Rhodium enalcarbenoids: direct synthesis of indoles by rhodium(II)-catalyzed [4+2] benzannulation of pyrroles

Enal-azomethine ylides: application in the synthesis of functionalized pyrroles

Rhodium-catalyzed [3 + 2] annulation of diazoenals and N-alkyl imines resulted in N-alkyl-pyrrole-3-carbaldehyde derivatives. The reaction involves thermal 6π-electrocyclization and aromatization of a new class of enal-azomethine ylides (EAYs). The EAYs derived from dihydroisoquinoline and 2H-azirine gave fused-pyrrole and pyridine derivatives, respectively. The synthetic importance of pyrrole products has been demonstrated by one-step synthesis of the biologically relevant pyrrolo[3,2-c]quinoline scaffold as well as pyrrolo[2,1-a]isoquinoline which is a core structure of lamellarin alkaloids.

Triple Nucleophilic Head-to-Tail Cascade Polycyclization of Diazodienals via Combination Catalysis: Direct Access to Cyclopentane Fused Aza-Polycycles with Six-Contiguous Stereocenters

Reported herein are the bench stable (2E,4E)-diazohexa-2,4-dienals (diazodienals) and their unprecedented polycyclization with aldimine and arylamines enabled by Rh(II)/Brønsted acid relay catalysis. This scalable and atom-economical reaction provides direct access to the biologically important azatricyclo[6.2.1.04,11]undecane fused polycycles having six-contiguous stereocenters. Mechanistic studies revealed that polycyclization proceeds through an unusual triple-nucleophilic cascade initiated by aldimine attack on remote Rh-carbenoid, 6π-electrocyclization of aza-trienyl azomethine ylide, stereoselective aza-Michael addition via iminium activation, and inverse electron-demand intramolecular aza Diels-Alder reaction. The π-π secondary interactions play a crucial role in the preorganization of reactive intermediates for the pericyclic reactions and, hence, the overall efficiency of the polycyclization.

Efficient Synthesis of Cyclohepta[b]indoles and Cyclohepta[b]indole-Indoline Conjugates via RCM, Hydrogenation, and Acid-Catalyzed Ring Expansion: A Biomimetic Approach

Cyclohepta[b]indoles, prevalent in natural products and pharmaceuticals, are conventionally accessed via metal or Lewis acid-mediated cycloadditions with prefunctionalized substrates. Our study introduces an innovative sequential catalytic assembly for synthesizing cyclohepta[b]indoles from readily available isatin derivatives. The process involves three catalytic sequences: ring-closing metathesis, catalytic hydrogenation, and acid-catalyzed ring expansion. The RCM of 2,2-dialkene-3-oxindoles, formed by butenyl Grignard addition to 3-allyl-3-hydroxy-2-oxindoles, yields versatile spirocyclohexene-3-oxindole derivatives. These derivatives undergo further transformations, including dibromination, dihydroxylation, epoxidation, Wacker oxidation at the double bond. Hydrogenation of spirocyclohexene-3-oxindole yields spirocyclohexane-3-oxindoles. Their subsequent acid-catalyzed ring expansion/aromatization, dependent on the acid catalyst, results in either cyclohepta[b]indoles or cyclohepta[b]indole-indoline conjugates, adding a unique synthetic dimension. The utility of this methodology is exemplified through the synthesis of an A-FABP inhibitor, showcasing its potential in pharmaceutical applications.

[3+3] Annulation of Diazoenals and α-Mercapto Ketones via Protic Sulfonium Ylides: Direct Synthesis of 2H-Thiopyrans, Innovative Progenitors for Unstudied 2H-Thiopyran-2-ones and 4H-Thiopyran-4-ones

Herein, we report a new Rh(II)/Sc(III)-catalyzed [3+3] annulation between diazoenals and α-mercapto ketones for the direct synthesis of 4-formyl-2H-thiopyrans. The reaction proceeds via protic sulfonium ylides derived from highly electrophilic Rh-enalcarbenoids, followed by regioselective intramolecular aldol condensation. Further studies revealed that 4-formyl-2H-thiopyrans are novel precursors for unstudied 2H-thiopyran-2-ones and 4H-thiopyran-4-ones. The 4H-thiopyran-4-ones were obtained via a novel O2/Et3N-mediated oxidative deformylation. This methodology was applied to the short synthesis of structurally complex pyrimidine-fused 2H-thiopyran via cascade Schmidt, Ritter, and intramolecular cyclization reactions.

Rh-Catalyzed Chemodivergent [3+3] Annulations of Diazoenals and α-Aminoketones: Direct Synthesis of Functionalized 1,2-Dihydropyridines and Fused 1,4-Oxazines

Novel rhodium-catalyzed [3+3] annulations of diazoenals and α-amino ketones has been disclosed here. The reactivity of diazoenals has been switched from carbenoid to vinylogous NH-insertion by altering acyclic to cyclic α-amino ketones. In this direction, we report an efficient strategy to synthesize 1,2-dihydropyridines (DHPs) and fused 1,4-oxazines. Mechanistic investigation revealed that the formyl group is necessary for carbenoid [3+3] annulation and the cyclohexyl group is the dictating factor for vinylogous NH- insertion. The synthetic utility of 1,2-dihydropyridines was demonstrated by synthesizing piperidine, pyrido[1,2-a]indole, and 2-pyridone scaffolds. Further, structural diversification of fused 1,4-oxazines resulted in the short synthesis of hexahydroquinolin-2(1H)-ones, hexahydro quinolines and tetrahydroquinolinones via ring opening rearrangement and a new oxidative deformylation, respectively.

Iodonium ylides: an emerging and alternative carbene precursor for C-H functionalizations

The metal-catalyzed successive activation and functionalization of arene/heteroarene is one of the most fundamental transformations in organic synthesis and leads to privileged scaffolds in natural products, pharmaceuticals, agrochemicals, and fine chemicals. Particularly, transition-metal-catalyzed C-H functionalization of arenes with carbene precursors via metal carbene migratory insertion has been well studied. As a result, diverse carbene precursors have been evaluated, such as diazo compounds, sulfoxonium ylides, triazoles, etc. In addition, there have been significant developments with the use of iodonium ylides as carbene precursors in recent years, and these reactions proceed with high efficiencies and selectivities. This review provides a comprehensive overview of iodonium ylides in C-H functionalizations, including the scope, limitations, and their potential synthetic applications.

Rhodium-Catalyzed Enal Transfer with N-Methoxypyridazinium Salts

Herein, we report a simple method for functionalized enals involving enal-transfer reaction of water-soluble N-methoxypyridazinium salts. This open-flask reaction proceeds under mild aqueous basic conditions through [2,3]-sigmatropic rearrangement of propargyl/allyl sulfur-ylides derived from in situ-generated Rh-(E)-enalcarbene. Various synthetically challenging allene- and allyl-functionalized (E)-enals with a γ-C(sp³) quaternary center were obtained in good to high yields. InCl₃-catalyzed cascade cyclization of allenyl-enal and aniline gave a valuable pyrrolo[1,2-a]quinoline motif.

Synthesis of functionalized indoles via cascade benzannulation strategies: a decade's overview

Indoles are one of the most prominent aromatic heterocycles in the organic chemistry field. Due to their widespread presence in various natural products, alkaloids, drugs, approved medicines, etc. the synthesis and functionalization of indoles are of great interest. This review emphasizes recent developments and techniques in the domino cascade cyclization process in the last decade starting from the various building blocks. In particular, this review depicts several intriguing benzannulation methods of creating a benzene ring on a pre-existing pyrrole nucleus in an inter/intramolecular fashion under metal-catalyzed/metal-free approaches. Different subsections focus on gradual timely developments in this complementary area and a detailed analysis of the mechanisms and reactivity patterns. As a complementary method, this review gives a significant incentive to various annulation strategies and also gives an overview of the remaining challenges and upcoming possibilities.

Synthesis of C7-Functionalized Indoles through an Aromaticity Destruction-Reconstruction Process

A process for the synthesis of C7-functionalized indoles using para-substituted 2-alkynylanilines as starting materials was reported. The process involves a dearomatization, an 1,2-addition by organic lithium or Grignard reagents, an aromatization-driven allylic rearrangement, and a cyclization. A variety of groups including alkyl, aryl, alkenyl, or alkynyl groups were selectively installed at the C7 site of indoles leading to the formation of 2,5,7-trisubstituted indoles.

New Histamine-Related Five-Membered N-Heterocycle Derivatives as Carbonic Anhydrase I Activators

A series of histamine (HST)-related compounds were synthesized and tested for their activating properties on five physiologically relevant human Carbonic Anhydrase (hCA) isoforms (I, II, Va, VII and XIII). The imidazole ring of HST was replaced with different 5-membered heterocycles and the length of the aliphatic chain was varied. For the most interesting compounds some modifications on the terminal amino group were also performed. The most sensitive isoform to activation was hCA I (K_A values in the low micromolar range), but surprisingly none of the new compounds displayed activity on hCA II. Some derivatives (1, 3a and 22) displayed an interesting selectivity for activating hCA I over hCA II, Va, VII and XIII.

Gold catalyzed efficient preparation of dihydrobenzofuran from 1,3-enyne and phenol

A gold catalyzed formal intermolecular [2+3] cyclo-coupling of 1,3-enynes with phenols was developed to prepare dihydrobenzofuran derivatives with the addition of 2,6-dichloropyridine N-oxide, in which, a highly ortho-selective phenol S_EAr functionalization was achieved by using 1,3-enynes as α-oxo vinyl gold carbenoid surrogates.

Annulative π-Extension (APEX) of Indoles to Pyrido[1,2-a]indoles Using 4-Oxo Peroxides as C4 Units

Annulative π-extension (APEX) of 3-substituted indoles to pyrido[1,2-a]indoles is developed by using 4-oxo peroxides as π-extending reagents, which are employed as versatile C4 building blocks. This transformation is initiated by Brønsted acid-mediated Hock rearrangement of the peroxyl group. Notably, the pyrido[1,2-a]indole products are obtained by elimination of the indole moiety from the corresponding dihydropyrido[1,2-a]indoles, which could be selectively formed at room temperature.

A one-pot "back-to-front" approach for the synthesis of benzene ring substituted indoles using allylboronic acids

Synthesis of only benzene ring functionalized indoles and poly-substituted carbazoles is reported via a one-pot triple cascade benzannulation protocol. Usage of differently substituted and readily accessible allylboronic acids as a 3-carbon annulating partner enables diverse aliphatic and aromatic substitution patterns, which is still a daunting task. This scalable synthetic protocol tolerates broad scope, thus enabling further downstream modifications. As an application, carbazole based natural products glycozoline and glycozolinol were synthesized.

Aromaticity-Driven Access to Cycloalkyl-Fused Naphthalenes

We report the efficient synthesis of cycloalkyl-fused naphthalenes through the [4 + 2]-cycloaddtion/decarboxylative aromatization of alkyne-tethered aryne insertion adducts. These scaffolds were difficult to synthesize using conventional reactions. The reaction proceeds via the formation of a benzopyrylium intermediate followed by intramolecular [4 + 2] cycloaddition and a subsequent decarboxylation pathway. This method is also compatible with allene-tethered substrates to afford similar products. In addition, the one-pot synthesis of polysubstituted naphthalenes via aryne insertion/benzannulation has also been developed in good yield.

Straightforward Construction and Functionalizations of Nitrogen-Containing Heterocycles Through Migratory Insertion of Metal-Carbenes/Nitrenes

Nitrogen-containing heterocycles are widely found in various biologically active substrates, pharmaceuticals, natural products and organic materials. Consequently, the continuous effort has been devoted towards the development of straightforward, economical, environmentally acceptable, efficient and ingenious methods for the synthesis of various N-containing heterocycles and their functionalizations. Arguably, one of the most prominent direct strategy is regioselective C-H bond functionalizations which provide the step and atom economical approaches in the presence of suitable coupling partners. In this context, site-selective migratory insertion of metal carbenes/nitrenes to the desired C-H bonds has proven as a useful tool to access various functionalized nitrogen heterocycles. In this personal account, we highlight some of our contemporary development toward constructing N-containing heterocycles and their direct functionalizations via transition metal catalysed C-H bond functionalizations based on migratory insertion of metal-carbenes and nitrenes.

Metal-Free [3+2] Annulation of Ynamides with Anthranils to Construct 2-Aminoindoles

A novel metal-free [3+2] annulation of ynamides with anthranils provides a facile, flexible, environmentally friendly, and atom-economical route to 2-aminoindoles. This synthetic process proceeds with efficiency, excellent regioselectivity, and wide functional group tolerance under mild conditions. Moreover, the obtained 2-aminoindole products represent a multifunctional platform for the construction of various 2-aminoindolyl frameworks.

Facile synthesis of 1,4-oxazines by ruthenium-catalyzed tandem N-H insertion/cyclization of α-arylamino ketones and diazo pyruvates

Herein, we report an efficient strategy for the rapid construction of 1,4-oxazines starting from simple α-amino ketones and diazo pyruvates under mild reaction conditions. This transformation is efficiently catalyzed by RuCl3 through a tandem N-H insertion/cyclization sequence via an enol formation. This reaction shows broad functional group tolerance, and the resulting 1,4-oxazine products show promising anticancer activities toward HCT116.

Gold(I)-catalyzed intramolecular cyclization/intermolecular cycloaddition cascade as a fast track to polycarbocycles and mechanistic insights

Metal carbene is an active synthetic intermediate, which has shown versatile applications in synthetic chemistry. Although a variety of catalytic methods have been disclosed for the generation of carbene species from different precursors, there is an increasing demand for the development of efficient and practical approaches for the in-situ formation of metal carbene intermediates with structural diversity and unrevealed reactivity. Herein we report a gold-catalyzed cascade protocol for the assembly of polycarbocyclic frameworks in high yields under mild reaction conditions. Mechanistic studies indicate that the unique β-aryl gold-carbene species, generated via gold-promoted 6-endo-dig diazo-yne cyclization, is the key intermediate in this reaction, followed by a [4 + 2]-cycloaddition with external alkenes. In comparison to the well-documented metal carbene cycloadditions, this carbene intermediate serves as a 4-C synthon in a cycloaddition reaction. A variety of elusive π-conjugated polycyclic hydrocarbons (CPHs) with multiple substituents are readily accessible from the initially generated products by a mild oxidation procedure.

Sequential Annulations to Interesting Novel Pyrrolo[3,2-c]carbazoles

Herein we report a significant, valuable extension of a recently implemented pyrrole benzannulation methodology that, employing versatile nitrodienes from our lab as useful C₄ building blocks, led to indole derivatives characterized by unusual patterns of substitution. The 6-nitro-7-arylindoles resulting from suitably derivatized, non-symmetric dienes are of foreseeable synthetic interest in search for new polyheterocyclic systems. As an example, pyrrolocarbazoles with a rarely reported ring fusion were synthesized with the classical Cadogan protocol. Furthermore, the proven easy reducibility of the nitro group to amine will surely open the way to further interesting elaborations.

RhCl3·3H2O-Catalyzed C7-Selective C-H Carbonylation of Indolines with CO and Alcohols

An attractive method for the synthesis of indoline-7-carboxylates through RhCl₃-catalyzed C-H carbonylation of indolines with CO and alcohols was developed. The copper-based oxidant and removable pyrimidyl directing group played important roles in achieving high-level yields of the title products. Based on control experiments, a possible catalytic cycle was proposed.

Photorearrangement of dihetarylethenes as a tool for the benzannulation of heterocycles

A general strategy for the preparative benzannulation of aromatic heterocycles via photocyclization of 1,2-dihetarylethenes was proposed for the first time. The strategy includes two steps, namely, modular assembly of dihetarylethenes from widely available 3-hetarylacetic acids and 2-bromo-1-hetarylethanones, and subsequent preparative photorearrangement (using a UV lamp at 365 nm as the light source). This approach is efficient for the annulation of a wide range of heterocycles and provides C-, N-, O- or S-substituents in the benzoheterocycles obtained. The photochemical step is a metal-, acid-, and oxidant-free reaction, which requires non-inert conditions, and can be easily monitored by NMR spectroscopy. Applicability of the proposed strategy was tested in the synthesis of a wide range of substituted carbazoles and benzo[b]thiophenes as well as on a gram-scale benzannulation of 3-indoleacetic acid. Our study disclosed how to overcome two notable obstacles to the successful photorearrangement of dihetarylethenes: undesired reactions associated with photogenerated singlet oxygen, and the instability of desired products. The first problem was successfully solved by the addition of DABCO, while development of an in situ alkylation protocol to trap unstable photoproducts allowed us to overcome the second issue.

Iron-Catalyzed Carbenoid-Transfer Reactions of Vinyl Sulfoxonium Ylides: An Experimental and Computational Study

A method for the generation of unprecedented vinyl carbenoids from sulfoxonium ylides has been developed and applied in the synthesis of a diverse array of heterocycles such as indolizines, pyrroles, 3-pyrrolin-2-ones, and furans. The reactions proceed by FeBr₂ catalysis under mild reaction conditions with a broad substrate scope. A reaction pathway involving iron carbenoids is proposed based on a series of control experiments and DFT calculations.

Annulative π-extension of indoles and pyrroles with diiodobiaryls by Pd catalysis: rapid synthesis of nitrogen-containing polycyclic aromatic compounds

A palladium-catalyzed one-step annulative π-extension (APEX) reaction of indoles and pyrroles that allows rapid access to nitrogen-containing polycyclic aromatic compounds is described. In the presence of palladium pivalate and silver carbonate, diverse indoles or pyrroles coupled with diiodobiaryls in a double direct C-H arylation manner to be transformed into the corresponding π-extended compounds in a single step. The newly developed catalytic system enables the use of various pyrroles and indoles as templates with a series of diiodobiaryls to provide structurally complicated and largely π-extended nitrogen-containing polycyclic aromatic compounds that are otherwise difficult to synthesize.

Selective synthesis of pyrrolo[1,2-a]azepines or 4,6-dicarbonyl indoles via tandem reactions of alkynones with pyrrole derivatives

Novel methodologies for the selective synthesis of pyrrolo[1,2-a]azepines or 4,6-dicarbonyl indoles starting from pyrrole derivatives and alkynones are described. When reactions were carried out with 1,2,4-trisubstituted N-propargyl pyrroles using a ZnI₂ catalyst, pyrrolo[1,2-a]azepines were obtained. Whereas 4,6-dicarbonyl indoles were produced selectively with 1,2-disubstituted pyrroles in the presence of silica gel. The reaction outcomes depend on the substituent pattern of the substrates and the nature of the catalysts chosen. Control reactions suggested that the formation of a conjugated enamine intermediate was crucial for both the processes. With easily accessible starting materials, inexpensive catalysts and an easy-to-handle procedure, this reaction has the potential to become a general protocol for the synthesis of pyrrolo[1,2-a]azepines or indoles.

Chemodivergent synthesis of multi-substituted/fused pyrroles via copper-catalyzed carbene cascade reaction of propargyl α-iminodiazoacetates

A novel cascade reaction of alkynyl-tethered α-iminodiazoacetates has been developed, which provides a general access to both multi-substituted and fused pyrroles in high yields with a broad substrate scope. The γ-imino carbene is proposed as the key intermediate in this divergent reaction and followed by unpresented transformations.

Facile synthesis of substituted diaryl sulfones via a [3 + 3] benzannulation strategy

A metal-free and efficient synthesis of highly substituted diaryl sulfones via a base-mediated [3 + 3] benzannulation strategy.

Rhodium-catalyzed cyclization of acceptor-substituted biphenyl α-diazoketones: a study of the substitution effect on chemoselectivity

A range of biphenyl α-diazoketones containing different α-electron withdrawing groups exhibits divergent chemoselectivity toward rhodium(ii) catalysis, delivering phenanthrols, benz[α]azulenones, aromatic ketones and/or 1,2-diketones in varying ratios.

Copper/amine-catalyzed formal regioselective [3 + 2] cycloaddition of an α,β-unsaturated O-acetyl oxime with enals

A novel copper/amine co-catalyzed formal regioselective [3 + 2] cycloaddition reaction of an O-acyl oxime with α,β-unsaturated aldehydes is developed.