Synthesis of Novel Heterocycles by Amide Activation and Umpolung Cyclization

PyrAtes: Modular Organic Salts with Large Stokes Shifts for Fluo-rescence Microscopy

The deployment of small-molecule fluorescent agents plays an ever-growing role in medicine and drug development. Herein, we complement the portfolio of powerful fluorophores, reporting the serendipitous discovery and development of a novel class with an imidazo[1,2-a]pyridinium triflate core, which we term PyrAtes. These fluorophores are synthesized in a single step from readily available materials (>60 examples) and display Stokes shifts as large as 240 nm, while also reaching NIR-I emissions at λmax as long as 720 nm. Computational studies allow the development of a platform for the prediction of λmax and λEm. Furthermore, we demonstrate the compatibility of these novel fluorophores with live cell imaging in HEK293 cells, suggesting PyrAtes as potent intracellular markers.

Fluorine-Assisted Rearrangement of Geminal Azidofluorides to Imidoyl Fluorides

Organoazide rearrangement constitutes versatile synthetic strategies but typically requires an extremely strong acid and/or a high reaction temperature. Our group recently discovered the remarkable accelerating effect of the geminal fluorine substituent that enables the facile rearrangement of azides into imidoyl fluorides without the aid of acid under much milder reaction conditions. The role of geminal fluorine was elucidated by both experimental and computational investigations. This new reactivity led to the development of a practical one-step tandem preparative method for potentially useful and bench-stable imidoyl fluorides from a wide range of structurally diverse geminal chlorofluorides. Our additional efforts to expand the reaction scope regarding the migrating group, halogen, and carbonyl function are described, and the synthetic utility of the imidoyl fluoride products was demonstrated in hopes of promoting the use of this under-appreciated functional group in the synthetic organic community.

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.

Photoinduced carbamoylation reactions: unlocking new reactivities towards amide synthesis

The preparation of amide-containing compounds is among the most interesting and challenging topics for the synthetic community. Such relevance is given by their reactive aspects explored in the context of organic synthesis and by the direct application of these compounds as pharmaceuticals and useful materials, and their key roles in biological structures. A simple and straightforward strategy for the amide moiety installation is the use of carbamoyl radicals - this nucleophilic one-electron intermediate is prone to undergo a series of transformations, providing a range of structurally relevant derivatives. In this review, we summarize the latest advances in the field from the perspective of photoinduced protocols. To this end, their synthetic applications are organized accordingly to the nature of the radical precursor (formamides through HAT, 4-substituted-1,4-dihydropyridines, oxamic acids, and N-hydroxyphthalimido esters), the mechanistic aspects also being highlighted. The discussion also includes a recent approach proceeding via photolytic C-S cleavage of dithiocarbamate-carbamoyl intermediates. By exploring fundamental concepts, this material aims to offer an understanding of the topic, which will encourage and facilitate the design of new synthetic strategies applying the carbamoyl radical.

Ir-Catalyzed Chemoselective Reductive Condensation Reactions of Tertiary Amides with Active Methylene Compounds

The catalytic reductive condensation reactions of tertiary amides with active methylene compounds leading to multifunctionalized non-N-containing products is described. The reactions proceed through sequential iridium-catalyzed hydrosilylation of the amides followed by acid-mediated condensation with the active methylene compounds. This scalable method is broad in scope and shows remarkable chemoselectivity for the amide group in the presence of several sensitive or even more reactive functionalities such as ester, cyano, nitro, silyl dienol ether, and ketone.

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

Unified Synthesis of Polycyclic Alkaloids by Complementary Carbonyl Activation*

A complementary dual carbonyl activation strategy for the synthesis of polycyclic alkaloids has been developed. Successful applications include the synthesis of tetracyclic alkaloids harmalanine and harmalacinine, pentacyclic indoloquinolizidine alkaloid nortetoyobyrine, and octacyclic β-carboline alkaloid peganumine A. The latter synthesis features a protecting-group-free assembly and an asymmetric disulfonimide-catalyzed cyclization. Furthermore, formal syntheses of hirsutine, deplancheine, 10-desbromoarborescidine A, and oxindole alkaloids rhynchophylline and isorhynchophylline have been achieved. Finally, a concise synthesis of berberine alkaloid ilicifoline B was completed.

Diastereoselective Synthesis of Functionalized 5-Amino-3,4-Dihydro-2H-Pyrrole-2-Carboxylic Acid Esters: One-Pot Approach Using Commercially Available Compounds and Benign Solvents

A novel three-step four-transformation approach to highly functionalized 5-amino-3,4-dihydro-2H-pyrrole-2-carboxylic acid esters, starting from commercially available phenylsulfonylacetonitrile, aldehydes, and N-(diphenylmethylene)glycine tert-butyl ester, was developed. The one-pot strategy delivered this class of amidines bearing, for the first time, three contiguous stereocenters, in good to high yield and diastereoselectivity. The entire sequence was carried out using diethyl carbonate and 2-methyl tetrahydrofuran as benign solvents, operating under metal-free conditions. The process could be conveniently scaled-up, and the synthetic utility of the products was demonstrated.

General Synthesis of Fully Substituted 4-Aminooxazoles from Amides and 1,4,2-Dioxazol-5-ones Based on Amide Activation and Umpolung Process

A general and efficient synthesis of fully substituted 4-aminodixazoles was developed based on the strategies of amide activation and umpolung reaction. In this method, 1,4,2-dioxazol-5-ones were introduced as a rare type of umpolung reagent bearing a nucleophilic N-atom that could be used well together with the activating agent Tf₂O. Because 1,4,2-dioxazol-5-ones played triple roles as an umpolung reagent, a substrate, and a weak base, the method proceeded smoothly under extremely convenient conditions.