Synthesis, structure-activity relationships, and pharmacological evaluation of pyrrolo[3,2,1-ij]quinoline derivatives: potent histamine and platelet activating factor antagonism and 5-lipoxygenase inhibitory properties. Potential therapeutic application in asthma

Biological properties of pyrroloquinoline and pyrroloisoquinoline derivatives

In this review, we summarize pyrroloquinoline and pyrroloisoquinoline derivatives (PQs and PIQs) that act on a broad spectrum of biological targets and are used as bacteriostatic, antiviral, plasmodial, anticancer, antidiabetic and anticoagulant agents. Many of these compounds play important roles in the study of DNA and its interactions, the regulation of the cell cycle and programmed cell death. This review involves twenty-five types of skeletally analogical compounds bearing pyrrole and (iso)quinoline scaffolds with different mutual annelations.

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.

Boryl radical catalysis enables asymmetric radical cycloisomerization reactions

The development of functionally distinct catalysts for enantioselective synthesis is a prominent yet challenging goal of synthetic chemistry. In this work, we report a family of chiral N-heterocyclic carbene (NHC)-ligated boryl radicals as catalysts that enable catalytic asymmetric radical cycloisomerization reactions. The radical catalysts can be generated from easily prepared NHC-borane complexes, and the broad availability of the chiral NHC component provides substantial benefits for stereochemical control. Mechanistic studies support a catalytic cycle comprising a sequence of boryl radical addition, hydrogen atom transfer, cyclization, and elimination of the boryl radical catalyst, wherein the chiral NHC subunit determines the enantioselectivity of the radical cyclization. This catalysis allows asymmetric construction of valuable chiral heterocyclic products from simple starting materials.

Metal-free polychloromethylation/cyclization of unactivated alkenes towards ring-fused tricyclic indolones and benzoimidazoles

Polychloromethylative cyclization of N-alkenyl indoles was developed under metal-free conditions to afford tricyclic pyridoindolones and pyrroloindolones in moderate to good yields. In the reaction, commercially available CHCl3 and CH2Cl2 were employed as tri- and dichloromethyl radical sources. Moreover, tri- and dichloromethylated polycyclic benzoimidazoles can also be obtained under standard conditions.

Pd-Catalyzed C-7 Arylation of Indolines with Aryltriazenes under Mild Conditions

A palladium-catalyzed direct C-H arylation of indolines at C-7 position has been achieved at near-ambient temperature. The reaction was carried out with aryltriazene as a stable aryl source and electron shuttle to sustainably release aryl radical in situ under the action of promoter, and pyrimidine as a detachable directing group for the synthesis of 7-arylindolines under oxidant- and ligand-free conditions. Notably, this catalytic system can also be applied to the direct and site-selective arylation of tetrahydroquinolines (C-8) and carbazoles (C-1).

Cp*CoIII-Catalyzed C(7)-H Bond Annulation of Indolines with Alkynes

An efficient protocol for the synthesis of biologically essential pyrroloquinolinones has been developed under Cp*Co^III catalysis, which involves a cascade reaction of C(7)-H alkenylation with alkynes followed by nucleophilic addition. A wide variety of internal alkynes including enyne, diyne, and ynamide and more challenging terminal alkynes were successfully employed for the annulation in good to excellent yield with high regioselectivity.

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.

Photocatalyzed intermolecular amination for the synthesis of hydrazonamides

A photocatalyzed intermolecular amination strategy for the synthesis of hydrazonamides is reported by a multi-component reaction of β-ketonitriles with N,N-disubstituted hydrazines.

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.

Chemoselective N–H functionalization of indole derivatives via the Reissert-type reaction catalyzed by a chiral phosphoric acid

An asymmetric N-alkylation of indole derivatives via the Reissert-type reaction was realized in the presence of 10 mol% chiral phosphoric acid.

Recent advances in radical transformations of internal alkynes

This review highlights the recent progress in the radical transformation of internal alkynes and focuses on the reaction mechanisms by carbon/heteroatom-centered triggered additions, and offers a comprehensive overview on the existing procedures and employed methodologies.

Cobalt(II) Porphyrin-Catalyzed Intramolecular Cyclopropanation of N-Alkyl Indoles/Pyrroles with Alkylcarbene: Efficient Synthesis of Polycyclic N-Heterocycles

A protocol on chemoselective cobalt(II) porphyrin-catalyzed intramolecular cyclopropanation of N-alkyl indoles/pyrroles with alkylcarbenes has been developed. The reaction enables the rapid construction of a range of nitrogen-containing polycyclic compounds in moderate to high yields from readily accessible materials. These N-containing polycyclic compounds can be converted into a variety of N-heterocycles with potential synthetic and biological interest. Compared to their N-tosylhydrazone counterparts, the use of bulky N-2,4,6-triisopropylbenzenesulfonyl hydrazones as carbene precursors allows cyclopropanation to occur under milder reaction conditions.

A Novel Pd-Catalysed Annulation Reaction for the Syntheses of Pyrroloindoles and Pyrroloquinolines

Pd-catalysed annulation reactions between indole derivatives and internal alkyne esters leading to various pyrrolo[1,2-a]indoles and pyrroloquinolines have been developed. The strategy involves an intermolecular addition of the indole nitrogen on to the internal alkyne ester followed by an intramolecular insertion of a vinyl-palladium complex into the carbonyl group. This method offers a facile and practical approach to pyrrolo[1,2-a]indoles and pyrroloquinolines.

Sodium salts of anionic chiral cobalt(III) complexes as catalysts of the enantioselective Povarov reaction

The sodium salts of anionic chiral cobalt(III) complexes (CCC(-) Na(+) ) have been found to be efficient catalysts of the asymmetric Povarov reaction of easily accessible dienophiles, such as 2,3-dihydrofuran, ethyl vinyl ether, and an N-protected 2,3-dihydropyrrole, with 2-azadienes. Ring-fused tetrahydroquinolines with up to three contiguous stereogenic centers were thus obtained in high yields, excellent diastereoselectivities (endo/exo up to >20:1), and high enantioselectivities (up to 95:5 e.r.).

Enantioselective N-alkylation of isatins and synthesis of chiral N-alkylated indoles

Asymmetric N-alkylations of isatins with enals were shown to be feasible via a prolinol-catalyzed iminium activation, and N-alkylated isatins were obtained in good yields and with excellent enantioselectivity. The biologically useful N-alkylated isatins also served as valuable synthetic precursors, and could be readily converted to chiral N-alkylated indole derivatives. The described method provides a novel entry to access optically enriched N-alkylated isatins and indole derivatives.

Enantioselective direct functionalization of indoles by Pd/sulfoxide-phosphine-catalyzed N-allylic alkylation

A general and efficient Pd/sulfoxide-phosphine complex-catalyzed direct asymmetric N-allylic alkylation of indoles has been developed to allow for the preparation of various N-allylated indoles in generally good yields with excellent enantioselectivities. This catalytic system could also be extended to other N-containing heterocycles with good results.

Ruthenium-catalyzed cyclization of N-carbamoyl indolines with alkynes: an efficient route to pyrroloquinolinones

A regioselective synthesis of substituted pyrroloquinolinones by a ruthenium-catalyzed oxidative cyclization of substituted N-carbamoyl indolines with alkynes is described.

Organocatalytic asymmetric intramolecular aza-Henry reaction: facile synthesis of trans-2,3-disubstituted tetrahydroquinolines

An enantio- and diastereoselective synthesis of trans-2-aryl-3-nitro-tetrahydroquinolines has been developed via an intramolecular aza-Henry reaction using tertiary amine thiourea as a catalyst.

C7-derivatization of C3-alkylindoles including tryptophans and tryptamines

A versatile strategy for C7-selective boronation of tryptophans, tryptamines, and 3-alkylindoles by way of a single-pot C2/C7-diboronation-C2-protodeboronation sequence is described. The combination of a mild iridium-catalyzed C2/C7-diboronation followed by an in situ palladium-catalyzed C2-protodeboronation allows efficient entry to valuable C7-boroindoles that enable further C7-derivatization. The versatility of the chemistry is highlighted by the gram-scale synthesis of C7-boronated N-Boc-L-tryptophan methyl ester and the rapid synthesis of C7-halo, C7-hydroxy, and C7-aryl tryptophan derivatives.

One-Pot Site-Selective Sonogashira Cross-Coupling–Heteroannulation of the 2-Aryl-6,8-Dibromoquinolin-4(1H)-Ones: Synthesis of Novel 6-H-Pyrrolo[3,2,1-ij]Quinolin-6-Ones

The 2-aryl-6,8-dibromoquinolin-4(1 H)-ones were subjected to site-selective Sonogashira cross-coupling with terminal acetylenes as models for C sp2-C sp bond formation in the presence of Pd/C–PPh3 and CuI as catalysts and K2CO3 as a base in dioxane to afford the 2-substituted 4-aryl-8-bromo-4 H-pyrrolo[3,2,1- ij]quinolin-6-ones. These were, in turn, subjected to Suzuki–Miyaura cross-coupling with 4-fluorophenylboronic acid as coupling partner to afford the 2-substituted 4,8-diaryl-4 H-pyrrolo[3,2,1- ij]quinolin-6-ones.

NH4PF6-promoted cyclodehydration of α-amino carbonyl compounds: efficient synthesis of pyrrolo[3,2,1-ij]quinoline and indole derivatives

NH₄PF₆ is an inexpensive, safe, and low-toxicity inorganic salt; it was found to promote the cyclodehydration of α-amino carbonyl compounds in the absence of metal reagents.

Bifunctional cinchona alkaloid-squaramide-catalyzed highly enantioselective aza-Michael addition of indolines to α,β-unsaturated ketones

An enantioselective aza-Michael addition of indolines to α,β-unsaturated ketones was achieved using a bifunctional cinchona alkaloid-derived chiral squaramide derivative. Various β-indolinyl ketone derivatives were obtained in good to excellent yields and with high enantioselectivity. DDQ or MnO2 oxidation of indoline derivatives provided convenient access to various enantioenriched N-substituted indole derivatives.