optimized in Deep Eutectic Solvents Using Mechanochemistry.

In the presence of different nucleophilic Knoevenagel competitors, cyclic and acyclic ketones have been shown to undergo highly chemoselective aldol reactions with aldehydes. In doing so, the substrate breadth for this emerging methodology has been significantly broadened. The method is also no longer beholden to proline-based catalyst templates, e.g., commercially available O-t-Bu-L-threonine is advantageous for acyclic ketones. The key insight was to exploit water-based mediums under conventional (in-water) and non-conventional (deep eutectic solvents) conditions. With few exceptions, high aldol-to-Knoevenagel chemoselectivity (>10:1) and good product profiles (yield, dr, and ee) were observed, but only in DESs (deep eutectic solvents) in conjunction with ball milling did short reaction times occur.

Evans’ Chiral Auxiliary‐Based Asymmetric Synthetic Methodology and Its Modern Extensions

AbstractAlthough the asymmetric catalysis has made a spurt of progress, the use of chiral auxiliaries remains crucial in asymmetric synthesis due to both the reliability and versatility of the methods, and the predictability of stereochemistry of the reactions. Up to date, Evans’ chiral non‐racemic oxazolidinone‐based asymmetric synthetic methodology is still widely used in asymmetric synthesis. More importantly, the Evans asymmetric synthetic methodology turned out to be a fruitful source of inspiration for the development of related asymmetric synthetic methodologies. However, although reviews on the application of Evans’ asymmetric synthetic methodology in both organic synthesis and medicinal chemistry continually to appear in the literature, a comprehensive review dedicating to the extensions of Evans’ chiral non‐racemic oxazolidinone‐based asymmetric methodology remains elusive. In this review, we summarize the extensions of the Evans asymmetric methodology, which cover: (1) the modification of the chiral oxazolidinone auxiliaries; (2) the extension of the Evans’ asymmetric aldol reaction from Evans’ syn‐aldol to other diastereomeric aldol adducts; (3) the extension of the asymmetric reaction types; (4) the extension of chiral imide‐type substrates to N‐alkenyl, N‐allenenyl and N‐alkynyl oxazolidinones; (5) the achiral oxazolidinone‐based asymmetric catalysis; (6) the catalytic transformation of Evans’ products; and (7) the straightforward transformations of Evans’ chiral products to other classes of compounds than the chiral carboxylic acids, esters, alcohols, and Weinreb amides.

Synthesis of Chiral Diazabicycloalkanes via Organocatalytic aza‐Michael/Aldol Reaction

AbstractWe introduce a strategy for the one‐step synthesis of chiral diazabicycloalkanes that delivers the annulated ring system in a single reaction step. The method entails a direct aza‐Michael/aldol reaction of enals in conjunction with modified oxopropanamides and establishes the chiral center via iminium‐enamine tandem catalysis. For this purpose, we developed simple diphenylethylendiamine frameworks as dual activation organocatalyst, effectively engaging both the substrate and reagent, thus leading to yields of 31–86% and enantioselectivities in the range of 80–98%. The final product features not only the diazabicycloalkane moiety but includes also additional functionalities, such as aldehyde motives that can be modified in situ without loss of enantioselectivity, and an sp2‐bearing methyl group. Our approach accommodates a wide array of enals with high functional group tolerance. Additionally, we demonstrated the adjustability of enantioselectivity and yield by modulating the acid cocatalyst‘s quantity, thus offering multiple optimization possibilities.

Synthesis of chiral functional polyhydroxylated arenes via Mukaiyama aldol reaction from Perlin aldehydes

Functionalized aryl polyhydroxylated compounds could be of great synthetic value for natural product synthesis. However, the synthesis of such compounds usually requires multi-step synthesis or the usage of sensitive reagents. We present here a practically simple route for the synthesis of such functionalized arylpolyols from glycal derived α,β-unsaturated 2,3-dideoxy aldehyde as well as α,β-saturated 2,3-dideoxy aldehyde (Perlin aldehydes) via Mukaiyama cross aldol condensation in the presence of silyl enol ether and TiCl4. It was observed that the nature of the electronic substitution of the silyl enol ether does not play any role in the yield of the desired products. Further functionalization of the products has also been shown.

Total Synthesis of (+)-Euphorikanin A via an Atropospecific Cascade.

A total synthesis of the ingenane-derived diterpenoid (+)-euphorikanin A is described. Key to the strategy is a stereocontrolled one-pot sequence consisting of transannular aldol addition reaction, hemiketal formation, and subsequent semipinacol rearrangement that efficiently leads to the complete euphorikanin skeleton. Atroposelective ring-closing olefin metathesis proved critical for the stereospecific cascade, leading to formation of a (Z)-bicyclo[7.4.1]tetradecenone core. An additional salient feature of the route is pyrolysis of a bis-methylxanthate to cleanly furnish the natural product.

Progress in Catalytic Asymmetric Reactions with 7-Azaindoline as the Directing Group

α-Substituted-7-azaindoline amides and α,β-unsaturated 7-azaindoline amides have emerged as new versatile synthons for various metal-catalyzed and organic-catalyzed asymmetric reactions, which have attracted much attention from chemists. In this review, the progress of research on 7-azaindoline amides in the asymmetric aldol reaction, the Mannich reaction, the conjugate addition, the 1,3-dipole cycloaddition, the Michael/aldol cascade reaction, aminomethylation and the Michael addition-initiated ring-closure reaction is discussed. The α-substituted-7-azaindoline amides, as nucleophiles, are classified according to the type of α-substituted group, whereas the α,β-unsaturated 7-azaindoline amides, as electrophiles, are classified according to the type of reaction.

Tetrameric cubane Al 9Ln 7 (Ln = Ce, Pr, Nd) clusters as aldol addition catalysts

Tetrameric cubane Al ₉Ln ₇ (Ln = Ce, Pr, Nd) clusters as aldol addition catalysts

Enantio‐ and Diastereoselective (Ipc)2BOTf‐Mediated Aldol Reactions of Morpholine Carboxamides

AbstractHighly enantio‐ and diastereoselective (Ipc)2BOTf mediated aldol reactions of morpholine carboxamides are described. A wide variety of α‐substituted N‐acyl morpholine carboxamides were successfully employed, including α‐bromo, α‐chloro, α‐vinyl and para‐methoxyphenyl morpholine carboxamides which provided the corresponding aldol products in moderate to excellent yields, and generally with high enantio‐ and diastereoselectivities.

Multiscale Modeling Approach for the Aldol Addition Reaction in Multicompartment Micelle-Based Nanoreactor.

Water has emerged as a versatile solvent for organic chemistry in recent years due to its abundance, low cost, and environmental friendliness. However, one of the most important reactions, the aldol reaction, in the presence of excess water exhibits low yields and poor enantioselectivities. In this regard, we have employed a multiscale modeling approach to investigate the aldol addition reaction catalyzed by l-proline in the hydrophobic compartment of multicompartment micelle (MCM) nanoreactor consisting of amphiphilic bottlebrush copolymer, which minimizes the water content at the reactive site. Through performing dissipative particle dynamics (DPD) simulation, it is found that the "clover-like" morphology of the MCM is formed from multiblock copolymer with a sequence of ethylene oxide-based hydrophilic blocks, styrene lipophilic blocks, l-proline catalyst blocks, and a pentafluorostyrene fluorophilic block in aqueous media. We find that the vicinity of the catalyst in the clover-like MCM has a low dielectric environment, which could facilitate the aldol addition reaction. Our DFT calculations demonstrate that the asymmetric aldol addition of l-proline-catalyzed acetone and 4-nitrobenzaldehyde is energetically more favorable under the low dielectric environment in MCM compared with other commonly used solvents such as DMSO, water, and vacuum condition.