Dihydroxyacetone variants in the organocatalytic construction of carbohydrates: mimicking tagatose and fuculose aldolases

Green Catalysis: Water as a Sustainable Medium in Organocatalyzed Reactions

The use of organocatalysts has increased significantly in recent years due to their tremendous applications in green solvents. Thus, using water as a solvent has evolved as a critical factor. Organocatalysts are efficient and sustainable agents for promoting chemical reactions in water. The literature has been extensively reviewed, and the use of various organocatalysts for three fundamental C-C bond-forming processes─the Aldol, Michael, and Mannich reactions in aqueous media─have been compiled in this study. Organocatalysts can overcome the limitations of conventional organic solvents by achieving high reaction rates and regioselectivity in water. This Review highlights the advantages of organocatalysts in aqueous media for these key reactions. It discusses the principles behind designing effective organocatalysts, focusing on their impact on selectivity, sustainability, and reaction efficiency. This study also summarizes the most significant advancements in sustainable organic reactions over the past decade.

Construction of three contiguous stereocenters through amine-catalyzed asymmetric aldol reactions

Three contiguous stereocenters were constructed by an amino acid-catalyzed asymmetric aldol reaction of α-siloxyketones with racemizable α-haloaldehydes via dynamic kinetic resolution. One-pot catalytic asymmetric synthesis of the highly functionalized products could also be accomplished by the α-bromination of simple aldehydes and the subsequent asymmetric aldol reaction.

Catalytically Active Multicompartment Micelles

This article presents the self-assembly behavior of multicompartment micelles (MCMs) in water into morphologies with multiple segregated domains and their use as supports for aqueous catalysis. A library of poly(norbornene)-based amphiphilic bottlebrush copolymers containing covalently attached l-proline in the hydrophobic, styrene, and pentafluorostyrene domains and a poly(ethylene glycol)-containing repeat unit as the hydrophilic block have been synthesized using ring-opening metathesis polymerization. Interaction parameter (χ) values between amphiphilic blocks were determined using a Flory-Huggins-based computational model. The morphologies of the MCMs are observed via cryogenic transmission electron microscopy and modeled using dissipative particle dynamic simulations. The catalytic activities of these MCM nanoreactors were systematically investigated using the aldol addition between 4-nitrobenzaldehyde and cyclohexanone in water as a model reaction. MCMs present an ideal environment for catalysis by providing control over water content and enhancing interactions between the catalytic sites and the aldehyde substrate, thereby forming the aldol product in high yields and selectivities that is otherwise not possible under aqueous conditions. Catalyst location, block ratio, and functionality have substantial influences on micelle morphology and, ultimately, catalytic efficiency. "Clover-like" and "core-shell" micelle morphologies displayed the best catalytic activity. Our MCM-based catalytic system expands the application of these nanostructures beyond selective storage of guest molecules and demonstrates the importance of micelle morphology on catalytic activity.

Total Synthesis of ( + )-Swainsonine, (–)- Swainsonine, ( + )-8--

A tactical combination of either ( S)- or ( R)-proline catalyzed aldol reaction followed by intramolecular reductive amination enabled the synthesis of a chiral pyrrolidine derivative with 3 contiguous stereocenters in only 2 synthetic steps, starting from achiral precursors. This product, obtainable in both enantiomeric forms, was further exploited as a common intermediate in total syntheses of the biologically active iminosugars: ( + )-swainsonine, (–)-swainsonine, ( + )-8- epi-swainsonine, and ( + )-dideoxy-imino-lyxitol.

Asymmetric Organocatalysis Enables Rapid Assembly of Portimine Precursor Chains

Sequential organocatalytic additions of 2-furanone and dihydroxyacetone derivatives to a crotonaldehyde lynchpin provide polyhydroxylated chains reminiscent of lactonized deoxo Kdn type sugars. Further homologation via Kulinkovich ring opening of the butyrolactone and acylation of the zinc homoenolate derived from the incipient cyclopropanol allows assembly of functionalized chain precursors to portimine. Our experiments probe the stability and reactivity of monosubstituted methylidene pyrrolines and generate advanced intermediates useful for exploring the biosynthesis and de novo synthesis of portimine.

Highly diastereo- and enantioselective organocatalytic synthesis of trifluoromethylated erythritols based on the in situ generation of unstable trifluoroacetaldehyde

Thus far, only a few methods for the asymmetric synthesis of erythritols bearing a trifluoromethyl group have been developed, and these methods present serious disadvantages such as the requirement of multiple steps for the preparation of their starting materials, low stereoselectivity, and the use of highly toxic reagents. Herein, we have developed a highly diastereo- and enantioselective organocatalytic method to synthesise erythritols bearing a trifluoromethyl group using (1) a commercially available organocatalyst to produce unstable trifluoroacetaldehyde in situ from its corresponding hemiacetal, followed by the simultaneous asymmetric carbon-carbon bond-forming reaction of the organocatalyst with an in situ-generated chiral enamine derived from 2,2-dimethyl-1,3-dioxane-5-one to obtain the corresponding aldol product in good yield (65-80%) with high diastereoselectivity (up to 94% de) and excellent enantioselectivity (up to >98% ee), (2) the highly diastereoselective reduction of the ketone moiety in the aldol product (up to 98% de), and (3) the deprotection of the acetal moiety.

A Simple and Efficient Protocol for Proline-Catalysed Asymmetric Aldol Reaction

The proline-catalysed asymmetric aldol reaction is usually carried out in highly dipolar aprotic solvents (dimethylsulfoxide, dimethylformamide, acetonitrile) where proline presents an acceptable solubility. Protic solvents are generally characterized by poor stereocontrol (e.g., methanol) or poor reactivity (e.g., water). Here, we report that water/methanol mixtures are exceptionally simple and effective reaction media for the intermolecular organocatalytic aldol reaction using the simple proline as the catalyst.

Novel chiral proline-based organocatalysts with amide and thiourea–amine units for highly efficient asymmetric aldol reaction in saturated brine without additives

A series of novel proline-based organocatalysts with amide and thiourea-amine units (7a–7f) were developed and evaluated in the asymmetric aldol reaction of 4-nitrobenzaldehyde with cyclohexanone. The organocatalyst (7c or 7d, 5 mol%) exhibited efficient catalytic activity to afford aldol products in high diastereoselectivity (up to >99:1), enantioselectivity (up to >99%), and yield (up to >96%) at 0 °C in saturated brine without adding an acid. Aldol products of benzaldehyde derivatives almost universally provide high diastereoselectivity and enantioselectivity.

Biocatalytic Production of Amino Carbohydrates through Oxidoreductase and Transaminase Cascades

Plant-derived carbohydrates are an abundant renewable resource. Transformation of carbohydrates into new products, including amine-functionalized building blocks for biomaterials applications, can lower reliance on fossil resources. Herein, biocatalytic production routes to amino carbohydrates, including oligosaccharides, are demonstrated. In each case, two-step biocatalysis was performed to functionalize d-galactose-containing carbohydrates by employing the galactose oxidase from Fusarium graminearum or a pyranose dehydrogenase from Agaricus bisporus followed by the ω-transaminase from Chromobacterium violaceum (Cvi-ω-TA). Formation of 6-amino-6-deoxy-d-galactose, 2-amino-2-deoxy-d-galactose, and 2-amino-2-deoxy-6-aldo-d-galactose was confirmed by mass spectrometry. The activity of Cvi-ω-TA was highest towards 6-aldo-d-galactose, for which the highest yield of 6-amino-6-deoxy-d-galactose (67 %) was achieved in reactions permitting simultaneous oxidation of d-galactose and transamination of the resulting 6-aldo-d-galactose.

Total Synthesis of the Trisaccharide Antigen of the Campylobacter jejuni RM1221 Capsular Polysaccharide via de Novo Synthesis of the 6-Deoxy-d- manno-heptose Building Blocks

A de novo approach utilizing the d-proline-catalyzed and LDA-promoted aldol reactions as key steps for the preparation of differentiated-protected 6-deoxy-d- manno-heptose building blocks was developed. PPh₃AuBAr₄^F-catalyzed glycosylation with the 6-deoxy-d- manno-heptosyl o-hexynylbenzoate as donor was demonstrated as a direct and practical method for the stereoselective synthesis of the β-linked 6-deoxy-d- manno-heptoside as the major product. Coupling of the 6-deoxy-α-d- manno-heptosyl H-phosphonate with the 3-hydroxyl disaccharide acceptor based on H-phosphonate chemistry was described for the construction of the trisaccharide skeleton with the acid-labile phosphodiester linkage. Finally, first total synthesis of the unique trisaccharide antigen of the capsular polysaccharide of Campylobacter jejuni RM1221 that belongs to HS:53 serotype complex was accomplished for further evaluation as vaccine candidate against C. jejuni RM1221 infection.

Copper-catalyzed asymmetric methylation of fluoroalkylated pyruvates with dimethylzinc

The catalytic asymmetric methylation of fluoroalkylated pyruvates is shown with dimethylzinc as a methylating reagent in the presence of a copper catalyst bearing a chiral phosphine ligand. This is the first catalytic asymmetric methylation to synthesize various α-fluoroalkylated tertiary alcohols with CF₃, CF₂H, CF₂Br, and n-C _n F_2n+1 (n = 2, 3, 8) groups in good-to-high yields and enantioselectivities. Axial backbones and substituents on phosphorus atoms of chiral phosphine ligands critically influence the enantioselectivity. Moreover, the methylation of simple perfluoroalkylated ketones is found to be facilitated by only chiral phosphines without copper.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Synthesis of D-manno-heptulose via a cascade aldol/hemiketalization reaction

A [4 + 3] synthesis of D-manno-heptulose is described. The cascade aldol/hemiketalization reaction of a C₄ aldehyde with a C₃ ketone provides the differentially protected ketoheptose building block, which can be further reacted to furnish target D-manno-heptulose.

Synthesis of 2-Keto-d- and l-gluconic Acid via Stereoselective Direct Aldol Reactions

Stereoselective direct aldol reaction between optically pure d- or l-glyceraldehyde and hydroxyacetylfuran is demonstrated as an efficient and straightforward methodology for the synthesis of six-carbon atom d- and l-arabino-hex-2-ulosonic acids. syn-Selective aldol reactions realized by using either tertiary amines or a dizinc aldol catalyst constitute two parallel routes to the de novo synthesis of orthogonally protected biologically relevant 2-keto-d- and l-gluconic acids.

Organocatalytic Synthesis of Higher-Carbon Sugars: Efficient Protocol for the Synthesis of Natural Sedoheptulose and d-Glycero-l-galacto-oct-2-ulose

Herein we report a short and efficient protocol for the synthesis of naturally occurring higher-carbon sugars-sedoheptulose (d-altro-hept-2-ulose) and d-glycero-l-galacto-oct-2-ulose-from readily available sugar aldehydes and dihydroxyacetone (DHA). The key step includes a diastereoselective organocatalytic syn-selective aldol reaction of DHA with d-erythrose and d-xylose, respectively. The methodology presented can be expanded to the synthesis of various higher sugars by means of syn-selective carbon-carbon-bond-forming aldol reactions promoted by primary-based organocatalysts. For example, this methodology provided useful access to d-glycero-d-galacto-oct-2-ulose and 1-deoxy-d-glycero-d-galacto-oct-2-ulose from d-arabinose in high yield (85 and 74 %, respectively) and high stereoselectivity (99:1).

Catalytic Asymmetric Synthesis of Tertiary Alcohols and Oxetenes Bearing a Difluoromethyl Group

The catalytic asymmetric ene reaction with difluoropyruvate as an electrophile in the presence of a dicationic palladium complex is shown. This is the reliable and practical catalytic asymmetric synthesis for various α-CF2H tertiary alcohols in high yields and enantioselectivities. The reaction with isobutene can be catalyzed efficiently under solvent-free conditions with low catalyst loading (up to S/C 2000). Furthermore, difluoropyruvate is applicable to the [2 + 2] cycloaddition reaction in high yields and enantioselectivities.

Synthesis of octitols and the respective amino-derivatives from 'organo-aldols'

Two diastereoisomeric keto-octoses, obtained in the reaction of 2,3:4,5-diacetone-D-arabinose with protected dihydroxyacetone catalyzed with L- or D-proline, were converted into octitols by stereoselective reduction of the carbonyl group with zinc borohydride and final deprotection. The study on the preparation of the respective amino-derivatives by reductive amination of these organo-adducts is presented; stereochemical aspects of these processes are discussed.

Amine-catalyzed direct aldol reactions of hydroxy- and dihydroxyacetone: biomimetic synthesis of carbohydrates

This article presents comprehensive studies on the application of primary, secondary, and tertiary amines as efficient organocatalysts for the de novo synthesis of ketoses and deoxyketoses. Mimicking the actions of aldolase enzymes, the synthesis of selected carbohydrates was accomplished in aqueous media by using proline- and serine-based organocatalysts. The presented methodology also provides direct access to unnatural L-carbohydrates from the (S)-glyceraldehyde precursor. Determination of the absolute configuration of all obtained sugars was feasible using a methodology consisting of concerted ECD and VCD spectroscopy.

Total synthesis of (+)-swainsonine and (+)-8-epi-swainsonine

Enantioselective total synthesis of (+)-swaisonine that hinges on a combination of organocatalyzed aldolization and reductive amination, affords the title compound in 9 steps, with 24% overall yield.

A tandem organocatalytic α-chlorination-aldol reaction that proceeds with dynamic kinetic resolution: a powerful tool for carbohydrate synthesis

A tandem, proline-catalyzed α-chlorination/aldol reaction is described that involves a dynamic kinetic resolution of α-chloroaldehyde intermediates. The resulting syn-chlorohydrins are produced with good to excellent diastereoselectivity in high enantiopurity and provide new opportunities for the synthesis of carbohydrates.

Aminohydroxyacetone synthons: versatile intermediates for the organocatalytic asymmetric aldol reaction

A practical method for the synthesis of 1,3-aminohydroxyacetone synthons was developed, and their utility in the organocatalytic asymmetric aldol reaction was demonstrated in a short synthesis of aza-sugars.