First total synthesis of (+/-)-strychnofoline via a highly selective ring-expansion reaction

Enantioselective formal total synthesis of dihydrospirotryprostatin B

Spirotryprostatins are representative members of medicinally interesting bioactive molecules of the spirooxindole natural products. In this communication, we present a novel enantioselective total synthesis of the spirooxindole alkaloid dihydrospirotryprostatin B. The synthesis takes advantage of copper-catalyzed tandem reaction of o-iodoanilide chiral sulfinamide derivatives with alkynone to rapidly construct the key quaternary carbon stereocenter of the natural product dihydrospirotryprostatin B.

A Formal Synthesis of (±)-Arborisidine

Herein, we report a formal synthesis of (±)-arborisidine via the creation of Jiao's intermediate with the critical caged structure. Starting from tryptamine, a Pictet-Spengler cyclization forged the piperidine ring, a Pd-catalyzed indole allylation and ring-closing metathesis protocol afforded a bridged aza-bicyclo[3.3.1]nonane moiety, and an intramolecular N-alkylation closed the final pyrrolidine ring. This study provides a new approach to the unique caged framework of arborisidine and relevant alkaloids.

Unified Strategy Enables the Collective Syntheses of Structurally Diverse Indole Alkaloids

Natural products and their analogues are significant sources of therapeutic lead compounds. However, synthetic strategies for generating large collections of these molecules remain a significant challenge. The most difficult step in their synthesis is the design of a common intermediate that can be easily transformed into natural products belonging to different families. This study demonstrates the evolution of synthetic tactics designed to assemble the functionalized piperidines present in indole alkaloids from a common intermediate. More importantly, we also report a previously unknown Ir- and Er-catalyzed dehydrogenative spirocyclization reaction that enables direct access to spirocyclic oxindole alkaloids. As a practical application, the asymmetric total syntheses of 29 natural alkaloids belonging to different families were accomplished by following a uniform synthetic route. The proposed methodology extends the capability of the iridium-catalyzed dehydrogenative coupling reaction to the realm of indole-alkaloid synthesis and provides new opportunities for the efficient preparation of natural product-like molecules.

Alkene carboamination/oxidative denitrogenation of 3-allyl-3-hydrazinylindolin-2-ones: one-pot entry to spirocyclopropyloxindoles

A one-pot protocol, consisting of a Pd-catalysed carboamination reaction, followed by N-deprotection and oxidative denitrogenation, has been developed for the synthesis of diversely substituted spirocyclopropyloxindoles, in yields up to 73% and with diastereoselectivity close to 1 : 1. Readily accessible starting materials, mild reaction conditions, an easy to operate one-pot procedure and good functional group tolerance make this transformation a versatile tool for the synthesis of substituted spirocyclopropyloxindoles. This protocol successfully works on the gram-scale and allows access to both diastereoisomers separately. A plausible mechanism was proposed, and a series of post-transformations were performed on the obtained products, showing their remarkable synthetic versatility.

Recent advances in microwave-assisted multicomponent synthesis of spiro heterocycles

Spiro heterocycle frameworks are a class of organic compounds that possesses unique structural features making them highly sought-after targets in drug discovery due to their diverse biological and pharmacological activities. Microwave-assisted organic synthesis has emerged as a powerful tool for assembling complex molecular architectures. The use of microwave irradiation in synthetic chemistry is a promising method for accelerating reaction rates and improving yields. This review provides insights into the current state of the art and highlights the potential of microwave-assisted multicomponent reactions in the synthesis of novel spiro heterocyclic compounds that were reported between 2017 and 2023.

Total Synthesis and Structural Plasticity of Kratom Pseudoindoxyl Metabolites

Mitragynine pseudoindoxyl, a kratom metabolite, has attracted increasing attention due to its favorable side effect profile as compared to conventional opioids. Herein, we describe the first enantioselective and scalable total synthesis of this natural product and its epimeric congener, speciogynine pseudoindoxyl. The characteristic spiro-5-5-6-tricyclic system of these alkaloids was formed through a protecting-group-free cascade relay process in which oxidized tryptamine and secologanin analogues were used. Furthermore, we discovered that mitragynine pseudoindoxyl acts not as a single molecular entity but as a dynamic ensemble of stereoisomers in protic environments; thus, it exhibits structural plasticity in biological systems. Accordingly, these synthetic, structural, and biological studies provide a basis for the planned design of mitragynine pseudoindoxyl analogues, which can guide the development of next-generation analgesics.

Natural-Product-Inspired Microwave-Assisted Synthesis of Novel Spirooxindoles as Antileishmanial Agents: Synthesis, Stereochemical Assignment, Bioevaluation, SAR, and Molecular Docking Studies

Leishmaniasis is a neglected tropical disease, and there is an emerging need for the development of effective drugs to treat it. To identify novel compounds with antileishmanial properties, a novel series of functionalized spiro[indoline-3,2'-pyrrolidin]-2-one/spiro[indoline-3,3'-pyrrolizin]-2-one 23a-f, 24a-f, and 25a-g were prepared from natural-product-inspired pharmaceutically privileged bioactive sub-structures, i.e., isatins 20a-h, various substituted chalcones 21a-f, and 22a-c amino acids, via 1,3-dipolar cycloaddition reactions in MeOH at 80 °C using a microwave-assisted approach. Compared to traditional methods, microwave-assisted synthesis produces higher yields and better quality, and it takes less time. We report here the in vitro antileishmanial activity against Leishmania donovani and SAR studies. The analogues 24a, 24e, 24f, and 25d were found to be the most active compounds of the series and showed IC₅₀ values of 2.43 µM, 0.96 µM, 1.62 µM, and 3.55 µM, respectively, compared to the standard reference drug Amphotericin B (IC₅₀ = 0.060 µM). All compounds were assessed for Leishmania DNA topoisomerase type IB inhibition activity using the standard drug Camptothecin, and 24a, 24e, 24f, and 25d showed potential results. In order to further validate the experimental results and gain a deeper understanding of the binding manner of such compounds, molecular docking studies were also performed. The stereochemistry of the novel functionalized spirooxindole derivatives was confirmed by single-crystal X-ray crystallography studies.

Asymmetric Synthesis of Spiro-3,3'-cyclopropyl Oxindoles via Vinylogous Michael Initiated Ring Closure Reaction

A novel organocatalytic cascade approach for the synthesis of spiro-cyclopropyl oxindole derivatives has been developed. The methodology is based on asymmetric vinylogous Michael addition of 4-nitroisoxazole derivatives to N-Boc isatylidene malonates followed by intramolecular alkylation. Its remarkable stereocontrol, wide substrate scope, and scalability highlight this new developed strategy. Moreover, this work represents the first example of vinylogous Michael initiated ring closure (MIRC) reaction for the synthesis of chiral 3,3'-cyclopropyl oxindole.

Palladium-Catalyzed Carbene Migratory Insertion/Carbonylation Cascade Reaction: Synthesis of 2-Indolones with a C3 All-Carbon Quaternary Center

An attractive palladium-catalyzed three-component reaction of ortho-amino aryl diazo esters, allyl carboxylates, and carbon monoxide (CO) has been developed. This catalytic system rendered domino carbene migratory insertion and carbonylation. Remarkably, 2-indolones 3 with a C3 all-carbon quaternary center can be selectively obtained in good to excellent yields via one-pot synthesis, in which two different C-C bonds and one C-N bond were formed in a straightforward manner.

Stereoselective synthesis of C3-tetrasubstituted oxindoles via copper catalyzed asymmetric propargylation

Herein, a copper catalyzed asymmetric propargylation of 2-oxindole-3-carboxylate esters with terminal propargylic esters is described. This strategy successfully provides a direct approach to constructing a broad range of chiral C3-tetrasubstituted oxindoles with contiguous tertiary and quaternary carbon stereocenters in high yields and excellent enantioselectivities (16 examples, up to 99% yield and 98% ee). Moreover, the diastereoisomers of the two newly formed stereocenters can be separated by silica gel chromatography, thereby providing a valuable stereoselective access to all four possible stereoisomers of C3-tetrasubstituted oxindoles.

Herein, a copper catalyzed asymmetric propargylation of 2-oxindole-3-carboxylate esters with terminal propargylic esters is described.

Atom-Economical, Catalyst-Free Hydrosulfonation of Densely Functionalized Alkenes: Access to Oxindole Containing Sulfones

An atom-economical hydrosulfonation of densely functionalized alkenes under catalyst-free conditions is described. Alkenes possessing hydroxy-oxindole moiety underwent hydrosulfonation on treatment with arylsulfinic acids in green media to afford the resulting...

Lewis acid-catalyzed [3 + 2] annulations of oxindole based spirocyclic donor-acceptor cyclopropanes with ynamides

[3 + 2] annulations of oxindole based spirocyclic donor-acceptor cyclopropanes and ynamides catalyzed by copper triflate have been developed for the synthesis of biologically important spirocyclopenteneoxindoles. These reactions tolerated a wide scope of substrates and provided the desired products in good to high yields (up to 90%) with up to >40 : 1 diastereoselectivities under mild conditions.

Stereoselective synthesis and applications of spirocyclic oxindoles

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

Synthesis of Spiro 3,3'-Cyclopropyl Oxindoles Via N-Bromosuccinimide-Mediated Ring-Closing and Contraction Cascade

An N-bromosuccinimide-mediated cascade reaction involving the cyclization/oxygen-migration/ring-contraction process of 3-(β, β-diaryl) indolylethanol was disclosed. A variety of spiro 3,3'-cyclopropyl oxindole derivatives were efficiently synthesized in good yields under mild reaction conditions. A possible mechanism was suggested based on intermediate isolation and computational studies.

Recent research progress of Uncaria spp. based on alkaloids: phytochemistry, pharmacology and structural chemistry

Medicinal plants are well-known in affording clinically useful agents, with rich medicinal values by combining with disease targets through various mechanisms. Plant secondary metabolites as lead compounds lay the foundation for the discovery and development of new drugs in disease treatment. Genus Uncaria from Rubiaceae family is a significant plant source of active alkaloids, with anti-hypertensive, sedative, anti-Alzheimer's disease, anti-drug addiction and anti-inflammatory effects. This review summarizes and discuss the research progress of Uncaria based on alkaloids in the past 15 years, mainly in the past 5 years, including biosynthesis, phytochemistry, pharmacology and structural chemistry. Among, focusing on representative compounds rhynchophylline and isorhynchophylline, the pharmacological activities surrounding the central nervous system and cardiovascular system are described in detail. On the basis of case studies, this article provides a brief overview of the synthesis and analogues of representative compounds types. In summary, this review provides an early basis for further searching for new targets and activities, discussing the mechanisms of pharmacological activity and studying the structure-activity relationships of active molecules.

Recent Advances in Stereoselective Ring Expansion of Spirocyclopropanes: Access to the Spirocyclic Compounds

Spirocyclopropane represents a privileged structural scaffold for accessing synthetic libraries of densely functionalized spirocarbo- and heterocyclic compounds. Due to the ubiquity of spirocyclic motifs as a potent pharmacophore in natural products and pharmaceuticals, recent years have witnessed significant advances in developing synthetic strategies that exploits carbon-carbon bond scission in spirocyclopropanes. This paper summarizes the recent developments in stereoselective ring expansion of spirocyclopropanes in diversity-oriented synthesis and highlights the synthetic as well as mechanistic rationale of those methodologies. This review also encompasses the applicability of the protocols in bioactive natural product syntheses.

Catalytic enantioselective synthesis of carbocyclic and heterocyclic spiranes via a decarboxylative aldol cyclization

The synthesis of a variety of enantioenriched 1,3-diketospiranes from the corresponding racemic allyl β-ketoesters via an interrupted asymmetric allylic alkylation is disclosed. Substrates possessing pendant aldehydes undergo decarboxylative enolate formation in the presence of a chiral Pd catalyst and subsequently participate in an enantio- and diastereoselective, intramolecular aldol reaction to furnish spirocyclic β-hydroxy ketones which may be oxidized to the corresponding enantioenriched diketospiranes. Additionally, this chemistry has been extended to α-allylcarboxy lactam substrates leading to a formal synthesis of the natural product (-)-isonitramine.

Stereocontrolled Synthesis of (±)-Melokhanine E via an Intramolecular Formal [3 + 2] Cycloaddition

A convergent sequence to access the indole alkaloid (±)-melokhanine E in 12-steps (8-step longest linear sequence) and an 11% overall yield is reported. The approach utilizes two cyclopropane moieties as reactive precursors to a 1,3-dipole and imine species to enable stereoselective construction of the core scaffold through a formal [3 + 2] cycloaddition. The natural product was evaluated for its antimicrobial activity based on isolation reports; however, no activity was observed. The reported efforts serve as a synthetic platform to prepare an array of alkaloids bearing this core structural motif.

Chameleon-Like Activating Nature of the Spirooxindole Group in Donor-Acceptor Cyclopropanes

The concept of a chameleon activating group is considered in the context of donor-acceptor cyclopropane chemistry. When spiro-conjugated with cyclopropane, oxindole can act as an acceptor or a donor depending on the electronic nature of vicinal substituents. This dichotomy is reflected in the alteration of chemoselectivity of spiro[oxindole-1,3'-cyclopropane] ring opening with nucleophiles.

Synthesis of polycyclic spirooxindoles via an asymmetric catalytic one-pot stepwise Aldol/chloroetherification/aromatization procedure

A general method for the synthesis of chiral pentacyclic spirooxindoles containing a tetrahydropyrano[2,3-b]indole scaffold through a one-pot stepwise sequence from 3-(3-indolomethyl)oxindole, paraformaldehyde and NCS is reported. Furthermore, the pentacyclic spirooxindoles could be transformed to bispirooxindole and other structurally diverse spirocyclic oxindoles.

Stereocontrolled [3+2] Cycloaddition of Donor-Acceptor Cyclopropanes to Iminooxindoles: Access to Spiro[oxindole-3,2'-pyrrolidines]

A novel stereocontrolled assembly of spiro[oxindole-3,2'-pyrrolidines] via [3+2]-cycloaddition of donor-acceptor cyclopropanes to electron-poor ketimines, iminooxindoles, was developed. The method allows for efficient employment of common readily available donor-acceptor cyclopropanes, functionalized with ester, keto, nitro, cyano etc. groups, and N-unprotected iminooxindoles. The stereospecificity of the initial S_N2-like imine attack on a cyclopropane molecule together with a high diastereoselectivity of further C-C bond formation facilitate a rapid access to spiro[oxindole-3,2'-pyrrolidines] in their optically active forms. Preliminary in vitro testing of the synthesized compounds against LNCaP (p53+) and PC-3 (p53-) cells revealed good antiproliferative activities and p53-selectivity indices for several compounds that are intriguing in terms of their further investigation as inhibitors of MDM2-p53 interaction.

An NHC-catalyzed, stereoselective α-functionalization of α,β-unsaturated carboxylic acids through in situ activation

An N-heterocyclic carbene (NHC)-catalyzed diastereo- and enantioselective α-functionalization of α,β-unsaturated carboxylic acids bearing γ-H to construct dihydrocoumarin was realized through in situ activation.

Lewis acid catalyzed annulation of spirocyclic donor–acceptor cyclopropanes with exo-heterocyclic olefins: access to highly functionalized bis-spirocyclopentane oxindole frameworks

Oxindole-activated spiro-DAC is used as synthon to prepare complex bis-spirocyclopentane oxindole frameworks via ring-enlargement.

Application of the Asymmetric Pictet-Spengler Reaction in the Total Synthesis of Natural Products and Relevant Biologically Active Compounds

Tetrahydroisoquinolines are the framework of numerous natural products predominantly alkaloids, an important and one of the most wide spread families of naturally occurring compounds in the plant kingdom. Tetrahydroisoquinolines are commonly constructed through an old reaction, the so-called Pictet−Spengler Reaction (PSR). In this reaction, a β-aryl ethylamine undergoes an acid mediated condensation with a suitable aldehyde or ketone, followed by ring closure. In this review, we aim to highlight the applications of the asymmetric variant of this old name reaction in the total synthesis of natural products, chiefly, alkaloids, which exhibit significant biological properties.

Nine-step total synthesis of (−)-strychnofoline

The asymmetric synthesis of an anti-cancer spirooxindole alkaloid, (−)-strychnofoline, has, for the first time, been accomplished in only nine steps.

Asymmetric ring-opening of cyclopropyl ketones with β-naphthols catalyzed by a chiral N,N′-dioxide–scandium(iii) complex

An efficient asymmetric ring-opening reaction of cyclopropyl ketones with 2-naphthols has been realized using a chiral N,N′-dioxide–Sc^III complex catalyst. A variety of chiral β-naphthol derivatives were obtained in excellent outcomes (up to 99% yield, 97% ee).

Ni-Al Bimetallic Catalyzed Enantioselective Cycloaddition of Cyclopropyl Carboxamide with Alkyne

A Ni-Al bimetallic catalyzed enantioselective cycloaddition reaction of cyclopropyl carboxamides with alkynes has been developed. A series of cyclopentenyl carboxamides were obtained in up to 99% yield and 94% ee. The bifunctional-ligand-enabled bimetallic catalysis proved to be an efficient strategy for the C-C bond cleavage of unreactive cyclopropanes.

Diastereo- and enantioselective [3 + 3] cycloaddition of spirocyclopropyl oxindoles using both aldonitrones and ketonitrones

Optically active spirocyclic compounds play an important role in drug discovery, and new synthetic strategies for the efficient generation of spiro stereocenters are in much demand. Here we report a catalytic enantioselective cycloaddition using spirocyclic donor–acceptor cyclopropanes as a promising approach for the generation of spiro stereocenters. A diastereo- and enantioselective [3 + 3] cycloaddition of spirocyclopropyl oxindoles with both aldonitrones and ketonitrones is developed. The key to reaction development is the activation of spirocyclopropyl oxindoles by a suitable electron-withdrawing N-protecting group. This activation approach offers the promise of a general solution to enable spirocyclopropyl oxindoles as synthons for catalytic enantioselective synthesis of spirocyclic oxindoles featuring a C3 spiro stereocenter, a prominent structural motif in drugs and pharmaceutically active compounds. This protocol also constitutes the catalytic enantioselective reaction using unactivated achiral ketonitrones to construct tetrasubstituted carbon stereocenters.

Chiral spirocyclic compounds are important structural motifs for drug discovery. Here, the authors report a synthetic route to spirocycles based on enantioselective cycloaddition of activated spirocyclopropyl oxindoles, which act as donor-acceptor cyclopropanes.

N-heterocyclic carbene-catalyzed synthesis of spirocyclopentene-oxindoles from bromoenals

Spirocyclopentene-oxindoles were synthesized in good yields with good diastereo- and enantioselectivitiesviathe N-heterocyclic carbene-catalyzed reaction of bromoenals and oxindoles.

Lanthanide amide-catalyzed one-pot functionalization of isatins: synthesis of spiro[cyclopropan-1,3′-oxindoles] and 2-oxoindolin-3-yl phosphates

A lanthanide-catalyzed one-pot reaction using readily accessible substrates was developed affording spiro[cyclopropan-1,3′-oxindoles] in good yields and with high stereoselectivity.

Organoiodine reagent-promoted intermolecular oxidative amination: synthesis of cyclopropyl spirooxindoles

An organoiodine-promoted intramolecular direct oxidative C(sp²)–N cross-coupling reaction was developed for the preparation of cyclopropyl spirooxindoles from readily available secondary cyclopropyl carboxamides.

Zinc triflate-mediated cyclopropanation of oxindoles with vinyl diphenyl sulfonium triflate: a mild reaction with broad functional group compatibility

The first use of Zn(OTf)₂ for the cyclopropanation of unprotected oxindoles with vinyl diphenyl sulfonium triflate to show high regioselectivity.