Total Synthesis of Isodihydrokoumine, (19Z)-Taberpsychine, and (4R)-Isodihydroukoumine N4-Oxide

Enantioselective Aminative Dearomatization of Indoles via Electrophilic 1,6-Addition of p-Quinone Diimides (p-QDIs)

Herein we report the first use of p-quinone diimide for the aminative dearomatization of 2,3-disubstituted indoles to furnish C3 aza-quaternary chiral indolenines. This approach, which proceeds via an electrophilic 1,6-addition of p-quinone diimide, allows the synthesis of an array of optically active aza-quaternary indolenines with high yields and excellent enantioselectivities. A one-pot approach of the same has also been established to further improve the synthetic accessibility of this protocol.

Recent progress in chemistry and bioactivity of monoterpenoid indole alkaloids from the genus gelsemium: a comprehensive review

Monoterpenoid indole alkaloids (MIAs) represent a major class of active ingredients from the plants of the genus Gelsemium. Gelsemium MIAs with diverse chemical structures can be divided into six categories: gelsedine-, gelsemine-, humantenine-, koumine-, sarpagine- and yohimbane-type. Additionally, gelsemium MIAs exert a wide range of bioactivities, including anti-tumour, immunosuppression, anti-anxiety, analgesia, and so on. Owing to their fascinating structures and potent pharmaceutical properties, these gelsemium MIAs arouse significant organic chemists' interest to design state-of-the-art synthetic strategies for their total synthesis. In this review, we comprehensively summarised recently reported novel gelsemium MIAs, potential pharmacological activities of some active molecules, and total synthetic strategies covering the period from 2013 to 2022. It is expected that this study may open the window to timely illuminate and guide further study and development of gelsemium MIAs and their derivatives in clinical practice.

Short, Scalable Access to Pyrrovobasine

A concise gram-scale synthesis of pyrrovobasine (1) is reported. Key transformations include a three-step decagram-scale synthesis of the tetracyclic compound, Mn-mediated direct radical cyclization, and the introduction of a naturally rare pyrraline structure. The synthesis is designed to be applicable to gram-scale synthesis using inexpensive and readily available reagents.

Synthesis of N-Vinyl Cinnamaldehyde Nitrones through Atropisomeric Quinoxaline-Derived N,N,O-Ligand-Promoted Chan-Lam Reaction

A type of quinoxaline-derived tridentate N,N,O-ligand was synthesized in good to excellent yields over three steps through iodination of 2-aryl indoles, sequential Kornblum-type oxidation with DMSO, and capture by 1,2-diaminobenzenes. The prepared atropisomeric N,N,O-ligand was successfully applied in the synthesis of N-vinyl cinnamaldehyde nitrones as only Z-isomers in good yields through the Chan-Lam reaction. The method features an easily accessed tunable tridentate N,N,O-ligand, broad substrate scope, good functional group tolerance, and high Z-isomer for N-vinyl nitrones.

Asymmetric Total Synthesis of (+)-Alstonlarsine A

The first asymmetric total synthesis of (+)-alstonlarsine A has been realized. The prominent features of the current synthesis include the following: (i) a Pd/self-adaptable ligand complex-catalyzed asymmetric allylic alkylation of 2-methyl-2-cyclopentenyl carbonate with 2-indolylsubstituted dimethyl malonate to establish the key stereocenter of C15, (ii) an intramolecular nitrile oxide-alkene [3 + 2] cycloaddition (INOC [3 + 2]) to construct the cyclohepta[b]indole backbone with the installment of the requisite stereochemistry of the all-carbon quaternary center of C20, and (iii) a late-stage interrupted Pictet-Spengler reaction (IPSR) to rapidly assemble the core structure of (+)-alstonlarsine A.

Ag(I)/PPh3-catalyzed diastereoselective syntheses of spiro[indole-3,4'-piperidine] derivatives via cycloisomerizations of tryptamine-ynamides

A Ag(I)/PPh₃-catalyzed chelation-controlled cycloisomerization of tryptamine-ynamide was developed to access the spiro[indole-3,4'-piperidine] scaffold in a racemic and diastereoselective manner. The diastereoselective products were achieved by a chiron approach. Density functional theory (DFT) calculations indicated that strong non-covalent effects between the substrate and catalyst/ligand complex stabilized the spiroindoleninium intermediate via cation-π-π interactions.

Structure units oriented approach towards collective synthesis of sarpagine-ajmaline-koumine type alkaloids

Sarpagine-Ajmaline-Koumine type monoterpenoid indole alkaloids represent a fascinating class of natural products with polycyclic and cage-like structures, interesting biological activities, and related biosynthetic origins. Herein we report a unified approach towards the asymmetric synthesis of these three types of alkaloids, leading to a collective synthesis of 14 natural alkaloids. Among them, akuammidine, 19-Z-akuammidine, vincamedine, vincarine, quebrachidine, vincamajine, alstiphylianine J, and dihydrokoumine are accomplished for the first time. Features of our synthesis are a new Mannich-type cyclization to construct the key indole-fused azabicyclo[3.3.1]nonane common intermediate, a SmI2 mediated coupling to fuse the aza-bridged E-ring, stereoselective olefinations to install either the 19-E or 19-Z terminal alkenes presented in the natural alkaloids, and an efficient iodo-induced cyclization to establish the two vicinal all-carbon quaternary centers in the Koumine-type alkaloids.

Solvent-free synthesis of nitrone-containing template as a chemosensor for selective detection of Cu(II) in water

A state-of-the-art method was developed for repurposing nitrone-containing compounds in the chemosensory field, the ability of the designed molecules to chelate metal cations was evaluated, and their unprecedented solubility in water was confirmed. A facile, rapid, and solvent-free method of synthesizing small molecular mass chemosensors was developed by using a modulative α-aryl-N-aryl nitrone template. α-(Z)-Imidazol-4-ylmethylen-N-phenyl nitrone (Nit1) and α-(Z)-2-pyridyl-N-phenyl nitrone (Nit2) were prepared in 15 min, isolated in less than 60 min with ca. 90% yield, and screened against nine metal cations. Nit1 is a small-molecular-mass compound (188 g mol^-1) that is water-soluble and has specificity for sensing Cu²⁺ with an association constant of K = 1.53 × 10¹⁰ and a limit of detection (LOD) of 0.06 ppm. These properties make Nit1 a competitive chemosensor for the detection of Cu²⁺ in aqueous solution. The nitrone-containing template used in this study is a step forward for new and small chemosensory entities.

Sempervirine Mediates Autophagy and Apoptosis via the Akt/mTOR Signaling Pathways in Glioma Cells

The potential antitumor effects of sempervirine (SPV), an alkaloid compound derived from the traditional Chinese medicine Gelsemium elegans Benth., on different malignant tumors were described in detail. The impact of SPV on glioma cells and the basic atomic components remain uncertain. This study aimed to investigate the activity of SPV in vitro and in vivo. The effect of SPV on the growth of human glioma cells was determined to explore three aspects, namely, cell cycle, cell apoptosis, and autophagy. In this study, glioma cells, U251 and U87 cells, and one animal model were used. Cells were treated with SPV (0, 1, 4, and 8 μM) for 48 h. The cell viability, cell cycle, apoptosis rate and autophagic flux were examined. Cell cycle, apoptotic, autophagy, and Akt/mTOR signal pathway-related proteins, such as CDK1, Cyclin B1, Beclin-1, p62, LC3, AKT, and mTOR were investigated by Western blot approach. As a result, cells induced by SPV led to G2/M phase arrest and apoptosis. SPV also promoted the effect of autophagic flux and accumulation of LC3B. SPV reduced the expression of p62 protein and induced the autophagic death of glioma cells. Furthermore, SPV downregulated the expressions of AKT and mTOR phosphorylated proteins in the mTOR signaling pathway, thereby affecting the onset of apoptosis and autophagy in U251 cells. In conclusion, SPV induced cellular G2/M phase arrest and blockade of the Akt/mTOR signaling pathway, thereby triggering apoptosis and cellular autophagy. The in vivo and in vitro studies confirmed that SPV inhibits the growth of glioma cancer.

Asymmetric Total Synthesis of Sarpagine and Koumine Alkaloids

We report here a concise, collective, and asymmetric total synthesis of sarpagine alkaloids and biogenetically related koumine alkaloids, which structurally feature a rigid cage scaffold, with L-tryptophan as the starting material. Two key bridged skeleton-forming reactions, namely tandem sequential oxidative cyclopropanol ring-opening cyclization and ketone α-allenylation, ensure concurrent assembly of the caged sarpagine scaffold and installation of requisite derivative handles. With a common caged intermediate as the branch point, by taking advantage of ketone and allene groups therein, total synthesis of five sarpagine alkaloids (affinisine, normacusine B, trinervine, N_a -methyl-16-epipericyclivine, and vellosimine) with various substituents and three koumine alkaloids (koumine, koumimine, and N-demethylkoumine) with more complex cage scaffolds has been accomplished.

Au(I)-Catalyzed Domino Cyclization of 1,6-Diynes Incorporated with Indole

We disclose herein a Au(I)-catalyzed domino cyclization of 1,6-diynes incorporated with indole. This protocol enabled the diastereoselective buildup of indole-fused azabicyclo[3.3.1]nonanes from linear precursors. Density functional theory calculations showed that the reaction proceeded via an unprecedented cascade dearomatization/rearomatization/dearomatization process. Independent gradient model analysis revealed that a noncovalent attractive interaction between the distal alkyne and the Au/proximal complex was responsible for the chemoselectivity of the first spirocyclization step.

Catalytic asymmetric formal [3+2] cycloaddition of isatogens with azlactones to construct indolin-3-one derivatives

A number of enantioenriched indolin-3-one derivatives were readily obtained by chiral guanidine-catalyzed [3+2] cycloaddition of isatogens with azlactones.

O-Isocyanates as Uncharged 1,3-Dipole Equivalents in [3+2] Cycloadditions

1,3-Dipoles are commonly used in [3+2] cycloadditions, whereas isoelectronic uncharged dipole variants remain underdeveloped. In contrast to conventional 1,3-dipoles, uncharged dipole equivalents form zwitterionic cycloadducts, which can be exploited to build further molecular complexity. In this work, the first cycloadditions of oxygen-substituted isocyanates (O-isocyanates) were studied experimentally and by DFT calculations. This unique cycloaddition strategy provides access to a novel class of heterocycle aza-oxonium ylides through intramolecular and intermolecular cycloadditions with alkenes. This allowed a systematic study of the reactivity of the transient aza-oxonium ylide intermediate, which can undergo N-O bond cleavage followed by nitrene C-H insertion, and the formation of β-lactams or isoxazolidinones upon varying the structure of the alkene or O-isocyanate reagents.

Diversification of Nucleophile-Intercepted Beckmann Fragmentation Products and Related Density Functional Theory Studies

The nucleophile-intercepted Beckmann fragmentation (NuBFr) has the potential to be broadly applicable to the synthesis of indoline alkaloid-based natural products. However, the reaction has not been widely adopted, in part, because of limitations associated with the availability of appropriate promoter-Nu reagents. We have devised a stereospecific Ag(I)-promoted reaction for functionalizing NuBFr products to give novel compositions of matter that may be useful in synthesis and medicinal chemistry. With unhindered amine nucleophiles, structurally unique [2.2.2]-bicycloamidines are generated. We also disclose for the first time detailed density functional theory studies, which shed light on the mechanism of the NuBFr and Ag-promoted substitution reaction that supports an unusual aziridinium ion as a key intermediate.

Regio- and Diastereoselective Indium-Catalyzed Conia-Ene Reaction of ortho-Alkynyl Diketopiperazines to Access Fused Diketopiperazinoindolines

An efficient synthesis of diketopiperazinoindolines through an indium-catalyzed intramolecular 5-exo-dig cyclization of ortho-alkynyl diketopiperazines has been reported. The formation of diketopiperazinoindolines proceeds via a regio- and diastereoselective Conia-ene reaction. This synthetic method opens a new door for easy access to functionalized fused diketopiperazinoindolines in high to excellent yields with exclusive Z diastereoselectivity.

Synthesis and Formation Mechanism of a Compound with an Unprecedented Skeleton: Dodecahydro-4,10:5,9-diepoxydipyrrolo[3,4-b:3',4'-f][1,5]diazocine

The reaction of N-(2-{[(tert-butyldimethylsilyl)oxy]imino}ethyl)-4-methyl-N-(3-phenylprop-2-yn-1-yl)benzenesulfonamide (6b) with BF3·OEt2 afforded a compound with an unprecedented dodecahydro-4,10 : 5,9-diepoxydipyrrolo[3,4-b:3',4'-f][1,5]diazocine skeleton (7) after aqueous work-up. The formation mechanism of meso-7 appears to involve dimerization of the hydrated forms (6aS)-C and (6aR)-C of the initial racemic product 9 via cation B generated by facile protonation at the C3a position of 9. Extensive computational studies revealed that the driving force of the facile hydration of 9 is probably release of the ring strain of 9, which arises in part from the bent sp2-hybridized C3a carbon.

Nucleophile-intercepted Beckmann fragmentation reactions

We describe the first examples of nucleophile-intercepted Beckmann fragmentations of indoline oximes. This reaction uses MsCl as a promoter to give cyano chlorides and is believed to proceed through an aziridinium intermediate via a double stereoinvertive process. Mechanistic insights have led to the further discovery that oxygen, nitrogen, and bromide nucleophiles can be employed for this fragmentation by the use of other promoters. We envision that these products may be useful in the syntheses of members of the akuammiline and koumine families of indoline alkaloids.

Asymmetric organocatalyzed reaction sequence of 2-hydroxy cinnamaldehydes and acyclic N-sulfonyl ketimines to construct diverse chiral bridged polycyclic aminals

2-Hydroxy cinnamaldehydes and acyclic N-sulfonyl ketimines were used as multisite substrates in an iminium catalysis triggered sequential process, leading to diverse chiral bridged polycyclic aminals in a highly regio- and stereoselective manner.