The Diastereoselective Synthesis of Pyrroloindolines by Pd-Catalyzed Dearomative Cycloaddition of 1-Tosyl-2-vinylaziridine to 3-Nitroindoles

Palladium-Catalyzed (3+2) and (4+2) Cycloaddition Reactions of Sulfamidate Imine-Derived Azadienes: Synthesis of Spirocyclic Pyrrolidines and Tetrahydroquinolines

Diastereoselective Pd-catalyzed (3+2) and (4+2) cycloaddition reactions of sulfamidate imine-derived 1-azadienes with zwitterionic N-dipoles derived from 1-tosyl-2-vinylaziridine and 4-vinylbenzoxazinone have been developed. These reactions provide highly functionalized azaspirocycles featuring three contiguous stereocenters. The sulfonyl imine moiety of the cycloadducts can be fully reduced to access valuable β-amino alcohols.

Magic Blue-Initiated SN2-Type Ring Opening of Activated Aziridines: Friedel-Crafts-Type Alkylation of Electron-Rich Arenes/Heteroarenes

A transition metal-free, atom-economical, and highly stereospecific synthetic approach to Friedel-Crafts-type alkylation of arenes/heteroarenes has been developed. The protocol involves the catalytic aminium radical-cation salt (Magic Blue)-initiated SN2-type nucleophilic ring opening of activated aziridines with arenes/heteroarenes to give the corresponding 2,2-diarylethylamines up to 99% yield and 85% ee (for nonracemic aziridines) in a very short reaction time. Moreover, on reaction with 1,3-dimethylindole and benzofuran, aziridines undergo domino-ring-opening cyclization (DROC) to give the various biologically significant heterocyclic scaffolds in moderate to good yields.

Palladium-Catalyzed Asymmetric Decarboxylation of 5-Vinyloxazolidine-2,4-Diones Triggering the Dearomatization of Electron-Deficient Indoles for the Synthesis of Chiral Highly Functionalized Pyrroloindolines

A catalyst system consisting of a chiral phosphoramidite ligand and Pd2(dba)3·CHCl3 causes the decarboxylation of 5-vinyloxazolidine-2,4-diones to generate amide-containing aza-π-allylpalladium 1,3-dipole intermediates, which are capable of triggering the dearomatization of 3-nitroindoles for diastereo- and enantioselective [3+2] cycloaddition, leading to the formation of a series of highly functionalized pyrroloindolines containing three contiguous stereogenic centers with excellent results (up to 99% yield, 88:12 dr, and 96% ee).

Pd-Catalyzed Intramolecular Dearomative [4 + 2] Cycloaddition of Naphthalenes with Arylalkynes

A Pd-catalyzed intramolecular dearomative [4 + 2] cycloaddition reaction of naphthalenes with arylalkynes is developed. The protocol provides a straightforward method to access a range of polycyclic dihydronaphthalenes containing two vicinal all-carbon stereocenters in moderate yields under mild conditions in an air atmosphere. The deuterium labeling experiment suggests a pathway involving electrophilic dearomatization followed by Friedel-Crafts cyclization. Several synthetic transformations of the product were conducted to demonstrate the utility of this reaction.

Copper-Catalyzed Annulation of Electrophilic Benzannulated Heterocycles with 2-Aminopyridine and 2-Aminoquinoline: Direct Access toward Polyring-Fused Imidazo[1,2-a]pyridines

We have developed a direct method for the synthesis of polyring-fused imidazo[1,2-a]pyridines via a copper-catalyzed annulation of electrophilic benzannulated heterocycles with 2-aminopyridine and 2-aminoquinoline. From 3-nitroindoles and 2-aminopyridine, we could synthesize tetracenes, viz., indole-fused imidazo[1,2-a]pyridines, and by starting from 2-aminoquinoline, we could generate pentacenes, viz., indolo-imidazo[1,2-a]quinolines. In addition, we could also extend the methodology toward the synthesis of benzothieno-imidazo[1,2-a]pyridines starting from 3-nitrobenzothiophene. Furthermore, the basic photophysical properties of these synthesized heteroacenes were evaluated.

Palladium-Catalyzed Dearomative [4 + 2]-Cycloaddition toward Hydrocarbazoles

An efficient method for the construction of hydrocarbazoles bearing three continuous sterically hindered stereocenters, two quaternary and one tertiary, via a highly diastereoselective palladium-catalyzed [4 + 2]-cycloaddition/dearomatization of 3-nitroindoles has been developed. The cycloaddition of 3-nitroindoles occurs at ambient conditions with a 1,4-zwitterionic intermediate, in situ generated from γ-methylidene-δ-valerolactones. The further synthetic utility of this method is demonstrated by the multifaceted transformations possible from the products. The catalytic asymmetric aspect of this transformation has also been explored.

Palladium-Catalyzed Carbonylative [5+1] Cycloaddition of N-Tosyl Vinylaziridines: Solvent-Controlled Divergent Synthesis of α,β- and β,γ-Unsaturated δ-Lactams

A palladium-catalyzed carbonylative [5+1] cycloaddition of N-tosyl vinylaziridines with CO has been developed. This protocol affords an efficient and practical approach for solvent-controlled divergent synthesis of α,β-unsaturated δ-lactams in dimethylformamide and β,γ-unsaturated δ-lactams in tetrahydrofuran in good to excellent yields. Significantly, the step- and atom-economical reactions are more regioselective toward [5+1] cycloaddition than toward [3+1] cycloaddition.

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.

Pd-Catalyzed [4 + 2] cycloaddition of methylene cyclic carbamates with dihydropyrazolone-derived alkenes: synthesis of spiropyrazolones

In this paper, a palladium-catalyzed [4 + 2] cycloaddition of 5-methylene-1,3-oxazinan-2-ones with 4-arylidene-2,4-dihydro-3H-pyrazol-3-ones has been developed to produce spiropyrazolones in high yields with excellent diastereoselectivities in nearly all cases. The cycloaddition reaction was scaled-up without significant loss of yield, and its synthetic utility has been demonstrated by further transformations of the products. The reaction type of N-Ts cyclic carbamates under palladium catalysis was extended to include [4 + 2] cycloaddition for the first time.

Dearomatization of 3-Nitroindoles Enabled Using Palladium-Catalyzed Decarboxylative [4 + 2] Cycloaddition of 2-Alkylidenetrimethylene Carbonates

A dearomatization process of 3-nitroindoles enabled using palladium-catalyzed decarboxylative [4 + 2] cycloaddition of either 2-alkylidenetrimethylene carbonates or 2-(hydroxymethyl)-3-arylallyl carbonates has been developed, affording a wide range of indoline-fused tetrahydropyrans in good yields with excellent diastereoselectivities. This reaction features a wide substrate scope and mild conditions and represents the first example of the application of π-allyl palladium 1,4-[O,C]-dipole species for the dearomative cycloaddition of electron-deficient heteroarenes.

Base-controlled dearomative [3 + 2] cycloadditions between 3-nitro-indoles and fumaric acid amide esters

The base-controlled dearomative [3 + 2] cycloaddition reaction between 3-nitroindoles and fumaric acid amide esters has been disclosed by using the dearomatization and aromatization strategy. Three kinds of diverse functionalized pyrrolo[2,3-b]indole derivatives were obtained respectively with excellent chemoselectivities and good diastereoselectivities using different bases.

Metal-Free Diastereo- and Enantioselective Dearomative Formal [3 + 2] Cycloaddition of 2-Nitrobenzofurans and Isocyanoacetate Esters

The diastereo- and enantioselective dearomative formal [3 + 2] cycloaddition of 2-nitrobenzofurans and α-aryl-α-isocyanoacetate esters provides tricyclic compounds bearing the 3a,8b-dihydro-1H-benzofuro[2,3-c]pyrrole framework with three consecutive stereogenic centers. The reaction was enabled by a cupreine-ether organocatalyst. The reaction products were obtained with almost full diastereoselectivity and with excellent enantiomeric excesses for a number of substituted 2-nitrobenzofurans and isocyanoacetates.

Acid-promoted formal [3 + 2] cyclization/N,O-ketalization of in situ generated ortho-alkynyl quinone methides: access to bridged 2,3-cyclopentanoindoline skeletons

An acid-catalyzed formal [3 + 2] cyclization/N,O-ketalization of in situ formed ortho-alkynyl quinone methides (o-AQMs) and tryptophol derivatives was developed.

Palladium-Catalyzed Dearomative Methoxyallylation of 3-Nitroindoles with Allyl Carbonates

Herein we report a Pd-catalyzed dearomative methoxyallylation of 3-nitroindoles with readily available allyl carbonates. Good yields (up to 86 %) and diastereoselectivity (up to >20:1 dr) are obtained for a wide range of substrates. The compatibility of gram-scale synthesis and the relatively low catalyst loading (down to 1 mol % of [Pd]) enhance the practicality of this method. The kinetic experiments indicate that the rate-determining step of this reaction is the nucleophilic attack of the alkoxide anion.

Dynamics in Catalytic Asymmetric Diastereoconvergent (3 + 2) Cycloadditions with Isomerizable Nitrones and α-Keto Ester Enolates

Reaction design in asymmetric catalysis has traditionally been predicated on a structurally robust scaffold in both substrates and catalysts, to reduce the number of possible diastereomeric transition states. Herein, we present the stereochemical dynamics in the Ni(II)-catalyzed diastereoconvergent (3 + 2) cycloadditions of isomerizable nitrile-conjugated nitrones with α-keto ester enolates. Even in the presence of multiple equilibrating species, the catalytic protocol displays a wide substrate scope to access a range of CN-containing building blocks bearing adjacent stereocenters with high enantio- and diastereoselectivities. Our computational investigations suggest that the enantioselectivity is governed in the deprotonation process to form (Z)-Ni-enolates, while the unique syn addition is mainly controlled by weak noncovalent bonding interactions between the nitrone and ligand.

Palladium-Catalyzed Formal (3 + 2) Cycloaddition Reactions of 2-Nitro-1,3-enynes with Vinylaziridines, -epoxides, and -cyclopropanes

A two-step Pd-catalyzed (3 + 2) cycloaddition/HNO₂ elimination reaction sequence has been developed to give novel cyclic 1,3-dien-5-yne systems from Pd-stabilized zwitterionic 1,3-dipoles and 2-nitro-1,3-enyne substrates. The process is highly atom-efficient and tolerates the reaction of 2-vinyloxirane, 1-tosyl-2-vinylaziridine, and diethyl 2-vinylcyclopropane-1,1-dicarboxylate derived 1,3-dipoles with a variety of 2-nitro-1,3-enyne substrates. The stereochemistry of the intermediate (3 + 2) cycloadducts was determined by single crystal X-ray analysis. Furthermore, a selective kinetic elimination of the cycloadduct with an antiperiplanar relationship between the NO₂ group and the participating hydrogen was demonstrated, allowing for efficient isolation of a single diastereoisomer of the cycloadduct.

The transition-metal-catalyzed stereoselective ring-expansion of vinylaziridines and vinyloxiranes

The transition-metal-aided stereoselective construction of sp3-carbon-rich heterocyclic scaffolds using strained-ring systems has received considerable attention in recent years due to the prominent presence of these scaffolds in myriad natural products, bioactive molecules, and pharmaceutical components. In this area, the catalytic ring-enlargement of vinylaziridines and vinyloxiranes plays a predominant role when synthesizing high sp3-content biorelevant heterocyclic compounds. This article aims to portray recent advancements in the ring-expansion of vinylaziridines and vinyloxiranes for accessing densely functionalized stereoselective heterocycles that have been developed over the past five years, with an emphasis on the substrate scopes and mechanistic insights into the key methodologies, and it is arranged based on the transition metals used and the ring sizes of the heterocyclic scaffolds.

A Solvent-free, Catalyst-free Formal [3+3] Cycloaddition Dearomatization Strategy: Towards New Fluorophores for Biomolecules Labelling

A general, sustainable dearomatization reaction for nitrogen-containing heterocycles was developed. Under solvent free conditions and without catalyst, the biorenewable methyl coumalate (MC) reacted as an efficient C₃ partner to convert nine types of basic aromatic rings into their pyrido[1,2-a] fused derivatives in good to excellent yields. The fluorescence properties of some of the products were harnessed to conjugate fluorescent tags to bovine serum albumin (BSA) and immunoglobulin G.

Unprecedented access to functionalized pyrrolo[2,1-a]isoquinolines from the domino reaction of isoquinolinium ylides and electrophilic benzannulated heterocycles

We have come across an unexpected reaction between electrophilic indoles and isoquinolinium methylides for accessing functionalized pyrrolo[2,1-a]isoquinolines. The reaction was found in general to yield the products in good yields. We also observed the formation of S-S-bridged bis-pyrrolo[2,1-a]isoquinolines from the reaction of 3-nitro benzothiophene and isoquinolinium methylides.

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.

Development of Dipolarophiles for Catalytic Asymmetric Cycloadditions through Pd-π-Allyl Zwitterions

The development of new and efficient methodology for the construction of optically active molecules is of great interest in both synthetic organic and medicinal chemistry fields. To this end, the personal account summarizes our studies on the development of electron-deficient alkenes, allenes, and alkynes containing single activator as new dipolarophiles for Pd-catalyzed asymmetric cycloaddition reactions. These new dipolarophiles can participate in Pd-catalyzed asymmetric [3+2] and [4+2] cycloadditions through Pd-π-allyl 1,3- and 1,4-zwitterions in-situ generated by the reaction of Pd(0) catalyst with vinyl aziridines, vinyl epoxides, vinyl cyclopropanes, 4-vinyl-1,3-dioxan-2-ones, and vinyl benzoxazinanones. These [3+2] and [4+2] cycloadditions provide efficient approaches to a wide range of enantiomerically enriched five- and six-membered ring compounds containing contiguous chiral centers with high to excellent chemo-, diastereo-, and enantioselectivities. The utilities of these protocols are demonstrated by transformation of the cycloadducts into other useful chiral building blocks. DFT calculations reveal the dissimilar reactivity of different electron deficient alkenes and rationalize the mechanism and stereo-control of the reaction. A Pd-catalyzed inverse [3+2] cycloaddition is disclosed.

Catalytic asymmetric preparation of pyrroloindolines: strategies and applications to total synthesis

Pyrroloindolines are widely present in natural products. In this review, we summarize state-of-the-art of catalytic asymmetric synthesis of pyrroloindolines, as well as related applications to natural products total synthesis.

Copper-catalyzed, N-auxiliary group-controlled switchable transannulation/nitration initiated by nitro radicals: selective synthesis of pyridoquinazolones and 3-nitroindoles

Within the scope of nitration reactions, the efficiency of sensitive heteroaromatics such as indoles is often eroded by various competitive oxidative decomposition pathways.

Recent advances in annulation reactions based on zwitterionic π-allyl palladium and propargyl palladium complexes

Carbocyclic and heterocyclic compounds could be constructed through the palladium-catalyzed annulation reactions of zwitterionic π-allyl palladium or propargyl palladium complexes with unsaturated electrophiles.

Visible-Light Photoredox-Catalyzed Formal [5 + 1] Cycloaddition of N-Tosyl Vinylaziridines with Difluoroalkyl Halides

A visible-light photoredox-catalyzed formal [5 + 1] cycloaddition of N-tosyl vinylaziridines with difluoroalkyl halides as unique C1 synthons was developed. The procedure provides an efficient and practical method to synthesize diverse pyridines in moderate to good yields. The reaction underwent a radical-initiated kinetically controlled ring-opening of vinylaziridines and involved a key α,β-unsaturated imine intermediate, followed by an E2 elimination, a 6π electrocyclization, and defluorinated aromatization.

B(C6F5)3-Catalyzed Highly Chemoselective Reduction of Isatins: Synthesis of Indolin-3-ones and Indolines

A chemo- and site-selective reduction reaction of isatin derivatives using catalyst B(C₆F₅)₃ and hydrosilanes is described. This transformation is operationally simple, proceeds under mild conditions, and is resistant to various functional groups. Thus, this efficient reaction using a combination of B(C₆F₅)₃ and BnMe₂SiH or B(C₆F₅)₃ and Et₂SiH₂ could potentially be utilized to produce various indolin-3-ones and indolines, without the need for multistep procedures and metal catalysis conditions.

Enantioselective [4+2] Annulation to the Concise Synthesis of Chiral Dihydrocarbazoles

A highly efficient phosphine-catalyzed enantioselective [4 + 2] annulation of allenoates with 3-nitroindoles or 3-nitrobenzothiophenes has been developed. The protocol represents a unique dearomatization–aromatization process to access functionalized dihydrocarbazoles or dihydrodibenzothiophenes with high optical purity (up to 97% ee) under mild reaction conditions. The synthetic utility of the highly enantioselective [4 + 2] annulation enables a concise synthesis of analgesic agent.

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High regio-, chemo-, and enantioselectivity

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Broad substrate scope

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Dearomatization/aromatization steps proceed under mild conditions

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Concise synthesis of chiral analgesic agent

Dearomative [4 + 2] annulations between 3-nitroindoles and enals through oxidative N-heterocyclic carbene catalysis

A novel intermolecular dearomative [4 + 2] annulation of 3-nitroindoles and enals under oxidative N-heterocyclic carbene catalysis has been developed. This protocol was also suitable for the oxidative cyclisation of 2-nitrobenzothiophenes with enals.

Enantioselective Dearomatization of Indoles by an Azoalkene-Enabled (3+2) Reaction: Access to Pyrroloindolines

The enantioselective dearomatization of indoles by an organocatalytic (3+2) reaction has been established. The reaction makes use of simple indole derivatives as substrates, and employs azoalkenes reaction partners. A wide range of pyrroloindolines containing an all-carbon quaternary stereogenic center were readily prepared in high yields and with excellent enantioselectivities.

Divergent Pd-catalyzed cross-coupling of allenyloxazolidinones to give chiral 1,3-dienes and vinyloxazolidinones

The divergent reactivity of 5-allenyloxazolidinones has been explored. This novel building block undergoes Pd(0)-catalyzed cross-coupling with boronic acids to form a wide range of chiral 1,3-dienes and pharmaceutically useful vinyloxazolidinones, the chemoselectivity being tightly controlled by a simple switch in additive.