alpha.,.beta. Dehydrogenation of carboxamides

Sulfonium Rearrangements Enable the Direct Preparation of Sulfenyl Imidinium Salts

Sulfenyl imidinium salts are a virtually unexplored class of intermediates in organic chemistry. Herein, we demonstrate how sulfonium rearrangements can be deployed to access these versatile synthetic intermediates, bearing three contiguous (and congested) stereogenic centers, with high levels of selectivity. The synthetic value of the scaffold was unraveled by selective transformations into a range of building blocks, including 1,4-dicarbonyl derivatives and sulfonolactones.

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.

Copper-Catalyzed Chemo-, Regio-, and Stereoselective Multicomponent 1,2,3-Trifunctionalization of Internal Alkynes

Herein, we report the first diaryliodonium salts promoted multicomponent 1,2,3-trifunctionalization of alkynes, where both the acetylenic bond and the adjacent nonactivated propargylic C(sp³)-H bond were functionalized synergistically to generate α-arylated enones with high chemo-, regio-, and stereoselectivity. A broad spectrum of diaryliodonium salts and internal alkynes could be utilized in this protocol, and a diverse collection of highly substituted and stereochemically defined linear and cyclic complex structures could be elaborated from the enone products.

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.

Recent Progress in Enolonium Chemistry under Metal-Free Conditions

Umpolung approach through inversion of the polarity of conventional enolates, has opened up an unprecedented opportunity in the cross-coupling via alkylation. The enolonium equivalents can be accessed either by hypervalent iodine reagents, activation/oxidation of amides, or the oxidation of alkynes. Under umpolung conditions, highly basic conditions required for classical enolate chemistry can be avoided, and they can couple with unmodified nucleophiles such as heteroatom donors and electron-rich arenes.

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.

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.

Chemoselective α-Sulfidation of Amides Using Sulfoxide Reagents

The direct α-sulfidation of tertiary amides using sulfoxide reagents under electrophilic amide activation conditions is described. Employing convenient and readily available reagents, selective functionalization takes place to generate isolable sulfonium ions en route to α-sulfide amides. Mechanistic studies identified activated sulfoxides as promoters of the desired transformation and enabled the extension of the methodology from benzylic to aliphatic amide substrates.

An α-Cyclopropanation of Carbonyl Derivatives by Oxidative Umpolung

The reactivity of iodine(III) reagents towards nucleophiles is often associated with umpolung and cationic mechanisms. Herein, we report a general process converting a range of ketone derivatives into α-cyclopropanated ketones by oxidative umpolung. Mechanistic investigation and careful characterization of side products revealed that the reaction follows an unexpected pathway and suggests the intermediacy of non-classical carbocations.

A Novel Class of 7-Membered Heterocyclic Compounds

The work presented herein describes the synthesis of a formerly inaccessible class of heterocyclic compounds. The reaction relies on α‐phthalimido‐amides, which are readily prepared from amino acids in 2 simple reactions steps. Under amide activation conditions in which classical keteniminium ions are not formed, the nitrile solvent is incorporated into the new fused 7‐membered ring system. Due to the absence of a keteniminium intermediate, the stereogenic information in the α‐position is fully retained.

Chemoselective formal β-functionalization of substituted aliphatic amides enabled by a facile stereoselective oxidation event

Aliphatic C-H functionalization is a topic of current intense interest in organic synthesis. Herein, we report that a facile and stereoselective dehydrogenation event enables the functionalization of aliphatic amides at different positions in a one-pot fashion. Derivatives of relevant pharmaceuticals were formally functionalized in the β-position in late-stage manner. A single-step synthesis of incrustoporine from a simple precursor further showcases the potential utility of this approach.

α-Arylation of Carbonyl Compounds through Oxidative C-C Bond Activation

A synthetically useful approach for the direct α-arylation of carbonyl compounds through a novel oxidative C-C bond activation is reported. This mechanistically unusual process relies on a 1,2-aryl shift and results in all-carbon quaternary centers. The transformation displays broad functional-group tolerance and can in principle also be applied as an asymmetric variant.

α-Fluorination of carbonyls with nucleophilic fluorine

Given the unique properties of fluorine, and the ability of fluorination to change the properties of organic molecules, there is significant interest from medicinal chemists in innovative methodologies that enable the synthesis of new fluorinated motifs. State-of-the-art syntheses of α-fluorinated carbonyl compounds invariably rely on electrophilic fluorinating agents, which can be strongly oxidizing and difficult to handle. Here we show that reversing the polarity of the enolate partner to that of an enolonium enables nucleophilic fluorinating agents to be used for direct chemoselective α-C-H-fluorination of amides. Reduction of these products enables facile access to β-fluorinated amines and the value of this methodology is shown by the easy preparation of a number of fluorinated analogues of drugs and agrochemicals. A fluorinated analogue of citalopram, a marketed antidepressant drug, is presented as an example of the preserved biological activity after fluorination.

Amide activation: an emerging tool for chemoselective synthesis

It is textbook knowledge that carboxamides benefit from increased stabilisation of the electrophilic carbonyl carbon when compared to other carbonyl and carboxyl derivatives. This results in a considerably reduced reactivity towards nucleophiles. Accordingly, a perception has been developed of amides as significantly less useful functional handles than their ester and acid chloride counterparts. However, a significant body of research on the selective activation of amides to achieve powerful transformations under mild conditions has emerged over the past decades. This review article aims at placing electrophilic amide activation in both a historical context and in that of natural product synthesis, highlighting the synthetic applications and the potential of this approach.

Chemoselective Intermolecular Cross-Enolate-Type Coupling of Amides

A new approach for the synthesis of 1,4-dicarbonyl compounds is reported. Chemoselective activation of amide carbonyl functionality and subsequent umpolung viaN-oxide addition generates an electrophilic enolonium species that can be coupled with a wide range of nucleophilic enolates. The method conveys broad functional group tolerance on both components, does not suffer from formation of homocoupling byproducts and avoids the use of transition metal catalysts.

Flexible and Chemoselective Oxidation of Amides to α-Keto Amides and α-Hydroxy Amides

A suite of flexible and chemoselective methods for the transition-metal-free oxidation of amides to α-keto amides and α-hydroxy amides is presented. These highly valuable motifs are accessed in good to excellent yields and stereoselectivities with high functional group tolerance. The utility of the method is showcased by the formal synthesis of a potent histone deacetylase inhibitor.

Metal-Free Formal Oxidative C-C Coupling by In Situ Generation of an Enolonium Species

Much contemporary organic synthesis relies on transformations that are driven by the intrinsic, so-called "natural", polarity of chemical bonds and reactive centers. The design of unconventionally polarized synthons is a highly desirable strategy, as it generally enables unprecedented retrosynthetic disconnections for the synthesis of complex substances. Whereas the umpolung of carbonyl centers is a well-known strategy, polarity reversal at the α-position of a carbonyl group is much rarer. Herein, we report the design of a novel electrophilic enolonium species and its application in efficient and chemoselective, metal-free oxidative C-C coupling.