Synthesis of C3-Methyl-Substituted Pyrroloindolines and Furoindolines via Cascade Dearomatization of Indole Derivatives with Methyl Iodide

Palladium-Catalyzed Asymmetric Decarboxylation of 5-Vinyloxazolidine-2,4-Diones Triggering the Dearomatization of Electron-Deficient Indoles for the Synthesis of Chiral Highly Functionalized Pyrroloindolines

A catalyst system consisting of a chiral phosphoramidite ligand and Pd2(dba)3·CHCl3 causes the decarboxylation of 5-vinyloxazolidine-2,4-diones to generate amide-containing aza-π-allylpalladium 1,3-dipole intermediates, which are capable of triggering the dearomatization of 3-nitroindoles for diastereo- and enantioselective [3+2] cycloaddition, leading to the formation of a series of highly functionalized pyrroloindolines containing three contiguous stereogenic centers with excellent results (up to 99% yield, 88:12 dr, and 96% ee).

Application of Metal-Free Dearomatization Reaction as a Sustainable Strategy to Direct Access Complex Cyclic Compounds

The highly efficient construction of complicated heterocyclic frameworks in an atom- and step-economic manner is still one of the cores of synthetic chemistry. Dearomatization reactions show the unique advantage for the construction of functionalized heterocycles and have attracted widespread attention over the past two decades. The metal-free approach has proved to be a green and sustainable paradigm for the synthesis of spirocyclic, polycyclic and heterocyclic scaffolds, which are widely present in natural products and bioactive molecules. In this review, the advances in the recent six years (2017-2023) in metal-free dearomatization reactions are highlighted. Emphasis is placed on developments in the field of organo-catalyzed dearomatization reactions, oxidative dearomatization reactions, Brønsted acid- or base-promoted dearomatization reactions, photoredox-catalyzed dearomatization reactions, and electrochemical oxidation dearomatization reactions.

Diastereoselective Synthesis of Indoline- and Pyrrole-Embedded Tetracycles via an Unprecedented Dearomative Indole-C3-Alkylation/Aza-Friedel-Crafts Cascade Reaction

Dearomative indole C3-alkylation─intramolecular iminium trapping cascade reaction of indole-C3-tethered nucleophiles is a well-known blueprint for accessing 2,3-fused indolines. In exploring this strategy, synthetic chemists have utilized diverse classes of electrophilic reagents. However, the tethered nucleophiles have mainly been limited to heteronucleophiles and enolates; exploitation of tethered arenes/heteroarenes remains unknown. We herein describe the first examples of pyrrole-intercepted dearomative indole C3-allylation and benzylation of indole-tethered pyrroles toward the synthesis of 2,3-cis-fused tetracyclic indolines featuring a C3 all-carbon quaternary stereocentre. Our methodology capitalizes on the capability of NaO^tBu/Et₃B combination to direct the intermolecular alkylation to take place regioselectively at the indole C3 position over the other reactive sites (indole N and C2 and pyrrole C2 positions) and leverages the high nucleophilicity of the pyrrole template for the concomitant aza-Friedel-Crafts ring closure that traditionally would require an additional acid-catalyzed synthetic step. This cascade reaction is accomplished with broad substrate scope and excellent yields and chemo-, regio-, and diastereoselectivities.

Organocatalytic cycloaddition of alkynylindoles with azonaphthalenes for atroposelective construction of indole-based biaryls

The axially chiral indole-aryl motifs are present in natural products and biologically active compounds as well as in chiral ligands. Atroposelective indole formation is an efficient method to construct indole-based biaryls. We report herein the result of a chiral phosphoric acid catalyzed asymmetric cycloaddition of 3-alkynylindoles with azonaphthalenes. A class of indole-based biaryls were prepared efficiently with excellent yields and enantioselectivities (up to 98% yield, 99% ee). Control experiment and DFT calculations illustrate a possible mechanism in which the reaction proceeds via a dearomatization of indole to generate an allene-iminium intermediate, followed by an intramolecular aza-Michael addition. This approach provides a convergent synthetic strategy for enantioselective construction of axially chiral heterobiaryl backbones.

There is great interest in methods for catalytic enantioselective construction of axially chiral compounds found in natural products. Here, the authors develop a cycloaddition strategy for atroposelective construction of indole-based biaryls via chiral phosphoric acid-catalysed cycloaddition.

Biocatalytic C3-Indole Methylation-A Useful Tool for the Natural-Product-Inspired Stereoselective Synthesis of Pyrroloindoles

Enantioselective synthesis of bioactive compounds bearing a pyrroloindole framework is often laborious. In contrast, there are several S-adenosyl methionine (SAM)-dependent methyl transferases known for stereo- and regioselective methylation at the C3 position of various indoles, directly leading to the formation of the desired pyrroloindole moiety. Herein, the SAM-dependent methyl transferase PsmD from Streptomyces griseofuscus, a key enzyme in the biosynthesis of physostigmine, is characterized in detail. The biochemical properties of PsmD and its substrate scope were demonstrated. Preparative scale enzymatic methylation including SAM regeneration was achieved for three selected substrates after a design-of-experiment optimization.

Iodine-promoted ring-opening methylation of benzothiazoles with dimethyl sulfite

A halogen-bond promoted ring-opening methylation of benzothiazoles has been developed using dimethyl sulphite as a methylating reagent in the presence of a base. This approach represents a simple and efficient synthesis of N-methyl-N-(o-methylthio)phenyl amides, and features direct construction of both N-Me and S-Me bonds in a one-pot reaction through the decomposition of easily prepared benzothiazoles.

KOH-mediated stereoselective alkylation of 3-bromooxindoles for the synthesis of 3,3′-disubstituted oxindoles with two contiguous all carbon quaternary centres

The stereoselective synthesis of 3,3′-disubstituted oxindoles having all-carbon quaternary stereocenters has been achieved using KOH as a base with an excellent diastereomeric ratio (98 : 2).

Bioinspired radical cyclization of tryptamines: synthesis of peroxypyrroloindolenines as potential anti-cancer agents

Inspired by the heme iron-catalyzed radical insertion of dioxygen to the tryptophan indole ring, herein we utilize alkylperoxy radical species as a coupling partner to trigger a peroxycyclization of readily accessible tryptophan derivatives and enable the first synthesis of peroxypyrroloindolenines.

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.