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

Cu(OTf)2 catalyzed Ugi-type reaction of N,O-acetals with isocyanides for the synthesis of pyrrolidinyl and piperidinyl 2-carboxamides

The Cu(OTf)₂ catalyzed Ugi-type reactions of N,O-acetals with isocyanides have been described for the synthesis of pyrrolidinyl and piperidinyl 2-carboxamides. 4-Hydroxy-5-substituted-prolinamides can be obtained in high diastereoselectivities (2,4-cis/trans > 19 : 1) and a stereoselective model was proposed for 2,4-cis selectivity. Moreover, 4-F-VH 032, a novel analog of the VHL ligand, was conveniently obtained by utilizing the present method.

The Ugi three-component reaction and its variants

A broad variety of α-aminoamide-based compounds have been synthesized via the three-component version of the Ugi reaction (U-3CR) or by any of its variants (Ugi-Zhu-3CR, Orru-3CR, Ugi-4C-3CR, Ugi-Joullié-3CR, GBB-3CR, Ugi-Reissert-3CR, and so on).

Iridium-catalyzed reductive Ugi-type reactions of tertiary amides

Amides are ubiquitous in the fine chemical, agrochemical and pharmaceutical industries, but are rarely exploited as substrates for homologous amine synthesis. By virtue of their high chemical stability, they are essentially inert to all but the harshest of chemical reagents and to the majority of chemical transformations routinely used in organic synthesis. Accordingly, the development of chemoselective carbon−carbon bond-forming methodologies arising from the functionalization of the amide functionality should find widespread use across academia and industry. We herein present our findings on a series of Ugi-type reactions of tertiary amides enabled by an initial chemoselective iridium-catalyzed partial reduction, followed by reaction with isocyanide and (thio)acetic acid or trimethylsilyl azide, thus providing a multicomponent synthesis of α-amino (thio)amide or α-amino tetrazole derivatives. The reductive Ugi-type reactions are amenable to a broad range of amides and isocyanides, and are applicable to late-stage functionalization of various bioactive molecules and pharmaceutical compounds.

Chemical transformation of amides is normally occurring under harsh conditions. Here, the authors report a mild iridium-catalyzed reductive Ugi-type coupling of tertiary amides, isocyanides and (thio)acetic acid or trimethylsilyl azide to give homologous, bioactive amine products.

Total synthesis of complex alkaloids by nucleophilic addition to amides

This mini review focuses on the recent progress of total synthesis of complex alkaloids based on the nucleophilic additions toN-alkoxyamides, tertiary amides and secondary amides.

Direct Regioselective Synthesis of Tetrazolium Salts by Activation of Secondary Amides under Mild Conditions

Tetrazolium salts are biologically active molecules that have found broad applications in biochemical assays. A regioselective synthesis of tetrazolium salts is described through a formal (3 + 2) cycloaddition. The possibility of employing simple amides and azides as starting material and the mild conditions allow a broad functional group tolerance.

Recent advances on the total synthesis of alkaloids in mainland China

Alkaloids are a large family of natural products that mostly contain basic nitrogen atoms. Because of their intriguing structures and important functions, they have long been popular targets for synthetic organic chemists. China's chemists have made significant progress in the area of alkaloid synthesis over past decades. In this article, selected total syntheses of alkaloids from research groups in mainland China during the period 2011–16 are highlighted.

One-pot synthesis of N-heterocycles and enimino carbocycles by tandem dehydrative coupling–reductive cyclization of halo-sec-amides and dehydrative cyclization of olefinic sec-amides

We report a versatile approach for the one-pot synthesis of substituted pyrrolidine, piperidine, indolizidine, and quinolizidine ring systems, and enimino carbocycles.

Metal-Free Intermolecular Coupling of Arenes with Secondary Amides: Chemoselective Synthesis of Aromatic Ketimines and Ketones, and N-Deacylation of Secondary Amides

The direct transformation of common secondary amides into aromatic ketimines and aromatic ketones with C-C bond formation is described. The reaction can also be used for N-deacylation of secondary amides to release amines. This method consists of in situ amide activation with triflic anhydride and intermolecular capture of the resulting highly electrophilic nitrilium intermediate with an arene. The reaction is applicable to various kinds of secondary amides (electrophiles), but only electron-rich and moderately electron-rich arenes can be used as nucleophiles. Thanks to the use of bench stable arenes instead of reactive and basic organometallics as nucleophiles, the reaction proceeded with high chemoselectivity at the secondary amido group in the presence of a series of sensitive functional groups such as aldehyde, ketone, ester, cyano, nitro, and tertiary amido groups. The reaction can be viewed as a Friedel-Crafts-type reaction using secondary amides as acylating agents or as an intermolecular version of the Bischler-Napieralski reaction.

One-Pot Reductive 1,3-Dipolar Cycloaddition of Secondary Amides: A Two-Step Transformation of Primary Amides

The one-pot reductive 1,3-dipolar cycloaddition of secondary aromatic N-(trimethylsilylmethyl)amides with reactive dipolarophiles is reported. The method relies on the in situ generation of nonstabilized NH azomethine ylide dipoles via amide activation with triflic anhydride, partial reduction with 1,1,3,3-tetramethyldisiloxane (TMDS), and desilylation with cesium fluoride (CsF). Running under mild conditions, the reaction tolerated several sensitive functional groups and provided cycloadducts in 71-93% yields. The use of less reactive dipolarophile methyl acrylate led to the cycloadduct in only 40% yield. A (Z) geometric intermediate of NH-azomethine 1,3-dipole was postulated to account for the observed higher yields and higher cis diastereoselectivity for the substrates bearing an electron-withdrawing group. This model features an unconventional cyclic transition state via carbanion-aryl ring interaction. Because the starting secondary amides can be prepared from common primary amides, the current method also constitutes a two-step transformation of primary amides.

Aza-Knoevenagel-type condensation of secondary amides: direct access to N-monosubstituted β,β-difunctionalized enamines

An efficient approach toN-monosubstituted β,β-difunctionalized enamines, a class of versatile building blocks for the synthesis of bioactive compounds, is reported.