One-Pot Gold-Catalyzed Synthesis of Azepino[1,2-a]indoles

Synthesis of 3-substituted 2,3-dihydropyrazino[1,2-a]indol-4(1H)-ones by sequential reactions of 2-indolylmethyl acetates with α-amino acids†

The synthesis of 2,3-dihydropyrazino[1,2-a]indol-4(1H)-ones from the sequential reaction of amino acid methyl esters with readily available indole-2-ylmethyl acetates is described. The reaction proceeds in situ under basic conditions of highly unstable and reactive 2-alkylideneindolenines followed by Michael-type addition of α-amino acid methyl esters/intramolecular cyclization.

The synthesis of 2,3-dihydropyrazino[1,2-a]indol-4(1H)-ones through the in situ generation of 2-methide-2H-indole intermediate I starting from 2-indolylmethyl acetates under basic conditions/nucleophilic Michael addition/cyclization cascade reaction.

Highly Efficient and Mild Gold (I) Catalyzed Synthesis of 3,8-Diarylidene-2,7-dioxaspiro[4.4]nonane-1,6-diones

The gold-catalyzed cyclization of 2,2-bis(3-arylprop-2-yn1-yl)malonic acid has been proposed as an efficient approach to substituted 3,8-dibenzyl-2,7-dioxaspiro[4.4]nonane-1,6-diones. The reaction proceeds smoothly in mild reaction conditions to give the desired products in quantitative yields in the presence of variously substituted starting materials. In addition, the synthesis of γ-arylidene spirobislactone bearing different substituents on the two aromatic rings has been achieved. This kind of compound could be of great interest in pharmaceutical science given the widespread presence of this scaffold in bioactive natural and synthetic products.

Synthesis of Polysubstituted 1,2-Dihydro-3H-pyrrolo[1,2-a]indol-3-ones through Domino Palladium-Catalyzed Reactions of Indol-2-ylmethyl Acetates with 1,3-Dicarbonyl Derivatives

A straightforward assembly of polysubstituted 1,2-dihydro-3H-pyrrolo[1,2-a]indol-3-ones through a domino palladium-catalyzed reaction of indol-2-ylmethyl acetates with 1,3-dicarbonyl derivatives is described. The key role of the features of the 1,3-dicarbonyls on the reaction outcome has been explored. The employment of 2-methylcyclohexan-1,3-dione as the dicarbonyl source could allow further challenging indole nucleus functionalizations.

Boosting Gold(I) Catalysis via Weak Interactions: New Fine-Tunable Impy Ligands

A series of modular ImPy-carbene-Au(I) complexes are synthesized and fully characterized both in the solid state and in solution. The presence of oligoaryl units (phenyl and thienyl rings) at the C5-position of the ImPy core (in close proximity to the gold center) imprints on the organometallic species fine-tunable and predictable catalytic properties. A marked accelerating effect was recorded in several [Au(I)]-catalyzed electrophilic activations of unsaturated hydrocarbons when a CF₃-containing aromatic ring was accommodated at the ImPy core.

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...

Recent Advances in Transition-Metal-Free (4+3)-Annulations

(4+3)-Annulations are incredibly versatile reactions which combine a 4-atom synthon and a 3-atom synthon to form both 7-membered carbocycles as well as heterocycles. We have previously reviewed transition-metal-catalyzed (4+3)-annulations. In this review, we will cover examples involving bases, NHCs, phosphines, Lewis and Brønsted acids as well as some rare examples of boronic acid catalysis and photocatalysis. In analogy to our previous review, we exclude annulations involving cyclic dienes like furan, pyrrole, cyclohexadiene or cyclopentadiene, as Chiu, Harmata, Fernándes and others have recently published reviews encompassing such substrates. We will however discuss the recent additions (2010–2020) to the literature on (4+3)-annulations involving other types of 4-atom-synthons.

1 Introduction

2 Bases

3 Annulations Using N-Heterocyclic Carbenes

3.1 N-Heterocyclic Carbenes (NHCs)

3.2 N-Heterocyclic Carbenes and Base Dual-Activation

4 Phosphines

5 Acids

5.1 Lewis Acids

5.2 Brønsted Acids

6 Boronic Acid Catalysis and Photocatalysis

7 Conclusion

A rhodium(ii) catalysed domino synthesis of azepino fused diindoles from isatin tethered N-sulfonyl-1,2,3-triazoles and indoles

An efficient and convenient protocol for the synthesis of a novel class of azepino fused diindoles from isatin tethered N-sulfonyl-1,2,3-triazoles and indoles has been disclosed. The reaction proceeds via denitrogenative aza-vinyl rhodium carbene formation to give a carbonyl ylide, which with indole results in 1,3-dipolar cycloaddition followed by sequential semipinacol rearrangement/ring expansion/oxidation to produce azepino fused diindoles. The reaction shows a broad substrate scope giving up to 81% yield. Furthermore, reversible catalytic hydrogenation and photophysical studies were carried out to demonstrate the application of these molecules.

Recent advances in the synthesis of indoles from alkynes and nitrogen sources

This review highlights ten years of success in the synthesis of indoles using alkynes and nitrogen sources as substrates.

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.

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.

Highly enantioselective synthesis of functionalized azepino[1,2-a]indolesviaNHC-catalyzed [3+4] annulation

The enantioselective [3+4] annulation of 3-formylindol-2-methylmalonates with 2-bromoenals catalyzed by NHCs is described to efficiently synthesise functionalized azepino[1,2-a]indoles.

Friedel–Crafts Chemistry. Part 53. Divergent and Diversity-Oriented Synthesis of Condensed Indole Scaffolds via Friedel–Crafts Ring Closure Approach

A series of indole-fused medium-sized N-heterocyclic systems 10a–h were prepared from laboratory-synthesized indole-based esters 9a–h via intramolecular Friedel–Crafts cyclizations induced by both trifluoromethanesulfonic acid and AlCl3/CH3NO2 catalysts under suitable conditions. The synthetic sequence to precursors 9a–h that started from simple N-methylindole-2-carboxylic acid involved conversion to aminoindoles 2a, b, reaction with α,β-unsaturated acid chlorides to yield acyclic amides 5a–d, ring closure to tricyclic lactams 6a–d, and carbonyl reduction to respective pyrido and azepino tricyclic amines 7a–d, which finally underwent N-alkylations with α- or β-haloesters to produce the required ester precursors. The structures of synthesized compounds without stereochemical implications are established using both spectral and analytical data.

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.

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%).

Gold(i)-catalysed cascade reactions in the synthesis of 2,3-fused indole derivatives

Unactivated alkenes and 1,3-unsubstituted indoles in gold(i)-catalysed hydroaminative/arylative cascade cyclizations.

Palladium(ii)-catalyzed intramolecular carboxypalladation–olefin insertion cascade: direct access to indeno[1,2-b]furan-2-ones

An efficient intramolecular carboxypalladation–olefin insertion cascade was developed for the synthesis of indeno[1,2-b]furan-2-ones.

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.

A catalytic diastereoselective formal [5+2] cycloaddition approach to azepino[1,2-a]indoles: putative donor-acceptor cyclobutanes as reactive intermediates

A catalytic formal [5+2] cycloaddition approach to the diastereoselective synthesis of azepino[1,2-a]indoles is reported. The reaction presumably proceeds through a Lewis acid catalyzed formal [2+2] cycloaddition of an alkene with an N-indolyl alkylidene β-amide ester to form a donor-acceptor cyclobutane intermediate, which subsequently undergoes an intramolecular ring-opening cyclization. Azepine products are formed in up to 92% yield with high degrees of diastereoselectivity (up to 34:1 d.r.).