Metal-free electrochemistry promoted radical cascade cyclization to access CF3-containing benzimidazo[2,1-a]isoquinolin-6(5H)-ones

Using CF3SO2Na as the CF3 radical source, an eco-friendly approach for electrochemistry-mediated radical cascade cyclization of N-methacryloyl-2-phenylbenzoimidazoles was described. This reaction features mild reaction conditions, readily available substrates, and moderate to good yields through the construction of two C-C bonds in one step.

Metal- and additive-free radical-triggered nitration/cyclization to construct indolo[2,1-α]isoquinoline and benzimidazo[2,1-a]isoquinolin-6(5H)-one derivatives using t-BuONO as nitro reagents

An efficient metal- and additive-free nitro radical-triggered addition/cyclization of 2-aryl-N-acryloyl indoles/2-arylbenzimidazoles for the synthesis of nitro-substituted indolo[2,1-α]isoquinoline and benzimidazo[2,1-a]isoquinolin-6(5H)-one derivatives has been developed. The commercially available and low-cost t-BuONO was used as a nitro reagent. Due to mild reaction conditions, a variety of functional groups could be tolerated to give the corresponding products in moderate to good yields. Moreover, this nitration process could be scaled-up and the nitro group could be readily converted into the amino group, which may find applications in synthetic and medicinal chemistry.

Visible-light-promoted radical cascade alkylation/cyclization: access to alkylated indolo/benzoimidazo[2,1-a]isoquinolin-6(5H)-ones

A new protocol is herein described for the direct generation of alkylated indolo/benzoimidazo[2,1-a]isoquinolin-6(5H)-one derivatives by using Hantzsch esters as alkylation radical precursors using a photoredox/K₂S₂O₈ system. This oxidative alkylation of active alkenes involves a radical cascade cyclization process and a sequence of Hantzsch ester single electron oxidation, C-C bond cleavage, alkylation, arylation and oxidative deprotonation.

Visible-Light-Induced Radical Cascade Reaction of 1-Allyl-2-ethynylbenzoimidazoles with Thiosulfonates to Assemble Thiosulfonylated Pyrrolo[1,2-a]benzimidazoles

A visible-light-induced radical domino reaction of 1-allyl-2-ethynylbenzoimidazoles with thiosulfonates was developed, which generated the thiosulfonylated pyrrolo[1,2-a]benzimidazoles in moderate to good yields. This reaction proceeded under transition-metal-free conditions with good functional group tolerance and high regioselectivity. The possible pathway involved thiosulfonates were activated through the energy transfer route promoted by photocatalysis.

The construction of benzimidazo[2,1-a]isoquinolin-6(5H)-ones from N-methacryloyl-2-phenylbenzoimidazoles through radical strategies

Benzimidazo[2,1-a]isoquinolin-6(5H)-one constitutes a structurally unique class of tetracyclic N-heterocycles that are found throughout a myriad of biologically active natural products, pharmaceutical compounds, and functional materials. Various synthetic routes for the preparation of benzimidazo[2,1-a]isoquinolin-6(5H)-ones have been reported. In particular, the use of N-methacryloyl-2-phenylbenzoimidazoles to construct benzimidazo[2,1-a]isoquinolin-6(5H)-ones through various radical strategies have attracted widespread attention due to the versatility and simple preparation of raw materials, as well as the step-economy and mild reaction conditions. Using representative examples, we highlight significant progress in the synthesis of benzimidazo[2,1-a]isoquinolin-6(5H)-ones, including the selection of the catalytic system, substrate scope, mechanistic understanding, and applications. The contents of this review focus on the development of C-, S-, P-, and Si-centered radical addition-intramolecular cyclization strategies.

The emergence of the C-H functionalization strategy in medicinal chemistry and drug discovery

Owing to the market competitiveness and urgent societal need, an optimum speed of drug discovery is an important criterion for successful implementation. Despite the rapid ascent of artificial intelligence and computational and bioanalytical techniques to accelerate drug discovery in big pharma, organic synthesis of privileged scaffolds predicted in silico for in vitro and in vivo studies is still considered as the rate-limiting step. C-H activation is the latest technology added into an organic chemist's toolbox for the rapid construction and late-stage modification of functional molecules to achieve the desired chemical and physical properties. Particularly, elimination of prefunctionalization steps, exceptional functional group tolerance, complexity-to-diversity oriented synthesis, and late-stage functionalization of privileged medicinal scaffolds expand the chemical space. It has immense potential for the rapid synthesis of a library of molecules, structural modification to achieve the required pharmacological properties such as absorption, distribution, metabolism, excretion, toxicology (ADMET) and attachment of chemical reporters for proteome profiling, metabolite synthesis, etc. for preclinical studies. Although heterocycle synthesis, late-stage drug modification, ¹⁸F labelling, methylation, etc. via C-H functionalization have been reviewed from the synthetic standpoint, a general overview of these protocols from medicinal and drug discovery aspects has not been reviewed. In this feature article, we will discuss the recent trends of C-H activation methodologies such as synthesis of medicinal scaffolds through C-H activation/annulation cascade; C-H arylation for sp²-sp² and sp²-sp³ cross-coupling; C-H borylation/silylation to introduce a functional linchpin for further manipulation; C-H amination for N-heterocycles and hydrogen bond acceptors; C-H fluorination/fluoroalkylation to tune polarity and lipophilicity; C-H methylation: methyl magic in drug discovery; peptide modification and macrocyclization for therapeutics and biologics; fluorescent labelling and radiolabelling for bioimaging; bioconjugation for chemical biology studies; drug-metabolite synthesis for biodistribution and excretion studies; late-stage diversification of drug-molecules to increase efficacy and safety; cutting-edge DNA encoded library synthesis and improved synthesis of drug molecules via C-H activation in medicinal chemistry and drug discovery.

Visible-Light-Driven Sulfonylation/Cyclization to Access Sulfonylated Benzo[4,5]imidazo[2,1-a]isoquinolin-6(5H)-ones

Visible-light-driven sulfonylation/cyclization of N-methacryloyl-2-phenylbenzoimidazoles has been successfully developed. Using commercially available sulfonyl chloride as sulfonylation reagent, a wide range of sulfonylated benzo[4,5]imidazo[2,1-a]isoquinolin-6(5H)-ones with potential antitumor activity were provided in acceptable to excellent yields. This method has the advantages of mild reaction conditions and outstanding functional group tolerance, and provides a new strategy for the development of potential antitumor lead compounds.

Visible-Light-Induced Deaminative Alkylation/Cyclization of Alkyl Amines with N-Methacryloyl-2-phenylbenzoimidazoles in Continuous-Flow Organo-Photocatalysis

Herein, we present a metal-free visible-light-induced eosin-y-catalyzed deaminative strategy for the sequential alkylation/cyclization of N-methacryloyl-2-phenylbenzoimidazoles with alkyl amine-derived Katritzky salts, which provides an efficient avenue for the construction of various benzo[4,5]imidazo[2,1-a]isoquinolin-6(5H)-one derivatives in moderate to excellent yields under mild reaction conditions. The key enabling feature of this novel reaction includes utilization of redox-active pyridinium salts from abundant and inexpensive primary amine feedstocks that were converted into alkyl radicals via C-N bond scission and subsequent alkylation/cyclization with N-methacryloyl-2-phenylbenzoimidazoles by the formation of two new C-C bonds. In addition, we implemented this protocol for a variety of amino acids, affording the products in moderate yields. Moreover, the novel, environmentally benign batch protocol was further carried out in a continuous-flow regime by utilizing a perfluoroalkoxy alkane tubing microreactor under optimized reaction conditions with a blue light-emitting diode light source, enabling excellent yields and a shorter reaction time (19 min) versus the long reaction time (16 h) of the batch reaction. The reaction displays excellent functional group tolerance, easy operation, scalability, mild reaction conditions, and broad synthetic utility.

Chemoselective Installation of Diverse Succinimides on Fused Benzimidazoles via Rhodium-Catalyzed C-H Activation/Annulation: Chemosensor for Heavy Metals

A novel Rh-catalyzed cascade C-H activation/annulation of 2-arylbenzimidazoles with maleimides is reported. Rapid chemoselective access to two structurally distinct succinimide-bearing benzoimidazoisoquinolinones is achieved, depending on the acidic and basic conditions. This atom- and step-economic strategy features a wide substrate scope, excellent functional group tolerance, and site-specific functionalization. Application of the methodology yields a novel benzimidazole-based probe as a fluorescent chemosensor for the nanomolar detection of Hg²⁺, Cu²⁺, and Fe³⁺ ions.

Peroxide-mediated synthesis of benzimidazo[2,1-a]isoquinoline-6(5H)-ones via cascade methylation/ethylation and intramolecular cyclization

A metal-free oxidative radical methylation/arylation of 2-arylbenzoimidazoles with DTBP as the oxidant and methyl radical source was developed. The reaction proceeds through a sequential methyl radical addition/cyclization pathway and affords a series of methyl functionalized benzimidazo[2,1-a]isoquinoline-6(5H)-ones in moderate to good yields. Besides, the ethylation/arylation of 2-arylbenzoimidazoles was also achieved with DTAP.

Copper-catalyzed radical cascade cyclization for synthesis of CF3-containing tetracyclic benzimidazo[2,1-a]iso-quinolin-6(5H)-ones

Here, a general copper-catalyzed radical cascade carbocyclization reaction with 2-arylbenzoimidazoles and a Togni reagent was realized. Structurally diverse CF3-containing tetracyclic core benzimidazo[2,1-a]isoquinoline-6(5H)-ones were obtained in moderate to good yields. The wide substrate scope, good functional group tolerance, and ease of scale-up of this method are expected to promote its potential applications in pharmacy and biotechnology.

Visible-light-driven cascade radical cyclization toward the synthesis of α-carbonyl alkyl-substituted benzimidazo[2,1-a]isoquinolin-6(5H)-one derivatives

A visible-light-driven cascade radical cyclization process of N-methacryloyl-2-phenylbenzimidazole has been established with α-carbonyl alkyl bromide. This protocol provides an efficient and practical method for the synthesis of various α-carbonyl alkyl-substituted benzimidazo[2,1-α]isoquinolin-6(5H)-ones in outstanding yields, mild reaction conditions and excellent functional group tolerance.

A visible-light-driven radical cyclization method has been developed for synthesis of α-carbonyl alkyl-substituted benzimidazo[2,1-a]isoquinolin-6(5H)-ones. This reaction is characterized by excellent yields and good functional group tolerance.

Lanthanide-Catalyzed Tandem Insertion of Secondary Amines with 2-Alkynylbenzonitriles: Synthesis of Aminoisoindoles

A lanthanide-catalyzed intermolecular hydroamination of 2-alkynylbenzonitriles with secondary amines has been disclosed, providing a streamlined access to a range of aminoisoindoles in moderate to excellent yields. The salient features of this reaction include high bond-formation efficiency, mild reaction conditions, 100 % atomic efficiency and good functional group tolerance. This methodology has also been successfully applied to the construction of other nitrogen-containing compounds, such as 5 H-imidazo[2,1-a]isoindoles and isoquinolines. A plausible mechanism for the formation of aminoisoindoles involving initial N-H activation by a lanthanide complex followed by C≡N insertion into a Ln-N bond to form an amidinate lanthanide intermediate, which undergoes the cyclization is proposed.

Rh(iii)-catalyzed sequential C–H activation and annulation: access to N-fused heterocycles from arylazoles and α-diazocarbonyl compounds

Valuable N-fused heterocycles were successfully obtained via Rh(iii)-catalyzed sequential C–H activation and annulation.

Synthesis of cycloalkyl substituted purine nucleosides via a metal-free radical route

An efficient route to synthesize cycloalkyl substituted purine nucleosides was developed. This metal-free C-H activation was accomplished by a tBuOOtBu initiated radical reaction. By adjusting the amount of tBuOOtBu and reaction time, the selective synthesis of C6-monocycloalkyl or C6,C8-dicycloalkyl substituted purine nucleosides could be realized. Furthermore, uracil and related nucleosides were also suitable substrates, giving the C5-cyclohexyl substituted uracil derivatives in good yields with excellent regioselectivities.

Redox condensation of o-halonitrobenzene with 1,2,3,4-tetrahydroisoquinoline: involvement of an unexpected auto-catalyzed redox cascade

A practical synthesis of fused benzimidazoles 5 has been developed by simply heating o-halonitrobenzenes 1 with tetrahydroisoquinolines 2. In this transformation, 2 played multiple roles as a building block, base and a double hydride donor in a cascade of uncatalyzed aromatic substitution, reduction of the nitro group, oxidation of the α-methylene group and condensation.

Palladium-catalyzed regioselective C-2 arylation of 7-azaindoles, indoles, and pyrroles with arenes

A palladium-catalyzed regioselective C-2 arylation of 7-azaindoles, indoles, and pyrroles with arenes has been developed, which features substrate dependent acid concentration.

Modification of purine and pyrimidine nucleosides by direct C-H bond activation

Transition metal-catalyzed modifications of the activated heterocyclic bases of nucleosides as well as DNA or RNA fragments employing traditional cross-coupling methods have been well-established in nucleic acid chemistry. This review covers advances in the area of cross-coupling reactions in which nucleosides are functionalized via direct activation of the C8-H bond in purine and the C5-H or C6-H bond in uracil bases. The review focuses on Pd/Cu-catalyzed couplings between unactivated nucleoside bases with aryl halides. It also discusses cross-dehydrogenative arylations and alkenylations as well as other reactions used for modification of nucleoside bases that avoid the use of organometallic precursors and involve direct C-H bond activation in at least one substrate. The scope and efficiency of these coupling reactions along with some mechanistic considerations are discussed.

Sequential decarboxylative azide-alkyne cycloaddition and dehydrogenative coupling reactions: one-pot synthesis of polycyclic fused triazoles

Herein, we describe a one-pot protocol for the synthesis of a novel series of polycyclic triazole derivatives. Transition metal-catalyzed decarboxylative CuAAC and dehydrogenative cross coupling reactions are combined in a single flask and achieved good yields of the respective triazoles (up to 97% yield). This methodology is more convenient to produce the complex polycyclic molecules in a simple way.

Catalytic C–H bond functionalisation of purine and pyrimidine nucleosides: a synthetic and mechanistic perspective

C–H bond functionalisation of heteroarenes, especially nucleosides, has received a lot of attention in the past few years. This review describes the state-of the art in this area with a global aspiration for possibly functionalising purine and pyrimidine moieties in complex biomolecular systems.

Pd-catalyzed oxidative C–H alkenylation for synthesizing arylvinyltriazole nucleosides

Pd-catalyzed oxidative C–H alkenylation: an effective method for synthesizing arylvinyltriazole nucleosides in good yields and with large functional group tolerance.

Metal-free TEMPO-promoted C(sp³)-H amination to afford multisubstituted benzimidazoles

An efficient TEMPO-air/cat. TEMPO-O2 oxidative protocol was developed to synthesize multisubstituted or fused tetracyclic benzimidazoles via a metal-free oxidative C-N coupling between the sp(3) C-H and free N-H of readily available N(1)-benzyl/alkyl-1,2-phenylenediamines.

Intramolecular arylation of benzimidazoles via Pd(II)/Cu(I) catalyzed cross-dehydrogenative coupling

Electron poor benzimidazole substrates were arylated via an intramolecular cross-dehydrogenative coupling (CDC) reaction. These CDC reactions were catalyzed by a Pd(II)/Cu(I) catalyst system, capable of producing moderate yields on a large library of substrates. The substrate scope consisted of tethered arene-benzimidazoles that upon coupling, produced a fused polycyclic motif.