Cooperative Noncovalent Interactions Controlling Amine-Catalyzed Aldol Reaction Pathways Catalyzed by the Bifunctional Amino Quaternary Phosphonium Ion

Members of a new class of bifunctional amino quaternary phosphonium salts have been synthesized and utilized as catalysts in aldol condensation reactions, as demonstrated herein. These secondary amines feature a phosphonium ion connected by a carbon chain, enabling the quaternary phosphonium ion to engage in distinct cooperative noncovalent interactions. These interactions work in tandem to stabilize different transition state complexes, exclusively controlling competing amine-catalyzed aldol pathways via the Mannich mechanism. Comprehensive mechanistic investigations were conducted through theoretical calculations. This study uncovers a proximity-driven catalytic mechanism in which the distance between the N and the P+ of the bifunctional catalyst emerges as a critical factor determining catalytic efficacy. The method has been demonstrated through its application to the total synthesis of several bioactive natural products.

Chiral π-Cu(ii)-catalyzed site-, exo/endo-, and enantioselective dearomative (3 + 2) cycloadditions of isoquinolinium ylides with enamides, dienamides, and a trienamide

Here, we report a highly effective dearomative (3 + 2) cycloaddition reaction between isoquinolinium ylides and α,β-enamides, α,β-γ,δ-dienamides, or an α,β-γ,δ-ε,ζ-trienamide, which is catalyzed by a chiral π-Cu(ii) complex (1-10 mol%) and proceeds in a site-selective, exo/endo-selective, and enantioselective manner. The (3 + 2) cycloaddition involving the α,β-enamides proceeds with high exo-selectivity and enantioselectivity. This method is applicable to various substrates including α-substituted, α,β-disubstituted, or β,β-disubstituted α,β-enamides, which are compounds with an intrinsically low reactivity. This method provides synthetic access to pyrroloisoquinoline derivatives with up to three chiral carbon centers, including those featuring fluorine and trifluoromethyl groups, as well as quaternary carbon centers. The (3 + 2) cycloaddition involving α,β-γ,δ-dienamides proceeds with high γ,δ-site-selectivity and enantioselectivity, whereby the exo/endo-selectivity depends on the substrates and ligands. Remarkably, the (3 + 2) cycloaddition of δ-phenyl-α,β-γ,δ-dienamide proceeds with high α,β-site-selectivity, exo-selectivity, and enantioselectivity. In a manner similar to the reaction with the α,β-γ,δ-dienamides, α,β-γ,δ-ε,ζ-trienamide furnishes a (3 + 2) cycloadduct with good ε,ζ-site-selectivity, endo-selectivity, and enantioselectivity.

Organocatalysis as an enabling tool for enantioselective ring-opening reactions of cyclopropanes

The rich reactivity profile of cyclopropanes has been extensively explored to trigger new organic transformations that enable unusual disconnective approaches to synthesize molecular motifs that are not easily reached through conventional reactions. In particular, the chemistry of cyclopropanes has received special attention in the last decade, with multiple new approaches that capitalize on the use of organocatalysis for the activation of the cyclopropane scaffold. This situation has also opened the possibility of developing enantioselective variants of many reactions that until now were only carried out in an enantiospecific or diastereoselective manner. Our group has been particularly active in this field, focusing more specifically on the use of aminocatalysis and Brønsted acid catalysis as major organocatalytic activation manifolds to trigger new unprecedented transformations involving cyclopropanes that add to the current toolbox of general methodologies available to organic chemists for the enantioselective synthesis of chiral compounds.

Electrophilic Fluorination of Silyl Dienol Ethers: a General and Selective Access to γ-Fluoro Enals

We describe the direct synthesis of γ-fluoro enals from the corresponding silyl dienol ethers. This simple process operates under mild conditions and is compatible with a wide range of functionalities. The high γ regioselectivity of this protocol was rationalized by means of theoretical calculations.

Enamine/Iminium-based Dual Organocatalytic Systems for Asymmetric Catalysis and Synthesis

The rational combination of two catalysts to expedite the construction of chiral complex biologically and pharmacologically relevant chiral compounds has widely gained momentum over the past decade. In particular, enamine or iminium catalysis ensuing from the activation of aldehyde or ketone by chiral amine catalysts in conjugation with other organocatalytic cycles has facilitated several asymmetric transformations to yield the enantioenriched products. Regardless of the considerable discussion on the various dual catalytic approaches, literature lacks a comprehensive review focusing on the enamine and iminium-based dual organocatalytic systems. Thus, this review article has discussed the noteworthy achievements in the field of asymmetric catalysis and synthesis catalyzed by the enamine and iminium-based dual organocatalytic systems.

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 Synthesis of Octahydrofuranoindole Core of Aspidosperma Alkaloids via a Diels-Alder/Reduction/Fluoroetherification Reaction Sequence

Herein, we disclose the enantioselective synthesis of novel tricyclic fluorooctahydrofuranoindole spirooxindoles bearing five contiguous stereocenters via an organocatalytic sequential Diels-Alder/Reduction/Fluoroetherifiction reaction strategy. The potential of the developed approach was witnessed by generating vast examples (up to 20 examples) of library molecules embedding natural product core with good yields and phenomenal diastereo- and enantioselectivities (up to 77 % overall yield, up to 99 % ee and 10 : 1 dr). The synthetic utility of our protocol was further demonstrated by synthesizing tricyclic iodooctahydroindole spirooxindole framework through sequential Diels-Alder/reduction/iodoetherification reaction in 65 % overall yield and excellent stereoselectivity (99 % ee and 4 : 1 dr).

Organocatalytic Asymmetric Inverse-Electron-Demand Diels-Alder Reaction between Alkylidene Pyrazolones and Allyl Ketones: Access to Tetrahydropyrano[2,3-c]pyrazoles

Herein we report a catalytic asymmetric inverse-electron-demand Diels-Alder reaction between alkylidene pyrazolones and allyl ketones. Allyl ketone gets activated by a bifunctional thiourea catalyst and acts as a dienolate in this reaction. The trisubstituted tetrahydropyrano[2,3-c]pyrazoles were obtained in moderate to good yields with high diastereo- and enantioselectivities. Few applications, including a decarbonylation reaction, have been demonstrated.

Enantioselective catalytic remote perfluoroalkylation of α-branched enals driven by light

Herein, we report a photochemical organocatalytic method for the asymmetric introduction of perfluoroalkyl fragments (including the valuable trifluoromethyl moiety) at the remote γ-position of α-branched enals. The chemistry exploits the ability of extended enamines (dienamines) to form photoactive electron donor-acceptor (EDA) complexes with perfluoroalkyl iodides, which under blue light irradiation generate radicals through an electron transfer mechanism. The use of a chiral organocatalyst, derived from cis-4-hydroxy-l-proline, secures a consistently high stereocontrol while inferring complete site selectivity for the more distal γ position of the dienamines.

Sterically Hindered and Deconjugative α-Regioselective Asymmetric Mannich Reaction of Meinwald Rearrangement-Intermediate

Compared to γ-addition, the α-addition of α-branched β,γ-unsaturated aldehydes faces larger steric hindrance and disrupts the π-π conjugation, which might be why very few examples are reported. In this article, a highly diastereo- and enantioselective α-regioselective Mannich reaction of isatin-derived ketimines with α-, β- or γ-branched β,γ-unsaturated aldehydes, generated in situ from Meinwald rearrangement of vinyl epoxides, is realized by using chiral N,N'-dioxide/Sc^III catalysts. A series of chiral α-quaternary allyl aldehydes and homoallylic alcohols with vicinal multisubstituted stereocenters are constructed in excellent yields, good d.r. and excellent ee values. Experimental studies and DFT (density functional theory) calculations reveal that the large steric hindrance of the ligand and the Boc (tButyloxy carbonyl) protecting group of imines are critical factors for the α-regioselectivity.

Trienamine catalysed unprecedented remote olefin E/Z isomerisation/[4 + 2]-cycloaddition reaction to access spirooxindole hexahydroindoles

Herein, for the first time, we report the asymmetric synthesis of an unexpected stereoisomer of spirohexahydroindole via a trienamine-catalysed remote olefin E/Z isomerisation/[4 + 2]-cycloaddition reaction. The reaction afforded a vast library of aesthetically pleasing spirooxindole hexahydroindole scaffolds with exceptional enantio- and diastereo-selectivities (up to 95% yield, 99% ee and >99 : 1 dr). In addition, we demonstrated the synthetic transformation of enantiomerically pure spirooxindole hexahydroindoles to synthesize alkyl homologated spirooxindole hexahydroindole and fluoro-pyranooctahydroindole moieties with four and seven contiguous stereocenters, respectively, in excellent yield and selectivities. We have also demonstrated the evidence for the proposed pathway through NMR investigations.

Highly selective γ-alkoxylation, γ-amination and γ-alkylation of unbiased enals by means of photoredox catalysis

We report herein a general and highly selective γ-functionalization protocol under visible light irradiation. This mild radical approach enables the expansion of the scope of application to unbiased enals and the introduction of a wide variety of alkoxy, amino and alkyl functionalities in the γ position with complete regioselectivity.

Asymmetric synthesis of the perhydroepoxyethanoindole core via sequential [4 + 2]-addition/reduction/fluoroannulation reactions

Herein, we present the sequential aminocatalytic [4 + 2]-addition/reduction and fluoroannulation reactions to afford a novel class of bridged fluoro-perhydroepoxyethanoindole spiropyrazolone and fluoro-perhydroepoxyethanoindole spirooxindole moieties with six contiguous stereocenters. An array of perhydroepoxyethanoindole core derivatives (up to 31 examples) mimicking aspidosperma alkaloids were obtained with moderate to good yields and excellent enantio- and diastereo-selectivities (up to 69% overall yield, up to 99.9% ee and up to >20 : 1 dr). Furthermore, we have also disclosed the synthesis of the unexpected tribromo derivative of hexahydroepoxyethanoindole spiropyrazolone in a moderate yield with excellent selectivity by employing the developed protocol in sequential bromoannulation reactions.

Aminocatalytic asymmetric [4 + 2]-annulation to access functionally rich hexahydrospiroindole pyrazolones

Herein, we report the [4 + 2]-annulation of in situ generated trienamine from 2-(E)-benzylidine-3-pyrrolidinyl acraldehyde with pyrazolone olefins for the first time. It is a robust protocol for the synthesis of biologically appealing functionally rich hexahydrospiroindole pyrazolones which is reflected in excellent yields, stereoselectivity, operational simplicity and broad substrate scope as demonstrated in this paper. Furthermore, we have also explored the synthetic applications of optically pure hexahydrospiroindole pyrazolone to construct biologically important alkylated hexahydrospiroindole pyrazolone and octahydrospiroindole pyrazolone with good yields and high selectivity.

Organocatalyzed Cross-Nucleophile Couplings: Umpolung of Catalytic Enamines

ConspectusThe concept of umpolung, or polarity reversal, introduced by Seebach and Corey nearly half a century ago, ushered a new paradigm into synthetic chemistry. Novel connections were able to be forged among functional groups that were typically inaccessible. Conceptually, an umpolung reaction is identified only upon retrosynthetic analysis. Stoichiometric examples have served as a platform to develop and refine elegant methodologies into catalytic processes. The advent of these unconventional arrangements of canonical synthons into new points of diversity has expanded the repertoire of the synthetic toolbox. Within this context, asymmetric organocatalyzed methodologies remain rare, and there are even fewer aminocatalyzed variants.Recent years have witnessed a renaissance in α-functionalizations of aldehydes, specifically in the context of oxidative umpolung strategies. Unlike previous open-shell approaches, application of a quinone-based oxidant in conjunction with an aminocatalyst leads to a discrete, substitutionally labile quinone adduct. These have proven to be valuable building blocks toward polar reactivity─auguring the advent of new avenues to construct tetrasubstituted tertiary stereocenters through the application of conventional nucleophiles to form C-C, C-N, C-O, and C-S bonds through an organocatalyzed cross-nucleophile coupling (organo-CNC) reaction. The resulting nonepimerizable stereocenter demonstrates high optical fidelity and provides a significant advancement in many applications that suffer from racemization, such as in vivo studies.This strategy harnesses a trifunctional aminocatalyst to promote an unusual S_N2 reaction at a highly congested center. The selection of the quinone oxidant and nucleophile converges to a continuum of reactivity ranging from enantioselective oxidation to stereoselective substitution. A remarkable aspect of these developments is the identification of an asymmetric S_N2 dynamic kinetic resolution (S_N2-DKR) manifold. These organo-CNC reactions are highly modular and demonstrate complete stereocontrol from the catalyst with minimal influence from incoming chiral nucleophiles. Leveraging this facet, these technologies have been extended to peptidic bioconjugations bearing bio-orthogonoal linker molecules.This Account aims to highlight the progress, from an internal perspective, toward directing the initial result into established methodologies. Within this construct, the underlying principles of each reaction will be disseminated with specific content on inherent challenges and opportunity. Combined, these will serve as an instructive tool to stimulate applications in cross-disciplinary interfaces.

Photochemical organocatalytic enantioselective radical γ-functionalization of α-branched enals

Reported herein is a rare example of asymmetric catalytic functionalisation of enals at the remote γ-position, proceeding via a radical path. The process requires visible light and exploits the synergistic actions of two distinct organocatalysts. A nucleophilic organic catalyst generates radicals upon S_N2-based activation of commercially available alkyl halides and blue light irradiation. Concomitantly, a chiral secondary amine catalyst triggers the formation of a dienamine from α-branched enals. This chiral dienamine intercepts the photogenerated radicals with excellent γ-selectivity and good stereocontrol.

Differentiating Catalysis in the Dearomative [4 + 2]-Cycloaddition Involving Enals and Heteroaromatic Aldehydes

In this paper, the application of differentiating catalysis in the [4 + 2]-cycloaddition between 2-alkyl-3-formylheteroarenes and α,β-unsaturated aldehydes is described. Within the developed approach, the same aminocatalyst is employed for the independent activation of both starting materials, differentiating their properties via LUMO-lowering and HOMO-rising principles. By the combination of dearomative dienamine activation with iminium ion chemistry high enantio- and diastereoselectivity of the doubly asymmetric process was accomplished. Selected transformations of products were also demonstrated.

Exploring the Role of Aminocatalysis in the Dearomatization and Regioselectivity of Heteroaromatic Aldehydes

Heteroaromatic aldehydes have recently received a lot of attention as a scaffold for aminocatalytic functionalization as they allow for the construction of remote stereocenters and highly complex heterocyclic compounds. In this paper, we employ computational methods (M06-2X/cc-pVTZ//M06-2X/6-31 + G(d,p) and MP2/cc-pVTZ//M06-2X/6-31 + G(d,p)) to examine the abilities of secondary amines to activate several model heteroaromatic aldehydes by promoting loss of aromaticity and formation of the reactive trienamine intermediate. The hyperhomodesmotic equations used to assess the energy penalty for dearomatization show that the formation of the iminium ion decreases the energy cost for dearomatization, especially when X = O and S. Furthermore, we also investigated the role that the catalyst and heteroatom may have on the orbital coefficients of the various positions of the trienamine intermediary in order to better understand and/or predict the regioselectivity these systems may showcase. Synergistic effects between the catalyst and the heteroatom of the aromatic ring were observed to increase electron density at the most remote positions of several of the model systems studied.

Extended Enolates: Versatile Intermediates for Asymmetric C-H Functionalization via Noncovalent Catalysis

Catalyst-controlled functionalization of unmodified carbonyl compounds is a relevant operation in organic synthesis, especially when high levels of site- and stereoselectivity can be attained. This objective is now within reach for some subsets of enolizable substrates using various types of activation mechanisms. Recent contributions to this area include enantioselective transformations that proceed via transiently generated noncovalent di(tri)enolate-catalyst coordination species. While relatively easier to form than simple enolate congeners, di(tri)enolates are ambifunctional in nature and so control of the reaction regioselectivity becomes an issue. This Minireview discusses in some detail this and other problems, and how noncovalent activation approaches based on metallic and metal free catalysts have been developed to advance the field.

Synthesis of 15N-labeled heterocycles via the cleavage of C-N bonds of anilines and glycine-15N

A nitrogen replacement process that directly incorporates the 15N atom of glycine-15N into anilines was reported. The process involves a Csp2-N bond cleavage of anilines driven by dearomatization and a Csp3-N bond cleavage of glycine-15N driven by aromatization. A variety of 15N-labeled aromatic heterocycles can be prepared via this process.

Organocatalytic Asymmetric Multicomponent Cascade Reaction for the Synthesis of Contiguously Substituted Tetrahydronaphthols

Isobenzopyrylium ions are unique, highly reactive, aromatic intermediates which are largely unexplored in asymmetric catalysis despite their high potential synthetic utility. In this study, an organocatalytic asymmetric multicomponent cascade via dienamine catalysis, involving a cycloaddition, a nucleophilic addition, and a ring-opening reaction, is disclosed. The reaction furnishes chiral tetrahydronaphthols containing four contiguous stereocenters in good to high yield, high diastereoselectivity (up to >20:1), and excellent enantioselectivity (93-98% ee). The obtained products are important synthetic intermediates, and it is demonstrated that they can be used for the generation of frameworks such as octahydrobenzo[h]isoquinoline and [2.2.2]octane scaffolds. Furthermore, mechanistic experiments involving oxygen-18-labeling studies and density functional theory calculations provide a vivid picture of the reaction mechanism. Finally, the bioactivity of 16 representative tetrahydronaphthol compounds has been evaluated in U-2OS cancer cells with some compounds showing a unique profile and a clear morphological change.

Asymmetric Synthesis of Tetrahydroisoquinoline Derivatives through 1,3-Dipolar Cycloaddition of C,N-Cyclic Azomethine Imines with Allyl Alkyl Ketones

A [3 + 2] 1,3-Dipolar cycloaddition of C,N-cyclic azomethine imines with allyl alkyl ketones has been achieved. The reaction proceeds under mild conditions and tolerates a wide range of functional groups. An array of tetrahydroisoquinoline derivatives is generally constructed with good diastereoselectivities and enantioselectivities (up to >25:1 dr, >95% ee). Moreover, the absolute configuration of the product was previously determined by using the quantum electronic circular dichroism calculation and ECD spectrum method.

Bifunctional amino sulfonamide-catalyzed asymmetric conjugate addition to alkenyl alkynyl ketimines as enone surrogates

A bifunctional amino sulfonamide-catalyzed asymmetric conjugate addition of aldehydes to alkenyl alkynyl ketimines as reactive surrogates for enones has been developed. Use of a phenylcyclopropane-based amino sulfonamide catalyst, which can activate and orient the ketimines through hydrogen bonding, affords the desired conjugate adducts with high chemo-, diastereo- and enantioselectivity.

Enantioselective H-bond-directed vinylogous iminium ion strategy for the functionalization of vinyl-substituted heteroaryl aldehydes

In this work the first H-bond-directed vinylogous iminium ion strategy has been developed as a convenient strategy for the γ,δ-functionalization of vinyl-substituted heteroaromatic aldehydes. Their reaction with α-mercaptoketones proceeds in a cascade manner involving 1,6-addition followed by intramolecular aldol reaction. Excellent stereoselectivities have been obtained as a result of the H-bond interactions controlling the outcome of the cyclization step. The application of the strategy for the synthesis of tricyclic compounds bearing furan, tetrahydrothiophene and dihydropyran moieties has also been demonstrated.

Design and application of intramolecular vinylogous Michael reaction for the construction of 2-alkenyl indoles

A base-mediated transformation based on a designed intramolecular vinylogous Michael addition (intra-VMA) is presented to access 3-substituted 2-alkenyl indole derivatives. The reaction represents the first example of the intra-VMA for the construction of indoles. A one-pot N-allylation of ortho-tosylamidocinnamates/congeners with γ-bromocrotonates followed by intra-VMA has been described to provide access to a diverse range of 2-alkenyl indole derivatives in reasonable to high yields. The synthetic value of the developed intra-VMA has been demonstrated by gram-scale synthesis of a representative indole derivative and also by the formal synthesis of MK-7246: a Merck's clinical CRTH2 antagonist.

α-Keto hydrazones in asymmetric aminocatalysis: reactivity through β-amino aza-dienamine intermediates

α-keto hydrazones activated by a bifunctional amino-thiourea catalyst generate β-amino aza-dienamine intermediates which react with nitroalkenes in highly enantioselective fashion.

Phosphine-catalysed (4+1) annulations of β′-acetoxy allenoate with β,γ-unsaturated carbonyl compounds

While β,γ-unsaturated carbonyl compounds have been widely used as γC- or αC-nucleophiles, their potential αC,αC-bisnucleophilic reactivity is still underdeveloped.

Regioselective and diastereodivergent organocatalytic asymmetric vinylogous Michael addition

Regioselective and diastereodivergent γ-position asymmetric vinylogous Michael addition (AVMA) of cyclohex-2-enones to nitroalkenes and γ′-AVMA between cyclohex-3-enones and nitroalkenes.

Organocatalytic asymmetric tandem α-functionalization/1,3-proton shift reaction of benzylidene succinimides with β-trifluoromethyl enones

A bifunctional thiourea-catalyzed asymmetric tandem α-functionalization/1,3-proton shift reaction of benzylidene succinimides with β-trifluoromethyl enones has been developed. A series of F₃C-containing chiral Rauhut-Currier type products were obtained in moderate to high yields (up to 98%) with excellent enantioselectivities (up to >99% ee). This represents the first example of benzylidene succinimides undergoing tandem α-functionalization/1,3-proton shift in catalytic enantioselective transformation.

Asymmetric Dearomative Cascade Multiple Functionalizations of Activated N-Alkylpyridinium and N-Alkylquinolinium Salts

An enantioselective cascade reaction of N-alkylpyridinium and -quinolinium salts with o-hydroxybenzylideneacetones to access fused polyheterocycles through cross dienamine-mediated addition followed by trapping of the dearomatized enamine-type intermediates and aminal formation has been developed. A cascade assembly of N-benzyl-4-methylpyridinium salt and cyclic 2,4-dienones is further disclosed to give bridged frameworks via repetitive dearomatization and aromatization activation.

Doubly vinylogous and doubly rearomative functionalization of 2-alkyl-3-furfurals

The manuscript describes a straightforward functionalization of 2-alkyl-3-furfurals via simple aminocatalytic conjugate addition. The reaction proceeds through the formation of dearomatized dienamine-like intermediate that undergoes 1,6-addition to 4-alkylidene-2,6-dialkylcyclohexa-2,5-dienones. This process can be described as doubly rearomative as it proceeds with the re-formation of both furan and phenyl aromatic moieties. Target products have been obtained in a highly stereoselective manner, providing an interesting example of 2-alkyl-3-furfural functionalization via doubly vinylogous Michael addition. The mechanism of the reaction has been studied by means of computational methods.

Lewis acid-catalyzed asymmetric reactions of β,γ-unsaturated 2-acyl imidazoles

The investigation of diverse reactivity of β,γ-unsaturated carbonyl compounds is of great value in asymmetric catalytic synthesis. Numerous enantioselective transformations have been well developed with β,γ-unsaturated carbonyl compounds as nucleophiles, however, few example were realized by utilizing them as not only nucleophiles but also electrophiles under a same catalytic system. Here we report a regioselective catalytic asymmetric tandem isomerization/α-Michael addition of β,γ-unsaturated 2-acyl imidazoles in the presence of chiral N,N′-dioxide metal complexes, delivering a broad range of optically pure 1,5-dicarbonyl compounds with two vicinal tertiary carbon stereocenters in up to >99% ee under mild conditions. Meanwhile, stereodivergent synthesis is disclosed to yield all four stereoisomers of products. Control experiments suggest an isomerization process involved in the reaction and give an insight into the role of NEt₃. In addition, Mannich reaction and sulfur-Michael addition of β,γ-unsaturated 2-acyl imidazoles proceed smoothly as well under the same catalytic system.

The investigation of reactivity of β,γ-unsaturated carbonyl compounds is of great synthetic value, especially in asymmetric transformations. Here, the authors report a catalytic asymmetric tandem isomerization/α-Michael addition of β,γ-unsaturated 2-acyl imidazoles in presence of chiral N,N′-dioxide metal catalysts.