Anodic C(sp3)-H Acyloxylation of Indolin-3-ones Enabled by Oxidant-Free Cross-Dehydrogenative C(sp3)-O Coupling

Electrochemical C-H Hydroxylation and Alkoxylation Reactions

The electrochemical C-H hydroxylation and alkoxylation reactions have emerged as sustainable platforms to construct valuable oxygenated compounds such as alcohols, phenols, and ethers. Compared with traditional approaches, these electrochemical strategies not only enhance the atom economy through bypassing the use of chemical oxidants but also unlock new reactivities by accessing reactive intermediates under mild conditions. In this review, we tried to provide an overview of the representative achievements in this field since 2020. The related transformations are classified into five parts according to the underlying mechanisms. Furthermore, the potential opportunities and challenges within this rapidly emerging field were also discussed. This review is not intended to be exhaustive but rather to illustrate the synthetic potential of electrochemical C-H hydroxylation and alkoxylation strategies in the sustainable and selective assembly of valuable oxygenated compounds.

Unusual Regio- and Chemoselectivity in Oxidation of Pyrroles and Indoles Enabled by a Thianthrenium Salt Intermediate

A dearomative oxidation of pyrroles to Δ3-pyrrol-2-ones is described, which employs a sulfoxide as oxidant, in conjunction with a carboxylic acid anhydride and a Brønsted acid additive. 3-substituted pyrroles undergo regioselective oxidation to give the product isomer in which oxygen has been introduced at the more hindered position. Regioselectivity is rationalized by a proposed mechanism that proceeds by initial thianthrenium introduction at the less-hindered pyrrole α-position, followed by distal attack of an oxygen nucleophile and subsequent elimination of thianthrene. The same reaction conditions are also able to effect a chemoselective oxidation of indoles to indolin-3-ones and additionally of indolin-3-ones to 2-hydroxyindolin-3-ones. Here again, the regio- and chemoselectivities are rationalized through the intermediacy of a thianthrenium salt.

Oxidative Transformation of 2-Furylanilines into Indolin-3-ones

Oxidation of 2-furylaninlies with m-CPBA followed by treatment with a base provides access to functionalized indolin-3-ones. The designed oxidative transformation utilizes an underassessed chemical behavior of furyl-containing amines to form a C-N bond via engaging a β-carbon atom of the furan core upon a ring-forming step, thereby providing an alternative disconnection toward nitrogen-containing heterocycles.

Progress in Electrochemically Empowered C-O Bond Formation: Unveiling the Pathway of Efficient Green Synthesis

(C-X) bonds (X=C, N, O) are the main backbone for making different skeleton in the organic synthetic transformations. Among all the sustainable techniques, electro-organic synthesis for C-X bond formation is the advanced tool as it offers a greener and more cost-effective approach to chemical reactions by utilizing electrons as reagents. In this review, we want to explore the recent advancements in electrochemical C-O bond formation. The electrochemically driven C-O bond formation represents an emerging and exciting area of research. In this context, electrochemical techniques offers numerous advantages, including higher yields, cost-efficient production, and simplified work-up procedures. This method enables the continuous and consistent formation of C-O bonds in molecules, significantly enhancing overall reaction yields. Furthermore, both intramolecular and intermolecular C-O bond forming reaction provided valuable products of O-containing acyclic/cyclic analogue. Hence, carbonyl (C=O), ether -O-), and ester (-COOR) functionalization in both cyclic/acyclic analogues have been prepared continuously via this innovative pathway. In this context, we want to discuss one-decade electrochemical synthetic pathways of various C-O bond contains functional group in chronological manner. This review focused on all the synthetic aspects including mechanistic path and has also mentioned overall critical finding regarding the C-O bond formation via electrochemical pathways.

Total Synthesis of Calanthoside, a Potential Hair Growth Stimulant: A Facile Synthetic Approach via One-Pot S- and O-Glucosidic Bond Formation

The first total synthesis of calanthoside (1), which exhibits potent proliferative activity against human hair follicle dermal papilla cells, has been achieved in seven steps with an overall yield of 43% on a gram scale starting from anthranilic acid (11). The synthetic strategy features a one-pot process involving thioglucoside bond formation via nucleophilic substitution reaction and enol-glucosylation for building the S-,O-bisdesmoside structure of 1. Moreover, the one-pot reaction showed broad substrate adaptability to several sugar donors other than d-glucose, thus affording S,O-bisglycoside intermediates in ∼84% yield.

KI-Catalyzed C(sp3)-H Amination and Acyloxylation of Indolin-3-ones Using Air as the Oxidant

We have developed an efficient and green strategy for the synthesis of C2-amino indolin-3-ones and C2-acyloxy indolin-3-ones via KI-catalyzed C(sp3)-H amination and acyloxylation of indolin-3-ones using air as the oxidant. The reaction provides straightforward access to 2-substituted indolin-3-ones by the direct functionalization of indolin-3-ones at the C2 position under mild conditions. Moreover, the conditions enable direct functionalization of a range of complex pharmaceuticals, providing attractive products for medicinal chemistry programs.

Metal-Free Oxidative Dearomatization-Alkoxylation/Acyloxylation of Indoles: Synthesis of 2-Monosubstituted Indolin-3-ones

A metal-free route for the preparation of 2-monosubstituted indolin-3-ones, including 2-alkoxyindolin-3-ones and 2-acyloxyindolin-3-ones from commercially available indoles, has been developed employing (bis(trifluoroacetoxy)iodo)benzene (PIFA) as an oxidant. The present protocol features mild reaction conditions, good tolerance with diverse functional groups, and a wide substrate scope, affording the desired products in good yields. This transformation is easy to scale up, and the desired products can be further modified. Most importantly, this method is suitable for the late-stage modification of bioactive molecules. Mechanism studies show that this transformation involves metal-free radical dearomatization and oxygenation. Furthermore, this method also provides a practical and efficient way to prepare indolin-3-ones from commercially available reagents in one step.

Oxidative Cross-Coupling of α-Amino Ketones with Alcohols Enabled by I2-Catalyzed C-H Hydroxylation

An I2-catalyzed oxidative cross-coupling of α-amino ketones with a wide range of alcohols is described. Using a combination of air and dimethyl sulfoxide (DMSO) as oxidants, the protocol allows an efficient synthesis of α-carbonyl N,O-acetals with high functional group tolerance and enables the late-stage introduction of α-amino ketones into biorelevant alcohols. Moreover, the present method can be used in the coupling of α-amino ketones with other kinds of nucleophiles, which demonstrates great generality for the functionalization of α-amino ketones. A preliminary mechanistic investigation suggests that C-H hydroxylation of α-amino ketones has been recognized as the key step followed by subsequent dehydration coupling.

Radical C(sp3)-S Coupling for the Synthesis of α-Amino Sulfides

A unique transition-metal-free radical thiolation of 2-azaallyl anions has been developed. Easily accessible thiosulfonates and 2-azaallyls undergo the tandem process of single-electron transfer and radical-radical coupling to construct C(sp³)-S bonds. This robust protocol enables a mild and chemoselective coupling between 2-azaallyl anions and thiosulfonates to access α-amino sulfides in 50-92% yields (25 examples). The scalability of this protocol was demonstrated by telescopic gram-scale experiments. Mechanistic studies provide significant evidence for this radical thiolation reaction.

Gold-catalyzed redox cycloisomerization/nucleophilic addition/reduction: direct access to 2-phosphoryl indolin-3-ones

An efficient gold(I)-catalyzed redox cycloisomerization/nucleophilic addition/reduction reaction of o-nitroalkynes with various H-phosphorus oxides is established. Through the intramolecular redox cyclization of o-nitroalkynes and subsequent intermolecular nucleophilic addition/reduction with no external reactant, a variety of arylphosphoryl and alkylphosphoryl indolin-3-ones with high functional-group compatibility are obtained in moderate to good yields. Mechanistic studies suggest that phosphorus nucleophiles mediate the cleavage of the N-O bond as a reductant.

Dearomative oxyphosphorylation of indoles enables facile access to 2,2-disubstituted indolin-3-ones

A highly efficient oxidative dearomatization of indoles with H-phosphorus oxides in the presence of TEMPO oxoammonium salt has been demonstrated. Through the intramolecular oxidative dearomatization of indoles and subsequent intermolecular nucleophilic addition with phosphorus nucleophile, a variety of structurally diverse arylphosphoryl and alkylphosphoryl indolin-3-ones were obtained in good yields with a broad substrate scope and high functional-group compatibility.

Synthesis of 2-monosubstituted indolin-3-ones by cine-substitution of 3-azido-2-methoxyindolines

We report herein the formal cine-substitution/hydrolysis of 3-azidoindole intermediates generated from 3-azido-2-methoxyindolines (AZINs). This protocol enables the introduction of both various carboxylic acid and alcohol into indolin-3-ones at the C2-position,...