A Copper-Mediated Oxidative Coupling Route to 3H- and 1H-Indoles fromN-Aryl-enamines

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Base-promoted, CBr4-mediated tandem bromination/intramolecular Friedel-Crafts alkylation of N-aryl enamines: a facile access to 1H- and 3H-indoles

Described here is a general and highly efficient method for the synthesis of 1H- and 3H-indoles. In the presence of CBr₄ and a suitable base, the cyclization of N-aryl enamines proceeds with high efficiency. Unlike previous intramolecular cross dehydrogenative coupling (CDC) of the same substrates, this process does not require the use of either a transition metal or a stoichiometric amount of oxidant. This method also features operational simplicity, easy scalability and good substrate tolerability. Control experiments indicate the reactions may proceed in a tandem sequence of bromination and intramolecular Friedel-Crafts alkylation in a simple one-pot procedure.

Synthesis of Dithiolethiones and Identification of Potential Neuroprotective Agents via Activation of Nrf2-Driven Antioxidant Enzymes

Oxidative stress is implicated in the pathogenesis of a wide variety of neurodegenerative disorders, and accordingly, dietary supplement of exogenous antioxidants or/and upregulation of the endogenous antioxidant defense system are promising for therapeutic intervention or chemoprevention of neurodegenerative diseases. Nrf2, a master regulator of the cellular antioxidant machinery, cardinally participates in the transcription of cytoprotective genes against oxidative/electrophilic stresses. Herein, we report the synthesis of 59 structurally diverse dithiolethiones and evaluation of their neuroprotection against 6-hydroxydopamine- or H₂O₂-induced oxidative damages in PC12 cells, a neuron-like rat pheochromocytoma cell line. Initial screening identified compounds 10 and 11 having low cytotoxicity but conferring remarkable protection on PC12 cells from oxidative-mediated damages. Further studies demonstrated that both compounds upregulated a battery of antioxidant genes as well as corresponding genes' products. Significantly, silence of Nrf2 expression abolishes cytoprotection of 10 and 11, indicating targeting Nrf2 activation is pivotal for their cellular functions. Taken together, the two lead compounds discovered here with potent neuroprotective functions against oxidative stress via Nrf2 activation merit further development as therapeutic or chemopreventive candidates for neurodegenerative disorders.

Copper(ii)-catalyzed synthesis of multisubstituted indoles through sequential Chan–Lam and cross-dehydrogenative coupling reactions

Starting from arylboronic acids and ester (Z)-3-aminoacrylates, one-pot syntheses of diverse indole-3-carboxylic esters have been described through copper(ii)-catalyzed sequential Chan–Lam N-arylation and cross-dehydrogenative coupling (CDC) reactions. The initial Chan–Lam arylation can proceed in DMF at 100 °C for 24 h to give ester (Z)-3-(arylamino)acrylate intermediates in the presence of Cu(OAc)₂/tri-tert-butylphosphine tetrafluoroborate, a catalytic amount of myristic acid as the additive, KMnO₄ and KHCO₃. Sequentially, these in situ arylated intermediates can undergo an intramolecular oxidative cross-dehydrogenative coupling process in mixed solvents (DMF/DMSO = 2 : 1) at 130 °C to give C3-functionalized multi-substituted indole derivatives.

One-pot syntheses of diverse indole-3-carboxylic esters have been described through copper(ii)-catalyzed sequential oxidative Chan–Lam N-arylation and cross-dehydrogenative coupling (CDC) reaction.

Copper-Catalyzed Synthesis of Multisubstituted Indoles through Tandem Ullmann-Type C-N Formation and Cross-dehydrogenative Coupling Reactions

Multisubstituted indoles were synthesized via a one-pot tandem copper-catalyzed Ullmann-type C-N bond formation/intramolecular cross-dehydrogenative coupling process at 130 °C in DMSO. The methodology allows practical and modular assembly of indoles in good to excellent yields from readily available aryl iodides and enamines.

One-Pot Synthesis of Benzo[b][1,4]oxazins via Intramolecular Trapping Iminoenol

A highly atom-efficient "one-pot" protocol has been developed to construct multisubstituted 2-hydroxy-benzo[b][1,4]oxazins starting from N-(2-hydroxylaryl)enaminones. This procedure comprises a PIDA-mediated intramolecular iminoenol tautomer trapping reaction, followed by Et₃N-promoted aerobic oxidative ring construction. In particular, an O₂ molecule from air served as the oxygen source of the hydroxyl group in the titled products. This reaction proceeded smoothly at room temperature under air atmosphere and metal-free conditions.

Photoredox-catalyzed procedure for carbamoyl radical generation: 3,4-dihydroquinolin-2-one and quinolin-2-one synthesis

A reductive approach for carbamoyl radical generation from N-hydroxyphthalimido oxamides under photoredox catalysis is outlined. This strategy was applied to the synthesis of 3,4-dihydroquinolin-2-ones via the intermolecular addition/cyclization of carbamoyl radicals with electron deficient olefins in a mild, redox-neutral manner. Under a general set of reaction conditions, diversely substituted 3,4-dihydroquinolin-2-ones, including spirocyclic systems can be prepared. By using chlorine-substituted olefins, aromatic quinolin-2-ones can also be accessed.

A Cu-Catalysed Radical Cross-Dehydrogenative Coupling Approach to Acridanes and Related Heterocycles

The synthesis of acridanes and related compounds through a Cu-catalysed radical cross-dehydrogenative coupling of simple 2-[2-(arylamino)aryl]malonates is reported. This method can be further streamlined to a one-pot protocol involving the in situ fomation of the 2-[2-(arylamino)aryl]malonate by α-arylation of diethyl malonate with 2-bromodiarylamines under Pd catalysis, followed by Cu-catalysed cyclisation.

The base-promoted synthesis of multisubstituted benzo[b][1,4]oxazepines

A variety of substituted benzo[b][1,4]oxazepines have been prepared via base-promoted intramolecular O-arylation from N-(2-haloaryl)enaminones under metal-free conditions.