Aerobic Palladium(II)-Catalyzed 5-endo-trig Cyclization: An Entry into the Diastereoselective C-2 Alkenylation of Indoles with Tri- and Tetrasubstituted Double Bonds

Construction and Single-Crystal Structures of N-Isoxazolin-5-ylcarbonylindole Derivatives, and Their Pesticidal Activities and Toxicology Study

To explore natural product-based pesticide candidates, a series of indole derivatives containing the isoxazoline skeleton at the N-1 position were synthesized by 1,3-dipolar [2 + 3] cycloaddition reaction. Their structures were characterized by melting points (mp), infrared (IR) spectra, proton nuclear magnetic resonance spectra (1H NMR), carbon-13 nuclear magnetic resonance spectra (13C NMR), and high resolution mass spectrometry (HRMS). The single-crystal structures of five compounds were presented. Against Tetranychus cinnabarinus Boisduval, compound 3b showed greater than 3.8-fold acaricidal activity of indole and good control effects under glasshouse conditions. Against Aphis citricola Van der Goot, compounds 3b and 3q exhibited 48.3- and 36.8-fold aphicidal activity of indole and 6-methylindole, respectively. Particularly, compound 3b showed good bioactivities against T. cinnabarinus and A. citricola. Against Eriosoma lanigerum Hausmann, compound 3h and 3i showed 2.1 and 1.9 times higher aphicidal activity compared to indole. Furthermore, the construction of the epidermal cuticle layer of 3b-treated carmine spider mites was distinctly damaged, which ultimately led to their death.

Palladium-Catalyzed Trans-Selective Synthesis of Spirocyclic Cyclobutanes Using α,α-Dialkylcrotyl- and Allylhydrazones

Herein, we demonstrate the use of E/Z mixtures of α,α-disubstituted crotylhydrazones to obtain spirocyclic vinylcyclobutanes in a diastereoselective fashion. We show 24 examples of a 1,1-insertion/4-exo-trig tandem process to produce these motifs. Additionally, spirocyclic alkylidene cyclobutanes can be obtained by using α,α-disubstituted allylated hydrazones (11 examples). In this study, we show that the aryl migrating group has a dramatic impact on the course of the reaction. Specifically, allylic C-H insertion products can be obtained in good yields using bromoenones as reaction partners. When Pd(0) is used with no aryl or alkenyl bromide, an intramolecular cyclopropanation reaction takes place to afford [2.1.0]-bicycles.

The Phosphinamide-Based Catalysts: Discovery, Methodology Development, and Applications in Natural Product Synthesis

In the total synthesis of natural products, synthetic efficiency has been an important driver for designing and developing new synthetic strategies and methodologies. To this end, the step, atom, and time economy and the overall yield are major factors to be considered. On the other hand, developing unified routes that can be used for synthesizing multiple molecules, specifically skeletally different classes of molecules, are also important aspects with which to be concerned. In the efforts toward efficient and flexible synthesis of structurally unique terpenoid and indole alkaloid natural products, we have designed and developed several phosphinamide-based new catalysts and reaction methodologies that have been compellingly demonstrated to be widely useful as strategic protocols for the diverse synthesis of various complex terpenoids and indole alkaloids. The important progress of these results will be summarized in this Account.In the first part, we present the stories of successful design and establishment of a novel method for the synthesis of P-stereogenic phosphinamides (P-SPhos) via a Pd-catalyzed C-H desymmetric enantioselective arylation, as well as the flexible derivatization of the P-stereogenic phosphinamides into various types of skeletally unique tricyclic and N,P-bidentate P-stereogenic compounds. Subsequently, the discovery of P-stereogenic phosphinamides as chiral organocatalysts for the desymmetric enantioselective reduction of cyclic 1,3-diketones and of phosphinamide-based cyclopalladium complex (C-Pd) as precatalysts for highly efficient Suzuki-Miyaura cross-coupling reaction of sterically congested nonactivated enolates is introduced. The notable features of the P-stereogenic phosphinamide-catalyzed desymmetric enantioselective reduction are highlighted by the broad substrate compatibility and excellent stereoselectivity, as well as most significantly, the good recoverability and reusability of catalysts. With regard to the sterically congested nonactivated enolates, such substrates are challenging for Suzuki cross-coupling reactions. We demonstrate that the phosphinamide-based cyclopalladium is a type of highly active precatalyst that allows the reaction to proceed under mild conditions and to be easily scaled up. Following the methodology development, the practical applications of these methods serving as strategic transformations are highlighted by the unified synthesis of four cyathane-type and two hamigeran-type terpenoids.In the second part, we describe the development of a robust method for oxidative Heck cross-coupling of indolyl amides by using the phosphinamide-based cyclopalladium as catalyst or phosphinamide as coligand. The method provides a general and straightforward method for diverse synthesis of indolyl δ-lactam derivatives, which present as a common core in a variety of Aspidosperma-derived indole alkaloids. The successful demonstration of this protocol for a concise and divergent synthesis of leuconodine-type indole alkaloids is also presented. We believe the results presented in this Account would have significant implications beyond our results and would find further applications in the field of synthetic methodology and natural product synthesis.

Electro-Oxidative C-N Bond Formation through Azolation of Indole Derivatives: An Access to 3-Substituent-2-(Azol-1-yl)indoles

A practical protocol to synthesize 3-substituent-2-(azol-1-yl)indole derivatives has been developed via an electrochemical oxidative cross coupling process under mild conditions. This electro-oxidative C-N bond formation strategy tolerates a range of functional groups and is amenable to gram scale synthesis. Moreover, this method was applied to the late-stage functionalization of bioactive molecules.

Design, synthesis and antitumor activity evaluation of Chrysamide B derivatives

Marine natural products derived from special or extreme environment provide an important source for the development of anti-tumor drugs due to their special skeletons and functional groups. In this study, based on our previous work on the total synthesis and structure revision of the novel marine natural product Chrysamide B, a group of its derivatives were designed, synthesized, and subsequently of which the anti-cancer activity, structure-activity relationships and cellular mechanism were explored for the first time. Compared with Chrysamide B, better anti-cancer performance of some derivatives against five human cancer cell lines (SGC-7901, MGC-803, HepG2, HCT-116, MCF-7) was observed, especially for compound b-9 on MGC-803 and SGC-7901 cells with the IC ₅₀ values of 7.88 ± 0.81 and 10.08 ± 1.08 μM, respectively. Subsequently, cellular mechanism study suggested that compound b-9 treatment could inhibit the cellular proliferation, reduce the migration and invasion ability of cells, and induce mitochondrial-dependent apoptosis in gastric cancer MGC-803 and SGC-7901 cells. Furthermore, the mitochondrial-dependent apoptosis induced by compound b-9 is related with the JAK2/STAT3/Bcl-2 signaling pathway. To conclude, our results offer a new structure for the discovery of anti-tumor lead compounds from marine natural products.

Fe-Catalyzed Sequential C(sp3)-H/N-H Annulation of 2-Methylindoles with Ethyl Trifluoropyruvate at Room Temperature: Construction of Pyrrolo[1,2-α]indoles

An efficient and benign iron-catalyzed room-temperature method was developed for direct sequential C(sp³)-H/N-H annulation to construct pyrroloindole scaffolds. This strategy features cheap and readily available raw materials and mild room-temperature reaction conditions and provides a green and practical method for the one-pot rapid synthesis of a wide range of diversely functionalized pyrrolo[1,2-α]indoles.

Palladium-catalysed 5-endo-trig allylic (hetero)arylation

A palladium-catalysed intramolecular allylic (hetero)arylation strategy for the synthesis of fused cyclopentenes incorporated with all-carbon quaternary and spiro centres is described. The method is straightforward, shows broad scope, proceeds in synthetically useful yields, and provides a rare means to construct complex cyclopentanoids. The reaction is believed to involve a kinetically unfavourable 5-endo-trig carbocyclisation of the tethered (π-allyl)palladium system. Further, this method was successfully applied as the key step in the total synthesis of diterpene natural products taiwaniaquinone H and dichroanone.

Recent advances in the synthesis of bridgehead (or ring-junction) nitrogen heterocycles via transition metal-catalyzed C–H bond activation and functionalization

An update on recent advances in the synthesis of bridgehead nitrogen fused heterocycles via transition metal-catalyzed C–H activation and functionalization is reported.

Palladium-catalysed Heck-type alkenylation reactions in the synthesis of quinolines. Mechanistic insights and recent applications

Recent developments in Pd(0)- and Pd(ii)-catalysed alkenylation reactions for the synthesis of quinolines focusing on mechanistic understanding.

One-Step Synthesis of Isoindolo[2,1-a]indol-6-ones via Tandem Pd-Catalyzed Aminocarbonylation and C-H Activation

A unified catalytic system for tandem Pd-catalyzed carbonylation and C-C cross-coupling via C-H activation was designed. The proposed cascade reaction allows a facile one-step construction of a tetracyclic isoindoloindole skeleton, in which three new C-C/C-N bonds are simultaneously formed. In detail, the carbonylation of aryl dibromides with indoles and C-H activation of in situ formed N-(2'-bromoaroyl)-indole provide biologically relevant 6H-isoindolo[2,1-a]indol-6-ones from commercially available substrates. The aminocarbonylation step in the proposed tandem reaction utilizes glyoxylic acid monohydrate as an environmentally friendly CO surrogate.

Pd-Catalyzed Aerobic Oxidative Heck Cross-Coupling for the Straightforward Construction of Indole δ-Lactams

The [6.5.6]-tricyclic indole δ-lactam represents a common key intermediate for the synthesis of a broad variety of structurally intriguing indole alkaloids. The development of a method for the versatile and straightforward construction of such structural motif is of great importance for potential synthetic applications. Herein, we present a co-ligand-prompted Pd-catalyzed 6-exo-trig intramolecular cyclization of indolyl amides via the aerobic oxidative Heck cross-coupling. The method provided a general and efficient way for the construction of [6.5.6]-tricyclic indole δ-lactams. A mechanistic study suggests that a Pd^(I)/Pd^(III) catalytic cycle should be responsible for effective coupling, which represents a mechanistically alternative pathway when compared with the Pd⁽⁰⁾/Pd^(II) cycle proposed for other related coupling reactions.

Oxidative organocatalytic chemoselective N-acylation of heterocycles with aromatic and conjugated aldehydes

The oxidative organocatalytic acylation of indoles with aldehydes serves as an alternative to sensitive acyl chlorides and coupling agents.

Palladium-Catalyzed Dehydrogenative Coupling: An Efficient Synthetic Strategy for the Construction of the Quinoline Core

Palladium-catalyzed dehydrogenative coupling is an efficient synthetic strategy for the construction of quinoline scaffolds, a privileged structure and prevalent motif in many natural and biologically active products, in particular in marine alkaloids. Thus, quinolines and 1,2-dihydroquinolines can be selectively obtained in moderate-to-good yields via intramolecular C–H alkenylation reactions, by choosing the reaction conditions. This methodology provides a direct method for the construction of this type of quinoline through an efficient and atom economical procedure, and constitutes significant advance over the existing procedures that require preactivated reaction partners.

Silver-Catalyzed Radical-Involved Cascade Cyclization of Diphenylphosphine with Cinnamamides: Access to 2-Phosphinoyl-3H-pyrrolo[1,2-a]indoles

A Ag-catalyzed phosphorus radical promoted cyclization of cinnamamides to give exclusively 2-phosphinoyl-3H-pyrrolo[1,2-a]indoles derivatives via a 5-endo-trig manner is described. This novel reaction leads to the simultaneous formation of C-C and C-P bonds in one step with excellent chemo- (only 5-endo) and diastereoselectivity (dr >20:1) under mild conditions in moderate to good yields.

One-pot synthesis of fluorescent 2,4-dialkenylindoles by rhodium-catalyzed dual C–H functionalization

A one-pot synthesis of fluorescent 2,4-dialkenylindoles by rhodium-catalyzed dual C–H bond alkenylation of indoles.

Reactivity of alkynylindole-2-carboxamides in [Pd]-catalysed C–H activation and phase transfer catalysis: formation of pyrrolo-diindolones vs. β-carbolinones

[Pd]-Catalysed reaction of 3-alkynylindole-2-carboxamidesviaC–H activation leads to pyrrolodiindolones; in contrast, under phase-transfer catalysis, the same precursors afford β-carbolinones.

Mechanistic insight into the pyridine enhanced α-selectivity in alkyne hydrothiolation catalysed by quinolinolate–rhodium(i)–N-heterocyclic carbene complexes

Highly Markovnikov-selective Rh^I–NHC–quinolinolate catalysts in alkyne hydrothiolation are reported.

Palladium-catalyzed, copper-mediated construction of benzene rings from the reactions of indoles with in situ generated enones

Construction of the benzene ring was efficiently realized through a palladium(ii)-catalyzed, copper(ii)-mediated domino dehydrochlorination/alkenylation/cycloaddition–oxidation sequence.