New Entry to Polycyclic Fused Indoles via Gold(I)-catalyzed Cascade Reaction

NQNHC Ligand-Enabled Cu(I)-Catalyzed Double Hydroamination: A Regio- and Chemoselective Bicyclization of o-Amino 1,6-Diyne

In this work, we have developed an efficient method for the intramolecular double hydroamination of aniline by employing o-amino 1,6-diyne as a potential starting material. This protocol enables easy access to bioactive motif 3,4-dihydro-1H-[1,4]oxazino[4,3-a]indole through an intramolecular cascade bicyclization and concomitant isomerization pathway in one pot. This transformation has been effectively achieved by utilizing a stereoelectronically tuned, π-accepting NHC-supported copper(I) system. During ligand optimization trials, naphthoquinone-annulated N-heterocyclic carbene, Nq(IDipp) [1,3-bis(2,6-diisopropylphenyl)-4,5-naphthoquino-imidazolidene]-supported copper(I) complexes of the type Nq(IDipp)CuX (X = Cl or I) were synthesized and fully characterized using various spectroscopic techniques. For this conversion, NHC plays a crucial role in providing the optimum electron density around the metal center. It is a highly regio- and chemoselective transformation with a high atom economy and uses cheap, environmentally benign copper-based catalysts. Furthermore, a plausible mechanism has been proposed on the basis of experimental observations and literature support.

Facile synthesis of the spiro-pyridoindolone scaffold via a gold-catalysed intramolecular alkynol cyclisation/hydroindolylation

A simple approach for the synthesis of pyridoindolone scaffolds with a spiroannulated tetrahydrofuran ring is described. The overall process comprises intramolecular sequential gold-catalysed 5-endo-dig alkynol cycloisomerization and subsequent addition of indole C2 to the in situ generated oxocarbenium cation.

Convenient synthesis of tricyclic N(1)-C(2)-fused oxazino-indolones via [Au(I)] catalyzed hydrocarboxylation of allenes

A new [Au(I)] catalyzed intramolecular hydrocarboxylation of allenes is presented as a valuable synthetic route to oxazino-indolones. The employment of 3,5-(CF3)2-C6H3-ImPyAuSbF6 as the optimal catalyst (5 mol%) was necessary to...

Gold-Catalyzed Domino Cycloisomerization/Alkoxylation: An Entry to 3,4-Dihydro-1H-[1,4]oxazino[4,3-a]indole

A novel and mild synthetic route for the preparation of functionalized polycyclic indole skeletons via a gold-mediated cycloisomerization/alkoxylation of 1,6-aldehyde-yne has been developed. This atom-economical catalytic process that associates IPrAu(MeCN)BF₄ and an alcohol demonstrated remarkable selectivity in accessing functionalized 3,4-dihydro-1H-[1,4]oxazino[4,3-a]indole derivatives of high synthetic utility (21 examples, yields of ≤96%) and could be optimized under asymmetric conditions with an enantiomeric excess of ≤86%.

Synthesis of Oxazino[4,3-a]indoles and biological applications

This review brings together the various pathways to the oxazino[4,3-a]indole motif over the last decades. Representative examples showing the scope of these processes will illustrate the synthetic pathways and the biological activity of the synthesized oxazinoindoles will be mentioned wherever possible.

An Apparent Umpolung Reactivity of Indole through [Au]-Catalysed Cyclisation and Lewis-Acid-Mediated Allylation

The sequential functionalization of indole C2 and C3 in an umpolung fashion was executed with a predesigned substrate and choice of reagents. The developed method comprises gold-catalysed alkynol cycloisomerisation/intramolecular addition of C2 of indole and subsequent BF₃ ⋅OEt₂ -mediated regioselective C3 allylation, resulting in the synthesis of the functionalized indoloisoquinolinone scaffold. The reaction involves 5-endo-alkynol cycloisomerisation and the dearomative addition of indole C2 to the intermediate oxocarbenium cation, which results in two equilibrating fused and spiropentacyclic intermediates, which upon treatment with allyl silane in the presence of BF₃ ⋅OEt₂ , undergo selective indole C3 allylation. Other nucleophiles, such as hydride, azide and indole, were also found to be compatible with this process.

Site-selective synthesis of 1,3-dioxin-3-ones via a gold(i) catalyzed cascade reaction

A novel gold(i)-catalyzed protocol for the synthesis of 4H-1,3-dioxin-3-ones is presented.

Gold-catalyzed enantioselective functionalization of indoles

This review documents the potential of enantioselective gold catalysis for the functionalization of indoles.

Cascade reaction based synthetic strategies targeting biologically intriguing indole polycycles

In this review, cascade reaction based synthesis strategies delivering biologically intriguing indole polycycles are presented.

Graphene Oxide Promotes Site-Selective Allylic Alkylation of Thiophenes with Alcohols

The graphene oxide (GO) assisted allylic alkylation of thiophenes with alcohols is presented. Mild reaction conditions and a low GO loading enabled the isolation of a range of densely functionalized thienyl and bithienyl compounds in moderate to high yields (up to 90%). The cooperative action of the Brønsted acidity, epoxide moieties, and π-surface of the 2D-promoter is highlighted as crucial in the reaction course of the present Friedel-Crafts-type protocol.

Synthesis of indole-fused heteroacenes by cascade cyclisation involving rhodium(ii)-catalysed intramolecular C–H amination

Indole-fused heteroacenes have been synthesised from cascade cyclisation of 2-[(2-azidophenyl)ethynyl]anilines and -phenols in a catalytic manner under redox-neutral conditions.

TBAF catalyzed one-pot synthesis of allenyl-indoles

The site selective synthesis of functionalized indoles is presented under environmentally convenient tetrabutylammonium fluoride catalysis.

Enantioselective gold catalyzed dearomative [2+2]-cycloaddition between indoles and allenamides

The highly enantioselective synthesis of densely functionalized 2,3-indoline-cyclobutanes by means of chiral gold catalysis is presented. Intermolecular formal [2+2]-cycloaddition reactions between substituted indoles and allenamides enabled direct access to methylenecyclobutane-fused indolines, featuring two consecutive quaternary stereogenic centers with excellent stereochemical control (dr > 20 : 1, ee up to 99%).

Metal-free enantioselective electrophilic activation of allenamides: stereoselective dearomatization of indoles

The effective and unprecedented chiral BINOL phosphoric acid catalyzed (1-10 mol%) dearomatization of indoles through electrophilic activation of allenamides (ee up to 94%), is documented. Besides the synthesis of 3,3-disubstituted indolenine cores, a dearomatization/hydrogen transfer cascade sequence is also presented as a new synthetic shortcut toward highly enantiomerically enriched indolines.

Taming gold(I)-counterion interplay in the de-aromatization of indoles with allenamides

A careful interplay between the π electrophilicity of a cationic Au(I) center and the basicity of the corresponding counterion allowed for the chemo- and regioselective inter- as well as intramolecular de-aromatization of 2,3-disubstituted indoles with allenamides. The silver-free bifunctional Lewis acid/Brønsted base complex [{2,4-(tBu)2C6H3O}3PAuTFA] assisted the formation of a range of densely functionalized indolenines under mild conditions.