Enantiodivergent Combination of Natural Product Scaffolds Enabled by Catalytic Enantioselective Cycloaddition

Pseudo-Natural Products: Expanding chemical and biological space by surpassing natural constraints

This review explores the recent advancements in the design and synthesis of pseudo-natural products (pseudo-NPs) by employing innovative principles and strategies, heralding a transformative era in chemistry and biology. Pseudo-NPs, produced through in silico fragmentation and the de novo recombination of natural product fragments, reveal compounds endowed with distinct biological activities. Their advantage lies in transcending natural product structures, fostering diverse possibilities. Research in this area over the past decade has yielded unconventional combinations of natural product fragments, leading to the identification of novel compounds possessing unique scaffolds and biological significance, thereby contributing to the discovery of new therapeutics. The pseudo-NPs exert potent biological effects through various signaling pathways. In chemical biology and medicinal chemistry, designing pseudo-NPs is an important strategy, harnessing molecular hybridization and bioinspired synthesis to generate diverse compounds with remarkable biological activities, underscoring their immense potential in drug discovery and development.

Access to 8-Azabicyclo[3,2,1]octanes via the [3 + 2] Cycloaddition of Oxidopyridinium Ions to Maleimides

Herein, we reported a unique and operationally simple method to assemble 8-azabicyclo[3,2,1]octanes by using oxidopyridinium ions and maleimides as synthons. The features of good to high yields and good functional group tolerance are achieved regularly under mild conditions. Of note, oxidopyridinium ions undergo a [3 + 2] cycloaddition on their C2 and C6 positions.

Oxidopyridinium Cycloadditions Revisited: A Combined Computational and Experimental Study on the Reactivity of 1-(2-Pyrimidyl)-3-oxidopyridinium Betaine

To investigate the effect of N-substituents on their reactivity and selectivity of oxidopyridinium betaines, we performed density functional theory (DFT) calculations of model cycloadditions with N-methylmaleimide and acenaphthylene. The theoretically expected results were compared with the experimental results. Subsequently, we demonstrated that 1-(2-pyrimidyl)-3-oxidopyridinium can be used for (5 + 2) cycloadditions with various electron-deficient alkenes, dimethyl acetylenedicarboxylate, acenaphthylene, and styrene. In addition, a DFT analysis of the cycloaddition of 1-(2-pyrimidyl)-3-oxidopyridinium with 6,6-dimethylpentafulvene suggested the possibility of pathway bifurcations involving a (5 + 4)/(5 + 6) ambimodal transition state, although only (5 + 6) cycloadducts were experimentally observed. A related (5 + 4) cycloaddition was observed in the reaction of 1-(2-pyrimidyl)-3-oxidopyridinium with 2,3-dimethylbut-1,3-diene.

Synergistic Catalysis between a Dipeptide Phosphonium Salt and a Metal-Based Lewis Acid for Asymmetric Synthesis of N-Bridged [3.2.1] Ring Systems

We present an unprecedented synergic catalytic route for the asymmetric construction of fluorinated N-bridged [3.2.1] cyclic members of tropane family via a bifunctional phosphonium salt/silver co-catalyzed cyclization process. A broad variety of substrates bearing an assortment of functional groups are compatible with this method, providing targeted compounds bearing seven-membered ring and four contiguous stereocenters in high yields with excellent stereoselectivities. The gram-scale preparations, facile elaborations and preliminary biological activities of the products demonstrate the application potential. Moreover, both experimental and computational mechanistic studies revealed that the cyclization proceeded via a "sandwich" reaction model with multiple weak-bond cooperative activations. Insights gained from our studies are expected to advance general efforts towards the catalytic synthesis of challenging chiral heterocyclic molecules.

Current applications of kinetic resolution in the asymmetric synthesis of substituted pyrrolidines

Chiral substituted pyrrolidines are key elements in various biologically active molecules and are therefore valuable synthetic targets. One traditional method towards enantiomerically pure compounds is the application of kinetic resolution. In this review, current KR methodology used in the synthesis of substituted pyrrolidines is surveyed, including enzymatic methods, cycloadditions and reduction of ketones.

Combination of Pseudo-Natural Product Design and Formal Natural Product Ring Distortion Yields Stereochemically and Biologically Diverse Pseudo-Sesquiterpenoid Alkaloids

We describe the synthesis and biological evaluation of a new natural product‐inspired compound class obtained by combining the conceptually complementary pseudo‐natural product (pseudo‐NP) design strategy and a formal adaptation of the complexity‐to‐diversity ring distortion approach. Fragment‐sized α‐methylene‐sesquiterpene lactones, whose scaffolds can formally be viewed as related to each other or are obtained by ring distortion, were combined with alkaloid‐derived pyrrolidine fragments by means of highly selective stereocomplementary 1,3‐dipolar cycloaddition reactions. The resulting pseudo‐sesquiterpenoid alkaloids were found to be both chemically and biologically diverse, and their biological performance distinctly depends on both the structure of the sesquiterpene lactone‐derived scaffolds and the stereochemistry of the pyrrolidine fragment. Biological investigation of the compound collection led to the discovery of a novel chemotype inhibiting Hedgehog‐dependent osteoblast differentiation

Natural product-informed exploration of chemical space to enable bioactive molecular discovery

The search for new bioactive molecules remains an open challenge limiting our ability to discover new drugs to treat disease and chemical probes to comprehensively study biological processes. The vastness of chemical space renders its exploration unfeasible by synthesis alone. Historically, chemists have tended to explore chemical space unevenly without committing to systematic frameworks for navigation. This minireview covers a range of approaches that take inspiration from the structure or origin of natural products, and help focus molecular discovery on biologically-relevant regions of chemical space. All these approaches have enabled the discovery of distinctive and novel bioactive small molecules such as useful chemical probes of biological mechanisms. This minireview comments on how such approaches may be developed into more general frameworks for the systematic identification of currently unexplored regions of biologically-relevant chemical space, a challenge that is central to both chemical biology and medicinal chemistry.

Stereoselective Synthesis of Tropanes via a 6π-Electrocyclic Ring-Opening/ Huisgen [3+2]-Cycloaddition Cascade of Monocyclopropanated Heterocycles

The synthesis of tropanes via a microwave-assisted, stereoselective 6π-electrocyclic ring-opening/ Huisgen [3+2]-cycloaddition cascade of cyclopropanated pyrrole and furan derivatives with electron-deficient dipolarophiles is demonstrated. Starting from furans or pyrroles, 8-aza- and 8-oxabicyclo[3.2.1]octanes are accessible in two steps in dia- and enantioselective pure form, being versatile building blocks for the synthesis of pharmaceutically relevant targets, especially for new cocaine analogues bearing various substituents at the C-6/C-7 positions of the tropane ring system. Moreover, the 2-azabicyclo[2.2.2]octane core (isoquinuclidines), being prominently represented in many natural and pharmaceutical products, is accessible via this approach.

Synthesis and evaluation of the performance of a small molecule library based on diverse tropane-related scaffolds

A unified synthetic approach was developed that enabled the synthesis of diverse tropane-related scaffolds. The key intermediates that were exploited were cycloadducts formed by reaction between 3-hydroxy-pyridinium salts and vinyl sulfones or sulfonamides. The diverse tropane-related scaffolds were formed by addition of substituents to, cyclisation reactions of, and fusion of additional ring(s) to the key bicyclic intermediates. A set of 53 screening compounds was designed, synthesised and evaluated in order to determine the biological relevance of the scaffolds accessible using the synthetic approach. Two inhibitors of Hedgehog signalling, and four compounds with weak activity against the parasite P. falciparum, were discovered. Three of the active compounds may be considered to be indotropane or pyrrotropane pseudo natural products in which a tropane is fused with a fragment from another natural product class. It was concluded that the unified synthetic approach had yielded diverse scaffolds suitable for the design of performance-diverse screening libraries.

Chemoselectivity issues of the asymmetric interaction between cyclohexanone, β-nitrostyrene, and benzoic acid under 5-aryl prolinate's organocatalysis

4-l-menthyloxycarbonyl 5-aryl prolinates were studied as organocatalysts of a novel three-component reaction of cyclohexanone, benzoic acid, and β-nitrostyrene. The presence of ortho-halogen atom in 5-aryl fragment of the catalyst is favored for driving the formation of chiral 7a-hydroxyoctahydro-2H-indol-2-one scaffold. 5-(o-Chlorophenyl) prolinate selectively afforded 3-phenyl-7a-hydroxyoctahydro-2H-indol-2-one with ee 63%, whereas 5-phenyl prolinate led to conjugation of β-nitrostyrene to cyclohexanone (the Michael adduct). Plausible chlorine effect is accounted for the specific interaction of the 5-aryl prolinate enamine intermediate with β-nitrostyrene in the transition state.

Principle and design of pseudo-natural products

Natural products (NPs) are a significant source of inspiration towards the discovery of new bioactive compounds based on novel molecular scaffolds. However, there are currently only a small number of guiding synthetic strategies available to generate novel NP-inspired scaffolds, limiting both the number and types of compounds accessible. In this Perspective, we discuss a design approach for the preparation of biologically relevant small-molecule libraries, harnessing the unprecedented combination of NP-derived fragments as an overarching strategy for the synthesis of new bioactive compounds. These novel 'pseudo-natural product' classes retain the biological relevance of NPs, yet exhibit structures and bioactivities not accessible to nature or through the use of existing design strategies. We also analyse selected pseudo-NP libraries using chemoinformatic tools, to assess their molecular shape diversity and properties. To facilitate the exploration of biologically relevant chemical space, we identify design principles and connectivity patterns that would provide access to unprecedented pseudo-NP classes, offering new opportunities for bioactive small-molecule discovery.

Guided by evolution: from biology oriented synthesis to pseudo natural products

This review provides an overview and historical context to two concepts for the design of natural product-inspired compound libraries and highlights the used synthetic methodologies.

Diastereodivergent Asymmetric 1,3-Dipolar Cycloaddition of Azomethine Ylides and β-Fluoroalkyl Vinylsulfones: Low Copper(II) Catalyst Loading and Theoretical Studies

A Cu^II -catalyzed asymmetric 1,3-dipolar cycloaddition using β-fluoroalkyl alkenyl arylsulfones as dipolarophiles and glycine/alanine iminoesters as azomethine ylide precursors has been developed. Remarkably, a catalyst loading as low as 0.5 mol % is highly efficient. Accordingly, a wide range of enantioenriched 3-fluoroalkyl pyrrolidines, as well as Δ² -pyrroline and pyrrole derivatives, are generated in good to excellent yields with high asymmetric induction. This synthetic approach is diastereodivergent in that exo-adducts could be converted into the corresponding exo'-adducts by 1,8-diazabicyclo[5.4.0]undec-7-ene mediated epimerization at C2 of the pyrrolidine core. The free-energy profiles from DFT calculations suggest the Michael addition of the 1,3-dipole to be the rate- and enantiodetermining step, and the origin of stereoselectivity is studied by means of the noncovalent interaction (NCI) analysis.

Copper(I)-Catalyzed Kinetic Resolution of exo-3-Oxodicyclopentadienes and endo-3-Oxodicyclopentadiene

The first example of highly efficient kinetic resolution of exo-3-oxodicyclopentadienes and endo-3-oxodicyclopentadiene has been developed by means of Cu(I)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylide. Compared with the existing methodologies for those synthetically important optically active convex molecules, the current protocol provides an alternative but more practical approach from the readily available racemic starting materials, which is free from the repetitive reduction/oxidation steps in the enzymatic resolution or the indispensable stoichiometric amount of chirality-induction reagents.

A AgOAc/quinine-derived aminophosphine complex as an efficient catalyst for diastereo- and enantioselective 1,3-dipolar cycloaddition of α,β-unsaturated 7-azaindoline amides and azomethine ylides

The Dixon’s catalyst system is effective for the asymmetric 1,3-dipolar cycloaddition.

Kinetic Resolution of Alkylidene Norcamphors via a Ligand-Controlled Umpolung-Type 1,3-Dipolar Cycloaddition

Development of a general catalytic and highly efficient method utilizing readily available precursors for the regio- and stereoselective construction of bioactive natural-product-inspired spiro architectures remains a formidable challenge in chemical research. Transition metal-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides produces numerous N-heterocycles, but reaction control with the regioselectivity opposite to the conventional fashion has rarely been demonstrated. Herein, we report a unique ligand-controlled Cu(I)-catalyzed umpolung-type 1,3-dipolar cycloaddition of azomethine ylide to realize efficient kinetic resolution of racemic alkylidene norcamphors with the concomitant construction of previously inaccessible spiro N-heterocycles with high levels of regio- and stereoselectivity. The success of this methodology relies on the strategy of kinetic resolution, and the serendipitous discovery of a unique ligand-enabled regiospecific cycloaddition, which not only provides evidence for the existence of the minor zwitterionic resonance form in metallated azomethine ylide but also diversifies the existing chemistry of azomethine ylide-involved 1,3-dipolar cycloadditions with rare polarity inversion.

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Kinetic resolution of racemic alkylidene norcamphors

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Spiro architectures incorporating norbornane and pyrrolidine scaffolds

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Unique ligand-enabled umpolung-type 1,3-dipolar cycloaddition

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.

A Highly Enantioselective Copper/Phosphoramidite-Thioether-Catalyzed Diastereodivergent 1,3-Dipolar Cycloaddition of Azomethine Ylides and Nitroalkenes

In contrast to the plethora of catalytic systems that enable access to any enantiomers of the chiral products by simply choosing between a pair of enantiomeric or pseudoenantiomeric chiral catalysts, few analogously effective protocols exist for the synthesis of compounds bearing multiple stereogenic centers with full control of the absolute and relative stereochemical configurations. Here, we report the application of our previously developed modular phosphoramidite-thioether ligands for the copper-catalyzed diastereodivergent asymmetric 1,3-dipolar cycloaddition of azomethine ylides and nitroalkenes. Our catalytic system enables wide substrate scope, great stereochemical control, and high reaction efficiency.

Highly efficient desymmetrization of cyclopropenes to azabicyclo[3.1.0]hexanes with five continuous stereogenic centers by copper-catalyzed [3 + 2] cycloadditions

An unprecedented copper-catalyzed desymmetrization/cycloaddition reaction of 1,1-disubstituted cyclopropenes provides an efficient access to azabicyclo[3.1.0]hexanes bearing five continuous carbon-stereogenic centers.

Synthesis of Pyrrole via a Silver-Catalyzed 1,3-Dipolar Cycloaddition/Oxidative Dehydrogenative Aromatization Tandem Reaction

Pyrroles are an important group of heterocyclic compounds with a wide range of interesting properties, which have resulted in numerous applications in a variety of fields. Despite the importance of these compounds, there have been few reports in the literature pertaining to the synthesis of pyrroles from simple alkenes using a one-pot sequential 1,3-dipolar cycloaddition/aromatization reaction sequence. Herein, we report the development of a benzoyl peroxide-mediated oxidative dehydrogenative aromatization reaction for the construction of pyrroles. We subsequently developed a one-pot tandem reaction that combined this new method with a well-defined silver-catalyzed 1,3-dipolar cycloaddition reaction, thereby providing a practical method for the synthesis of multisubstituted pyrroles from easy available alkenes. The mechanism of this oxidative dehydrogenative aromatization reaction was also examined in detail.

Natural product inspired compound collections: evolutionary principle, chemical synthesis, phenotypic screening, and target identification

Natural products have been an excellent and abundant source of therapeutics for many decades. To expand on their success, and explore areas of chemical space not covered by biosynthesis, the synthesis of natural product-inspired compound collections has emerged as a viable strategy. Herein we describe the principles behind biology-oriented synthesis and related approaches, the requirements for development of novel chemistry and how phenotypic screens are a very fruitful way to explore the bioactivity of compounds made using these approaches. Finally, we summarize state-of-the-art techniques to identify the biological targets of any hits identified.

Copper(i)-catalyzed asymmetric exo-selective [3+2] cycloaddition of azomethine ylides with β-trifluoromethyl β,β-disubstituted enones

A copper-catalyzed asymmetricexo-selective dipolar cycloaddition of β-trifluoromethyl β,β-disubstituted enones with azomethine ylides was developed, which provides an efficient access to valuable pyrrolidines bearing a trifluoromethylated all-carbon quaternary stereocenter.

The facile and stereoselective synthesis of pyrrolidine β-amino acids via copper(i)-catalyzed asymmetric 1,3-dipolar cycloaddition

A highly efficient copper(i)-catalyzed asymmetric 1,3-dipolar cycloaddition was developed, which provides a straightforward and efficient pathway to highly functionalized free pyrrolidine β-amino acids.

Enantioselective construction of tricyclic pyrrolidine-fused benzo[b]thiophene 1,1-dioxide derivatives via copper(i)-catalyzed asymmetric 1,3-dipolar cycloaddition

A facile Cu(i)-catalyzed asymmetric 1,3-dipolar cycloaddition is developed, affording chiral tricyclic pyrrolidine-fused benzo[b]thiophene derivatives in excellent diastereo- and enantioselectivities.

One-pot sequential reaction to 2-substituted-phenanthridinones from N-methoxybenzamides

A one-pot synthesis of 2-bromo/chloro-phenanthridinones via an amidation of arenes followed by a regioselective halogenation reaction has been developed using a hypervalent iodine reagent.