Diastereo- and Enantioselective Construction of Dihydrobenzo[e]indole Scaffolds via Catalytic Asymmetric [3 + 2] Cycloannulations

Regioselective C3Alkylation of Indoles for the Synthesis of Bis(indolyl)methanes and 3-Styryl Indoles

Herein, we describe an efficient transition-metal-free regioselective C3alkylation of indoles for the synthesis of bis(indolyl)methanes and 3-styryl indoles. Nitrobenzene is employed as the oxidant to oxidize the alcohols in the presence of a strong base and the reaction avoids the use of transition metals such as Ru and Mn. The protocol provides a favorable route to access biologically active compounds such as arundine, vibrindole A, and turbomycin B.

One-Pot Synthesis of 1,3,4-Oxadiazines from Acylhydrazides and Allenoates

The framework of 1,3,4-oxadiazine is crucial for numerous bioactive molecules, but only a limited number of synthetic methods have been reported for its production. In 2015, Wang's group developed a 4-(dimethylamino)pyridine (DMAP)-catalyzed [2 + 4] cycloaddition of allenoates with N-acyldiazenes, which provided an atom-efficient route for 1,3,4-oxadiazines. However, the practicality of this method was limited by the instability of N-acyldiazenes as starting materials. Building upon our ongoing research about the aerobic oxidation of hydrazides and their synthetic applications, we hypothesized that aerobic oxidative cycloadditions using acylhydrazides instead of N-acyldiazenes may provide a more practical synthetic route for 1,3,4-oxadiazines. In this manuscript, we describe a one-pot synthetic protocol for 1,3,4-oxadiazines from acylhydrazides and allenoates. The developed one-pot protocol consists of aerobic oxidations of acylhydrazides into N-acyldiazenes using NaNO₂ and HNO₃, followed by the DMAP-catalyzed cycloaddition of allenoate with the generated N-acyldiazenes. A variety of 1,3,4-oxadiazines were produced in good to high yields. In addition, the practicality of the developed method was demonstrated by a gram-scale synthesis of 1,3,4-oxadiazine.

Transition-Metal-Free Alkylative Aromatization of Tetralone Using Alcohol/Amino Alcohol towards the Synthesis of Bioactive Naphthol and Benzo[e/g]indole Derivatives

Herein, we report alkylative aromatization of tetralone for the synthesis of bioactive naphthols and benzo[e/g]indole derivatives using alcohols in the presence of NaOH via an aerobic oxidative cross-coupling protocol. This is a general and transition-metal-free method, which uses an inexpensive base, avoids inert conditions, and furnishes water and hydrogen peroxide as the byproducts. Moreover, this method demonstrated with wide substrate scope and obtained exclusive regioselectivity.

Catalytic asymmetric amination of azlactones with azobenzenes

We reported an efficient asymmetric amination of azlactones with N-aryl-N-aroyldiazenes through a chiral N,N'-dioxide-based Lewis acid catalyst. The multicoordination ability of Nd(III) enabled it to simultaneously activate and to locate the two reactants for N-selective addition. Hydrazine-bearing azlactone derivatives were obtained in moderate to good yields with high enantioselectivity.

Organocatalytic cycloaddition of alkynylindoles with azonaphthalenes for atroposelective construction of indole-based biaryls

The axially chiral indole-aryl motifs are present in natural products and biologically active compounds as well as in chiral ligands. Atroposelective indole formation is an efficient method to construct indole-based biaryls. We report herein the result of a chiral phosphoric acid catalyzed asymmetric cycloaddition of 3-alkynylindoles with azonaphthalenes. A class of indole-based biaryls were prepared efficiently with excellent yields and enantioselectivities (up to 98% yield, 99% ee). Control experiment and DFT calculations illustrate a possible mechanism in which the reaction proceeds via a dearomatization of indole to generate an allene-iminium intermediate, followed by an intramolecular aza-Michael addition. This approach provides a convergent synthetic strategy for enantioselective construction of axially chiral heterobiaryl backbones.

There is great interest in methods for catalytic enantioselective construction of axially chiral compounds found in natural products. Here, the authors develop a cycloaddition strategy for atroposelective construction of indole-based biaryls via chiral phosphoric acid-catalysed cycloaddition.

Organocatalytic cycloaddition-elimination cascade for atroposelective construction of heterobiaryls

The first chiral phosphoric acid (CPA) catalyzed cycloaddition–elimination cascade reaction of 2-naphthol- and phenol-derived enecarbamates with azonaphthalenes has been established, providing a highly atroposelective route to an array of axially chiral aryl-C3-benzoindoles in excellent yields with excellent enantioselectivities. The success of this strategy derives from the stepwise process involving CPA-catalyzed asymmetric formal [3 + 2] cycloaddition and subsequent central-to-axial chirality conversion by elimination of a carbamate. In addition, the practicality of this reaction had been verified by varieties of transformations towards functionalized atropisomers.

An organocatalytic asymmetric cycloaddition–elimination cascade reaction of aryl enecarbamates with azonaphthalenes has been developed to access axially chiral heterobiaryls in excellent yields and enantioselectivities.

Organocatalytic Asymmetric [2 + 4] Cycloadditions of 3-Vinylindoles with ortho-Quinone Methides

Catalytic asymmetric [2 + 4] cycloadditions of 3-vinylindoles with ortho-quinone methides and their precursors were carried out in the presence of chiral phosphoric acid to afford a series of indole-containing chroman derivatives with structural diversity in overall high yields (up to 98%), good diastereoselectivities (up to 93:7 dr) and moderate to excellent enantioselectivities (up to 98% ee). This approach not only enriches the chemistry of catalytic asymmetric cycloadditions involving 3-vinylindoles but is also useful for synthesizing chiral chroman derivatives.

Redox-Neutral Rhodium(III)-Catalyzed Chemospecific and Regiospecific [4+1] Annulation between Indoles and Alkenes for the Synthesis of Functionalized Imidazo[1,5-a]indoles

Exploiting internal alkenes embedded with an oxidizing function/leaving group as a rare and unconventional one-carbon unit, a redox-neutral rhodium(III)-catalyzed chemo- and regiospecific [4+1] annulation between indoles and alkenes for the synthesis of functionalized imidazo[1,5-a]indoles has been achieved. Internal alkenes employed here can fulfill an unusual [4+1] annulation rather than normal [4+2] annulation/C-H alkenylation. This method is characterized by excellent chemo- and regioselectivity, broad substrate scope, good functional group tolerance, good to high yields, and redox-neutral conditions.

Advances in organocatalytic asymmetric reactions of vinylindoles: powerful access to enantioenriched indole derivatives

This review summarizes advances in vinylindole-based organocatalytic asymmetric reactions since 2008 and includes the applications of some methodologies in the total synthesis of natural products, points out remaining challenges in this research area.

Catalytic Asymmetric Formal [3+2] Cycloaddition of Azoalkenes with 3-Vinylindoles: Synthesis of 2,3-Dihydropyrroles

Chiral phosphoric acid-catalyzed highly enantioselective formal [3 + 2] cycloaddition reaction of azoalkenes with 3-vinylindoles has been established. Under mild conditions, the projected cycloaddition proceeded smoothly, affording a variety of 2,3-dihydropyrroles in high yields and excellent enantioselectivities, and also in a diastereospecific manner. As opposed to the common 4-atom synthons in the previous literature reports, azoalkenes served as 3-atom synthons. Besides, the observed selectivity was supported by primary theoretical calculation. The unique chemistry of azoalkenes disclosed herein will empower asymmetric synthesis of nitrogen-containing ring structural motifs in a broader context.

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Chiral phosphoric acid catalyzed formal [3 + 2] cycloaddition reaction

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2,3-Dihydropyrroles were enantioselectively synthesized

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Azoalkenes served as 3-atom synthons

Enantioselective dearomative [3 + 2] annulation of 3-hydroxy chromanones with azonaphthalenes

Enantioselective dearomative [3 + 2] annulation of 3-hydroxy chromanones with azonaphthalenes provided a large variety of chromanone fused indolines in moderate to good yields with generally good enantioselectivities.

Construction of chiral chroman scaffolds via catalytic asymmetric (4 + 2) cyclizations of para-quinone methide derivatives with 3-vinylindoles

The title reaction has been established in the presence of chiral phosphoric acid, affording chiral chroman derivatives bearing an indole moiety in high yields and with moderate to good stereoselectivities.

Chiral phosphoric acid catalyzed asymmetric arylation of indolesvianucleophilic aromatic substitution: mechanisms and origin of enantioselectivity

Asymmetric arylation of indolesvianucleophilic aromatic substitution can be effectively achieved using chiral phosphoric acid as catalyst, where the mechanisms and origin of enantioselectivity were explored theoretically.

A Rh(ii)/phosphoric acid co-catalyzed three-component reaction of diazo-ketones with alcohols and azonaphthalenes: access to indole derivatives via a formal [3 + 2]-cycloaddition

Rh(ii)/Phosphoric Acid co-catalyzed muti-component annulation was well established as a new efficient route to construct 1-amino-indole derivatives via dearomatization/rearomatization/cyclization process with in situ formed oxonium ylide.

NaOH-Promoted Chemoselective Cascade Cyclization of Cyclopropyl Esters with Unsaturated Imines: Access to Bioactive Cyclopenta[c]pyridine Derivatives

A chemoselective cascade cycloaddition reaction is developed for green and efficient access to cyclopenta[c]pyridine derivatives. Simple and inexpensive NaOH is used as the sole catalyst for this process. The δ-carbon of cyclopropyl ester is activated as a nucleophilic carbon to initiate highly chemoselective cascade reactions. Cyclopenta[c]pyridines bearing various substituents are afforded in excellent yields. Preliminary studies on the bioactivities of the afforded products show promising antibacterial activities for potential applications in plant protections.

A tandem dearomatization/rearomatization strategy: enantioselective N-heterocyclic carbene-catalyzed α-arylation

In this study, the first example of carbene-catalyzed tandem dearomatization/rearomatization reaction of azonaphthalenes with α-chloroaldehydes is described.

In this study, the first example of the carbene-catalyzed tandem dearomatization/rearomatization reaction of azonaphthalenes with α-chloroaldehydes is described. This protocol enables the efficient assembly of chiral dihydrocinnolinone derivatives in good yields with excellent enantioselectivities (up to 99% ee). Moreover, this strategy enables not only the highly enantioselective NHC-catalyzed nucleophilic aromatic substitution, but also a formal Csp²–Csp³ bond formation.

Annulation Reactions of In-Situ-Generated N-(Het)aroyldiazenes with Isothiocyanates Leading to 2-Imino-1,3,4-oxadiazolines

A novel annulation reaction of N-(het)aroyldiazenes and isothiocyanates has been established. This transformation involves a sequential cyclization and desulfurization/intramolecular rearrangement to produce 2-imino-1,3,4-oxadiazolines. The less-stable N-(het)aroyldiazenes can be conveniently generated in situ by I₂-mediated oxidation of hydrazides, which allows a one-pot synthesis of the products directly from readily accessible hydrazide and isothiocyanate substrates. This operationally simple synthetic process requires no use of malodorous isocyanides and can be conveniently conducted on a gram scale.

Asymmetric synthesis of atropisomeric pyrazole via an enantioselective reaction of azonaphthalene with pyrazolone

The first catalytic asymmetric reaction of azonaphthalene with pyrazolone has been established. A wide range of axially chiral pyrazole derivatives have been achieved in good yields (68–99%) with excellent enantioselectivities (83–98% ee) by utilizing chiral phosphoric acid as a catalyst.

Regio- and stereoselective [3 + 2] cycloaddition reaction: access to isoxazole-dispirobisoxindoles featuring three contiguous stereocenters

A novel methodology toward the diversity-oriented asymmetric construction of densely functionalized isoxazole-dispirobisoxindoles was developed.