A one-pot synthesis of 7-phenylindolo[3,2-a]carbazoles from indoles and β-nitrostyrenes, via an unprecedented reaction sequence

Discovery of a potent melatonin-based inhibitor of quinone reductase-2 with neuroprotective and neurogenic properties

5-Methoxy-3-(5-methoxyindolin-2-yl)-1H-indole (3), whose structure was unambiguously elucidated by X-ray analysis, was identified as a multi-target compound with potential application in neurodegenerative diseases. It is a low nanomolar inhibitor of QR2 (IC50 = 7.7 nM), with greater potency than melatonin and comparable efficacy to the most potent QR2 inhibitors described to date. Molecular docking studies revealed the potential binding mode of 3 to QR2, which explains its superior potency compared to melatonin. Furthermore, compound 3 inhibits hMAO-A, hMAO-B and hLOX-5 in the low micromolar range and is an excellent ROS scavenger. In phenotypic assays, compound 3 showed neuroprotective activity in a cellular model of oxidative stress damage, it was non-toxic, and was able to activate neurogenesis from neural stem-cell niches of adult mice. These excellent biological properties, together with its both good in silico and in vitro drug-like profile, highlight compound 3 as a promising drug candidate for neurodegenerative diseases.

Total Synthesis and Antibacterial Activity of Marine Tris-indole Alkaloid Tulongicin A

Bis-indole alkaloids from marine sponges are an intriguing class of natural products with a variety of activities. However, only a preliminary biological study of tulongicin A (5), a related previously isolated marine tris-indole alkaloid, has been conducted. In this study, we accomplished the first asymmetric total synthesis of 5 via the construction of an imidazoline-linked bis-indolylmethane skeleton using a Friedel-Crafts-type reaction. Our synthesis enabled a detailed study of the antibacterial profile of 5. Compound 5 displayed bactericidal activity against Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA) strains.

Palladium-Catalyzed Synthesis of Linked Bis-Heterocycles─Synthesis and Investigation of Photophysical Properties

A palladium-catalyzed domino C-N coupling/Cacchi reaction is reported. Design of photoluminescent bis-heterocycles, aided by density functional theory calculations, was performed with synthetic yields up to 98%. The photophysical properties of the products accessed via this strategy were part of a comprehensive study that led to broad emission spectra and quantum yields of up to 0.59. Mechanistic experiments confirmed bromoalkynes as competent intermediates, and a density functional theory investigation suggests a pathway involving initial oxidative addition into the cis C-Br bond of the gem-dihaloolefin.

Stereospecific Synthesis of Unprotected, α,β-Disubstituted Tryptamines and Phenethylamines from 1,2-Disubstituted Alkenes via a One-Pot Reaction Sequence

Unprotected, α,β-disubstituted tryptamines and phenethylamines are obtained by a one-pot, metal-free sequence that proceeds by the in situ formation of aziridinium salts followed by Friedel-Crafts reaction with electron-rich (hetero)arenes. Both steps are facilitated by hexafluoroisopropanol as the solvent. The one-pot sequence was effective for diversely substituted indoles and 1,3,5-trimethoxybenzene, for cyclic and acyclic alkenes, and proceeded in a stereospecific fashion for both (E)- and (Z)-1,2-disubstituted alkenes. Moreover, one-pot morpholine addition to an aziridinium salt provided a diamine.

Deuteration of Indole Compounds: Synthesis of Deuterated Auxins, Indole-3-acetic Acid-d5 and Indole-3-butyric Acid-d5

In this study, we describe a practical and facile synthesis of deuterium-labeled indoles via acid-catalyzed hydrogen-deuterium exchange. 3-Substituted indoles were efficiently deuterated through treatment with 20 wt % D₂SO₄ in CD₃OD at 60-90 °C. A deuterium incorporation reaction of 3-unsubstituted indoles was accomplished through treatment with CD₃CO₂D at 150 °C. The in situ preparation of a 20 wt % D₂SO₄/CH₃OD/D₂O solution enabled a large-scale and low-cost synthesis of auxins, indole-3-acetic acid-d5 and indole-3-butyric acid-d5.

Facile and Mild Access to Fluorescent Ladder-Type Indolo[3,2-a]carbazoles via Cascade Annulation

A set of fluorescent ladder-type indolo[3,2-a]carbazoles were rationally developed via cascade annulation of indoles and nitroolefins under mild reaction conditions. Diverse functional groups were tolerated. Moreover, structure–photophysical properties relationships (SPPR) of indolo[3,2-a]carbazoles were observed to be simply tuned by changing the electronic nature of the peripheral substituents.

Benzannulation strategies for the synthesis of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines

This review presents a critical and authoritative analysis of several exciting benzannulation approaches developed in the past decade for the construction of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines.

Straight access to highly fluorescent angular indolocarbazoles via merging Au- and Mo-catalysis

A straightforward and efficient synthesis of the two less explored types of indolocarbazoles has been developed giving rise to highly fluorescent compounds with fluorescence quantum yields around 0.7.

Novel racemosin B derivatives as new therapeutic agents for aggressive breast cancer

Carbazole derivatives show anti-cancer activity and are of great interest for drug development. In this study, we synthesized and analyzed several new alkylamide derivatives of racemocin B, a natural indolo[3,2-a]carbazole molecule originally isolated from the green alga Caulerpa racemose. Several alkylamide derivatives were found to exhibit moderate to strong growth inhibition against human breast cancer cell lines. They induced G2/M cell cycle arrest and apoptosis in the aggressive triple-negative breast cancer cell line MDA-MB-231. Among these derivatives, compound 25 with the lowest IC₅₀ induced cell death by suppressing autophagy. This was accompanied by inhibition of autophagic flux and accumulation of autophagy protein 1 light chain 3, LC3II, and p62. The novel alkylamide derivative offers a potential new treatment for human breast cancer.

Chemistry and Properties of Indolocarbazoles

The indolocarbazoles are an important class of nitrogen heterocycles which has evolved significantly in recent years, with numerous studies focusing on their diverse biological effects, or targeting new materials with potential applications in organic electronics. This review aims at providing a broad survey of the chemistry and properties of indolocarbazoles from an interdisciplinary point of view, with particular emphasis on practical synthetic aspects, as well as certain topics which have not been previously accounted for in detail, such as the occurrence, formation, biological activities, and metabolism of indolo[3,2- b]carbazoles. The literature of the past decade forms the basis of the text, which is further supplemented with older key references.

Reactions of 3,3,3-Trihalogeno-1-nitropropenes with Arenes in the Superacid CF3SO3H: Synthesis of ( Z)-3,3,3-Trihalogeno-1,2-diarylpropan-1-one Oximes and Study on the Reaction Mechanism

3,3,3-Trihalogeno-1-nitropropenes C(Hal₃)CH═CH(NO₂) (Hal = F, Cl, Br) in reaction with arenes in the superacid CF₃SO₃H (TfOH) at room temperature in 1 h afford 3,3,3-trihalogeno-1,2-diarylpropan-1-one oximes C(Hal₃)CH(Ar)-C(Ar)═NOH (CHal₃-oximes) in yields of 23-99%. Such CHal₃-oximes having one ortho-substituent in the aryl ring exist as atropoisomers in solutions at room temperature. Several cationic intermediates of this reaction were studied by means of NMR and DFT calculations, which proves the detailed reaction mechanism of the formation of CHal₃-oximes in TfOH. CHal₃-oximes (for Hal = Cl, Br) with DBU in DMF at microwave or thermal activation are cyclized into 5-halogeno-3,4-diarylisoxazoles in yields of 37-59%. CHal₃-oximes under the conditions of Beckmann rearrangement with PCl₅ in benzene at room temperature in 24 h are turned at first into imidoyl chlorides (yields of 94-96%), which undergo transformation into the corresponding benzamides PhCONHCHPh(CHal₃) on silica gel (yields of 46-47%).

Metal-Free Oxidative Cross Coupling of Indoles with Electron-Rich (Hetero)arenes

A new method for the synthesis of bi-heteroaryls is reported, based on the umpolung of indoles with benziodoxol(on)e hypervalent iodine reagents (IndoleBX). The oxidative coupling of IndoleBX with an equimolar amount of electron-rich benzenes, indoles, pyrroles, and thiophenes proceeded under mild transition-metal-free conditions. Functionalized non-symmetrical bi-indolyl heterocycles were accessed efficiently. Introduction of a new type of C2-substituted indole benziodoxole reagents further allowed extending the scope of the reaction to NH unprotected and C3-alkylated indoles. The obtained bi-heterocycles are important building blocks in synthetic and medicinal chemistry, and could be easily transformed into more complex heterocyclic systems.

Facile synthesis of indolo[3,2-a]carbazoles via Pd-catalyzed twofold oxidative cyclization

A rapid and efficient route has been developed for the synthesis of 9-methoxycarbonylindolo[3,2-a]carbazole derivatives. The key steps in this approach involved an aromatic amination and an oxidative biaryl coupling. Via the present route, indolo[3,2-a]carbazole derivatives are available in 3-4 steps based on commercially available starting materials.

Metal-free transannulation reaction of indoles with nitrostyrenes: a simple practical synthesis of 3-substituted 2-quinolones

3-Substituted 2-quinolones are obtained via a novel, metal-free transannulation reaction of 2-substituted indoles with 2-nitroalkenes in polyphosphoric acid. The reaction can be used in conjunction with the Fisher indole synthesis offering a practical three-component heteroannulation methodology to produce 2-quinolones from arylhydrazines, 2-nitroalkenes and acetophenone.