Rh(III)-Catalyzed C–C/C–N Coupling of Imidates with α-Diazo Imidamide: Synthesis of Isoquinoline-Fused Indoles

Novel access to α-carbolines with biological applications

In recent years, the 9H-pyrido[2,3-b]indole nuclei, also named α-carboline which is found in many organic compounds such as natural products, pharmaceuticals, and materials, have intensively stimulated the research of new synthetic pathways. After a brief report published in 2015 describing novel accesses and biological applications of α-carbolines, this update reports between 2015 and 2023 on the emergence of original syntheses to this heterocyclic nucleus. Examples representing these processes are described and the biological activities of α-carbolines are mentioned when they have been prepared for therapeutic purposes.

Tunable Rh(III)-Catalyzed C(sp2)-H Bond Functionalization of Aryl Imidates with Cyclic 1,3-Diones: Strategic Use of Directing Groups

A tunable Rh(III)-catalyzed C(sp2)-H bond functionalization of aryl imidates with cyclic 1,3-diones was developed. With suitable and straightforward reaction condition adjustments, the C-H bond functionalization of diverse aryl imidates with cyclic 1,3-diones occurred smoothly and precisely at room temperature. Accompanied by different directing group transformations, a series of corresponding aryl nitriles, hydrophenanthridin-1(2H)-ones, spiro isoindoles, or hydrophenanthridine-1,6(2H,5H)-diones were synthesized in good yields to provide a rational directing group utilization strategy for the Rh(III)-catalyzed C(sp2)-H bond activation. Control experiments and primary mechanistic studies revealed that solvent effects and functional group electronic effects might influence the reaction's selectivity.

Synthesis of 1H-Isoindoles via Ruthenium(II)-Catalyzed Cyclization of Benzimidates with Alkenes

A Ru(II)-catalyzed efficient synthesis of 1H-isoindoles via the cyclization of benzimidates with alkenes has been demonstrated. This methodology exhibits high compatibility with various functionalized activated and unactivated olefins containing different sensitive functional groups. This protocol provides an effective method for synthesizing various 1H-isoindole derivatives in decent to excellent yields. Notably, the ortho-alkenylation of benzimidates with unactivated alkenes was achieved. A potential reaction mechanism has been suggested that involves C-H activation, 1,2 insertion, and β-hydride elimination subsequent to aza-Michael addition.

N-Heterocyclic Carbene-Catalyzed Atroposelective Synthesis of Axially Chiral α-Carbolinones via Heterocycle Formation

An NHC-catalyzed atroposelective synthesis of axially chiral α-carbolinones from α,β-unsaturated iminoindole derivatives and α-chloroaldehydes was developed. The reaction proceeds through a cascade process including [4 + 2] annulation and then oxidative dehydrogenation with concomitant central-to-axial chirality conversion under mild conditions. The developed method opens a new avenue to efficiently access axially chiral α-carbolinones in moderate to good enantioselectivities.

Rh(III)-Catalyzed Spiroannulation Reaction of N-Aryl Nitrones with 2-Diazo-1,3-indandiones: Synthesis of Spirocyclic Indole-N-oxides and Their 1,3-Dipolar Cycloaddition with Maleimides

An efficient strategy for the preparation of spirocyclic indole-N-oxide compounds through a Rh(III)-catalyzed [4 + 1] spiroannulation reaction of N-aryl nitrones with 2-diazo-1,3-indandiones as C1 synthons under extremely mild conditions is presented. From this reaction, 40 spirocyclic indole-N-oxides were easily obtained in up to 98% yield. In addition, the title compounds could be successfully used for the construction of structurally intriguing maleimide-containing fused polycyclic scaffolds via a diastereoselective 1,3-dipolar cycloaddition reaction with maleimides.

Ru(II)-Catalyzed C-H Alkenylation of Benzimidates with Unactivated Olefins: A Route to ortho-Alkenylated Benzonitriles

A Ru(II)-catalyzed C-H alkenylation of benzimidates with unactivated alkenes providing ortho-alkenylated benzonitriles in good to excellent yields in a highly regio- and stereoselective manner is described. In the reaction, an imidate group converted into a nitrile under the reaction conditions. The alkenylation reaction was compatible with various substituted benzimidates as well as functionalized unactivated olefins, including ibuprofen-, neproxen-, coumarin-, and cholesterol-substituted alkenes. A feasible reaction mechanism was proposed to account for the present alkenylation reaction.

Palladium-catalyzed cascade cyclization of allenamide with 2-iodoanilines to access functionalized indoloquinolines

A novel and efficient palladium-catalyzed cascade cyclization to indoloquinoline derivatives in one pot has been developed by using allenamide derivatives and 2-iodoanilines as the key building blocks. The process involved two cyclizations: intramolecular cyclization/π-allylic substitution and intramolecular 6-endo Heck cyclization. Furthermore, dihydrobenzofuro[2,3-b]quinoline derivatives could also be achieved via this strategy using allenyl ethers instead of allenamides. The readily available substrates, mild conditions, high efficiency and step economy make this strategy a promising method in the synthesis of polycyclic motifs.

Synthesis of Isoquinolone, 1,2-Benzothiazine, and Naphtho[1',2':4,5]imidazo[1,2-a]pyridine Derivatives via Rhodium(III)-Catalyzed (4 + 2) Annulation

In this study, we report a novel and efficient synthetic method to construct isoquinolone scaffold via the Rh(III)-catalyzed (4 + 2) annulation of benzamide with an unreported coupling reagent methyl 2-chloroacrylate. Accordingly, other valuable 1,2-benzothiazine and naphtho[1',2':4,5]imidazo[1,2-a]pyridine derivatives are also obtained through a similar synthetic protocol. Thus, our developed method is highlighted by high yield and reaction versatility.

Direct Synthesis of Biphenyl-2-carbonitriles by Rh(III)-Catalyzed C-H Hiyama Cross-Coupling in Water

This method represents an efficient rhodium(III)-catalyzed o-C-H arylation of readily available benzimidate derivatives with diverse arylsilanes in water as a sustainable solvent, enabling the straightforward synthesis of potentially useful biphenyl-2-carbonitrile derivatives. This silicon-based protocol employs benzimidates as both an efficacious directing group and the source of a nitrile group.

TfOH-Catalyzed Cascade C-H/N-H Chemo-/Regioselective Annulation of Indole-2-carboxamides with Benzoquinones for the Construction of Anticancer Tetracyclic Indolo[2,3-c]quinolinones

An efficient TfOH-catalyzed cascade C-H/N-H annulation of indole-2-carboxamides with benzoquinones has been developed for the synthesis of tetracyclic indolo[2,3-c]quinolinones. This reaction exhibits excellent chemo-/regioselectivity, achieving functionalization of the C-3 of indole and N-H of the amide moiety to form the new C-C and C-N bonds. Various expected products were synthesized from readily available starting materials in good to high yields with a wide substrate scope and good functional group tolerance. Among all synthetic products, 3d showed the most potent cytotoxicity toward the 4T1 cancer cell line with an IC₅₀ value of 0.62 ± 0.05 μM. In vivo study demonstrated that 3d remarkably suppressed 4T1 xenograft tumor growth without body weight loss.

Annulation-Triggered Denitrogenative Transformations of 2-(5-Iodo-1,2,3-triazolyl)benzoic Acids

The ability of [1,2,3]triazolobenzoxazinones to act as a source of "hidden" diazo group was discovered. These diazo precursors can be easily prepared by the intramolecular cyclization of 2-(5-iodo-1,2,3-triazolyl)benzoic acids. The Cu-catalyzed capture of the hidden diazo group allows for further functionalization through the denitrogenative pathway. The transformations proceed via the formation of either diazoimine or diazoamide intermediates. Novel routes to various anthranilamides as well as thiolated benzoxazinones were developed using the one-pot cyclization/diazo capture procedure.

Transition-Metal-Catalyzed C-H Bond Functionalization of Arenes/Heteroarenes via Tandem C-H Activation and Subsequent Carbene Migratory Insertion Strategy

The past decade has witnessed tremendous developments in transition-metal-catalyzed C-H bond activation and subsequent carbene migratory insertion reactions, thus assisting in the construction of diverse arene/heteroarene scaffolds. Various transition-metal catalysts serve this purpose and provide efficient pathways for an easy access to substituted heterocycles. A brief introduction to metal-carbenes has been provided along with key mechanistic pathways underlying the coupling reactions. The purpose of this review is to provide a concise knowledge about diverse directing group-assisted coupling of varied arenes/heteroarenes and acceptor-acceptor/donor-acceptor diazo compounds. The review also highlights the synthesis of various carbocycles and fused heterocycles through diazo insertion pathways, via C-C, C-N and C-O bond forming reactions. The mechanism usually involves a C-H activation process, followed by diazo insertion leading to subsequent coupling.

Cascade C–N bond cleavage of amides/intramolecular amination reactions: an atom economical way to α-cabolin-4-ones

A novel one-pot procedure for the synthesis of 2,3-disubstituted α-carbolin-4-ones through successive C–N bond cleavage and intramolecular amination reactions involving a 1,2-acyl migration has been developed.

A direct synthesis method towards spirocyclic indazole derivatives via Rh(iii)-catalyzed C–H activation and spiroannulation

A rhodium(iii)-catalyzed [4 + 1] spiroannulation of N-aryl phthalazine-diones (pyridazine-diones) with diazo compounds to construct spirocyclic indazole derivatives with diverse structures is described.

Recent advances in the synthesis of indole embedded heterocycles with 3-diazoindolin-2-imines

The preparations, reactivity and synthetic applications of 3-diazoindolin-2-imines, a valuable class of α-diazo amidines, are reviewed.

Sulfoximines-Assisted Rh(III)-Catalyzed C-H Activation and Intramolecular Annulation for the Synthesis of Fused Isochromeno-1,2-Benzothiazines Scaffolds under Room Temperature

A mild and facile Cp*Rh(III)-catalyzed C–H activation and intramolecular cascade annulation protocol has been proposed for the furnishing of highly fused isochromeno-1,2-benzothiazines scaffolds using S-phenylsulfoximides and 4-diazoisochroman-3-imine as substrates under room temperature. This method features diverse substituents and functional groups tolerance and relatively mild reaction conditions with moderate to excellent yields. Additionally, retentive configuration of sulfoximides in the conversion has been verified.

Rh-Catalyzed C-H activation/intramolecular condensation for the construction of benzo[f]pyrazolo[1,5-a][1,3]diazepines

A novel and mild Rh(iii)-catalyzed C-H activation/intramolecular condensation of 1-aryl-1H-pyrazol-5-amines with cyclic 2-diazo-1,3-diketones was developed, giving access to various important benzo[f]pyrazolo[1,5-a][1,3]diazepine scaffolds through sequential C-C/C-N bond formation in a one-pot procedure under additive- and oxidant-free conditions. Furthermore, 3-([1,1'-biphenyl]-2-ylamino)-2-ethoxycyclohex-2-enones can be obtained in good yields by constructing C-O and C-N bonds through 1,1'-insertion, dehydration, and isomerization processes.

Synthesis of 8-Alkoxy-5H-isochromeno[3,4-c]isoquinolines and 1-Alkoxy-4-arylisoquinolin-3-ols through Rh(III)-Catalyzed C-H Functionalization of Benzimidates with 4-Diazoisochroman-3-imines and 4-Diazoisoquinolin-3-ones

Rh(III)-catalyzed C-H activation/annulation of benzimidates with 4-diazoisochroman-3-imines furnished 8-alkoxy-5H-isochromeno[3,4-c]isoquinolines in moderate to excellent yields with a broad range of substrate scope. The reaction was carried out under mild reaction conditions and could be scaled up with practical usage. Similar reaction between benzimidates and 4-diazoisoquinolin-3-ones provided 1-alkoxy-4-arylisoquinolin-3-ols in excellent yields. Moreover, the synthesized products could be conveniently transformed to the corresponding heterocycles with a 1,8-naphthyridinone or isochromenopyridinone core, which are privileged structures in medicinal chemistry.

Synthesis of rhodium(iii)-catalyzed isoquinoline derivatives from allyl carbonates and benzimidates with hydrogen evolution

A novel Rh(iii)-catalyzed cascade C-H activation/cyclization approach to access isoquinoline derivatives from benzimidates and available allyl carbonates with the liberation of H2 has been realized. Allyl carbonates were first used as a versatile and universal C2 synthon to synthesize this biological activity skeleton via an efficient and practical process just within 1 h.

Rhodium-catalyzed C–H activation/annulation of amidines with 4-diazoisochroman-3-imines toward isochromeno[3,4-c]isoquinolines

Rhodium-catalyzed C–H activation/annulation of amidines with 4-diazoisochroman-3-imines toward 8-amino-5H-isochromeno[3,4-c]isoquinolines with good functional group tolerance.

Rhodium-catalyzed biheteroaryl-2-carbonitrile synthesis via double C–H activation

Rhodium(iii)-catalyzed double C–H activation and in situ dealcoholization to generate biheteroaryl-2-carbonitriles have been developed via a CDC mechanism, in which benzimidates act as both directing groups and the precursors of nitrile groups.

Imidates: an emerging synthon for N-heterocycles

This review highlights the recent application of imidates as building blocks for the synthesis of saturated and un-saturated N-heterocycles via C–N annulation reactions under acid/base/metal-catalyzed/radical-mediated reaction conditions.

Preparation of spiro[imidazolidine-4,3′-indolin]-2′-imines via copper(i)-catalyzed formal [2 + 2 + 1] cycloaddition of 3-diazoindolin-2-imines and triazines

We report a facile and efficient synthesis of spiro[imidazolidine-4,3′-indolin]-2′-imines via a copper(i)-catalyzed cascade reaction of 3-diazoindolin-2-imines with 1,3,5-triazines.

Controllable Rh(III)-Catalyzed C-H Arylation and Dealcoholization: Access to Biphenyl-2-carbonitriles and Biphenyl-2-carbimidates

A controllable Rh(III)-catalyzed C-H arylation and dealcoholization of benzimidates with arylboronic esters was developed, delivering various biphenyl-2-carbonitriles and biphenyl-2-carbimidates by simply tuning the reaction conditions. This approach features high efficiency, good functional group tolerance, and easy operation. It also provides an alternative pathway to thoroughly exploit the directing group in transition-metal-catalyzed C-H activations.

Rh(III)-Catalyzed C-C Coupling of Diverse Arenes and 4-Acyl-1-sulfonyltriazoles via C-H Activation

4-Acyl-1-sulfonyltriazoles act as versatile carbene reagents in Cp*Rh(III)-catalyzed ortho-selective coupling with arenes via C-H activation. The coupling led to olefination with possible cyclization, depending on the nature of the arene.