Useful Access to Uncommon Thiazolo[3,2-a]indoles

A practical and environmentally benign protocol for the assembly of poly substituted-thiazolo[3,2-a]indoles from 3-alkylated indoline-2-thiones and 2-halo-ketones has been developed. This metal-free approach consists in a complete chemo/regioselective formal [3 + 2] annulation that occurs in air, at 60 °C, and in water as the sole reaction medium. The opportunity to vary the substitution pattern up to six different positions, odorless manipulation of sulfurylated compounds, very easy product isolation, and mild reaction conditions are the main synthetic features of this method. The scaled-up experiment and the successive transformations of the products further demonstrate the utility of this chemistry.

Diastereoselective Synthesis of Pyrroloquinolines via N-H Functionalization of Indoles with Vinyl Sulfonium Salts

A [2 + 4]/[1 + 2] annulation approach was successfully established to construct pyrroloquinoline-fused cyclopropane in a highly diastereoselective fashion (>20:1 dr). The tetracyclic 1,7-fused indoles were efficiently obtained from readily available starting materials under mild conditions. This methodology displays impressive substrate generality with two reaction components. The products resulting from this doubly annulative strategy are useful synthetic intermediates.

Photoredox catalysis enabling decarboxylative radical cyclization of γ,γ-dimethylallyltryptophan (DMAT) derivatives: formal synthesis of 6,7-secoagroclavine

An unusual photoredox-catalyzed radical decarboxylative cyclization cascade reaction of γ,γ-dimethylallyltryptophan (DMAT) derivatives containing unactivated alkene moieties has been developed, providing green and efficient access to various six-, seven-, and eight-membered ring 3,4-fused tricyclic indoles. This type of cyclization, which was hitherto very difficult to comprehend in ergot biosynthesis and to accomplish by more conventional procedures, enables the synthesis of ergot alkaloid precursors. In addition, this work describes a mild, environmentally friendly method to activate, reductively and oxidatively, natural carboxylic acids for decarboxylative C-C bond formation by exploiting the same photocatalyst.

Rh(III)-Catalyzed Double C-H Functionalization of Indoles with Cyclopropenones via Sequential C-H/C-C/C-H Bond Activation

An unprecedented Rh(III)-catalyzed double C-H functionalization of indoles with cyclopropenones via sequential C-H/C-C/C-H bond activation has been developed. This procedure represents the first example for assembling of cyclopenta[b]indoles utilizing cyclopropenones as 3C synthons. This powerful approach shows excellent chemo- and regioselectivity, wide functional group tolerance, and good reaction yields.

Recent Advances in the Synthesis of 3,n-Fused Tricyclic Indole Skeletons via Palladium-Catalyzed Domino Reactions

3,n-fused (n = 4-7) tricyclic indoles are pervasive motifs, embedded in a variety of biologically active molecules and natural products. Thus, numerous catalytic methods have been developed for the synthesis of these skeletons over the past few decades. In particular, palladium-catalyzed transformations have received much attention in recent years. This review summarizes recent developments in the synthesis of these tricyclic indoles with palladium-catalyzed domino reactions and their applications in the total synthesis of representative natural products.

Synthetic Study of Dragmacidin E: Enantioselective Construction of the Seven-Membered Ring-Fused Indole Skeleton with Contiguous Stereocenters

We developed an enantioselective synthetic method of constructing a seven-membered ring-fused indole skeleton with contiguous stereocenters for the synthesis of dragmacidin E. Introduction of chirality at the benzylic position was achieved by Ir-catalyzed asymmetric hydrogenation. After construction of the tricyclic molecular framework using Pd-catalyzed cascade cyclization, the tetrasubstituted carbon center was created using the Ag nitrene-mediated C-H amination reaction. The developed method provided access to the functionalized seven-membered ring-fused indole skeleton with a hydroxymethyl branch in the tetrasubstituted carbon.

Anti-proliferating and apoptosis-inducing activity of chemical compound FTI-6D in association with p53 in human cancer cell lines

Compounds with 3,4-fused tricyclic indole (FTI) frameworks are attractive scaffolds for drug discovery. We synthesized FTI-6D, a compound with this framework, which was cytotoxic in several human cancer cell lines. FTI-6D induced apoptosis via activation of the p53 downstream mitochondria-related apoptotic pathway, characterized by an increased ratio of pro-apoptotic Bcl-2 family members to anti-apoptotic members. This change was followed by caspase-9 and caspase-3 cleavage and activation in two cancer cell lines, RKO and AGS. The anti-proliferating effect of FTI-6D was remarkably detected in eight cancer cells with wild-type TP53 (TP53_wt), including RKO and AGS, but not in seven cancer cells with mutated TP53 (TP53_mut). Additionally, p53 protein levels increased after FTI-6D treatment in TP53_wt cancer cells, and the cytotoxic effect of FTI-6D was decreased by TP53 knockdown. Accordingly, the expression of p53 downstream genes involved in apoptotic signaling pathways, such as BBC3 and TP53INP1, and those involved in cell growth inhibition, such as CDKN1A, was upregulated in TP53_wt cancer cells. These results suggest that the anti-proliferative and apoptosis-inducing activities of FTI-6D rely on p53 and the corresponding signaling processes. This study demonstrated that FTI-6D shows anti-cancer activity against TP53_wt cancer cells. FTI-6D may have potential as a prototype compound for a new drug to utilize a functional p53 pathway in TP53_wt cancer cells.

Synthesis, Antibacterial Activity, and Mechanisms of Novel Indole Derivatives Containing Pyridinium Moieties

The development of effective antibacterial agents equipped with novel action modes and unique skeletons starting from natural compounds serves as an important strategy in the modern pesticide industry. Disclosed here are a series of novel indole derivatives containing pyridinium moieties and their antibacterial activity evaluation against two prevalent phytopathogenic bacteria, Xanthomonas oryzae pv. oryzicola (Xoc) and X. oryzae pv. oryzae (Xoo). A three-dimensional (3D)-QSAR model was adopted to discover higher activity like title compounds based on the Xoc antibacterial activity of the tested compounds. Compound 43 was consequently designed, and it displayed higher antibacterial activity as expected with the half-maximal effective concentration EC₅₀ values of 1.0 and 1.9 μg/mL for Xoo and Xoc, respectively, which were better than those of the commercial drug thiodiazole copper (TC) (72.9 and 87.5 μg/mL). Under greenhouse conditions, the results of a rice in vivo pot experiment indicated that the protective and curative activities of compound 43 against rice bacterial leaf streak (BLS) and rice bacterial blight (BLB) were 45.0 and 44.0% and 42.0 and 39.3%, respectively, which were better than those of the commercial agent thiodiazole copper (38.0 and 37.9%, 38.6 and 37.0%) as well. Scanning electron microscopy images, defense enzyme activity tests, and proteomic techniques were utilized in a preliminary mechanism study, suggesting that compound 43 shall modulate and interfere with the physiological processes and functions of pathogenic bacteria.

Pd/Et3N·HI-Catalyzed Intramolecular C-H Alkylation to Access [a]-Annulated Indoles via Highly Regioselective Ring-Opening of Epoxides

A Pd/Et₃N·HI-catalyzed intramolecular C-H alkylation of indoles with epoxides was achieved to furnish N-fused indole frameworks (5-, 6-, and 7-membered rings) bearing an alcohol group. The conversion proceeded smoothly in the presence of a catalytic amount of Et₃N·HI together with a palladium catalyst and exhibited great functional group tolerance. The employment of Et₃N·HI not only avoids the prior preparation of alkyl halide substrates but also is the key to the high chemoselective ring-opening of epoxides. Preliminary mechanism explorations strongly support a radical pathway, and the catalytic cycle was established tentatively according to the mechanism investigation experiments.

Visible-light-induced acylation/cyclization of active alkenes: facile access to acylated isoquinolinones

Nitrogen heterocycles, especially polycyclic compounds, are significant skeletons in valuable molecules. Herein, we developed an efficient and practical visible-light-induced acylation/cyclization of active alkenes with acyl oxime derivatives for constructing acylated indolo/benzimidazo-[2,1,a]isoquinolin-6(5H) ones. This reaction was compatible with various functional groups and a series of fused indole/imidazole skeletons were prepared in up to 95% yield at room temperature.

Stereodivergent synthesis of enantioenriched azepino[3,4,5-cd]-indoles via cooperative Cu/Ir-catalyzed asymmetric allylic alkylation and intramolecular Friedel-Crafts reaction

The development of enantioselective annulation reactions using readily available substrates for the construction of structurally and stereochemically diverse heterocycles is a compelling topic in diversity-oriented synthesis. Herein, we report efficient catalytic asymmetric formal 1,3-dipolar (3 + 4) cycloadditions of azomethine ylides with 4-indolyl allylic carbonates for the construction of azepino[3,4,5-cd]-indoles fused with a challenging seven-membered N-heterocycle, a frequently occurring tricyclic indole scaffold in bioactive compounds and pharmaceuticals. Through cooperative Cu/Ir-catalyzed asymmetric allylic alkylation followed by intramolecular Friedel-Crafts reaction, an array of azepino[3,4,5-cd]-indoles were obtained in good yields with excellent diastereo-/enantioselective control. More importantly, the full stereodivergence of this transformation was established via synergistic catalysis followed by acid-promoted epimerization, and up to eight stereoisomers of the cycloadducts bearing three stereogenic centers could be predictably achieved from the same set of starting materials for the first time. Quantum mechanical computations established a plausible mechanism for the synergistic Cu/Ir catalysis to stereodivergently introduce two vicinal stereocenters whose stereochemical information is remotely delivered across the fused azepine ring to control the third chiral center. Epimerization of the last center involves protonation-enabled reversal of the thermodynamically controlled relative configuration.

Regioselectivity of the SEAr-based cyclizations and SEAr-terminated annulations of 3,5-unsubstituted, 4-substituted indoles

Indole-3,4- and 4,5-fused carbo- and heterocycles are ubiquitous in bioactive natural products and pharmaceuticals, and hence, a variety of synthetic approaches toward such compounds have been developed. Among these, cyclization and annulation of 3,5-unsubstituted, 4-substituted indoles involving an electrophilic aromatic substitution (S_EAr) as the ring closure are particularly attractive, because they avoid the use of 3,4- or 4,5-difunctionalized indoles as starting materials. However, since 3,5-unsubstituted, 4-substituted indoles have two potential ring-closure sites (indole C3 and C5 positions), such reactions in principle can furnish either or both of the indole 3,4- and 4,5-fused ring systems. This Commentary will briefly highlight the issue by summarizing recent relevant literature reports.

Access to Polycyclic Indole-3,4-Fused Nine-Membered Ring via Cascade 1,6-Hydride Transfer/Cyclization

A cascade aldimine condensation/1,6-hydride transfer/Mannich-type cyclization of indole-derived phenylenediamine with aldehydes was developed for one-step construction of a polycyclic indole-3,4-fused skeleton. Aldehyde serves as a key to start the whole process, including 1,6-hydride transfer enabled δ-C(sp³)-H activation of the secondary amine. The challenges of construction of medium-sized rings are addressed via hydride transfer chemistry.

Trifluoroethanol-promoted ring-opening cyclization of 4-(2-oxiranylmethoxy)indoles: access to 4,5-fused indoles

Herein, we report that the trifluoroethanol-mediated ring-opening cyclization of readily accessible 4-(2-oxiranylmethoxy)indoles takes place in a diastereoselective and 6-endo fashion to generate pyrano[2,3-e]indol-3-ols in high yields. This regioselective cyclization at the indole C-5 position requires the presence of a π-activating aryl substituent on the reacting epoxide carbon atom, but remains uninfluenced by the electronic nature of the indole-N-substituent. Interestingly, blocking the C-5 position of the indole unit directs the reaction to generate oxepino[4,3,2-cd]indol-3-ols via 7-endo epoxide-arene cyclization.

Mechanistic Studies of the Pd- and Pt-Catalyzed Selective Cyclization of Propargyl/Allenyl Complexes

Following the discovery of an unusual transition-metal-catalyzed reaction, the elucidation of the underlying mechanism is essential to understand the characteristic reactivity of the metal. We previously reported a synthetic method for tricyclic indoles using Pt-catalyzed Friedel-Crafts-type C-H coupling. In this reaction, the Pt catalyst selectively formed a seven-membered ring, but the Pd catalyst only afforded a six-membered ring. However, the reasons for the different selectivities caused by Pd and Pt were unclear. We performed density functional theory (DFT) calculations and experimental studies to reveal the origin of the different behaviors of the two metals. The calculations revealed that the formation of the six- and seven-membered rings proceeds via η¹-allenyl and η³-propargyl/allenyl complexes, respectively. A molecular orbital analysis of the η³-propargyl/allenyl complex revealed that, for the platinum complex, the energy required to convert the unoccupied molecular orbital on the reactive carbon into the lowest unoccupied molecular orbital (LUMO) was lower than that for the palladium complex. In addition, DFT calculations revealed that the combination of platinum and bis[2-(diphenylphosphino)phenyl] ether (DPEphos) reduced the activation energy of the seven-membered cyclization in comparison with palladium or PPh₃. Additional experimental studies, including NMR studies and stoichiometric reactions, support the aforementioned examination.

Hexafluoroisopropanol-Mediated Intramolecular Ring-Opening Cyclization of Indolyl-N-Tethered Epoxides: Tether-Length-Controlled Synthesis of 1,7- and 1,2-Fused Indoles

Despite having the capability to construct benzo-fused heterocycles in complete atom economy and high chemo-, regio-, enantio-, and diastereoselectivities, intramolecular Friedel-Crafts epoxide arene cyclization (IFCEAC) remains underutilized in organic synthesis. The wide adaptation of this powerful Csp²-Csp³ bond-forming reaction, therefore, requires a broad understanding of the substrate scope to better impact heterocycle synthesis. Along this line, we investigated the applicability of IFCEAC for the synthesis of 1,7- and 1,2-fused indoles. In this article, we report the results of our systematic investigation into the scope and limitations of the first examples of the hexafluoro-2-propanol (HFIP)-mediated IFCEAC of readily accessible indolyl-N-tethered epoxides. We observed that the nature and position of the indole and epoxide substituents and the tether length separating these two reacting moieties have strong effects on the cyclization. This mild and transition-metal-free protocol delivered pyrrolo[3,2,1-ij]quinolin-5-ols in moderate to good yields from substrates bearing both a methylene linker that connects the indole and epoxide moieties and an electron-rich indole carbocyclic ring. Notably, the reactions required the presence of a π-activating aryl substituent on the reacting epoxide carbon atom. Interestingly, replacing the methylene tether with an ethylene unit resulted in regioswitching, which delivered the corresponding tetrahydropyrido[1,2-a]indol-8-ols in good to high yields. We could also successfully extend this methodology to pyrrolyl-N-tethered epoxides for a very high-yielding synthesis of tetrahydroindolizin-7-ols.

Iridium-Catalyzed Diastereo- and Enantioselective [4 + 3] Cycloaddition of 4-Indolyl Allylic Alcohols with Azomethine Ylides

An unprecedented iridium-catalyzed asymmetric [4 + 3] cycloaddition of racemic 4-indolyl allylic alcohols with azomethine ylides is reported. The ability of acid promoter zinc triflate to perform multiple roles is the key factor for the success of this strategy. This method provides scalable and efficient access to biologically important azepino[3,4,5-cd] indoles in good yields with generally excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). Mild reaction conditions, easily accessible substrates and chiral catalyst, and broad substrate scope highlight the practicality of this methodology.

A novel and robust heterogeneous Cu catalyst using modified lignosulfonate as support for the synthesis of nitrogen-containing heterocycles

A waste biomass, sodium lignosulfonate, was treated with sodium 2-formylbenzenesulfonate, and the phenylaldehyde condensation product was then used as a robust supporting material to immobilize a copper species. The so-obtained catalyst was characterized by many physicochemical methods including FTIR, EA, FSEM, FTEM, XPS, and TG. This catalyst exhibited excellent catalytic activity in the synthesis of nitrogen-containing heterocycles such as tricyclic indoles bearing 3,4-fused seven-membered rings, 2‑arylpyridines, aminonaphthalenes and 3-phenylisoquinolines. In addition, this catalyst showed to be recyclable and could be reused several times without significant loss in activity during the course of the reaction process.

Metal-Free Synthesis of Indolopyrans and 2,3-Dihydrofurans Based on Tandem Oxidative Cycloaddition

The synthesis of versatile scaffold indolopyrans based on C-C radical-radical cross-coupling under metal-free conditions is described. The reaction involving single electron transfer between coupling partners followed by cage collapse allows highly selective cross-coupling while employing only equimolar amounts of coupling partners. Moreover, the mechanistic manifold was expanded for the functionalization of enamines to give the stereoselective synthesis of 2,3-dihydrofurans. This iodine-mediated oxidative coupling features mild conditions and fast reaction kinetics.

Synthesis of 3,4-Fused Tricyclic Indoles through Cascade Carbopalladation and C-H Amination: Development and Total Synthesis of Rucaparib

3,4-Fused tricyclic indole scaffolds are ubiquitous in bioactive natural products and pharmaceuticals. A new protocol for the synthesis of 3,4-fused tricyclic indoles has been developed through cascade carbopalladation and C-H amination with N,N-di-tert-butyldiaziridinone. The protocol allows access to a range of 3,4-fused tricyclic indoles, including those containing various linkers and fused with medium-sized rings. Rucaparib can be synthesized via this reaction, providing an advantageous synthetic method for the FDA-approved cancer medicine.

4-Aminoindoles as 1,4-bisnucleophiles for diversity-oriented synthesis of tricyclic indoles bearing 3,4-fused seven-membered rings

A straightforward access to tricyclic indoles bearing 3,4-fused seven-membered rings was established by using 4-aminoindoles as 1,4-bisnucleophiles in three-component reactions.