Base-mediated [4+2] annulation of electron-deficient nitrobenzoheterocycles and α,α-dicyanoalkenes in water: Facile access to structurally diverse functionalized dibenzoheterocyclic compounds

Base-Promoted and Copper(I)-Catalyzed Tandem Cyclization-C(sp2)-N Coupling of Vinyl Malononitriles with Ortho-Nitrochalcones: Access to Acridones and their Fused Derivatives

An efficient Cs2CO3-promoted and copper(I)-catalyzed double cyclization of ortho-nitrochalcones with vinyl malononitriles for the de novo access to a variety of tri- and tetra-substituted acridones and their fused derivatives with a value-added CN group has been developed for the first time. This one-pot operation proceeds through a Michael-cyclization-aromatization, followed by regioselective ipso-amination via nucleophilic aromatic substitution (SNAr) reaction, resulting in two C═C bonds and a C-N bond for acridone ring synthesis. This economic strategy based on 100% carbon atoms ensures the successive formation of two rings in a one-pot operation, good to high yields, a wide range of substrates, and good tolerance of functionalities. In addition, acridones were converted into several value-added acridones and acridines, highlighting the synthetic versatility and usefulness of the prepared acridone derivatives.

Three-component diels-alder reaction through palladium carbene migratory insertion enabled dearomative C(sp3)-H bond activation

Owning to the versatile nature in participation of Diels-Alder (D-A) reactions, the development of efficient approaches to generate active ortho-quinodimethanes (o-QDMs) has gained much attention. However, a catalytic method involving coupling of two readily accessible components to construct o-QDMs is lacking. Herein, we describe a palladium carbene migratory insertion enabled dearomative C(sp3)-H activation to form active o-QDM species through the cross-coupling of N-tosylhydrazones with aryl halides. The in situ generated o-QDM intermediates were trapped efficiently by 3-nitroindoles and N-sulfonylaldimines to provide dihydroindolo[2,3-b]carbazole derivatives and indole alkaloids modularly. To our knowledge, this reaction represents a rare example on three-component D-A cycloaddition through in situ generation of conjugated dienes by the coupling two readily available materials. We anticipate such a reaction mode could find broad application on diversity oriented six-membered ring construction. Deuterium labeling experiments and density functional theory calculations support a pathway through reversible C(sp3)-H activation to generate heterocyclic o-QDMs.

Expeditious synthesis of CF3-phenanthridones through a base-mediated cross-conjugated vinylogous benzannulation (VBA)

Herein, we report a mild, efficient, and rapid approach for the preparation of CF3-phenanthridones through a cross-conjugated vinylogous [4 + 2] benzannulation of easily accessible 4-methyl-3-trifluoroacetylquinolones and nitro-olefins. The present transformation is superior to previous approaches for obtaining CF3-phenanthridones, in that it proceeds exclusively with the assistance of a simple base, eliminating the need for transition metal catalysts or oxidants. The strong electron-withdrawing nature of the CF3-group present in the quinolone moiety promotes the formation of a reactive cross-conjugated vinylogous enolate.

Dibenzo-Fused Heterocycles: A Decade Update on the Syntheses of Carbazole, Dibenzofuran, and Dibenzothiophene

Polycyclic heterocycles are the most common and critical structural motifs found in a variety of natural products, medicines, fertilizers, and advanced materials. Because of their widespread use in biologically active compounds and material chemistry, functionalised dibenzo heterocyclic compounds, especially dibenzofuran, dibenzothiophene, and carbazole derivatives, garnered much attention over time. Scientists are especially interested in elucidating more efficient techniques for developing these industrially essential compounds. Dibenzo-fused heterocycles can rapidly be synthesised using highly efficient transition metal-catalysed strategies as well as by economic metal-free reaction conditions. This review includes a detailed overview of the most recent significant synthetic techniques, both metal-catalysed and metal-free, to produce these industrially significant and medicinally important dibenzo-fused heterocycles.

Divergent Transformations of 2-Nitro-1H-benzo[f]chromenes in Reactions with Alkylidenemalononitriles: Access to Naphtho[2,1-b]furans via Base-Mediated Pyran Ring Contraction

The action of 2-(1-arylethylidene)malononitriles on 2-nitro-1H-benzo[f]chromenes in the presence of Et3N and MoO3·2H2O results in naphtho[2,1-b]furans containing an allylidenemalononitrile unit in the α-position. The reaction proceeds with contraction of the pyran ring via a cascade carba-Michael addition/retro-oxa-Michael reaction/tautomerization/SN2/oxidation process. In contrast, the reaction of 2-nitro-1H-benzo[f]chromenes with the cyclic Knoevenagel adduct derived from 1-indanone and malononitrile leads to dihydroindeno[1,2-c]xanthenes. The possibility of further transformations of naphtho[2,1-b]furan derivatives as useful precursors and their optical properties were also investigated.

Annulation reactions of electrophilic benzannulated heterocycles towards heteroacenes

The current review describes different annulation strategies reported with electrophilic benzannulated heterocycles for accessing heteroacenes. For the past two decades, the chemistry of electrophilic benzannulated heterocycles was extensively investigated, and several dipolar cycloadditions, metal and organo-catalyzed transformations were introduced for the generation of fused heterocycles. In this review, we have collected all the reports where the annulation of electrophilic benzannulated heterocycles results in a fully aromatic system, viz. heteroacenes with tri-, tetra-, and pentacyclic rings. We reviewed every paper on the synthesis of fused heterocycles that was accessible and categorized the review into several parts based on the electrophilic benzannulated heterocycle used in the heteroacene synthesis such as electrophilic indole, electrophilic benzothiophene, and so forth. The generality and mechanistic postulates of each methodology are highlighted. In addition, we have also tried to feature the advantages or shortcomings of each method and have mentioned the possible applications of these methodologies for accessing heteroacenes for material applications.

Base-Mediated Annulation of Electrophilic Benzothiophene with Naphthols and Phenols: Accessing Benzothiophene-Fused Heteroacenes

A base-mediated annulation of 2-nitrobenzothiophenes with naphthols was realized for the synthesis of hitherto unknown class of heteroacenes, namely benzothieno[2,3-b]naphthofurans. By using naphthols with a hydroxyl group positioned at 1st or 2nd position, we could synthesize two positional isomers, benzothieno[2,3-b]naphtho[2,1-d]furans or benzothieno[2,3-b]naphtho[2,3-d]furans. The annulation was found to be general with a range of substituted 2-nitrobenzothiophenes and naphthols. This heteroannulation of benzothiophene was extended using a range of phenols affording the corresponding benzothieno[2,3-b]benzofurans in moderate yields. The basic photophysical properties of these heteroacenes were evaluated, and we also demonstrated the applicability of this annulation on the gram scale.

Cu(OAc)2/DABCO-mediated domino reaction of vinyl malononitriles with cyclic sulfamidate imines: access to 6-hydroxyaryl-2-aminonicotinonitriles

A novel Cu(II)-salt/DABCO-mediated one-pot access to a myriad of highly substituted biologically relevant 2-aminonicotinonitriles possessing a resourceful phenolic moiety with satisfactory yields is reported. This method involves cyclic sulfamidate imines as 1C1N sources and different kinds of acyclic/cyclic vinyl malononitriles as 4C sources for pyridine synthesis via a vinylogous Mannich-cycloaromatization sequence process, creating two new C-N bonds under mild conditions. Importantly, this de novo strategy is applicable to gram-scale syntheses, underlining the method's practicability and allowing for a wide range of substrates with excellent functional group tolerance.

Recent advances in the synthesis of cyclic compounds using α,α-dicyanoolefins as versatile vinylogous nucleophiles

This article provides a review of the applications of α,α-dicyanoolefins as versatile vinylogous nucleophiles in the synthesis of various cyclic compounds, covering the literature from the past 13 years.

Reactivity of 3-nitroindoles with electron-rich species

The indol(in)e building block is one of the "privileged-structures" for the pharmaceutical industry since this fragment plays a central role in drug discovery. While the electron-rich character of the indole motif has been investigated for decades, exploiting the electrophilic reactivity of 3-nitroindole derivatives has recently been put at the heart of a wide range of new, albeit challenging, chemical reactions. In particular, dearomatization processes have considerably enriched the scope of C2[double bond, length as m-dash]C3 functionalizations of these scaffolds. This feature article showcases this remarkable electrophilic reactivity of 3-nitroindoles with electron-rich species and highlights their value in generating diversely substituted indol(in)es. This compilation underlines how these heteroaromatic templates have gradually become model substrates for electron-poor aromatic compounds in dearomatization strategies.

Synthesis of Benzothienobenzofurans via Annulation of Electrophilic Benzothiophenes with Phenols

We have developed a metal-free, mild, and green synthetic route toward benzothieno[3,2-b]benzofurans by the annulation of 3-nitrobenzothiophene with phenols. The reaction was found to be general with a range of substituted phenols. In addition, we could extend the methodology for the synthesis of pentacenes and could demonstrate the synthesis in gram-scale. Moreover, we extended the strategy for the synthesis of benzothieno[2,3-b]benzofurans by starting from 2-nitrobenzothiophenes.

Dearomative [3 + 2] cycloaddition reaction of nitrobenzothiophenes with nonstabilized azomethine ylides

A highly diastereoselective dearomative [3 + 2] 1,3-dipolar cycloaddition reaction of nitrobenzothiophenes with an in situ-generated nonstabilized azomethine ylides has been developed. The transformation provides a series of functionalized fused tricyclic benzo[4,5]thieno[2,3-c]pyrroles in good yields (up to 92%) under mild reaction conditions. In addition, a gram-scale experiment and the synthetic transformation of the cycloadduct further highlighted the synthetic utility. The relative configurations of the typical products were clearly confirmed by X-ray crystallography.