Preparation of Indolenines via Nucleophilic Aromatic Substitution

Electrochemical reaction of indole-tethered alkynes enabling stereoselective synthesis of iodovinyl spiroindolenine-cyclopentanes

A novel electrochemical iodination/spirocyclization reaction of 3-alkynyl indoles with n-Bu4NI as coupling partner and electrolyte is developed, which affords iodovinyl spiroindolenine-cyclopentanes as products in excellent yields and high E-stereoselectivities. This reaction tolerates a wide range of 3-alkynyl indoles and iodide sources, which represents the first example of the electrosynthesis of spiroindolenine-cycloalkanes.

Photoredox-Catalyzed Nucleophilic Aromatic Substitution of Halophenols with Azoles via Oligomeric Phenylene Oxide Radicals

Nucleophilic aromatic substitution (SNAr) reactions are widely employed in organic synthesis yet typically require the use of electron-deficient arenes for efficient reactivity. Herein, we report a photocatalytic protocol for formal SNAr of electron-rich 4-halophenols with azole nucleophiles under mild, redox-neutral conditions. The transformation proceeds via a two-stage mechanism consisting of initial halophenol oligomerization to produce a key oligo(phenylene oxide) intermediate and its subsequent breakdown through SNAr with the azole enabled by photoredox-catalyzed arene umpolung. Reaction monitoring, stoichiometric control experiments, and luminescence quenching data implicate phenoxyl radicals and Brønsted acid-activated oligo(phenylene oxide) radicals as the reactive species in the oligomerization and the SNAr stages, respectively. The synthetic utility of this method is demonstrated across 17 (pseudo)halophenols bearing a variety of leaving groups (F, Cl, Br, OMs, and OTs) and 22 azole examples.

Synthesis and photodynamic activity of new 5-[(E)-2-(3-alkoxy-1-phenyl-1H-pyrazol-4-yl)ethenyl]-2-phenyl-3H-indoles

A series of new indole-pyrazole hybrids 8a-m were synthesized through the palladium-catalyzed ligandless Heck coupling reaction from easily accessible unsubstituted, methoxy- or fluoro-substituted 4-ethenyl-1H-pyrazoles and 5-bromo-3H-indoles. These compounds exerted cytotoxicity to melanoma G361 cells when irradiated with blue light (414 nm) and no cytotoxicity in the dark at concentrations up to 10 µM, prompting us to explore their photodynamic effects. The photodynamic properties of the example compound 8d were further investigated in breast cancer MCF-7 cells. Evaluation revealed comparable anticancer activities of 8d in both breast and melanoma cancer cell lines within the submicromolar range. The treatment induced a massive generation of reactive oxygen species, leading to different types of cell death depending on the compound concentration and the irradiation intensity.

Organo-photoredox-Catalyzed Selective Mono- and Bis-C-H Alkylation of Electron-Rich (Hetero)Arenes

Herein, we disclose a simple strategy for the C-H alkylation of electron-rich (hetero)arenes with alkyl bromides employing visible-light-mediated organo-photocatalytic SET processes. The generality of this method has been evidenced by the inclusion of a variety of alkyl radicals (α-alkyl-carbonyl, benzyl, cyanomethyl) as well as diverse biologically active electron-rich arenes and (hetero)arenes under mild conditions. The extent of alkylation with alkyl bromides was found to be controlled by introducing Zn(OAc)2 as a bromide scavenger, ensuring the blocking of potential bromo-arene byproduct formation under photoredox conditions. In addition, a sequential C-H alkylation strategy for selective bis-alkylation has also been developed via chronological incorporation of different alkyl radical precursors in one pot quite efficiently.

Synthesis of Indoles Using the Electrophilic Potential of Diazirines

The electrophilic potential of diazirines has been utilized to obtain N-substituted diaziridines that are directly hydrolyzed to produce monosubstituted hydrazines. The hydrazines can undergo the Fisher process with enolizable carbonyls to yield multiple indole derivatives in moderate to high yields. The N-metalated diaziridine intermediates can undergo isomerization prior to electrophilic substitution, to form N,N-disubstituted hydrazones. The latter react with enolizable carbonyls to produce N-protected indole derivatives in a single step. This protocol was used to efficiently synthesize indomethacin, an anti-inflammatory drug.

Copper-catalyzed construction of (Z)-benzo[cd]indoles: stereoselective intramolecular trans-addition and SN-Ar reaction

Substituted benzo[cd]indoles are one of the most attractive frameworks because of their wide range of biological and optical activities. Herein, a copper-catalyzed one-step synthesis of biologically important polysubstituted benzo[cd]indoles starting from 8-alkynyl-1-naphthylamine derivatives is reported. In this protocol, many substituents tolerated the reaction conditions and produced (Z)-benzo[cd]indoles in good yields. Preliminary mechanistic studies indicated that the reaction proceeds via a stereoselective intramolecular trans-addition and S_N-Ar reaction with high selectivity and high yields. The synthesized polysubstituted (Z)-benzo[cd]indoles possess sulfonamide building blocks, which make them candidates for bioactive molecules.

Diastereoselective synthesis of indolenine-based spiro[pyrazolone-4,2'-pyrrolidine] scaffolds via 1,3-dipolar cycloaddition of 4-aminopyrazolones, aldehydes, and indolenines

In this work, we report a one-pot [3 + 2] cycloaddition of 4-aminopyrazolones, indolenines, and aldehydes. The reaction utilized in situ generated azomethine ylides as 1,3-dipoles and 2-alkenylindolenines as dipolarophiles affording indolenine-derived spiro[pyrazolone-4,2'-pyrrolidine] scaffolds with four contiguous stereocenters with excellent yields (up to 95%) and diastereoselectivities (up to >20 : 1 dr) under simple conditions. The in situ generation of azomethine ylides and dipolarophiles in one pot is a unique feature of this process.

Et3SiH + KO t Bu provide multiple reactive intermediates that compete in the reactions and rearrangements of benzylnitriles and indolenines

The combination of potassium tert-butoxide and triethylsilane is unusual because it generates multiple different types of reactive intermediates simultaneously that provide access to (i) silyl radical reactions, (ii) hydrogen atom transfer reactions to closed shell molecules and to radicals, (iii) electron transfer reductions and (iv) hydride ion chemistry, giving scope for unprecedented outcomes. Until now, reactions with this reagent pair have generally been explained by reference to one of the intermediates, but we now highlight the interplay and competition between them.

Dialkylation of Indoles with Trichloroacetimidates to Access 3,3-Disubstituted Indolenines

2-Substituted indoles may be directly transformed to 3,3-dialkyl indolenines with trichloroacetimidate electrophiles and the Lewis acid TMSOTf. These reactions provide rapid access to complex indolenines which are present in a variety of complex natural products and medicinally relevant small molecule structures. This method provides an alternative to the use of transition metal catalysis. The indolenines are readily transformed into spiroindoline systems which are privileged scaffolds in medicinal chemistry.