Recent Advances in Squaramide‐Catalyzed Asymmetric Mannich Reactions

Deciphering the Underlying Mechanism of Anion Binding by Asymmetrical Squaramide-Based Dipeptides

Anions are involved in many important processes, which has led to growing interest in designing new molecules to bind them effectively. Squaramides have gained considerable attention as effective anion receptors due to their dual hydrogen bond donor capability. Combining squaramide with biomolecules is a promising approach for designing and developing biomimetic receptors for anions with enhanced H-bonding abilities, particularly due to their functional versatility. The present study explores the mechanism of interaction of H2PO4- and HSO4- anions with three asymmetrical squaramide-based dipeptide receptors, emphasizing the role of noncovalent interactions. The conformational states of the receptors and the amino acids of the dipeptide with varying side chain lengths are the two major factors that influence these interactions. The conformational analysis of the receptors and their anion complexes performed using conformer-rotamer ensemble sampling tool (CREST) and molecular dynamics (MD) simulations, shows that the anti/anti conformations are the most abundant. Following the MD simulations, density functional theory (DFT) was used to perform electronic structure calculations on the 1:1 receptor-anion complexes. Our findings indicate that the N-H···O and O-H···O═C interactions primarily drive the formation of the receptor-anion complexes. Energy decomposition analysis based on absolutely localized molecular orbitals (ALMO-EDA) highlighted the role of the electrostatic energy (ΔEelst) in stabilizing the receptor-anion complexes. Further confirmation of the intermolecular N-H···O and O-H···O═C interactions in the complexes was attained through several analytic tools. This outcome lays a foundation for designing and developing more efficient and selective dipeptide-based anion receptors.

Asymmetric Organocatalytic [3 + 2]-Cycloaddition of N-Alkoxy-4-oxo-acrylamides with Isatin-Derived Ketimines: Access to Enantioselective Synthesis of Spirooxindole-imidazolidinones

The spirooxindole framework is a privileged motif in numerous biologically significant natural products and has been demonstrated as a crucial core scaffold in a variety of medicinally significant compounds. Herein, we achieved a highly efficient enantioselective [3 + 2]-cycloaddition reaction of N-alkoxy-4-oxo-acrylamides with isatin-derived ketimines. This method enabled the precise synthesis of chiral spirooxindole-imidazolidinone derivatives, which are notable for their intricate structures and the presence of chiral quaternary centers. Our approach utilized an organocatalytic strategy with a hydrogen-bonding bifunctional squaramide-based catalyst. This reaction demonstrated impressive results, achieving high yields and exceptional enantioselectivities of >99% ee for the majority of substrates, even when employing only 2 mol % of the catalyst.

General Chiral Catalysis: A Cinchona Thiourea-Pyridoxazoline Scaffold as Both Organocatalyst and Chiral Ligand for an Enantioselective Mannich Reaction between α-Aminomaleimides and Benzothiazolimines

For proof of a new concept of general chiral catalysis, a series of new bifunctional chiral catalysts integrated with both cinchona alkaloid thiourea and pyridine-oxazoline scaffolds were devised and prepared. Using as independent organocatalysts, a new Mannich reaction between α-aminomaleimides and benzothiazolimines with acceptable enantioselectivities (up to 75% ee) has been disclosed. Served as a chiral ligand, the new organocatalyst synergically works with Cu(OTf)2 to catalyze the reaction in excellent enantioselectivities (up to 96% ee) with good yields under mild conditions even in a scale-up preparation. Both the substrates and the final multifunctional chiral adducts may provide a possibility for the development of new pharmaceutical entities and chiral ligands.

Enantioselective Addition of 1,3,5,7-Tetramethyl-BODIPYs to Isatins by Bifunctional Quinine-Based Squaramides

This work describes the development of the first enantioselective addition reaction between 1,3,5,7-tetramethyl-BODIPYs and isatin derivatives. The reaction utilizes bifunctional quinine/squaramide organocatalysts and affords nine novel chiral BODIPY dyes under mild conditions, with enantioselectivities reaching up to 60%. The synthesized BODIPY-oxindoles exhibit high fluorescence emissions, consistent with their parent BODIPYs, and display tunable colors. A representative example demonstrates a remarkably high quantum yield of 0.78 compared to fluorescein. Notably, the newly created carbon-stereocenter on the isatin skeleton induces detectable asymmetry in the electronically decoupled BODIPY chromophore. This is confirmed by the presence of Cotton effects in the visible region of the electronic circular dichroism (ECD) spectra. Density Functional Theory calculations suggested that the model oxindole 3aa adopts an (R) absolute stereochemical configuration, unveiling key interactions between the catalyst and substrates.

Organocatalyzed Asymmetric Conjugate Addition of Cyclic β-Keto Esters to (E)-β-Nitroacrylate Derivatives

A diaminomethylenemalononitrile organocatalyst efficiently promoted the asymmetric conjugate addition of cyclic β-keto esters to (E)-β-nitroacrylate derivatives, yielding the corresponding β-nitro esters derivatives with excellent enantioselectivities (up to >99 % ee). This is the first successful example of highly stereoselective conjugate addition of cyclic β-keto esters to (E)-β-nitroacrylate derivatives to obtain anti-isomers.

Highly Enantioselective Construction of Chiral Eight-Membered Cyclic Ethers through Tandem Cyclization of Ynones and Dicarbonyl Compounds

The enantioselective synthesis of eight-membered cyclic ether has always been a challenge in organic synthesis. Herein, we reported a highly enantioselective tandem cyclization reaction of alkyne ketone and dioxypyridines mediated by chiral bifunctional catalysts. This reaction generates two adjacent stereocenters using an atomeconomic manner, providing a simple and effective method for the one-step synthesis of highly enantioselective eight-membered cyclic ethers.

Organocatalyzed Synthesis of γ-Alkenyl Butenolides via Asymmetric Direct Vinylogous Conjugate Addition-Elimination of Substituted Furanone Derivatives to β-Phenylsulfonylenones

A diaminomethylenemalononitrile organocatalyst efficiently promoted the asymmetric direct vinylogous conjugate addition of α-angelica lactone derivatives to β-phenylsulfonylenones, affording the corresponding γ-alkenyl γ-butenolides in high yields with excellent enantioselectivities (up to 97% ee) after the elimination of the phenylsulfonyl group. This study reports the first successful example of a stereoselective reaction using β-phenylsulfonylenone as the direct alkenyl donor.

A Squaramide-Based Organocatalyst as a Novel Versatile Chiral Solvating Agent for Carboxylic Acids

A squaramide-based organocatalyst for asymmetric Michael reactions has been tested as a chiral solvating agent (CSA) for 26 carboxylic acids and camphorsulfonic acid, encompassing amino acid derivatives, mandelic acid, as well as some of its analogs, propionic acids like profens (ketoprofen and ibuprofen), butanoic acids and others. In many cases remarkably high enantiodifferentiations at 1H, 13C and 19F nuclei were observed. The interaction likely involves a proton transfer from the acidic substrates to the tertiary amine sites of the organocatalyst, thus allowing for pre-solubilization of the organocatalyst (when a chloroform solution of the substrate is employed) or the simultaneous solubilization of both the catalyst and the substrate. DOSY experiments were employed to evaluate whether the catalyst-substrate ionic adduct was a tight one or not. ROESY experiments were employed to investigate the role of the squaramide unit in the adduct formation. A mechanism of interaction was proposed in accordance with the literature data.

Enantioselective synthesis of spirooxindole-pyran derivatives via a remote inverse-electron-demand Diels-Alder reaction of β,γ-unsaturated amides

Novel construction methods for obtaining 3,4'-pyran spirooxindole heterocyclic skeletons have always been the focus of attention. Herein, we report a highly enantioselective inverse-electron-demand oxa-Diels-Alder cycloaddition reaction of a β,γ-unsaturated pyrazole amide and a N-diphenyl isatin-derived oxodiene using a bifunctional catalyst. In addition, large-scale experiments confirmed the reliability of the reaction. The resultant products of this study can be further transformed.

Cinchona-Based Hydrogen-Bond Donor Organocatalyst Metal Complexes: Asymmetric Catalysis and Structure Determination

In this study, we describe the synthesis of cinchona (thio)squaramide and a novel cinchona thiourea organocatalyst. These catalysts were employed in pharmaceutically relevant catalytic asymmetric reactions, such as Michael, Friedel-Crafts, and A3 coupling reactions, in combination with Ag(I), Cu(II), and Ni(II) salts. We identified several organocatalyst-metal salt combinations that led to a significant increase in both yield and enantioselectivity. To gain insight into the active catalyst species, we prepared organocatalyst-metal complexes and characterized them using HRMS, NMR spectroscopy, and quantum chemical calculations (B3LYP-D4/def2-TZVP), which allowed us to establish a structure-activity relationship.

Facile Synthesis of Quinoline-Substituted 3-Hydroxy-2-oxindoles and 3-Amino-2-oxindoles via a Palladium-Catalyzed Cascade Intramolecular Cyclization/Intermolecular Nucleophilic Addition Reaction

An efficient cascade intramolecular cyclization/intermolecular nucleophilic addition reaction of allenyl benzoxazinone with isatin or isatin-derived ketimine has been established by using Pd0-π-Lewis base catalysis. A series of 3-hydroxy-2-oxindoles and 3-amino-2-oxindoles with quaternary carbon atoms at the C3 position were synthesized in good yields under mild conditions through this protocol.

Enantioselective Synthesis of Biphenyl-Bridged ϵ-Sultams by Organocatalytic Mannich Reactions of Cyclic N-Sulfonylimines with Unactivated Ketones

A highly enantioselective Mannich reaction of biphenyl-bridged seven-membered cyclic N-sulfonylimines with methyl alkyl ketones is disclosed in this study. The reaction was performed under organocatalysis by using a quinine-derived primary amine as the catalyst in combination with a Brønsted acid as the co-catalyst. High yields (up to 89 %) and excellent enantioselectivities (up to 97 % ee) were observed. For methyl alkyl ketones containing a larger alkyl substituent, specific regioselective addition to the C=N bond is favored at the methyl group. On the contrary, ketones containing a smaller alkyl substituent or hydroxyacetone substrates gave major syn selective Mannich products at the methylene group.

Mechanochemical asymmetric three-component Mannich reaction involving unreactive arylamines

We report here a mechanochemical protocol for an asymmetric three-component Mannich reaction involving unreactive arylamines with simple cyclic ketones and arylaldehydes catalyzed by (S)-proline with a chiral diol. In this mechanochemical protocol, ball milling enables reaction acceleration and enantioselectivity control. The reported asymmetric three-component Mannich reactions usually involve reactive arylamines such as p-anisidine and phenylamine, while the catalytic asymmetric Mannich reactions involving unreactive arylamines in solution did not proceed smoothly or gave low yields and enantioselectivities. However, the use of ball-milling techniques overcomes the deficiency of the batch systems in solution and avoids the use of toxic organic solvents. The desired products were obtained in moderate-to-good yields (49%-80%) with good-to-high enantioselectivities (up to 99% ee). This is the first example of a mechanochemically activated catalytic asymmetric three-component Mannich reaction involving unreactive arylamines.

Asymmetric Acyclic 1,3-Difunctionalization of Vinyl Carbenes via Site-Selective Vinylogous Mannich-Type Interception of Oxonium Ylides

A novel and highly stereoselective acyclic 1,3-difunctionalization of vinyl metal carbene species has been developed via Rh(II)/chiral phosphoric acid co-catalyzed three-component reactions of vinyldiazoacetates with alcohols and imines. This innovative approach features excellent regio-, diastereo-, and enantioselectivities, demonstrating a broad scope and functional group compatibility. Notably, this is the first example of three-component asymmetric acyclic 1,3-difunctionalization with in situ-formed vinyl metal carbenes.

The vital use of isocyanide-based multicomponent reactions (MCR) in chemical synthesis

Multicomponent (MCRs) reactions are classified as one-pot reaction where more than two starting materials are employed to form a single product that contains the building blocks of the starting components. MCRs are considered a convenient approach in synthetic chemistry and have many advantages over the traditional one or two-component reaction, by reducing the number of sequential multiple steps required and often producing better yields. This chapter dissects the use of isocyanide-based MCRs and the elegant chemistry that they offer to build useful scaffolds in the chemical synthetic field. In addition MCRs are considered as one of the recognisable options for increasing “greenness” during the synthesis of pharmaceutical and industrial products.

Enantioselective Synthesis of Spiro Heterocyclic Compounds Using a Combination of Organocatalysis and Transition-Metal Catalysis

Over the last ten years, the combination of organocatalysis with transition metal (TM) catalysis has become one of the most important toolboxes used for synthesizing optically pure compounds containing chiral quaternary centers, including spiro heterocyclic molecules. The dominant method in the enantioselective synthesis of spiro heterocyclic compounds based on synergistic catalysis includes chiral aminocatalysis and NHC catalysis, as already established covalent organocatalytic strategies. Another area of organocatalysis widely combined with TM catalysis producing enantiomerically enriched spiro heterocyclic compounds is non-covalent catalysis, dominated by chiral phosphoric acids, thiourea, and squaramide derivatives. This review article aims to summarize enantioselective methods used for constructing spirocyclic heterocycles based on a combination of organocatalysis and transition metal catalysis.

Boron-Catalyzed α-Functionalizations of Carboxylic Acids

Catalytic, chemoselective, and asymmetric α-functionalizations of carboxylic acids promise up-grading simple feedstock materials to value-added functional molecules, as well as late-stage structural diversifications of multifunctional molecules, such as drugs and their leads. In this personal account, we describe boron-catalyzed α-functionalizations of carboxylic acids developed in our group (five reaction types). The reversible boron carboxylate formation is key to the acidification of the α-protons and enolization using mild organic bases, allowing for chemoselective and asymmetric bond formations of carboxylic acids. The ligand effects on reactivity and stereoselectivity, substrate scopes, and mechanistic insights are summarized.

Squaramide-Tethered Sulfonamides and Coumarins: Synthesis, Inhibition of Tumor-Associated CAs IX and XII and Docking Simulations

(1) Background: carbonic anhydrases (CAs) are attractive targets for the development of new anticancer therapies; in particular, CAs IX and XII isoforms are overexpressed in numerous tumors. (2) Methods: following the tail approach, we have appended a hydrophobic aromatic tail to a pharmacophore responsible for the CA inhibition (aryl sulfonamide, coumarin). As a linker, we have used squaramides, featured with strong hydrogen bond acceptor and donor capacities. (3) Results: Starting from easily accessible dimethyl squarate, the title compounds were successfully obtained as crystalline solids, avoiding the use of chromatographic purifications. Interesting and valuable SARs could be obtained upon modification of the length of the hydrocarbon chain, position of the sulfonamido moiety, distance of the aryl sulfonamide scaffold to the squaramide, stereoelectronic effects on the aromatic ring, as well as the number and type of substituents on C-3 and C-4 positions of the coumarin. (4) Conclusions: For sulfonamides, the best profile was achieved for the m-substituted derivative 11 (Ki = 29.4, 9.15 nM, CA IX and XII, respectively), with improved selectivity compared to acetazolamide, a standard drug. Coumarin derivatives afforded an outstanding selectivity (Ki > 10,000 nM for CA I, II); the lead compound (16c) was a strong CA IX and XII inhibitor (Ki = 19.2, 7.23 nM, respectively). Docking simulations revealed the key ligand-enzyme interactions.

Asymmetric oxidative Mannich reactions promoted by photocatalysis and electrochemistry

The asymmetric Mannich reaction is an essential method in contemporary organic chemistry. As a representative of clean and green synthesis methods, photochemical and electrochemical oxidation strategies have re-emerged in recent years, providing new ideas for asymmetric Mannich reactions. Numerous chiral β-amino carbonyl compounds have been accessed in satisfactory yields with excellent enantioselectivity via such novel asymmetric oxidative Mannich reactions. This minireview highlights plentiful advances in asymmetric oxidative Mannich reactions that rely on photoredox or anodic-oxidation and covers the literature from 2014 to date. Furthermore, the future development of this field is envisaged.

Enantioselective reaction of N-cyano imines: decarboxylative Mannich-type reaction with malonic acid half thioesters

The enantioselective reaction of imines bearing a cyano group as an activating group with malonic acid half thioesters gave chiral cyanamide derivatives with high enantioselectivity. The density functional theory (DFT) calculation clarified the stereochemical outcome and importance of the N-cyano group for imines.

An overview of the applications of chiral phosphoric acid organocatalysts in enantioselective additions to CO and CN bonds

Since 2004, chiral phosphoric acids (CPAs) have emerged as highyl efficient organocatalysts, providing excellent results in a wide reaction scope. In this review, the applications of CPA for enantioselective additions to CO and CN bonds are covered.

Catalysts’ evolution in the asymmetric conjugate addition of nitroalkanes to electron-poor alkenes

This review provides a journey of the catalyst usage for the enantioselective conjugate addition of nitroalkanes to electron-poor olefins from the early attempts to the latest achievements. Selected applications are also reported.

Organocatalytic diastereo- and enantioselective conjugate addition of pyrazol-3-ones to 3-trifluoroethylidene oxindoles with a newly developed squaramide catalyst

An efficient organocatalytic conjugated addition reaction of pyrazol-3-ones with 3-trifluoroethylidene oxindoles has been developed for the synthesis of enantioenriched triflouromethylated indolin-2-ones bearing adjacent tertiary chiral centers in good yields and good to excellent diastereo- and enantioselectivities. The use of a newly developed chiral spirobiindane-derived squaramide catalyst is essential in achieving high diastereo- and enantioselectivities.

Organocatalytic diastereo- and enantioselective conjugate addition of pyrazol-3-ones to 3-trifluoroethylidene oxindoles with a newly developed squaramide catalyst has been developed.

Asymmetric Henry reaction of trifluoromethyl enones with nitromethane using a N,N-dibenzyl diaminomethylenemalononitrile organocatalyst

A novel N,N -dibenzyl diaminomethylenemalononitrile organocatalyst efficiently promoted asymmetric Henry reactions of trifluoromethyl enones with nitromethane, affording corresponding highly functionalized products in high yields with excellent enantioselectivities (up to 90% ee). This study is the first to report the successful example of the asymmetric 1,2-additions of nitromethane to trifluoromethyl enones.

Organocatalytic Asymmetric Synthesis of Spirooxindole Embedded Oxazolidines

The first organocatalytic asymmetric synthesis of spirooxindole embedded oxazolidines has been developed via a domino reaction involving hemiaminal formation, followed by an unprecedented aza-Michael reaction between isatin derived N-Boc ketimines and γ-hydroxy enones. A quinine derived bifunctional squaramide catalyst was found to be efficient for this reaction, and the products were obtained in good diastereoselectivity and with high enantioselectivity.

A zinc/PyBisulidine catalyzed asymmetric Mannich reaction of N-tosyl imines with 3-acyloxy-2-oxindoles

A Zn-PyBisulidine catalyzed asymmetric Mannich reaction of 3-acyloxy-2-oxindoles has been developed. Various quaternary substituted 3-acyloxy-2-oxindoles bearing vicinal amino alcohol motifs were obtained in good to excellent yields with moderate to excellent dr and excellent enantioselectivities. The utility of this reaction was demonstrated by the easy removal of the acyl group to give C3-hydroxy derivatives and their application as a key skeleton of the ligand for the Ni-catalyzed enantioselective Henry reaction.

Regiodivergent Organocatalytic Reactions

Organocatalysts are abundantly used for various transformations, particularly to obtain highly enantio- and diastereomeric pure products by controlling the stereochemistry. These applications of organocatalysts have been the topic of several reviews. Organocatalysts have emerged as one of the very essential areas of research due to their mild reaction conditions, cost-effective nature, non-toxicity, and environmentally benign approach that obviates the need for transition metal catalysts and other toxic reagents. Various types of organocatalysts including amine catalysts, Brønsted acids, and Lewis bases such as N-heterocyclic carbene (NHC) catalysts, cinchona alkaloids, 4-dimethylaminopyridine (DMAP), and hydrogen bond-donating catalysts, have gained renewed interest because of their regioselectivity. In this review, we present recent advances in regiodivergent reactions that are governed by organocatalysts. Additionally, we briefly discuss the reaction pathways of achieving regiodivergent products by changes in conditions such as solvents, additives, or the temperature.

Diaminomethylenemalononitrile as a Chiral Single Hydrogen Bond Catalyst: Application to Enantioselective Conjugate Addition of α-Branched Aldehydes

An improved diaminomethylenemalononitrile organocatalyst, bearing a N,N-disubstituted structure, promoted enantioselective conjugate addition reaction of α-branched aldehydes with vinyl sulfone, affording adducts with excellent enantioselectivities (up to 96% ee). Mechanistic studies revealed that the diaminomethylenemalononitrile motif holds the vinyl sulfone substrate using a single hydrogen bond accompanied by multiple weak interactions, including electrostatic C-H⋅⋅⋅O interactions.

Stereodivergent Synthesis of Enantioenriched 2,3-Disubstituted Dihydrobenzofurans via a One-Pot C-H Functionalization/Oxa-Michael Addition Cascade

A one-pot rhodium-catalyzed C-H functionalization/organocatalyzed oxa-Michael addition cascade reaction has been developed. This methodology enables the stereodivergent synthesis of diverse 2,3-disubstituted dihydrobenzofurans with broad functional group compatibility in good yields with high levels of stereoselectivity under exceptionally mild conditions. The full complement of stereoisomers of chiral 2,3-disubstituted dihydrobenzofurans and 3,4-disubstituted isochromans could be accessed at will by appropriate permutations of the two chiral catalysts. The current work provides a rare example of two chiral catalysts independently controlling two contiguous stereogenic centers subsequently via a two-step reaction in a single operation.

Asymmetric Catalytic Ketimine Mannich Reactions and Related Transformations

The catalytic enantioselective ketimine Mannich and its related reactions provide direct access to chiral building blocks bearing an α-tertiary amine stereogenic center, a ubiquitous structural motif in nature. Although ketimines are often viewed as challenging electrophiles, various approaches/strategies to circumvent or overcome the adverse properties of ketimines have been developed for these transformations. This review showcases the selected examples that highlight the benefits and utilities of various ketimines and remaining challenges associated with them in the context of Mannich, allylation, and aza-Morita-Baylis-Hillman reactions as well as their variants.

Rapid umpolung Michael addition of isatin N,N′-cyclic azomethine imine 1,3-dipoles with chalcones

3,3-Disubstituted oxindoles were prepared rapidly in moderate to excellent yields with promising dr values by the t-BuONa-promoted Michael addition.

Recent advances in organocatalytic asymmetric multicomponent cascade reactions for enantioselective synthesis of spirooxindoles

Catalytic asymmetric MCCRs for enantioselective synthesis of spirooxindoles by using chiral phosphoric acids, amines, bifunctional thiourea/squaramides and metal-based reagents as catalysts.