Asymmetric total syntheses of (+)- and (-)-versicolamide B and biosynthetic implications

Inverse Electron-Demanding Diels-Alder Reactions in the Chemical Synthesis of Prenylated Indole Alkaloids Containing a Bicycle[2.2.2]diazaoctane Moiety: A Theoretical Study

The Diels-Alder reaction is believed to be a key step in the biosynthesis of prenylated indole alkaloids containing a bicycle[2.2.2]diazaoctane moiety. Many chemical syntheses of bicyclic structures by Diels-Alder reactions have been reported, but the reaction mechanism remains underexplored. We have carried out DFT calculations on both acid- and base-promoted Diels-Alder reactions in these syntheses and reveal that the reactions occur through an inverse-electron demand mechanism. We hope that the new mechanism is helpful for the mechanistic understanding of the biosynthesis of this class of important natural products.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Discovery of Pinostrobin as a Melanogenic Agent in cAMP/PKA and p38 MAPK Signaling Pathway

Melanogenesis is the process of melanin synthesis to protect the skin against ultraviolet radiation and other external stresses. The loss of skin pigmentation is closely related to depigmented skin disorders. The melanogenic effects of pinostrobin, an active flavanone found in honey, were evaluated. B16F10 cells were used for melanin content, tyrosinase activity, and the expression of melanogenesis-related markers. Moreover, computational simulations were performed to predict docking and pharmacokinetics. Pinostrobin increased melanin levels and tyrosinase activity by stimulating the expression of melanogenic regulatory factors including tyrosinase, tyrosinase-related protein (TRP) 1 and microphthalmia transcription factor (MITF). Specifically, the phosphorylation of cAMP response element binding (CREB) involved in the MITF activation was augmented by pinostrobin. Moreover, the compound upregulated the β-catenin by cAMP/PKA-mediated GSK-3β inactivation. Co-treatment with a PKA inhibitor, inhibited melanin production, tyrosinase activity, and expression of MITF, p-CREB, p-GSK-3β and p-β-catenin, demonstrating that pinostrobin-stimulated melanogenesis was closely related to cAMP/PKA signaling pathway. Furthermore, the combination of pinostrobin and a specific p38 inhibitor, showed that MITF upregulation by pinostrobin was partly associated with the p38 signaling pathway. Docking simulation exhibited that the oxygen group at C-4 and the hydroxyl group at C-5 of pinostrobin may play an essential role in melanogenesis. In silico analysis revealed that pinostrobin had the optimal pharmacokinetic profiles including gastrointestinal absorption, skin permeability, and inhibition of cytochrome (CYP) enzymes. From the present results, it might be suggested that pinostrobin could be useful as a potent and safe melanogenic agent in the depigmentation disorder, vitiligo.

Unified total synthesis of the brevianamide alkaloids enabled by chemical investigations into their biosynthesis

The bicyclo[2.2.2]diazaoctane alkaloids are a vast group of natural products which have been the focus of attention from the scientific community for several decades. This interest stems from their broad range of biological activities, their diverse biosynthetic origins, and their topologically complex structures, which combined make them enticing targets for chemical synthesis. In this article, full details of our synthetic studies into the chemical feasibility of a proposed network of biosynthetic pathways towards the brevianamide family of bicyclo[2.2.2]diazaoctane alkaloids are disclosed. Insights into issues of reactivity and selectivity in the biosynthesis of these structures have aided the development of a unified biomimetic synthetic strategy, which has resulted in the total synthesis of all known bicyclo[2.2.2]diazaoctane brevianamides and the anticipation of an as-yet-undiscovered congener.

Identification of anti-Parkinson's Disease Lead Compounds from Aspergillus ochraceus Targeting Adenosin Receptors A2A

Two novel alkaloids compounds together with fifteen know metabolites were identified from Aspergillus ochraceus. The stereochemistry features of the new molecules were determined via HRESIMS, NMR, ECD, and XRD analyses. Amongst these, compounds two compounds exhibited potential efficacy as anti-Parkinson's disease with the EC50 values of 2.30 and 2.45 μM, respectively. ADMET prediction showed that these compounds owned favorable drug-like characteristics and safe toxicity scores towards CNS drugs. Virtual screening analyses manifested that the compounds exhibited not only robust and reliable interactions to adenosine receptors A2A , but also higher binding selectivity to A2A receptors than to A1 and A3 receptors. Molecular dynamics simulation demonstrated the reliability of molecular docking results and the stability of the complexes obtained with the novel compounds and A2A receptors in natural environments. It is the first time that anti-PD lead compounds have been identified from Aspergillus ochraceus and targeting adenosine A2A receptors.

Flavin-Dependent Monooxygenases NotI and NotI' Mediate Spiro-Oxindole Formation in Biosynthesis of the Notoamides

The fungal indole alkaloids are a unique class of complex molecules that have a characteristic bicyclo[2.2.2]diazaoctane ring and frequently contain a spiro-oxindole moiety. While various strains produce these compounds, an intriguing case involves the formation of individual antipodes by two unique species of fungi in the generation of the potent anticancer agents (+)- and (-)-notoamide A. NotI and NotI' have been characterized as flavin-dependent monooxygenases that catalyze epoxidation and semi-pinacol rearrangement to form the spiro-oxindole center within these molecules. This work elucidates a key step in the biosynthesis of the notoamides and provides an evolutionary hypothesis regarding a common ancestor for production of enantiopure notoamides.

A unified strategy to reverse-prenylated indole alkaloids: total syntheses of preparaherquamide, premalbrancheamide, and (+)-VM-55599

A full account of our studies toward reverse-prenylated indole alkaloids that contain a bicyclo[2.2.2]core is described. A divergent route is reported which has resulted in the synthesis of preparaherquamide, (+)-VM-55599, and premalbrancheamide. An intramolecular Dieckmann cyclization between an enolate and isocyanate was used to forge the bicyclo[2.2.2]diazaoctane core that is characteristic of these molecules. The pentacyclic indole scaffold was constructed through a one-pot Hofmann rearrangement followed by Fischer indole synthesis. The utilization of our previously reported indole peripheral functionalization strategy also led to natural products including malbrancheamides B, C, stephacidin A, notoamides F, I and R, aspergamide B, and waikialoid A. Ultimately, the divergent route that we devised provided access to a wide range of prenylated indole alkaloids that are differently substituted on the cyclic amine core.

Organocatalytic Aza-Michael/Michael Cyclization Cascade Reaction: Enantioselective Synthesis of Spiro-oxindole Piperidin-2-one Derivatives

A simple, direct, and highly enantioselective synthesis of spiro-oxindole piperidin-2-one derivatives was achieved through an aza-Michael/Michael cyclization cascade sequence using a squaramide catalyst. The desired products were obtained in excellent yields (up to 99%) and good to high stereoselectivities (up to >20:1 dr and up to 99% ee) under mild conditions.

One-pot, catalyst-free synthesis of novel spiro[indole-3,4′-pyrano[2′,3′:4,5]pyrimido [2,1-b][1,3]benzothiazole] derivatives

The present report describes one-pot three-component condensation of isatins, malononitrile, and 2-hydroxy-4 H-pyrimido[2,1- b][1,3]benzothiazol-4-one in water–ethanol mixture at reflux to develop an efficient one-pot protocol for the synthesis of novel spiro[indole-3,4′-pyrano[2′,3′:4,5]pyrimido[2,1- b][1,3]benzothiazole] derivatives. The significant features of this protocol are short reaction times, avoidance of toxic catalysts, and provision of excellent yields, no column chromatographic purification, and use of ethanol-water as an environmentally benign solvent. The molecular structure of 4a has been supported by single-crystal X-ray diffraction.

Electrooxidation enables highly regioselective dearomative annulation of indole and benzofuran derivatives

The dearomatization of arenes represents a powerful synthetic methodology to provide three-dimensional chemicals of high added value. Here we report a general and practical protocol for regioselective dearomative annulation of indole and benzofuran derivatives in an electrochemical way. Under undivided electrolytic conditions, a series of highly functionalized five to eight-membered heterocycle-2,3-fused indolines and dihydrobenzofurans, which are typically unattainable under thermal conditions, can be successfully accessed in high yield with excellent regio- and stereo-selectivity. This transformation can also tolerate a wide range of functional groups and achieve good efficiency in large-scale synthesis under oxidant-free conditions. In addition, cyclic voltammetry, electron paramagnetic resonance (EPR) and kinetic studies indicate that the dehydrogenative dearomatization annulations arise from the anodic oxidation of indole into indole radical cation, and this process is the rate-determining step.

Enriching the chemical space of polycyclic heterocycles is of high value in chemical biology. Here, the authors report an electrooxidative protocol for the regioselective dearomative annulation of indoles and benzofurans under undivided cell conditions and obtain five- to eight-membered fused heterocycles.

Pd-Catalyzed de Novo Assembly of Diversely Substituted Indole-Fused Polyheterocycles

Here we describe a facile, tandem synthetic route for indolo[3,2-c]quinolinones, a class of natural alkaloid analogues of high biological significance. A Ugi four-component reaction with indole-2-carboxylic acid and an aniline followed by a Pd-catalyzed cyclization yields tetracyclic indoloquinolines in good to moderate yields. Commercially available building blocks yield highly diverse analogues in just two simple steps.

Isolation, Chiral-Phase Resolution, and Determination of the Absolute Configurations of a Complete Series of Stereoisomers of a Rearranged Acetophenone with Three Stereocenters

A synthesis-inspired chemical investigation of the leaves of Melicope ptelefolia led to the isolation of evodialones A-D (1-4), four rearranged acetophenone stereoisomers possessing a prenylated acylcyclopentenone skeleton with three stereogenic carbons. Evodialones C and D (3 and 4) are new minor constituents. The chiral-phase HPLC resolution gave (+)-1-4 and (-)-1-4, eight enantiomers forming a complete stereoisomer library. Their absolute configurations were elucidated via extensive spectroscopic data and a modified Mosher's method. The relationship between the chiral structures and their NMR and ECD data is discussed. Compounds (±)-1, -2, and -4 have significant protective effects on high-glucose-induced oxidative stress in human vein endothelial cells.

Structural and stereochemical diversity in prenylated indole alkaloids containing the bicyclo[2.2.2]diazaoctane ring system from marine and terrestrial fungi

Covering: up to February 2017 Various fungi of the genera Aspergillus, Penicillium, and Malbranchea produce prenylated indole alkaloids possessing a bicyclo[2.2.2]diazaoctane ring system. After the discovery of distinct enantiomers of the natural alkaloids stephacidin A and notoamide B, from A. protuberus MF297-2 and A. amoenus NRRL 35660, another fungi, A. taichungensis, was found to produce their diastereomers, 6-epi-stephacidin A and versicolamide B, as major metabolites. Distinct enantiomers of stephacidin A and 6-epi-stephacidin A may be derived from a common precursor, notoamide S, by enzymes that form a bicyclo[2.2.2]diazaoctane core via a putative intramolecular hetero-Diels-Alder cycloaddition. This review provides our current understanding of the structural and stereochemical homologies and disparities of these alkaloids. Through the deployment of biomimetic syntheses, whole-genome sequencing, and biochemical studies, a unified biogenesis of both the dioxopiperazine and the monooxopiperazine families of prenylated indole alkaloids constituted of bicyclo[2.2.2]diazaoctane ring systems is presented.

Novel protein science enabled by total chemical synthesis

Chemical synthesis is a well-established method for the preparation in the research laboratory of multiple-tens-of-milligram amounts of correctly folded, high purity protein molecules. Chemically synthesized proteins enable a broad spectrum of novel protein science. Racemic mixtures consisting of d-protein and l-protein enantiomers facilitate crystallization and determination of protein structures by X-ray diffraction. d-Proteins enable the systematic development of unnatural mirror image protein molecules that bind with high affinity to natural protein targets. The d-protein form of a therapeutic target can also be used to screen natural product libraries to identify novel small molecule leads for drug development. Proteins with novel polypeptide chain topologies including branched, circular, linear-loop, and interpenetrating polypeptide chains can be constructed by chemical synthesis. Medicinal chemistry can be applied to optimize the properties of therapeutic protein molecules. Chemical synthesis has been used to redesign glycoproteins and for the a priori design and construction of covalently constrained novel protein scaffolds not found in nature. Versatile and precise labeling of protein molecules by chemical synthesis facilitates effective application of advanced physical methods including multidimensional nuclear magnetic resonance and time-resolved FTIR for the elucidation of protein structure-activity relationships. The chemistries used for total synthesis of proteins have been adapted to making artificial molecular devices and protein-inspired nanomolecular constructs. Research to develop mirror image life in the laboratory is in its very earliest stages, based on the total chemical synthesis of d-protein forms of polymerase enzymes.

Rapid Generation of Molecular Complexity by Chemical Synthesis: Highly Efficient Total Synthesis of Hexacyclic Alkaloid (-)-Chaetominine and Its Biosynthetic Implications

The efficiency becomes a key issue in today's natural product total synthesis. While biomimetic synthesis is one of the most elegant strategies to achieve synthetic efficiency and thus to approach the ideal synthesis, most biogenetic pathways are unknown or unconfirmed. In this account, we demonstrate, through the shortest and also the most efficient asymmetric total syntheses of the hexacyclic alkaloid (-)-chaetominine to date, that on the basis of biogenetic thinking, one can develop quite efficient bio-inspired total synthesis, which in turn serves to suggest and chemically validate plausible biosynthetic routes for the natural product. The synthetic strategy thus developed is also inspiring for the development of other synthetic methods and efficient total synthesis of other natural products.

Saroclides A and B, Cyclic Depsipeptides from the Mangrove-Derived Fungus Sarocladium kiliense HDN11-112

Two new depsipeptides (1 and 2), together with three known related compounds, pestalotin (3), pestalotiopyrone L (4), and PC-2 (5), were discovered in the extract of a mangrove derived fungus Sarocladium kiliense HDN11-112. The structures of saroclides A and B were established by interpretation of extensive NMR spectroscopic data and X-ray crystallographic analysis. Compound 1 was also produced by Simplicillium lamellicola HDN13-430. Compounds 1 and 2 were inactive against five cancer cell lines and four pathogenic microorganisms, while compound 1 showed a lipid-lowering effect.

Enantiomeric Mixtures in Natural Product Chemistry: Separation and Absolute Configuration Assignment

Chiral natural product molecules are generally assumed to be biosynthesized in an enantiomerically pure or enriched fashion. Nevertheless, a significant amount of racemates or enantiomerically enriched mixtures has been reported from natural sources. This number is estimated to be even larger since the enantiomeric purity of secondary metabolites is rarely checked in the natural product isolation pipeline. This latter fact may have drastic effects on the evaluation of the biological activity of chiral natural products. A second bottleneck is the determination of their absolute configurations. Despite the widespread use of optical rotation and electronic circular dichroism, most of the stereochemical assignments are based on empirical correlations with similar compounds reported in the literature. As an alternative, the combination of vibrational circular dichroism and quantum chemical calculations has emerged as a powerful and reliable tool for both conformational and configurational analysis of natural products, even for those lacking UV-Vis chromophores. In this review, we aim to provide the reader with a critical overview of the occurrence of enantiomeric mixtures of secondary metabolites in nature as well the best practices for their detection, enantioselective separation using liquid chromatography, and determination of absolute configuration by means of vibrational circular dichroism and density functional theory calculations.

NHC-catalyzed [4+2] cycloaddition reactions for the synthesis of 3′-spirocyclic oxindoles via a C–F bond cleavage protocol

A chiral NHC-catalyzed cycloaddition of γ-fluoroenals is developed. The nucleophilic γ-carbon generated via C–F bond cleavage undergoes highly enantioselective cycloaddition (up to >99% ee) to isatins and provides 3′-spirocyclic oxindoles in good yields (up to 91%).

Enantioselective inhibitory abilities of enantiomers of notoamides against RANKL-induced formation of multinuclear osteoclasts

The marine-derived Aspergillus protuberus MF297-2 and the terrestrial A. amoenus NRRL 35600 produce enantiomeric prenylated indole alkaloids. Investigation of biological activities of the natural and synthetic derivatives revealed that (-)-enantiomers of notoamides A and B, 6-epi-notoamide T, and stephacidin A inhibited receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL)-induced osteoclastogenic differentiation of murine RAW264 cells more strongly than their respective (+)-enantiomers. Among them, (-)-6-epi-notoamide T was the most potent inhibitor with an IC₅₀ value of 1.7μM.

Isolation of amoenamide A and five antipodal prenylated alkaloids from Aspergillus amoenus NRRL 35600

A new prenylated alkaloid, Amoenamide A (6), was isolated from the fungus Aspergillus amoenus NRRL 35600. Previously, 6 was postulated to be a precursor of Notoamide E4 (21) converted from Notoamide E (16), which was a key precursor of the prenylated indole alkaloids in the fungi of the genus Aspergillus. We previously succeeded in the isolation of two pairs of antipodes, Stephacidin A (1) and Notoamide B (2), from A. amoenus and A. protuberus MF297-2 and expected the presence of other antipodes in the culture of A. amoenus. We here report five new antipodes (7-11) along with a new metabolite (12), which was isolated as a natural compound for the first time, from A. amoenus.

Waspergillamide A, a Nitro depsi-Tetrapeptide Diketopiperazine from an Australian Mud Dauber Wasp-Associated Aspergillus sp. (CMB-W031)

Chemical profiling of extracts from a mud dauber wasp-associated fungus, Aspergillus sp. (CMB-W031), revealed a remarkably diverse array of secondary metabolites, with many biosynthetic gene clusters being transcriptionally responsive to specific culture conditions. Chemical fractionation of a jasmine rice cultivation yielded many known fungal metabolites, including the highly cytotoxic (-)-stephacidin B and an unprecedented nonribosomal peptide synthase derived nitro depsi-tetrapeptide diketopiperazine, waspergillamide A (1). All structures were assigned by detailed spectroscopic analysis and, where appropriate, chemical degradation and Marfey's analysis.

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.

Oxidative radical cyclizations of diketopiperazines bearing an amidomalonate unit. Heterointermediate reaction sequences toward the asperparalines and stephacidins

A novel approach to the diazabicyclo[2.2.2]octane core of prenylated bridged diketopiperazine alkaloids is described by direct oxidative cyclizations of functionalized diketopiperazines mediated by ferrocenium hexafluorophosphate or the Mn(OAc)₃•2H₂O/Cu(OTf)₂ system. Divergent reaction pathways take place depending on the substitution pattern of the substrates and the oxidation conditions such as temperature or the presence or absence of persistent radical TEMPO. For ester-substituted diketopiperazines, the ester group exerts a significant influence on the reaction outcome and stereochemistry of the radical cyclizations.

Total synthesis of (-)-depyranoversicolamide B

Starting from easily prepared (R)-C3-isoprenylated pyrroloindoline, the C3-isoprenylated indolyl diketopiperazine is prepared by an efficient reductive opening of the pyrrolo ring, and undergoes biomimetic Diels-Alder reaction to generate an anti-adduct as a sole stereoisomer. Oxidation of the indoline moiety to oxindole completes the synthesis of (-)-depyranoversicolamide B.

Intermolecular Diels-Alder Cycloaddition for the Construction of Bicyclo[2.2.2]diazaoctane Structures: Formal Synthesis of Brevianamide B and Premalbrancheamide

A stereoselective intermolecular Diels-Alder cycloaddition of an intermediate pyrazinone with both achiral and chiral acrylate-derived dienophiles provides rapid access to the bicyclo[2.2.2]diazaoctane core shared among several prenylated indole alkaloids. The product derived from cycloaddition with 2-nitroacrylate required an additional five to six synthetic operations to intercept established precursors to premalbrancheamide and brevianamide B. The chemistry detailed in this manuscript constitutes a formal total synthesis (12 steps each) of these [2.2.2]diazabicyclic natural products from proline methyl ester.

Taichunamides: Prenylated Indole Alkaloids from Aspergillus taichungensis (IBT 19404)

Seven new prenylated indole alkaloids, taichunamides A-G, were isolated from the fungus Aspergillus taichungensis (IBT 19404). Taichunamides A and B contained an azetidine and 4-pyridone units, respectively, and are likely biosynthesized from notoamide S via (+)-6-epi-stephacidin A. Taichunamides C and D contain endoperoxide and methylsulfonyl units, respectively. This fungus produced indole alkaloids containing an anti-bicyclo[2.2.2]diazaoctane core, whereas A. protuberus and A. amoenus produced congeners with a syn-bicyclo[2.2.2]diazaoctane core. Plausible biosynthetic pathways to access these cores within the three species likely arise from an intramolecular hetero Diels-Alder reaction.

Marine natural products

This review covers the literature published in 2013 for marine natural products (MNPs), with 982 citations (644 for the period January to December 2013) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1163 for 2013), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Toxic indole alkaloids avrainvillamide and stephacidin B produced by a biocide tolerant indoor mold Aspergillus westerdijkiae

Toxic Aspergillus westerdijkiae were present in house dust and indoor air fall-out from a residence and a kindergarten where the occupants suffered from building related ill health. The A. westerdijkiae isolates produced indole alkaloids avrainvillamide (445 Da) and its dimer stephacidin B (890 Da). It grew and sporulated in presence of high concentrations of boron or polyguanidine (PHMB, PHMG) based antimicrobial biocides used to remediate mold infested buildings. The boar sperm cells were used as sensor cells to purify toxins from HPLC fractions of the fungal biomass. Submicromolar concentrations (EC50 0.3-0.4 μM) blocked boar spermatozoan motility and killed porcine kidney tubular epithelial cells (PK-15). Plate grown hyphal mass of the A. westerdijkiae isolates contained 300-750 ng of avrainvillamide and 30-300 ng of stephacidin B per mg (wet weight). The toxins induced rapid (30 min) loss of boar sperm motility, followed (24 h) by loss of mitochondrial membrane potential (ΔΨm). Apoptotic cell death was observed in PK-15 cell monolayers, prior to cessation of glucose uptake or loss of ΔΨm. Avrainvillamide and stephacidin B were 100-fold more potent towards the porcine cells than the mycotoxins stephacidin A, ochratoxin A, sterigmatocystin and citrinin. The high toxicity of stephacidin B indicates a role of nitrone group in the mechanism of toxicity. Avrainvillamide and stephacidin B represent a new class of toxins with possible a threat to human health in buildings. Furthermore, the use of biocides highly enhanced the growth of toxigenic A. westerdijkiae.

Asperlones A and B, dinaphthalenone derivatives from a mangrove endophytic fungus Aspergillus sp. 16-5C

Racemic dinaphthalenone derivatives, (±)-asperlone A (1) and (±)-asperlone B (2), and two new azaphilones, 6″-hydroxy-(R)-mitorubrinic acid (3) and purpurquinone D (4), along with four known compounds, (−)-mitorubrinic acid (5), (−)-mitorubrin (6), purpurquinone A (7) and orsellinic acid (8), were isolated from the cultures of Aspergillus sp. 16-5C. The structures were elucidated using comprehensive spectroscopic methods, including 1D and 2D NMR spectra and the structures of 1 further confirmed by single-crystal X-ray diffraction analysis, while the absolute configuration of 3 and 4 were determined by comparing their optical rotation and CD with those of the literature, respectively. Compounds 1, 2 and 6 exhibited potent inhibitory effects against Mycobacterium tuberculosis protein tyrosine phosphatase B (MptpB) with IC₅₀ values of 4.24 ± 0.41, 4.32 ± 0.60 and 3.99 ± 0.34 μM, respectively.

A novel one-pot multi-component synthesis of 3,3’-disubstituted oxindole and spirooxindole scaffolds via Sn-catalyzed C(sp3)–H functionalization of azaarenes by sequential Knoevenagel–Michael-cyclization reaction

Sn-catalyzed C(sp³)–H bond functionalization of 2-methyl azaarenes/2-(azaaryl)methanes has been achieved for the first time in a one-pot reaction with isatin and active methylene compounds.