Intermolecular Dehydrative [4 + 2] Aza-Annulation of N-Arylamides with Alkenes: A Direct and Divergent Entrance to Aza-Heterocycles

Rhodium-Catalyzed [4 + 2] Cascade Annulation to Easy Access N-Substituted Indenoisoquinolinones

An efficient approach for the synthesis of N-substituted indenoisoquinolinones via rhodium(III)-catalyzed C-H bond activation/subsequent [4 + 2] cyclization starting from easily available 2-phenyloxazolines and 2-diazo-1,3-indandiones has been developed. A series of indeno[1,2-c]isoquinolinones were obtained in up to 93% yield through C-H functionalization, followed by intramolecular annulation, elimination, and ring-opening in a "one pot manner" under mild reaction conditions. This protocol features excellent atom- and step-economy and provides a novel strategy for the synthesis of N-substituted indenoisoquinolinones and a chance to study their biological activities.

HFIP-Empowered One-Pot Synthesis of C4-Aryl-Substituted Tetrahydroquinolines with Propargylic Chlorides and Anilines

Transition-metal-free, practical one-pot synthesis of C4-aryl-substituted tetrahydroquinolines from simple anilines and readily accessible propargylic chlorides has been developed. Activation of the C-Cl bond by 1,1,1,3,3,3-hexafluoroisopropanol turned out to be the key interaction, which allowed C-N bond formation under an acidic medium. Propargylated aniline is formed as an intermediate via propargylation, and subsequential cyclization and reduction gave 4-arylated tetrahydroquinolines. To demonstrate the synthetic utility, total syntheses of aflaquinolone F and I have been accomplished.

Visible Light-Mediated Cyclisation Reaction for the Synthesis of Highly-Substituted Tetrahydroquinolines and Quinolines

Condensation of 2-vinylanilines and conjugated aldehydes followed by an efficient light-mediated cyclisation selectively yields either substituted tetrahydroquinolines with typically high dr, or in the presence of an iridium photocatalyst the synthesis of quinoline derivatives is demonstrated. These atom economical processes require mild conditions, with the substrate scope demonstrating excellent site selectivity and functional group tolerance, including azaarene-bearing substrates. A thorough experimental mechanistic investigation explores multiple pathways and the key role that imine and iminium intermediates play in the absorption of visible light to generate reactive excited states. The synthetic utility of the reactions is demonstrated on gram scale quantities in both batch and flow, alongside further manipulation of the medicinally relevant products.

Challenges and Breakthroughs in Selective Amide Activation

In contrast to ketones and carboxylic esters, amides are classically seen as comparatively unreactive members of the carbonyl family, owing to their unique structural and electronic features. However, recent decades have seen the emergence of research programmes focused on the selective activation of amides under mild conditions. In the past four years, this area has continued to rapidly develop, with new advances coming in at a fast pace. Several novel activation strategies have been demonstrated as effective tools for selective amide activation, enabling transformations that are at once synthetically useful and mechanistically intriguing. This Minireview comprises recent advances in the field, highlighting new trends and breakthroughs in what could be called a new age of amide activation.

Challenges and Breakthroughs in Selective Amide Activation

In contrast to ketones and carboxylic esters, amides are classically seen as comparatively unreactive members of the carbonyl family, owing to their unique structural and electronic features. However, recent decades have seen the emergence of research programmes focused on the selective activation of amides under mild conditions. In the past four years, this area has continued to rapidly develop, with new advances coming in at a fast pace. Several novel activation strategies have been demonstrated as effective tools for selective amide activation, enabling transformations that are at once synthetically useful and mechanistically intriguing. This Minireview comprises recent advances in the field, highlighting new trends and breakthroughs in what could be called a new age of amide activation.

Triflic Anhydride (Tf2O)-Activated Transformations of Amides, Sulfoxides and Phosphorus Oxides via Nucleophilic Trapping

Trifluoromethanesulfonic anhydride (Tf2O) is utilized as a strong electrophilic activator in a wide range of applications in synthetic organic chemistry, leading to the transient generation of a triflate intermediate. This versatile triflate intermediate undergoes nucleophilic trapping with diverse nucleophiles to yield novel compounds. In this review, we describe the features and applications of triflic anhydride in organic synthesis reported in the past decade, especially in amide, sulfoxide, and phosphorus oxide chemistry through electrophilic activation. A plausible mechanistic pathway for each important reaction is also discussed.

1 Introduction

2 Amide Chemistry

2.1 Carbon Nucleophiles

2.2 Hydrogen Nucleophiles

2.3 Nitrogen Nucleophiles

2.4 Oxygen and Sulfur Nucleophiles

2.5 hosphorus Nucleophiles

2.6 A Vilsmeier-Type Reagent

2.7 Umpolung Reactivity in Amides

3 Sulfoxide Chemistry

3.1 Oxygen Nucleophiles

3.2 Carbon Nucleophiles

3.3 Nitrogen Nucleophiles

3.4 Thionium Reagents

4 Phosphorus Chemistry

4.1 Hendrickson’s Reagent

4.2 Diaryl Phosphine Oxides

4.3 Phosphonates, Phosphates and Phosphinates

5 Conclusion and Outlook

An Efficient and Straightforward Approach for Accessing Thioesters via Palladium-Catalyzed C-N Cleavage of Thioamides

We first report the coupling of activated thioamides with alcohols to efficiently form thioesters via palladium-catalyzed C-N cleavage strategy. The new approach employs the thioamides as thioacylating reagent to give...

Rhodium-Catalysed Regioselective [4+2]-type Annulation of 1-H-Indazoles with Propargyl Alcohols: A direct entry to 6-alkenylindazolo[3,2-a]isoquinolines

An efficient method for the synthesis of 6-vinylindazolo[3,2-a]isoquinolines via rhodium(III)-catalysed C-H bond activation/subsequent [4+2] cyclization starting from easily available 3-aryl-1-H-indazoles and propargyl alcohols has been developed. A series of vinyl-annulated...

Synthesis of 2-trifluoromethylquinolines through rhodium-catalysed redox-neutral [3 + 3] annulation between anilines and CF3-ynones using traceless directing groups

Rhodium-catalysed [3 + 3]-cycloaddition of acetanilides with CF3-ynones was achieved through a traceless directing-group strategy, which delivered 2-trifluromethylquinolines exhibiting favorable blue emissions.

Tf2O-Mediated Cyclization of 7-Enamides: Bioinspired Construction of Fused Eight-Membered Carbocyclic Enimines and Enones

In this paper, we document the construction of functionalized and fused eight-membered carbocycles by the triflic anhydride-mediated cyclization of 7-enamides. Taking advantage of the high electrophilicity of the nitrilium ion intermediates, generated in situ from secondary N-(2,6-dimethyl)anilides, the nonactivated, trisubstituted alkene-nitrilium cyclization reactions proceeded smoothly to afford nonconjugated β,γ-enimines (for fused 6/6/8 ring systems), conjugated α,β-enimines (for 6/5/8), or fused 5/8 ring systems in good yields. When the cyclization reactions were followed by one-pot acidic hydrolysis, the reaction led directly to the corresponding α,β-enones. For some substrates, the reaction afford an efficient access to pendent cyclic β,γ-enimines/enones.

Chemoselektive γ-Oxidation von β,γ-ungesättigten Amiden mit TEMPO

Ein chemoselektives und robustes Protokoll zur γ‐Oxidation von β,γ‐ungesättigten Amiden wird dargelegt. Bei dieser Methode ermöglicht elektrophile Amidaktivierung eine bei ungesättigten Amiden bisher selten angewendete regioselektive Reaktion mit TEMPO, die zu γ‐aminoxylierten α,β‐ungesättigten Amiden führt. Radikalische Zyklisierungen und Oxidationen der synthetisierten Produkte untermauern die Nützlichkeit der hergestellten Verbindungen.

Nickel-Catalyzed Asymmetric Reductive Dicarbamoylation of Alkenes

Herein we report a nickel-catalyzed asymmetric reductive dicarbamoylation of alkenes, in which tethered carbamoyl chlorides and isocyanates serve as distinct electrophilic carbamoylating agents, providing new access to chiral oxindoles bearing an amide-substituted quaternary stereogenic center.

Chemoselective γ-Oxidation of β,γ-Unsaturated Amides with TEMPO

A chemoselective and robust protocol for the γ‐oxidation of β,γ‐unsaturated amides is reported. In this method, electrophilic amide activation, in a rare application to unsaturated amides, enables a regioselective reaction with TEMPO resulting in the title products. Radical cyclisation reactions and oxidation of the synthesised products highlight the synthetic utility of the products obtained.

Chemoselective Reactions of Isocyanates with Secondary Amides: One-Pot Construction of 2,3-Dialkyl-Substituted Quinazolinones

A facile method for the preparation of 2,3-dialkyl-substituted quinazolinones from readily available N-arylamides and commercial isocyanates was developed. This one-pot procedure involves the chemoselective activation of the secondary amide with Tf₂O/2-Br-Pyr, the sequential addition of isocyanate, and cyclization. The mild reaction is general for a wide range of substrates and can be run on a gram scale.

Synthesis of difluorinated 3-oxo-N,3-diarylpropanamides from 4-arylamino coumarins mediated by Selectfluor

An expeditious approach to synthesize difluorinated 3-oxo-N,3-diarylpropanamides from 4-arylamino coumarins has been accomplished in the presence of Selectfluor, which plays the dual role of a mild oxidant and a source of fluorine.

A one pot protocol to convert nitro-arenes into N-aryl amides

A two-step one pot, experimentally simple protocol, based on readily available and inexpensive reagents allowed the conversion of nitro-arenes directly to N-aryl amides. A metal-free reduction of the nitro group, mediated by trichlorosilane, followed by the addition of an anhydride afforded the corresponding N-aryl carboxyamide, that was isolated after a simple aqueous work up in good-excellent yields. When the methodology was applied to the reaction with γ-butyrolactone, the desired N-aryl butanamide derivative was obtained, featuring a chlorine atom at the γ-position, a functionalized handle that can be used for further synthetic manipulation of the reaction product. Such an intermediate has already been employed as a key advanced precursor of pharmaceutically active compounds.

Triflic anhydride mediated synthesis of 3,4-dihydroquinazolines: a three-component one-pot tandem procedure

A one-pot three-component tandem reaction involving a key Pictet-Spengler-like annulation step has been developed, providing an efficient method for the synthesis of 3,4-dihydroquinazolines in moderate to good yields from amides, aldehydes, and amines. The multicomponent triflic anhydride mediated reaction tolerates the installation of numerous functional groups, affording extensive diversity about the heterocyclic scaffold.

Simple and efficient synthesis of bicyclic enol-carbamates: access to brabantamides and their analogues

A simple and short synthesis of bicyclic enol-carbamates with high E/Z selectivity and the synthesis of brabantamide A analogue are presented.

Recent advances and prospects in the metal-free synthesis of quinolines

Metal-free synthesis of quinolines has recently gained attention, and this review focuses on the recent advances in the metal-free synthesis of quinolines.

Chemoselective Synthesis of α-Amino-α-cyanophosphonates by Reductive Gem-Cyanation-Phosphonylation of Secondary Amides

A novel approach to α-amino-α-cyanophosphonates has been developed. The method features a Tf₂O-mediated reductive geminal cyanation/phosphonylation of secondary amides. Mild reaction conditions, high bond-forming efficiency, inexpensive readily available starting materials, and good to excellent yields with wide functional group compatibility constitute the main advantages of this method. The protocol can be run on a gram scale.

Direct amide synthesis via Ni-mediated aminocarbonylation of arylboronic acids with CO and nitroarenes

Ni metal-mediated aminocarbonylation based on readily available arylboronic acids, nitroarenes, and carbon monoxide was achieved to prepare a variety of aryl amides.