Hypoiodite-Catalyzed Chemoselective Tandem Oxidation of Homotryptamines to Peroxy- and Epoxytetrahydropyridoindolenines

Catalyst-free coupling of peroxypyrroloindolenines with amines to afford stable peroxyindolenines

Here we report a highly efficient method for coupling of peroxypyrroloindolenines with amines under catalyst-free conditions to obtain stable C2-N peroxyindolenines in high yields with remarkable functional group tolerance. Initial studies have shown that compound 13 exhibits potent inhibition of the B16/F10 cell line with an IC50 value of 2.18 μM.

Recent advances in the oxidative activation of the C2-C3 π bond of indoles and its applications

The oxidative C2-C3 π bond activation strategy is the most efficient tool to synthesize oxygen-containing indoline, which frequently appears in natural products with various biological activities as structural units. Recently, the oxidation-induced cascade strategy through oxygenation activation of the indolic C2-C3 π bond of indoles has received much attention for its use in efficiently establishing complex indoline with oxygen-containing molecular architectures, and holds tremendous potential in the total synthesis of indole alkaloids. It can be carried out using potential activated indole radical cations or imine cation intermediates produced via oxidative C2-C3 π bond activation of indole with various nucleophiles or ring-forming reagents by employing simple and non-decorated indoles as starting substrates. Herein, we have reviewed recent advances in the oxidation-induced cascade strategies connecting intra-cyclization or inter-annulation reactions, nucleophilic or radical additions and rearrangement via the oxidative C2-C3 π bond activation of indoles over the past two decades, providing diverse oxygen-containing indolines such as indoxyls, indoline oxygen-heterocycles and indolones. The features and mechanisms of different types of oxidation-induced cascade reactions have been summarized and represented, and examples have been given of their asymmetric reactions and applications in the total synthesis of indole alkaloids.

Advances in radical peroxidation with hydroperoxides

Organic peroxides have become sought-after functionalities, particularly following the multi-tone consumption in polymer production and success in medicinal chemistry. The selective introduction of a peroxide fragment at different positions on the target molecule is a priority in the modern reaction design. The pioneering Kharasch-Sosnovsky peroxidation became the basic universal platform for the development of peroxidation methods, with its great potential for rapid generation of complexity due to the ability to couple the resulting free radicals with a wide range of partners. This review discusses the recent advances in the radical Kharasch-type functionalization of organic molecules with OOR fragment including free-component radical couplings. The discussion has been structured by the type of the substrate of radical peroxidation: C(sp 3 )-H substrates; aromatic systems; compounds with unsaturated C-C or C-Het bonds.

Radical-Initiated Dearomative Annulation of Tryptamine-Derived Isocyanides: Selective Synthesis of CF3-Substituted β-Aza-spiroindolines and β-Carbolines

A mild approach for synthesizing CF3-substituted β-aza-spiroindolines and β-carbolines from tryptamine-derived isocyanides via site-selective radical annulations has been reported. The crucial role of C2 substituents in the selective annulation process has been clarified. The approach shows good generality and practical applicability, highlighting its effectiveness and versatility.

I2-catalyzed synthesis of 3-aminopyrrole with homopropargylic amines and nitrosoarenes

The synthesis of 3-aminopyrrole using the amination reagent nitrosoarenes and homopropargylic amines catalyzed by I2 through cyclization and amination has been developed. This protocol features excellent functional group tolerance and mild reaction conditions, yielding 3-aminopyrroles in moderate to good yields without a metal catalyst. This method realizes the construction and amination of the 3-aminopyrroles in which nitrosoarenes serve as the amine source and oxidant.

NBS-induced intramolecular annulation reactions for the divergent synthesis of fused- and spirocyclic indolines

An NBS-induced intramolecular annulation of 3-(1H-indol-3-yl)-N-alkoxypropanamide is described. The reactions proceed well and quickly under mild conditions with the help of a base. It was found that C2-substituents on the indole ring in 3-(1H-indol-3-yl)-N-alkoxypropanamide have a great influence upon the reaction. By using C2-methyl- and C2-phenyl-3-(1H-indol-3-yl)-N-alkoxypropanamide as templates, practical protocols for the divergent synthesis of fused- and spirocyclic indoline compounds were studied and established.

Radical bicyclization of 1,6-enynes with sulfonyl hydrazides by the use of TBAI/TBHP in the aqueous phase

A novel 5-exo-dig/6-endo-trig bicyclization of 1,6-enynes with sulfonyl hydrazides in the aqueous phase using the cheap and available tetrabutylammonium iodide (TBAI)-tert-butyl hydroperoxide (TBHP) combined system is reported. The resulting reaction of diverse nitrogen- and oxygen-polyheterocycles displays high chemical selectivity, high step-economy, and a moderate substrate scope. Moreover, iodosulfonylation can be realized by modulating the structure of the 1,6-enynes.

Chemodivergent Photocatalyzed Heterocyclization of Hydrazones and Isothiocyanates for the Selectivity Synthesis of 2-Amino-1,3,4-thiadiazoles and 1,2,4-Triazole-3-thiones

A photocatalytic chemodivergent reaction for the selectivity formation of C-S and C-N bonds in a controlled manner was proposed. The reaction medium, either neutral or acidic, is critical to dictate the formation of 2-amino-1,3,4-thiadiazoles and 1,2,4-triazole-3-thiones from isothiocyanates and hydrazones. This is a practical protocol to achieve the chemoselectivity under mild and metal-free conditions.

Base-Controlled Synthesis of Fluorescent Acridone Derivatives via Formal (4 + 2) Cycloaddition

A transition-metal-free formal (4 + 2) cycloaddition for the direct assembly of acridone derivatives has been developed from simple and easily accessible o-aminobenzamides and 2-(trimethylsilyl)aryl triflates. The base played an important role in the selective controlled synthesis of N-H and N-aryl acridones. A preliminary study on the fluorescence properties of N-aryl acridones demonstrated that they could be used as fluorescent materials with a broad emission range.

In Situ Hypoiodite-Catalyzed Oxidative Rearrangement of Chalcones: Scope and Mechanistic Investigation

It is highly desirable to avoid using rare or toxic metals for oxidative reactions in the synthesis of pharmaceuticals and fine chemicals. Hypervalent iodine compounds are environmentally benign alternatives, but their catalytic use has been quite limited. Herein, the protocol for in situ hypoiodite-catalyzed oxidative rearrangement of chalcones is first realized under mild and metal-free conditions, which provided a nontoxic, environmental-benign, and catalytic alternative to the thallium-based protocol. Also, the applicability and effectiveness of this catalytic protocol got well demonstrated via gram-scale synthesis and product derivatization. What is more, control and NMR tracking experiments were performed to figure out the possible catalytic species and intermediates.

Hypoiodite-Catalyzed Oxidative Umpolung of Indoles for Enantioselective Dearomatization

Here we report the oxidative umpolung of 2,3-disubstituted indoles toward enantioselective dearomative aza-spirocyclization to give the corresponding spiroindolenines using chiral quaternary ammonium hypoiodite catalysis. Mechanistic studies revealed the umpolung reactivity of C3 of indoles by iodination of the indole nitrogen atom. Moreover, the introduction of pyrazole as an electron-withdrawing auxiliary group at C2 suppressed a competitive dissociative racemic pathway, and enantioselective spirocyclization proceeded to give not only spiropyrrolidines but also four-membered spiroazetidines that are otherwise difficult to access.