Synthesis of Carbazolequinones by Formal [3 + 2] Cycloaddition of Arynes and 2-Aminoquinones

Harnessing the benzyne insertion consequence to enable π-extended pyrido-acridine and quinazolino-phenanthridine

Distinct protocols have been devised for the preparation of hybrid heterocyclic scaffolds like π-extended pyrido-acridines and quinazolino-phenanthridines duly materialized through Rh(III)- and Pd(II)-mediated catalytic courses commencing from acridine and quinazolimine scaffolds. Interestingly, the parent compounds (acridines and quinazolimines) are actualized from 2-aminobenzonitrile and anthranilic acid, where 2-aminobenzonitrile acts as the 1,4-dipolarophilic species and anthranilic acid as the benzyne precursor. The molecular assembly of acridine suggests the participation of two benzyne units. In addition, the structural motif of the quinazolimine ring features one benzyne unit. Further, indolizine ring containing the enaminonitrile skeleton upon exposure to benzyne forms an indolizine fused quinoline ring, decorated with three benzyne units.

Practical Access to Fused Carbazoles via Oxidative Benzannulation and their Photophysical Properties

An aryne annulation strategy for the synthesis of fused carbazoles is developed using indolyl β-ketonitrile in a cascade manner. The reaction sequence involves aryne-mediated [2 + 2] cycloaddition cleavage and intramolecular Michael addition, followed by oxidation under transition-metal-free reaction conditions. Subsequently, conversion of benzo[b]carbazole-6-carbonitrile to carbazole quinone is observed upon prolongation of the reaction time. Furthermore, these materials exhibit high quantum efficiency, which promotes the light-emitting diode applications.

Chemodivergence in Fluorine Source-Controlled Cascade Reaction of Aryne Precursors to Synthesize Pyrrolo[3,4-b]indoles and 3-Arylated Maleimides

Reactions allowing chemodivergence prove to be attractive strategies in synthetic organic chemistry. We herein described a highly practical, transition-metal-free, highly regioselective and chemodivergent cascade reaction controlled by fluorine sources, which involved a [3 + 2] cycloaddition or C-arylation process between aryne precursors and 3-aminomaleimides. These two pathways led to a wide scope of structurally diverse pyrrolo[3,4-b]indoles (19 examples) and 3-arylated maleimides (25 examples) in good-to-excellent yields. Furthermore, the reaction could be scaled up, and several synthetic transformations were accomplished for the preparation of functionalized molecules and might provide new opportunities for the discovery of N-heterocyclic drugs.

Advances in the synthesis of nitrogen-containing heterocyclic compounds by in situ benzyne cycloaddition

Nitrogen-containing heterocyclic compounds are prevalent in various natural products, medicines, agrochemicals, and organic functional materials. Among strategies to prepare nitrogen-containing heterocyclic compounds, pathways involving benzyne intermediates are attractive given that they can readily assemble highly diverse heterocyclic compounds in a step-economical manner under transition-metal-free conditions. The synthesis of nitrogen-containing heterocyclic compounds from benzyne intermediates offers an alternative strategy to the conventional metal-catalyzed activation approaches. In the past years, chemists have witnessed the revival of benzyne chemistry, mainly attributed to the wide application of various novel benzyne precursors. The cycloaddition of benzynes is a powerful tool for the synthesis of nitrogen-containing heterocyclic compounds, which can be constructed by [n + 2] cyclization of benzyne intermediates in situ generated from benzyne precursors under mild reaction conditions. This review focuses on the application of cycloaddition reactions involving in situ benzynes in the construction of various nitrogen-containing heterocyclic compounds.

Regioselective synthesis of 6-nitroindole derivatives from enaminones and nitroaromatic compounds via transition metal-free C-C and C-N bond formation

Few methods are known for the synthesis of nitroindole derivatives. A simple and practical Cs₂CO₃-promoted method for the synthesis of 6-nitroindole derivatives from enaminones and nitroaromatic compounds has been developed. Two new C-C and C-N bonds were formed in a highly regioselective manner under transition metal-free conditions.

Exploiting Iminoquinones as Electrophilic at Nitrogen "N+" Synthons for C-N Bond Construction

New methods for C-N bond construction exploiting the N-centered electrophilic character of iminoquinones are reported. Iminoquinones, generated in situ via the condensation of o-vinylanilines with benzoquinones, undergo acid-catalyzed cyclization to afford N-arylindoles in excellent yields. Under similar reaction conditions, homoallylic amines react analogously to afford N-arylpyrroles. Additionally, organometallic nucleophiles are shown to add to the nitrogen atom of N-alkyliminoquinones to provide amine products. Finally, iminoquinones are shown to be competent electrophiles for copper-catalyzed hydroamination.

Construction of Bridged-Ring-Fused Naphthalenone Derivatives Through an Unexpected Zn(OTf)2-Catalyzed Cascade Transformation

An unexpected cascade transformation of aminonaphthoquinones with N-substituents bearing a p-methoxybenzyl ether into bridged-ring-fused naphthalenone derivatives is reported. This cascade transformation was initiated by a catalytic amount of Zn(OTf)₂ and involved with subsequent functional group migration and cyclization. The process proceeded through the cleavage of two bonds and the formation of three new bonds in one pot and was proven to be efficient and tolerant to various substituents.

Tandem Diels-Alder/oxidation-aromatization reactions involving 2-styrylchromones and arynes

Transition-metal-free Diels-Alder and tandem Diels-Alder/oxidation-aromatization reactions of 2-styrylchromones with arynes have been demonstrated under mild reaction conditions with a wide range of substrates. The tandem process afforded functionalized benzo[c]xanthone derivatives in moderate to good yields. This efficient protocol allows rapid access to either dihydrobenzo[c]xanthone and benzo[c]xanthone derivatives selectively by simply changing the atmosphere of the reaction.

Metal- and Oxidant-Free Modular Approach To Access N-Alkoxy Oxindoles via Aryne Annulation

An unprecedented metal- and oxidant-free (intermolecular) approach to access N-alkoxy oxindoles via [3 + 2] cycloadition of in situ generated electrophilic species viz. aryne and (putative) aza-oxyallyl cation is reported. This approach is amenable to both C3-unsubstituted as well as C3-substituted oxindoles. A one-pot manipulation further makes this reaction highly practical. The versatility of this approach was demonstrated through valuable synthetic transformations.

Stevens rearrangement of thioethers with arynes: a facile access to multi-substituted β-keto thioethers

An effective method for the synthesis of multi-substituted β-keto thioethers via Stevens rearrangement of simple β-keto thioethers with arynes has been developed. In these reactions, successive C-S/C-H/C-C bonds were formed in one pot under mild and transition-metal free conditions to afford multi-substituted β-keto thioethers in moderate to good yields.

Total Synthesis of Kanamienamide

Kanamienamide is a novel enol ether containing enamide with a single digit micromolar inhibitory activity against cancer cell lines. An efficient and convergent total synthesis of kanamienamide has been developed for the first time, which features a Cu-mediated amide coupling with vinyl iodide at the late stage. Other key transformations include Evans asymmetric alkylation, CBS asymmetric reduction, ring-closing metathesis reaction, and Stork-Zhao-Wittig olefination. This strategy is amenable for facile analogue preparation and SAR studies.

Insertion of Arynes into P-O Bonds: One-Step Simultaneous Construction of C-P and C-O Bonds

The insertion of arynes into P-O bonds for the preparation of o-hydroxy-substituted arylphosphine oxides, -phosphinates, and -phosphonates is described. This novel reaction leads to the simultaneous formation of C-P and C-O bonds in one step with good yields and regioselectivities under mild and transition-metal-free conditions. The easy follow-up transformations of the resulting o-hydroxyl group extend these reactions to the facile construction of other ortho-substituted arylphosphorus compounds.