Asymmetric Catalysis of Diels–Alder Reactions with in Situ Generated Heterocyclic ortho-Quinodimethanes

Cancer-Stem-Cell Phenotype-Guided Discovery of a Microbiota-Inspired Synthetic Compound Targeting NPM1 for Leukemia

The human microbiota plays an important role in human health and disease, through the secretion of metabolites that regulate key biological functions. We propose that microbiota metabolites represent an unexplored chemical space of small drug-like molecules in the search of new hits for drug discovery. Here, we describe the generation of a set of complex chemotypes inspired on selected microbiota metabolites, which have been synthesized using asymmetric organocatalytic reactions. Following a primary screening in CSC models, we identified the novel compound UCM-13369 (4b) whose cytotoxicity was mediated by NPM1. This protein is one of the most frequent mutations of AML, and NPM1-mutated AML is recognized by the WHO as a distinct hematopoietic malignancy. UCM-13369 inhibits NPM1 expression, downregulates the pathway associated with mutant NPM1 C+, and specifically recognizes the C-end DNA-binding domain of NPM1 C+, avoiding the nucleus-cytoplasm translocation involved in the AML tumorological process. The new NPM1 inhibitor triggers apoptosis in AML cell lines and primary cells from AML patients and reduces tumor infiltration in a mouse model of AML with NPM1 C+ mutation. The disclosed phenotype-guided discovery of UCM-13369, a novel small molecule inspired on microbiota metabolites, confirms that CSC death induced by NPM1 inhibition represents a promising therapeutic opportunity for NPM1-mutated AML, a high-mortality disease.

Shaping Chirality via Stereoselective, Organocatalytic [4+2] Cycloadditions involving Heterocyclic ortho-Quinodimethanes

Polycyclic compounds bearing a complex heterocyclic core such as an aromatic heterocycle "fused" with one or more functionalized rings, are widespread leading molecules in the domain of synthetic organic chemistry and pharmaceuticals. Although many synthetic methodologies have been devised to access achiral, fused heteroaromatic scaffolds, or related chiral variants adorned with out-of-cycle stereogenic elements, equally efficient strategies to afford chiral heterocycles featuring in-cycle stereocenters, exist to a lesser extent and presently represent a growing field of investigation. The mild, organocatalytic generation of elusive ortho-quinodimethane intermediates (oQDMs), derived from suitable heteroaromatic carbonyl- or carbonyl-like pronucleophiles has recently proved successful in the synthesis of such peculiar chiral architectures via stereoselective [4+2] cycloadditions. This review provides an overview of the most important advances attained in this field over the last decade.

Exo-Selective Diels-Alder Reactions

The Diels-Alder reaction stands as one of the most pivotal transformations in organic chemistry. Its efficiency, marked by the formation of two carbon-carbon bonds and up to four new stereocenters in a single step, underscores its versatility and indispensability in synthesizing natural products and pharmaceuticals. The most significant stereoselectivity feature is the "endo rule". While this rule underpins the predictability of the stereochemical outcomes, it also underscores the challenges in achieving the opposite diastereoselectivity, making the exo-Diels-Alder reactions often considered outliers. This review delves into recent examples of exo-Diels-Alder reactions, shedding light on the factors inverting the intrinsic tendency. We explore the roles of steric, electrostatic, and orbital interactions, as well as thermodynamic equilibriums in influencing exo/endo selectivity. Furthermore, we illustrate strategies to manipulate these factors, employing approaches such as bulky substituents, s-cis conformations, transient structural constraints, and innovative control physics. Through these analyses, our aim is to provide a comprehensive understanding of how to predict and design exo-Diels-Alder reactions, paving the way for new diastereoselective catalyst systems and expanding the chemical scope of Diels-Alder reactions.

DNDMH-mediated direct nitration of aryl alkenes

N-nitro type reagents have been demonstrated as mild nitration tools in recent years. This work presents an exploration of direct nitration of aryl alkenes mediated by DNDMH, a novel N-nitro type reagent developed in our previous study. It exhibits herein a new property of DNDMH as an effective direct nitration reagent for aryl alkenes, through probably the delivery of nitro radicals with the aid of TEMPO and Cu(OAc)2.

Metal-Free C(sp3)-H Bond Arylation of 3-Methylindole Derivatives via 3-Indole Imine Methides

Direct arylation of the benzylic C(sp3)-H bond is one of the most straightforward strategies for the construction of multi-aryl methanes, owing to the extraordinary step and atom economy. In this paper, we developed the first metal-free arylation of the C(sp3)-H bond in 3-methylindoles, thereby providing rapid access to a range of diaryl- and triarylmethanes with two indole rings. Mechanistically, 3-indole imine methide serves as the key intermediate. Water plays a crucial role in this process, likely serving as a proton shuttle to facilitate the key 1,3-proton transfer step in this reaction and, thus, enhance the reaction efficiency.

Asymmetric (4+n) Cycloadditions of Indolyldimethanols for the Synthesis of Enantioenriched Indole-Fused Rings

Catalytic asymmetric construction of chiral indole-fused rings has become an important issue in the chemical community because of the significance of such scaffolds. In this work, we have accomplished the first catalytic asymmetric (4+2) and (4+3) cycloadditions of 2,3-indolyldimethanols by using indoles and 2-naphthols as suitable reaction partners under the catalysis of chiral phosphoric acids, constructing enantioenriched indole-fused six-membered and seven-membered rings in high yields with excellent enantioselectivities. In addition, this approach is used to realize the first enantioselective construction of challenging tetrahydroindolocarbazole scaffolds, which are found to show promising anticancer activity. More importantly, theoretical calculations of the reaction pathways and activation mode offer an in-depth understanding of this class of indolylmethanols. This work not only settles the challenges in realizing catalytic asymmetric cycloadditions of indolyldimethanols but also provides a powerful strategy for the construction of enantioenriched indole-fused rings.

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).

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.

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.

Copper-catalyzed multicomponent reactions for the efficient synthesis of diverse spirotetrahydrocarbazoles

In the presence of copper sulfate, three- or four-component reactions of 2-methylindole, aromatic aldehydes and various cyclic dienophiles in refluxing toluene afforded diverse spirotetrahydrocarbazoles. This reaction is an important development of the Levy reaction by using 2-methylindole to replace ethyl indole-2-acetate and successfully provides facile access to important polysubstituted spiro[carbazole-3,3'-indolines], spiro[carbazole-2,3'-indolines], spiro[carbazole-3,5'-pyrimidines] and spiro[carbazole-3,1'-cycloalkanes] in satisfactory yields and with high diastereoselectivity.

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.

Recent Advances in the Nitration of Olefins

Nitroolefins are important synthetic intermediates in the field of organic synthesis as well as in medicinal chemistry. The high reactivity of nitroalkenes due to the polarized double bond which enables them to act as Michael acceptor in conjugate addition reactions, or as a dienophile in cycloaddition makes it an essential synthetic handle for accessing complex molecules. The classical method to prepare nitroolefins is indeed the Henry nitroaldol reaction, where a carbonyl compound and nitroalkane are condensed in presence of base. Direct nitration of olefin, on the other hand, serves as a useful alternative as olefins are abundant, have broad commercial availability and easy to manipulate. In this context, numerous methods have been developed over the last few decades, focusing on direct nitration of styrene and aliphatic olefins. Furthermore, thorough literature search revealed that implementation of this class of reactions are gaining momentum as a preferred pathway to access nitroolefins, despite the presence of a powerful technique such as Henry reaction. In this review, we aim to cover recent advances in direct olefin nitration and their importance in accessing biorelevant molecules, total synthesis targets and future outlook in this specific research area.

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.

Synthesis of Chiral Polycyclic Tetrahydrocarbazoles by Enantioselective Aminocatalytic Double Activation of 2-Hydroxycinnamaldehydes with Dienals

An efficient aminocatalytic enantioselective double-activation strategy has been developed that combines several different aminocatalytic modes in a cascade process, such as iminium ion, vinylogous iminium ion, trienamine, and dienamine activations. By using this strategy, 2-hydroxycinnamaldehydes worked well with various dienals via [4 + 2] cycloaddition and the oxa-Michael reaction-initiated cascade, respectively, leading to chiral polycyclic tetrahydrocarbazole and chromane derivatives with excellent diastereo- and enantioselectivities.

Diastereoselective synthesis of spiro[carbazole-3,5'-pyrimidines] and spiro[carbazole-3,1'-cyclohexanes] via four-component reaction

Functionalized spiro[carbazole-3,5'-pyrimidines] and spiro[carbazole-3,1'-cyclohexanes] were efficiently synthesized in satisfactory yields with high diastereoselectivity by CuSO4 catalyzed multicomponent reaction of indole-2-acetate, aromatic aldehyde and 1,3-dimethylbarbituric acid or dimedone. The reaction was finished with sequential Diels-Alder reaction of both in situ generated indole-2,3-quinodimethane and a dienophile. Additionally, the initially formed spiro[carbazole-3,5'-pyrimidines] were converted to dehydrogenated spiro[carbazole-3,5'-pyrimidines] by DDQ oxidation. The initially formed spiro[carbazole-3,1'-cyclohexanes] were converted to δ-valerolactone-substituted carbazoles by a DDQ promoted Baeyer-Villiger oxidation process.

Water Modulated Diastereoselective Synthesis of cis/trans-Spiro[indoline-3,6'-naphtho[2,3-c]carbazoles]

p-TsOH catalyzed Diels-Alder reaction of 2-(1-alkylindol-3-yl)naphthalene-1,4-diones and 3-phenacylideneoxindoles showed fascinating diastereoselectivity. The reaction with the hydrated p-TsOH afforded trans-isomers of dihydrospiro[indoline-3,6'-naphtho[2,3-c]carbazoles] as major products. Alternatively, the reaction with anhydrous p-TsOH under a Dean and Stark apparatus predominately gave cis-isomer of dihydrospiro[indoline-3,6'-naphtho[2,3-c]carbazoles]. On the other hand, the similar p-TsOH catalyzed reaction of 2-(indol-3-yl)naphthalene-1,4-diones with 3-arylideneindolin-2-ones afforded cis/trans-isomers of dihydrospiro[indoline-3,6'-naphtho[2,3-c]carbazoles]. Additionally, the p-TsOH catalyzed reaction of 2-(indol-3-yl)naphthalene-1,4-diones with 2-arylidene-1,3-indanediones gave the expected spiro[indene-2,6'-naphtho[2,3-c]carbazoles] in satisfactory yields.

Multicomponent Reaction for Diastereoselective Synthesis of Spiro[carbazole-3,4'-pyrazoles] and Spiro[carbazole-3,4'-thiazoles]

In the presence of copper sulfate, the three-component reaction of aromatic aldehydes, ethylindole-3-acetate and 4-arylidene-5-methyl-2-phenylpyrazol-3-ones, in refluxing toluene afforded spiro[carbazole-3,4'-pyrazoles] in good yields with high diastereoselectivity. More importantly, the similar CuSO₄ promoted the four-component reaction of two molecular aromatic aldehydes with ethylindole-3-acetate and 5-methyl-2-phenyl-pyrazol-3-one resulted in 2,4-diarylspiro[carbazole-3,4'-pyrazoles] in satisfactory yields. Additionally, CuSO₄ promoted the four-component reaction of two molecular aromatic aldehydes, ethylindole-3-acetate and 2-phenylthiazol-4-one, in refluxing toluene gave 2,4-diarylspiro[carbazole-3,4'-thiazoles] with diastereomeric ratios in the range of 3:1 to 20:1.

Highly enantioselective tandem cycloisomerization/Diels-Alder reaction of 2-(1-alkynyl)-2-alken-1-ones and enals: dual catalysis with platinum and amines

Herein, we disclosed a highly efficient strategy of enantioselective synthesis of 2,3-furan-fused carbocycles bearing three-contiguous stereocenters. This transformation is catalyzed by dual catalysis of PtCl4/chiral amines via tandem dehydrogenative annulation/Diels-Alder reaction of 2-(1-alkynyl)-2-alken-1-ones and enals. The in situ generation of the furan-based ortho-quinodimethane intermediates and the iminium activation of enals are crucial to this transformation.

Enantioselective Construction of the Cycl[3.2.2]azine Core via Organocatalytic [12 + 2] Cycloadditions

The first enantioselective [12 + 2] cycloaddition has been developed for the construction of a chiral cycl[3.2.2]azine core, a tricyclic moiety with a central ring-junction nitrogen atom, by an operationally simple one-step organocatalytic process. The reaction concept builds upon aminocatalytically generated 12π-components derived from 5H-benzo[a]pyrrolizine-3-carbaldehydes reacting with different electron-deficient 2π-components and affording the complex scaffold of benzo[a]cycl[3.2.2]azine (indolizino[3,4,5-ab]isoindole) with excellent enantio- and diastereoselectivity in good yields. The developed reaction is robust toward diverse substituent patterns and has been extended to different classes of electron-deficient 2π-components by minor variations in reaction setup. The obtained [12 + 2] cycloadducts are electron-deficient in nature, and their reaction with nucleophiles have been demonstrated. The enantioselective [12 + 2] cycloaddition with α,β-unsaturated aldehydes as the electron-deficient 2π-components relies upon an unconventional, simple aminocatalyst. In order to understand the high stereoselectivity of the [12 + 2] cycloaddition for this simple catalyst, combined experimental and computational investigations were performed. The investigations point to activation of both the 5H-benzo[a]pyrrolizine-3-carbaldehyde and the α,β-unsaturated aldehyde by the aminocatalyst and that the reaction proceeds by a stepwise cycloaddition, where especially the ring-closure is crucial for the stereochemical outcome. For other electron-deficient 2π-components, such as α,β-unsaturated ketoesters and nitroolefins, a more sterically demanding aminocatalyst has been applied and the corresponding [12 + 2] cycloadducts are obtained with excellent stereoselectivity. The [12 + 2] cycloaddition with vinyl sulfones afforded fully unsaturated systems, which display photoluminescence properties and for which quantum yields have been evaluated.

Organocatalytic asymmetric allylic alkylation of 2-methyl-3-nitroindoles: a route to direct enantioselective functionalization of indole C(sp3)-H bonds

We here described a direct catalytic asymmetric functionalization of 2-methylindoles using organocatalysis. An efficient asymmetric allylic alkylation reaction with respect to 2-methyl-3-nitroindoles and racemic Morita-Baylis-Hillman carbonate has been achieved by using a chiral biscinchona alkaloid catalyst, which provided the functionalized indole derivatives in good yields and enantioselectivities.

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.

Remote asymmetric conjugate addition catalyzed by a bifunctional spiro-pyrrolidine-derived thiourea catalyst

A novel spiro-pyrrolidine (SPD)-derived bifunctional thiourea catalyst has been developed and used in a stereoselective conjugate addition of furfurals to β,γ-unsaturated α-ketoesters.

Advances in organocatalytic asymmetric reactions of vinylindoles: powerful access to enantioenriched indole derivatives

This review summarizes advances in vinylindole-based organocatalytic asymmetric reactions since 2008 and includes the applications of some methodologies in the total synthesis of natural products, points out remaining challenges in this research area.

Recent advances of chromone-based reactants in the catalytic asymmetric domino annulation reaction

Chiral polycyclic chromanones are important heterocyclic frameworks that constitute the core structures of many natural products and bioactive molecules.

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.

Stereoselective synthesis and applications of spirocyclic oxindoles

This review summaries recent synthetic developments towards spirocyclic oxindoles and applications as valuable medicinal and synthetic targets.

Facile syntheses of tetrahydroquinolines and 1,2-dihydroquinolines via vinylogous cascade hydride transfer/cyclization

The medicinally significant 3-monosubstituted tetrahydroquinolines and 1,2-dihydroquinolines were controllably constructed via redox-neutral vinylogous cascade condensation/[1,5]-hydride transfer/cyclization in EtOH.

DDQ dehydrogenative Diels–Alder reaction for the synthesis of functionalized spiro[carbazole-1,3′-indolines] and spiro[carbazole-1,5′-pyrimidines]

Reactive 3-vinylindoles were generated by in situ DDQ oxidative dehydrogenation of 3-(indol-3-yl)-1,3-diphenylpropan-1-ones, and underwent sequential Diels–Alder reaction to give novel heterocyclic spirocarbazoles.

Unlocking Access to Enantiopure Fused Uracils by Chemodivergent [4+2] Cross-Cycloadditions: DFT-Supported Homo-Synergistic Organocatalytic Approach

The discovery of chemical methods enabling the construction of carbocycle-fused uracils which embody a three-dimensional and functional-group-rich architecture is a useful tool in medicinal chemistry oriented synthesis. In this work, an unprecedented amine-catalyzed [4+2] cross-cycloaddition is documented; it involves remotely enolizable 6-methyluracil-5-carbaldehydes and β-aryl enals, and chemoselectively produces two novel bicyclic and tricyclic fused uracil chemotypes in good yields with a maximum level of enantiocontrol. In-depth mechanistic investigations and control experiments support an intriguing homo-synergistic organocatalytic approach, where the same amine organocatalyst concomitantly engages both aldehyde partners in a stepwise eliminative [4+2] cycloaddition, whose vinylogous iminium ion intermediate product may diverge-depending upon conditions-to either bicyclic targets by hydrolysis or tricyclic products by a second homo-synergistic trienamine-mediated stepwise [4+2] cycloaddition.

Enantioselective [4 + 2] Cycloaddition Reaction of Vinylquinolines with Dienals Enabled by Synergistic Organocatalysis

An unprecedented organocatalytic enantioselective [4 + 2] cycloaddition reaction of vinyl quinolines with dienals is achieved with the synergistic activation of CH₃SO₃H and a chiral aminocatalyst. The power of the process is demonstrated by its high efficiency of the production of new synthetically and biologically valued chiral quinoline architectures in high yields and with excellent enantioselectivities.