Silver(I)-Catalyzed Dearomatization of Alkyne-Tethered Indoles: Divergent Synthesis of Spirocyclic Indolenines and Carbazoles

Cu(OTf)2/HFIP catalyzed regioselective cycloisomerization of indole-C3-functionalized alkynols to carbazoles

We report here a simple and atom economic cycloisomerization reaction of indole-tethered alkynols for constructing diverse carbazoles using Cu(OTf)2/HFIP as the excellent promoter system. The reaction proceeds through a one-pot, domino process of spiro cyclization and 1,2-migration followed by aromatization to deliver carbazoles.

A Sequential Transition Metal and Organocatalytic Approach to the Enantioselective Synthesis of C2-Spiroindoline Systems

We report herein an organocatalyzed enantioselective spirocyclization strategy to access valuable C2-spiroindoline scaffolds bearing a quaternary stereocenter via an aza-Michael addition reaction, wherein the acid additive plays the role of dual functionality. The substrates for this key step were put together by an exo-selective, Pd-catalyzed γ-arylation of silyldienol ethers of the corresponding cyclohexenones. A close alliance between a low catalyst loading and a slow reaction rate yields C2-spiroindolines with good enantioselectivity.

Iodine-Catalyzed Regioselective Synthesis of Diphenyl-Substituted Carbazoles via [4 + 2] Annulation of β-Formyl Ketones with Indoles

The [4 + 2] annulation of β-formyl ketones with an indole has been developed for the regioselective synthesis of diphenyl-substituted carbazoles in the presence of a catalytic amount of iodine. The 1,4-dicarbonyl compound containing a phenyl group at the α-position of an aldehyde group reacts more readily with indoles to form carbazole derivatives. Using this method, a variety of carbazole derivatives can be readily accessed under mild reaction conditions.

Trends in carbazole synthesis - an update (2013-2023)

The interest of scientists in the carbazole core has risen steadily over the last 30 years, particularly over the last decade given its presence in several active pharmaceutical ingredients, functional materials and a wide range of biologically active natural products. The continuous development of more efficient, more (regio-)selective and "greener" methodologies to access the carbazole core is thus imperative. This review compares and evaluates synthetic strategies towards the carbazole core that have been reported since 2013, with a focus on their applicability towards the total synthesis of carbazole-containing natural products.

Rh(I)-Catalyzed Cascade Carbonylative Cyclization of Propargyl α-Diazoindolacetates for Construction of Carbazoles

A Rh(I)-catalyzed carbene migration/carbonylation/cyclization (MCC) strategy has been established for the construction of diverse functionalized carbazoles from propargyl α-diazoindolacetates. Rh(I)-stabilized carbene with different electrophilic properties displays specific reactivity toward alkyne and CO during the transformation, ensuring the smooth progress of the tandem cyclization. Other heteroaryl scaffolds were achieved simultaneously through this cascade protocol, thus offering a straightforward pathway toward functionalized polycyclic aromatic molecule synthesis.

Catalyst- and Substrate-Controlled Regiodivergent Synthesis of Carbazoles through Gold-Catalyzed Cyclizations of Indole-Functionalized Alkynols

A wide variety of regioselectively substituted carbazole derivatives can be synthesized by the gold-catalyzed cyclization of alkynols bearing an indol-3-yl and an additional group at the homopropargylic positions. The regioselectivity of the process can be controlled by both the oxidation state of the gold catalyst and the electronic nature of the substituents of the alkynol moiety. The 1,2-alkyl migration in the spiroindoleninium intermediate, generated after indole attack to the activated alkyne, is favored with gold(I) complexes and for electron-rich aromatic substituents at the homopropargylic position, whereas the 1,2-alkenyl shift is preferred when using gold(III) salts and for alkyl or non-electron-rich aromatic groups.

Electrosynthesis of Spiro-indolenines via Dearomative Arylation of Indoles in Batch and Continuous Flow

An electrosynthesis of spiro-indolenines in batch and continuous flow was achieved through dearomative arylation of indoles with good functional group compatibility. User-friendly undivided cells were used under catalyst- and oxidant-free conditions. Moreover, the use of a flow electrolysis cell gave high daily productivity and excellent scale-up potential under less supporting electrolyte and higher substrate concentration conditions.

Direct Regioselective Hydro(hetero)arylation/Cyclocondensation Reactions of β-(2-Aminophenyl)-α,β-ynones by Means of Transition-Metal Catalysis/Brønsted Acid Synergism: Experimental Results and Computational Insights

Experimental results and computational insights explain the key role of transition-metal catalysis/Brønsted acid synergism in the achievement of the sequential regioselective direct heteroarylation/cyclocondensation reactions of β-(2-aminophenyl)-α,β-ynones with a variety of electron-rich aromatic heterocyclic/arenes to afford quinoline-(hetero)aromatic hybrids. The first approach to the synthesis of 4-(1H-pyrrol-2-yl)quinolines is described. The effectiveness of various transition metals is compared.

Selectivity, Speciation, and Substrate Control in the Gold-Catalyzed Coupling of Indoles and Alkynes

A convenient and mild protocol for the gold-catalyzed intermolecular coupling of substituted indoles with carbonyl-functionalized alkynes to give vinyl indoles is reported. This reaction affords 3-substituted indoles in high yield, and in contrast to the analogous reactions with simple alkynes which give bisindolemethanes, only a single indole is added to the alkyne. The protocol is robust and tolerates substitution at a range of positions of the indole and the use of ester-, amide-, and ketone-substituted alkynes. The use of 3-substituted indoles as substrates results in the introduction of the vinyl substituent at the 2-position of the ring. A combined experimental and computational mechanistic study has revealed that the gold catalyst has a greater affinity to the indole than the alkyne, despite the carbon–carbon bond formation step proceeding through an η²(π)-alkyne complex, which helps to explain the stark differences between the intra- and intermolecular variants of the reaction. This study also demonstrated that the addition of a second indole to the carbonyl-containing vinyl indole products is both kinetically and thermodynamically less favored than in the case of more simple alkynes, providing an explanation for the observed selectivity. Finally, a highly unusual gold-promoted alkyne dimerization reaction to form a substituted gold pyrylium salt has been identified and studied in detail.

Cu(i)-catalyzed cross-coupling of primary amines with 2,2′-dibromo-1,1′-biphenyl for the synthesis of polysubstituted carbazole

A Cu(i)-catalyzed cross-coupling of primary amines with 2,2′-dibromo-1,1′-biphenyl for the synthesis of polysubstituted carbazole has been achieved. This protocol provides an efficient strategy for the synthesis of carbazole using cheap copper catalysts with diamine ligand, and it provides convenient access to a series of carbazole derivatives in moderate yields.

This protocol provides an efficient strategy for the synthesis of carbazole using cheaper copper catalysts with diamine ligand. A series of carbazole derivatives can be obtained in moderate yields.

HFIP-mediated three-component imidization of electron-rich arenes with in situ formed spiroindolenines for facile construction of 2-arylspiroindolenines

The three-component reaction of o-aminobenzaldehydes with 5-hydroxyindole and electron-rich arenes has been achieved through HFIP-mediated cascade hydride transfer/dearomative cyclization/CDC-type imidization at room temperature under air.

Copper-Catalyzed Annulation of Indolyl α-Diazocarbonyl Compounds Leads to Structurally Rearranged Carbazoles

Indolyl α-diazocarbonyl compounds have proven to be effective starting materials for the construction of various 2,3-ring fused indole frameworks. Activation of the diazo functional group under metal catalysis generates a spiro-cyclic indolenine-type intermediate which rearranges to provide two distinct carbazoles upon oxidation. The current study investigates the effects of the catalyst as well as the substituents on the migratory group involved in controlling the selectivity of the rearrangement.

Rh(III)-Catalyzed [5 + 1] Annulation of Indole-enaminones with Diazo Compounds To Form Highly Functionalized Carbazoles

A novel Rh(III)-catalyzed C-H activation/annulation cascade of indole-enaminones with diazo compounds was reported to construct diversely functionalized carbazole frameworks. The most notable characteristic is that this transformation could smoothly furnish a novel [5 + 1] cyclization product with good to excellent yields (up to 95%), accompanied by the thorough removal of acetyl and N,N-dimethyl groups of two substrates from the target products, rather than the normally expected [4 + 2] cyclization products.

A Thiol-Mediated Three-Step Ring Expansion Cascade for the Conversion of Indoles into Functionalized Quinolines

An operationally simple, high yielding three-step cascade process is described for the direct conversion of indole-tethered ynones into functionalized quinolines. A single "multitasking" thiol reagent is used to promote a three-step dearomatizing spirocyclization, nucleophilic substitution, and one-atom ring expansion reaction cascade under remarkably mild conditions. In addition, a novel route to thio-oxindoles is described, which was discovered by serendipity.

Heteroarene-tethered Functionalized Alkyne Metamorphosis

Heteroarene-tethered functionalized alkynes are multipotent synthons in organic chemistry. This detailed Review described herein offers a thorough discussion of the metamorphosis of heteroarene-tethered functionalized alkynes, an area which has earned much attention over the past decade in the straightforward synthesis of architecturally complex heterocyclic scaffolds in atom and step economic manner. Depending upon the variety of functionalized alkynes, this Review is divided into multiple sections. Amongst the vast array of synthetic transformations covered, dearomatizing spirocyclizations and cascade spirocyclization/rearrangement are of great interest. Synthetic transformations involving the heteroarene-tethered functionalized alkynes with scope, challenges, limitations, mechanism, their application in the total synthesis of natural products and future perceptions are surveyed.

Direct conversion of secondary propargyl alcohols into 1,3-di-arylpropanone via DBU promoted redox isomerization and palladium assisted chemoselective hydrogenation in a single pot operation

Palladium(ii)acetate is found to be an efficient catalyst for the single-step conversion of secondary propargyl alcohols to 1,3-diarylpropanone derivatives under mild basic conditions.

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.

Dearomatization-Rearomatization Strategy for Synthesizing Carbazoles with 2,2'-Biphenols and Ammonia by Dual C(Ar)-OH Bond Cleavages

Carbazole is an essential building block in various pharmaceuticals, agrochemicals, natural products, and materials. For future sustainability, it is highly desirable to synthesize carbazole derivatives directly from renewable resources or cheap raw materials. Phenolic compounds are a class of degradation products of lignin. On the other hand, ammonia is a very cheap industrial inorganic chemical. Herein, an efficient dearomatization-rearomatization strategy has been developed to directly cross-couple 2,2'-biphenols with ammonia by dual C(Ar)-OH bond cleavages. This strategy provides a greener pathway to synthesize valuable carbazole derivatives from phenols.

Iridium-Catalyzed Enantioselective Intermolecular Indole C2-Allylation

The enantioselective intermolecular C2-allylation of 3-substituted indoles is reported for the first time. This directing group-free approach relies on a chiral Ir-(P, olefin) complex and Mg(ClO4 )2 Lewis acid catalyst system to promote allylic substitution, providing the C2-allylated products in typically high yields (40-99 %) and enantioselectivities (83-99 % ee) with excellent regiocontrol. Experimental studies and DFT calculations suggest that the reaction proceeds via direct C2-allylation, rather than C3-allylation followed by in situ migration. Steric congestion at the indole-C3 position and improved π-π stacking interactions have been identified as major contributors to the C2-selectivity.

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.

Visible-light-induced intramolecular charge transfer in the radical spirocyclisation of indole-tethered ynones

Indole-tethered ynones form an intramolecular electron donor–acceptor complex that can undergo visible-light-induced charge transfer to promote thiyl radical generation from thiols. This initiates a novel radical chain sequence, based on dearomatising spirocyclisation with concomitant C–S bond formation. Sulfur-containing spirocycles are formed in high yields using this simple and mild synthetic protocol, in which neither transition metal catalysts nor photocatalysts are required. The proposed mechanism is supported by various mechanistic studies, and the unusual radical initiation mode represents only the second report of the use of an intramolecular electron donor–acceptor complex in synthesis.

Indole-tethered ynones form an intramolecular electron donor–acceptor complex that can undergo visible-light-induced charge transfer to promote thiyl radical generation from thiols.

A Brønsted Acid Catalyzed Cascade Reaction for the Conversion of Indoles to α-(3-Indolyl) Ketones by Using 2-Benzyloxy Aldehydes

A Brønsted acid catalyzed, operationally simple, scalable route to several functionalized α-(3-indolyl) ketones has been developed and the long-standing regioisomeric issue has been eliminated by choosing appropriate carbonyls. A readily available and cheap bottle reagent was used as the catalyst. This protocol was also applicable to the synthesis of densely functionalized α-(3-pyrrolyl) ketones. A detailed mechanistic study confirmed the involvement of enolether as a reaction intermediate. Several postsynthetic modifications along with easy access to β-carboline, tryptamines, tryptophols, and spiro-indolenine proclaim the synthetic utility of this powerful building block. On the basis of this concept, functionalized carbazoles were constructed by a cascade annulation strategy.

Copper-Catalyzed Cascade Cyclization of Indolyl Homopropargyl Amides: Stereospecific Construction of Bridged Aza-[n.2.1] Skeletons

Catalytic cycloisomerization-initiated cascade cyclizations of terminal alkynes have received tremendous interest, and been widely used in the facile synthesis of a diverse array of valuable complex heterocycles. However, these tandem reactions have been mostly limited to noble-metal catalysis, and are initiated by an exo-cyclization pathway. Reported herein is an unprecedented copper-catalyzed endo-cyclization-initiated tandem reaction of indolyl homopropargyl amides, where copper catalyzes both the hydroamination and Friedel-Crafts alkylation process. This method allows the practical and atom-economical synthesis of valuable bridged aza-[n.2.1] skeletons (n=3-6) with wide substrate scope, and excellent diastereoselectivity and enantioselectivity by a chirality-transfer strategy. Moreover, the mechanistic rationale for this novel cascade cyclization is also strongly supported by control experiments, and is distinctively different from the related gold catalysis.

Substituent-Guided Palladium-Ene Reaction for the Synthesis of Carbazoles and Cyclopenta[ b]indoles

An efficient palladium-catalyzed intramolecular Trost-Oppolzer type Alder-ene strategy was developed for the synthesis of carbazoles and cyclopenta[ b]indoles from easily accessible(3-allyl-1 H-indol-2-yl)methyl acetates. This strategy was extended for the synthesis of naphthalenes and dibenzobenzofurans as well. In addition, a short synthesis of antibacterial and antifungal natural product glycozoline and its analogues was also achieved.

Synthesis of cyano-substituted carbazoles via successive C-C/C-H cleavage

A highly efficient protocol for the synthesis of polysubstituted cyano carbazoles has been developed. This process is realized through the tandem reactions of Cs2CO3 promoted C-C σ-bond activation of α-cyano ketones followed by Fe-catalyzed selective C-H and/or C-C bond activations. Two of the sp2 C-H bonds are functionalized, and two of the C-C σ-bonds are cleaved involving a 1,2-acyl migration during the reaction process.

Synthesis of Cyclobutane-Fused Angular Tetracyclic Spiroindolines via Visible-Light-Promoted Intramolecular Dearomatization of Indole Derivatives

An intramolecular dearomatization of indole derivatives based on visible-light-promoted [2+2] cycloaddition was achieved via energy transfer mechanism. The highly strained cyclobutane-fused angular tetracyclic spiroindolines, which were typically unattainable under thermal conditions, could be directly accessed in high yields (up to 99%) with excellent diastereoselectivity (>20:1 dr) under mild conditions. The method was also compatible with diverse functional groups and amenable to flexible transformations. In addition, DFT calculations provided guidance on the rational design of substrates and deep understanding of the reaction pathways. This process constituted a rare example of indole functionalization by exploiting visible-light-induced reactivity at the excited states.

Recent advances in the synthesis of carbazoles from indoles

Synthesis of carbazoles using indoles as precursors through CH activation/annulation.

Platinum-catalyzed selective N-allylation of 2,3-disubstituted indoles with allylic acetates in water

Herein, we have demonstrated that the platinum-catalyzed selective N-allylation of 2,3-disubstituted indoles proceeds in water.