TBHP Mediated Substrate Controlled Oxidative Dearomatization of Indoles to C2/C3-Quaternary Indolinones

Recent advances in oxidative dearomatization involving C-H bonds for constructing value-added oxindoles

Exploring three-dimensional chemical space is an important research objective of organic synthetic chemistry. Oxidative dearomatization (ODA) is one of the most important and powerful tools for realizing this goal, because it changes and removes aromatic structures from aromatic compounds to increase levels of saturation and stereoisomerism by direct addition reactions between functional groups with aromatic cores under oxidative conditions. As a hot topic in indole chemistry, the synthetic value of the oxidative dearomatization of indoles has been well recognized and has witnessed rapid development recently, since it could provide convenient and unprecedented access to fabricate high-value-added three-dimensional oxindole skeletons, such as C-quaternary indolones, polycycloindolones and spiroindolones, and be widely applied to the total synthesis of these oxindole alkaloids. Therefore, this article provides a review of recent developments in oxidative dearomatization involving the C-H bonds of indoles. In this article, the features and mechanisms of different types of ODA reactions of indoles are summarized and represented, and asymmetric synthesis methods and their applications are illustrated with examples, and future development trends in this field are predicted at the end.

Synthesis, α-glucosidase inhibition, α-amylase inhibition, and molecular docking studies of 3,3-di(indolyl)indolin-2-ones

The synthesized 3,3-di(indolyl)indolin-2-ones 1a-p showed desired higher α-glucosidase inhibitory activities and lower α-amylase inhibitory activities than standard drug acarbose. Particularly, compound 1i showed favorable higher α-glucosidase % inhibition of 67 ± 13 and lower α-amylase % inhibition of 51 ± 4 in comparison to acarbose with % inhibition activities of 19 ± 5 and 90 ± 2, respectively. Docking studies of selected 3,3-di(indolyl)indolin-2-ones revealed key interactions with the active sites of both α-glucosidase and α-amylase, further supporting the observed % inhibitory activities. Furthermore, the binding energies are consistent with the % inhibition values. The results suggest that 3,3-di(indolyl)indolin-2-ones may be developed as suitable Alpha Glucosidase Inhibitors (AGIs) and the lower α-amylase activities should be advantageous to reduce the side effects exhibited by commercial AGIs.

Metal-free synthesis of C2-quaternary indolinones by (NH4)2S2O8 mediated oxidative dearomatization of indoles

An efficient metal-free, (NH₄)₂S₂O₈ mediated oxidative dearomatization of indoles for the construction of C2-quaternary indolinones was disclosed. A series of C2-quaternary indolinones derivatives with good functional group tolerance were obtained in moderate to excellent yields. This methodology provides an alternative approach for the direct generation of all-carbon quaternary centers at the C2 position of indoles. This catalytic approach represents a step-economic and convenient strategy for the oxidative dearomatization of indoles.

An efficient metal-free, (NH₄)₂S₂O₈ mediated oxidative dearomatization of indoles for construction of C2-quaternary indolinones was disclosed which provides an approach for generation of all-carbon quaternary centers at the C2 position of indoles.

A general approach to 2,2-disubstituted indoxyls: total synthesis of brevianamide A and trigonoliimine C

The indoxyl unit is a common structural motif in alkaloid natural products and bioactive compounds. Here, we report a general method that transforms readily available 2-substituted indoles into 2,2-disubstituted indoxyls via nucleophile coupling with a 2-alkoxyindoxyl intermediate and showcase its utility in short total syntheses of the alkaloids brevianamide A (7 steps) and trigonoliimine C (6 steps). The developed method is operationally simple and demonstrates broad scope in terms of nucleophile identity and indole substitution, tolerating 2-alkyl substituents and free indole N-H groups, elements beyond the scope of most prior approaches. Spirocyclic indoxyl products are also accessible via intramolecular nucleophilic trapping.

A three-component iodine-catalyzed oxidative coupling reaction: a heterodifunctionalization of 3-methylindoles

A metal-free method for the synthesis of heterodifunctional indole derivatives is developed through TBHP/KI-mediated oxidative coupling. The reaction constructs C-O and C-C bonds in succession with the help of tert-butyl peroxy radicals generated by the TBHP/KI catalytic system, enabling the direct realization of the heterodifunctionalization of indole in one pot. The product of this reaction is a novel heterodifunctional compound. This work might provide a new effective method for the synthesis of polycyclic indole compounds.

Ru-catalyzed oxidation and C–C bond formation of indoles for the synthesis of 2-indolyl indolin-3-ones under mild reaction conditions

Herein, we described a ruthenium catalyzed oxidation and C–C bond formation reaction of 2-alkyl or 2-aryl substituted indoles using tert-butyl hydroperoxide (TBHP) as oxidant. Coupled with cascade transformation, it provided a mild catalytic oxidation system for the synthesis of 2-indolylindolin-3-ones. The reaction could readily occur using RuCl3·3H2O as catalyst, and the target product was obtained with medium to high yield.

Oxidative trimerization of indoles via water-assisted visible-light photoredox catalysis and the study of their anti-cancer activities

Incorporation of water has been revealed to successfully facilitate visible-light photoredox catalysis of indole leading to increased production of C2-quaternary indolinone. The water-promoted photoreaction of indole under catalyst-free conditions by a household compact fluorescence light was also demonstrated. The antiproliferative activity of the synthesized indolinones was evaluated against three human cancer cell lines.

Oxidative Coupling of 3-Oxindoles with Indoles and Arenes

A highly efficient method for the oxidative coupling of 2-substituted 3-oxindoles with aromatic compounds to form 2,2-disubstituted indolin-3-ones with broad scope is described. This work utilized oxygen as the terminal oxidant and a base-metal catalyst under mild conditions instead of toxic/precious-metal reagents and higher-molecular-weight oxidants. Quaternary structures were produced in modest-to-excellent yields (up to 96 %) without prefunctionalization.

Metal-free synthesis of 2,2-disubstituted indolin-3-ones

A straightforward method for the synthesis of indolin-3-ones bearing a C2-quaternary functionality is reported. This cross-coupling reaction allows the facile synthesis of a series of 2,2-disubstituted indolin-3-ones in the absence of a metal catalyst in up to 94% yields.