Electrochemical and photochemical reaction of isatins: a decade update

This review presents the latest progress in photochemical and electrochemical reactions involving isatins. Isatin and its functionalized scaffolds e.g., oxindoles, spirooxindoles, and quinolines are privileged heterocycles as they are largely present in several agrochemical, natural products, and pharmaceuticals. Thus, the functionalization of isatins using sustainable approaches, i.e., electro- and photochemical methods is of recent research interest worldwide. In this review, we have discussed most of the important reactions of isatins based on types of bond formation involved under electro- and photochemical conditions over the last decade. The reaction mechanism for each reaction has been discussed in detail to offer an inclusive guide to readers. Lastly, a summary of current challenges and the future outlook toward the development of effective electrochemical and photochemical methods for the reaction of isatins is also presented.

Visible-light promoted catalyst-free (VLCF) multi-component synthesis of spiro indolo-quinazolinone-pyrrolo[3,4-a]pyrrolizine hybrids: evaluation of in vitro anticancer activity, molecular docking, MD simulation and DFT studies

A new and highly efficient visible-light-promoted catalyst free (VLCF) strategy for neat and clean synthesis of spiro indolo-quinazolinone-pyrrolo[3,4-a]pyrrolizine hybrids (6a-d) has been introduced. We have performed visible-light triggered 1,3-Dipolar cycloaddition reaction of maleimide (5a-d) with azomethine ylide generated in situ derived from tryptanthrin (3) and L-proline (4) to obtain desired products (6a-d) in good to excellent yield. Authentication and characterization of product was done using various spectroscopic techniques such as IR, 1H NMR, 13C NMR, Mass spectrometry and single crystal XRD analysis. To explain the reaction spontaneity, product stability, reactivity as well as possible mode of the interaction a quantum chemical investigation was performed and depicted through DFT studies. The synthesized compound 6a was also evaluated for anti-proliferative activity against a panel of five cancer cell lines (MCF-7, MDA-MB-231, HeLa, PC-3 and Ishikawa) and normal human embryonic kidney (HEK-293) cell line by using MTT assay. Compound 6a showed very good in vitro anti-proliferative activity (IC50  = 6.58-17.98 μM) against four cancer cell lines and no cytotoxicity against normal HEK-293. In order to evaluate the anticancer potential of compounds 6a-d, molecular docking was performed against wild type and mutant EGFR. The results suggest that all the compounds occupied the active site of both enzymes, with a strong binding energy (-10.2 to -11.5 kcal/mol). These results have been confirmed by molecular dynamics simulation by evaluating root mean square deviation (RMSD) and root mean square fluctuation (RMSF), along with principal component analysis (PCA).Communicated by Ramaswamy H. Sarma.

Tryptanthrin Analogs Substoichiometrically Inhibit Seeded and Unseeded Tau4RD Aggregation

Microtubule-associated protein tau is an intrinsically disordered protein (IDP) that forms characteristic fibrillar aggregates in several diseases, the most well-known of which is Alzheimer's disease (AD). Despite keen interest in disrupting or inhibiting tau aggregation to treat AD and related dementias, there are currently no FDA-approved tau-targeting drugs. This is due, in part, to the fact that tau and other IDPs do not exhibit a single well-defined conformation but instead populate a fluctuating conformational ensemble that precludes finding a stable "druggable" pocket. Despite this challenge, we previously reported the discovery of two novel families of tau ligands, including a class of aggregation inhibitors, identified through a protocol that combines molecular dynamics, structural analysis, and machine learning. Here we extend our exploration of tau druggability with the identification of tryptanthrin and its analogs as potent, substoichiometric aggregation inhibitors, with the best compounds showing potencies in the low nanomolar range even at a ~100-fold molar excess of tau4RD. Moreover, conservative changes in small molecule structure can have large impacts on inhibitory potency, demonstrating that similar structure-activity relationship (SAR) principles as used for traditional drug development also apply to tau and potentially to other IDPs.

Visible-Light-Induced and Iodoform-Promoted Functionalization of Ether with Secondary Sulfonyl Amides

An iodoform-promoted functionalization of ether with secondary sulfonyl amides under visible-light irradiation was developed toward synthesis of hemiaminal skeleton with good to excellent isolated yields. The characterization of the isolated ether and iodoform complex revealed regioselective hydrogen atom transfer to initiate carbon radical formation and enabled the amination reaction with the sulfonamide.

Synthesis of Luminescent Indolo[2,1-b]quinazolin-6(12H)-ones via a Sequential Ugi/Iodine-Promoted Cyclization/Staudinger/Aza-Wittig Reaction

A new efficient synthesis of indolo[2,1-b]quinazolin-6(12H)-ones via a sequential Ugi/iodine-promoted cyclization/Staudinger/aza-Wittig reaction was developed. The acid catalyzed three-component reactions of 2-azidobenzaldehydes, 2-[2-(trimethylsilyl)ethynyl]benzenamines (or o-aminoacetophenones), and isocyanides gave Ugi-3CR intermediates, which reacted subsequently with I2/DMSO and triphenylphosphine to produce indolo[2,1-b]quinazolin-6(12H)-ones in good overall yields. The obtained indolo[2,1-b]quinazolin-6(12H)-ones were all colored in bright red or orange. Their luminescent property was studied preliminarily and some of them showed high molar absorption coefficients, strong fluorescence emission intensity, and good absolute light quantum yields.

Photoinduced Copper-Catalyzed 1,2-Difunctionalization of 1,3-Dienes with Aryl Diselenides

Described herein is a photoinduced copper-catalyzed 1,2-difunctionalization of 1,3-dienes. The selenium atom radical was generated by the visible light irradiation of diselenides, triggering radical addition with 1,3-dienes to form allyl radical intermediate. Subsequent rapid Z/E isomerization allowed for thermodynamically favorable intermediate formation and enabled copper catalyzed stereoselective functionalization with various nucleophiles.

Visible-Light-Mediated Three-Component Radical Iodosulfonylative Cyclization of Enynes

An efficient three-component radical iodosulfonylative cyclization of enynes is described. The visible-light irradiation of iodoform with sulfinates enables sulfonyl radical generation under catalyst- and oxidant-free conditions and triggers the radical addition, cyclization and iodination cascade reactions, giving various vinyl iodide containing sulfones in moderate to good yields.

Copper-Catalyzed Bromo-cyanomethylative Cyclization of Enynes

A copper-catalyzed bromo-cyanomethylative cyclization of 1,6-enynes is demonstrated. The treatment of 2-bromoacetonitrile with CuI enables the alkyl radical generation and triggers the radical addition/cyclization/bromination sequence, giving various vinyl C-Br bonds containing functionalized heterocycles in good yields.

Tryptanthrin and Its Derivatives in Drug Discovery: Synthetic Insights

Tryptanthrin is a golden-yellow, naturally occurring alkaloid that can be obtained from multiple sources and through different synthetic methodologies. This tetracyclic compound displays several relevant biological activities. The potential of this tetracyclic alkaloid has been widely explored, and several researchers have focused their attention on expanding the variety of tryptanthrin derivatives by using a range of synthetic strategies. In this short review, we aim to address recent developments in the synthesis of the tryptanthrin core, as well as the development of new strategies employed by synthetic organic chemists to obtain novel tryptanthrin derivatives with potential biological activity, using different tools from the chemists’ toolbox, such as photocatalysis, solvent-free approaches, and multicomponent reactions.

Copper(i)-catalyzed synthesis of natural alkaloid tryptanthrin and its derivatives

A facile one-pot synthesis of natural alkaloid tryptanthrin and its derivatives from isatins catalyzed by CuI in the presence of KHCO3 has been developed.

Three-Component Radical Iodonitrosylative Cyclization of 1,6-Enynes under Metal-Free Conditions

A three-component, metal-free radical cascade iodonitrosylative cyclization reaction was described. The nitroso radical was generated from tert-butyl nitrite and triggered the radical addition/cyclization/iodination/oxidation sequences. A variety of 1,6-enynes were tested and proved to be compatible, delivering various highly functionalized hetero- and all-carbon cycles and nitro and vinyl C-I bonds containing pyrrolidines, tetrahydrofuran, and cyclopentane in moderate to excellent isolated yields.

Visible-light-induced ligand to metal charge transfer excitation enabled phosphorylation of aryl halides

We herein described a visible light induced nickel(II)-catalyzed cross-coupling of secondary phosphine oxides with aryl halides. The Ni(I) species and chlorine atom radical Cl˙ were generated via the ligand to metal charge transfer (LMCT) process of the NiCl2(PPh3)2, which allows nickel(IV)-phosphorus species in situ formation, giving various tertiary phosphine oxides under photocatalyst-free conditions.

Stereo- and Regioselective cis-Hydrophosphorylation of 1,3-Enynes Enabled by the Visible-Light Irradiation of NiCl2(PPh3)2

Described herein is a stereo- and regioselective cis-hydrophosphorylation reaction of the internal alkyne of 1,3-enynes that accesses various 1,3-dienes in good isolated yields. The visible-light irradiation of NiCl₂(PPh₃)₂ allows the generation of highly reactive nickel(II)-phosphorus species that subsequently migrate into the internal alkyne of the 1,3-enynes and protonate the resulting vinyl nickel species, leading to various phosphinoyl 1,3-butadienes under mild reaction conditions.

Convenient Novel Method to Access N-Benzylated Isatoic Anhydride: Reaction Behavior of Isatoic Anhydride with 4-Chlorobenzyl Chloride in the Presence of Bases

Sodium hydride, potassium carbonate, and other bases are commonly used for N-alkylation of heterocyclic compounds. This report reveals the problems associated with N-benzylation of isatoic anhydride and identifies the plausible byproduct structures formed during the reaction. Subsequently, a novel breakthrough methodology has been developed using diisopropylamine and tetra butyl ammonium bromide. It gives excellent yields >88% in a short reaction time (2 h) at 30 °C with no byproducts, saving on processes as the pure product is directly obtained.

Visible-Light Mediated Diarylselenylative Cyclization of 1,6-Enynes

We herein described a selenylative cyclization reaction of enynes by the utilization of diselenides as radical sources. The visible-light irradiation of the reaction mixture enables the generation of the selenium atom radical to trigger the radical addition/cyclization/selenation sequences. Both terminal alkyne and internal alkyne derived 1,6-enynes were tested and suitable for the current synthetic protocol, delivering various kinds of selenium-containing cycles in good yields.

Recent advances in cyclization reactions of isatins or thioisatins via C–N or C–S bond cleavage

This review summarizes recent developments on cyclization reactions induced by the C–N or C–S bond cleavage of isatins or thioisatins in the last 5 years, which produce fused products instead of spiro compounds.

Divergent Synthesis of Methylisatoid and Tryptanthrin Derivatives by Ph3P-I2-Mediated Reaction of Isatins with and without Alcohols

A novel phosphonium-mediated reaction of isatins is described. In the presence of alcohol, the reaction proceeds to furnish C-12 modified tryptanthrin derivatives. Without alcohol, self-dimerization of isatins gives rise to tryptanthrin and its analogs. This divergent and step-economic approach provides a facile access to diverse indoloquinazoline structures including the natural alkaloids, methylisatoid and cephalanthrin B, in high yields from simple precursors under mild and metal-free reaction conditions.

Copper-Catalyzed Bromodifluoroacetylative Cyclization of Enynes

A copper-catalyzed bromodifluoroacetylative cyclization reaction is described. The treatment of bromodifluoroacete derivatives by CuI and B₂Pin₂ enables difluoroalkyl radical generation and triggers the radical addition/cyclization/bromination sequences. Bromodifluoroacetyl-derived ester, amide, and ketone were compatible and gave various vinyl C-Br bonds containing functionalized heterocycles in good yields.

Recent advances in photocatalytic manipulations of Rose Bengal in organic synthesis

This review highlights the recent advances in photocatalytic manipulations of Rose Bengal in organic synthesis.

Visible-Light-Induced Organic Photochemical Reactions through Energy-Transfer Pathways

Visible-light photocatalysis is a rapidly developing and powerful strategy to initiate organic transformations, as it closely adheres to the tenants of green and sustainable chemistry. Generally, most visible-light-induced photochemical reactions occur through single-electron transfer (SET) pathways. Recently, visible-light-induced energy-transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible-light-induced EnT reactions.

Direct construction of benzimidazo[l,2-c]quinazolin-6-ones via metal-free oxidative C–C bond cleavage

A highly regioselective protocol has been developed for the synthesis of benzimidazo[l,2-c]quinazolin-6-ones via C–C bond cleavage and triple C–N bond formation.

Photoredox-catalyzed direct aminoalkylation of isatins: diastereoselective access to 3-hydroxy-3-aminoalkylindolin-2-ones analogues

A practical protocol for the direct aminoalkylation of isatins with tetrahydroisoquinolines and other aminesviaa photoredox catalyzed radical–radical cross-coupling process is described.