Weinreb Amides as Privileged Acylating Agents for Accessing α-Substituted Ketones

Isomerization of E-Cinnamamides into Z-Cinnamamides Using a Recycling Photoreactor

The photocatalytic synthesis of thermodynamically less-stable Z-alkenes has received considerable research attention in recent years. In this study, a recycling photoreactor was applied to the photoisomerization of E-alkenes (cinnamamide and Weinreb amide derivatives) to produce Z-alkenes. The closed-loop recycling system comprises an immobilized photosensitizer to achieve rapid photoisomerization and a high-performance liquid chromatography instrument for separation of the Z/E diastereomers. After 4-10 cycles, the desired pure Z-alkenes were obtained efficiently. In the photoreactor system, a photosensitizer (thioxanthone) was covalently immobilized on silica gel via amide bonding, which led to an enhanced photocatalytic activity compared to the parent thioxanthone. This recycling photoreactor shows promise as an alternative system for the production of Z-alkenes.

Counterintuitive chemoselectivity in the reduction of carbonyl compounds

The reactivity of carbonyl functional groups largely depends on the substituents on the carbon atom. Reversal of the commonly accepted order of reactivity of different carbonyl compounds requires novel synthetic approaches. Achieving selective reduction will enable the transformation of carbon resources such as plastic waste, carbon dioxide and biomass into valuable chemicals. In this Review, we explore the reduction of less reactive carbonyl groups in the presence of those typically considered more reactive. We discuss reductions, including the controlled reduction of ureas, amides and esters to aldehydes, as well as chemoselective reductions of carbonyl groups, including the reduction of ureas over carbamates, amides and esters; the reduction of amides over esters, ketones and aldehydes; and the reduction of ketones over aldehydes.

Tertiary Amides as Directing Groups for Enantioselective C-H Amination using Ion-Paired Rhodium Complexes

Enantioselective C-H amination at a benzylic methylene is a vital disconnection towards chiral benzylamines. Here we disclose that butyric and valeric acid-derived tertiary amides can undergo highly enantioselective benzylic amination using an achiral anionic Rh complex that is ion-paired with a Cinchona alkaloid-derived chiral cation. A broad scope of compounds can be aminated encompassing numerous arene substitutions, amides, and two different chain lengths. Excellent tolerance of ortho substituents was observed, which has not been achieved before in asymmetric intermolecular C-H amination with Rh. We speculate that the tertiary amide group of the substrate engages in hydrogen bonding interactions directly with the chiral cation, enabling a high level of organisation at the transition state for C-H amination. This is in contrast with our previous work where a substrate bearing a hydrogen bond donor was required. Control experiments led to the discovery that methyl ethers also function as proficient directing groups under the optimised conditions, potentially also acting as hydrogen bond acceptors. This finding has the promise to dramatically expand the applicability of our ion-paired chiral catalysts.

Base-mediated homologative rearrangement of nitrogen-oxygen bonds of N-methyl-N-oxyamides

Due to the well known reactivity of C(O)-N functionalities towards canonical C1-homologating agents (e.g. carbenoids, diazomethane, ylides), resulting in the extrusion of the N-centered fragment en route to carbonyl compounds, formal C1-insertions within N-O bonds still remain obscure. Herein, we document the homologative transformation of N-methyl-N-oxyamides - with high tolerance for diverse O-substituents - into N-acyl-N,O-acetals. Under controlled basic conditions, the N-methyl group of the same starting materials acts as a competent precursor of the methylene synthon required for the homologation. The logic is levered on the formation of an electrophilic iminium ion (via N-O heterolysis) susceptible to nucleophilic attack by the alkoxide previously expulsed. The procedure documents genuine chemocontrol and flexibility, as judged by the diversity of substituents placed on both amide and nitrogen linchpins. The mechanistic rationale was validated through experiments conducted on D-labeled materials which unambiguously attributed the origin of the methylene fragment to the N-methyl group of the starting compounds.

Stereospecific Acylative Suzuki–Miyaura Cross-Coupling: General Access to Optically Active α-Aryl Carbonyl Compounds

A novel strategy for the stereospecific Pd-catalyzed acylative cross-coupling of enantiomerically enriched alkylboron compounds has been developed. The protocol features an extremely high level of enantiospecificity to allow facile access to synthetically challenging and valuable chiral ketones and carboxylic acid derivatives. The use of a sterically encumbered and electron-rich phosphine ligand proved to be crucial for the success of the reaction. Furthermore, on the basis of experimental and computational studies, a unique mechanism for the transmetalation, assisted by the noncovalent interactions of the C(sp³)-based organoboron reagent, has been identified.

Methyltrimethoxysilane (MTM) as a Reagent for Direct Amidation of Carboxylic Acids

Methyltrimethoxysilane [MTM, CH₃Si(OMe)₃] has been demonstrated to be an effective, inexpensive, and safe reagent for the direct amidation of carboxylic acids with amines. Two simple workup procedures that provide the pure amide product without the need for further purification have been developed. The first employs an aqueous base-mediated annihilation of MTM. The second involves simple product crystallization from the reaction mixture providing a low process mass intensity direct amidation protocol.

Discovery of Novel TRPM8 Blockers Suitable for the Treatment of Somatic and Ocular Painful Conditions: A Journey through pKa and LogD Modulation

Transient receptor potential melastatin 8 (TRPM8) is crucially involved in pain modulation and perception, and TRPM8 antagonists have been proposed as potential therapeutic approaches for pain treatment. Previously, we developed two TRPM8 antagonists and proposed them as drug candidates for topical and systemic pain treatment. Here, we describe the design and synthesis of these two TRPM8 antagonists (27 and 45) and the rational approach of modulation/replacement of bioisosteric chemical groups, which allowed us to identify a combination of narrow ranges of pK_a and LogD values that were crucial to ultimately optimize their potency and metabolic stability. Following the same approach, we then pursued the development of new TRPM8 antagonists suitable for the topical treatment of ocular painful conditions and identified two new compounds (51 and 59), N-alkoxy amide derivatives, that can permeate across ocular tissue and reduce the behavioral responses induced by the topical ocular menthol challenge in vivo.

Synthesis of Sulfoxonium Ylides from Amides by Selective N-C(O) Activation

The direct synthesis of sulfoxonium ylides from amides by selective N-C(O) cleavage is presented. The reaction proceeds through the nucleophilic addition of dimethylsulfoxonium methylide to the amide bond in acyclic twisted amides under exceedingly mild room temperature conditions. A variety of amides can be employed, and the protocol can be applied to the late-stage derivatization of pharmaceuticals. Mechanistic studies outline the relative order of reactivity of amides.

Concise Synthesis of Both Enantiomers of Pilocarpine

Furan-2-carboxylic acid was used as a starting material for the synthesis of dehydro-homopilopic acid. Esterification, hydrogenation and enzymatic hydrolysis followed by the reduction of Weinreb amides and a single-step attachment of a 1-methyl-imidazole residue allowed for the concise synthesis of both enantiomers of pilocarpine.

Synthesis of biaryl ketones by arylation of Weinreb amides with functionalized Grignard reagents under thermodynamic control vs. kinetic control of N,N-Boc2-amides

A highly efficient method for chemoselective synthesis of biaryl ketones by arylation of Weinreb amides (N-methoxy-N-methylamides) with functionalized Grignard reagents is reported. This protocol offers rapid entry to functionalized biaryl ketones after Mg/halide exchange with i-PrMgCl·LiCl under operationally-simple and practical reaction conditions. The scope of the method is highlighted in >40 examples, including bioactive compounds and pharmaceutical derivatives. Collectively, this transition-metal-free approach offers a major advantage over the recently established cross-coupling of amides by oxidative addition of N-C(O) bonds. Considering the utility of amide acylation reactions in modern synthesis, we expect that this method will be of broad interest.

A Fast and General Route to Ketones from Amides and Organolithium Compounds under Aerobic Conditions: Synthetic and Mechanistic Aspects

We report that the nucleophilic acyl substitution reaction of aliphatic and (hetero)aromatic amides by organolithium reagents proceeds quickly (20 s reaction time), efficiently, and chemoselectively with a broad substrate scope in the environmentally responsible cyclopentyl methyl ether, at ambient temperature and under air, to provide ketones in up to 93 % yield with an effective suppression of the notorious over-addition reaction. Detailed DFT calculations and NMR investigations support the experimental results. The described methodology was proven to be amenable to scale-up and recyclability protocols. Contrasting classical procedures carried out under inert atmospheres, this work lays the foundation for a profound paradigm shift of the reactivity of carboxylic acid amides with organolithiums, with ketones being straightforwardly obtained by simply combining the reagents under aerobic conditions and with no need of using previously modified or pre-activated amides, as recommended.

Nickel-Catalyzed Synthesis of Dialkyl Ketones from the Coupling of N-Alkyl Pyridinium Salts with Activated Carboxylic Acids

While ketones are among the most versatile functional groups, their synthesis remains reliant upon reactive and low-abundance starting materials. In contrast, amide formation is the most-used bond-construction method in medicinal chemistry because the chemistry is reliable and draws upon large and diverse substrate pools. A new method for the synthesis of ketones is presented here that draws from the same substrates used for amide bond synthesis: amines and carboxylic acids. A nickel terpyridine catalyst couples N-alkyl pyridinium salts with in situ formed carboxylic acid fluorides or 2-pyridyl esters under reducing conditions (Mn metal). The reaction has a broad scope, as demonstrated by the synthesis of 35 different ketones bearing a wide variety of functional groups with an average yield of 60±16 %. This approach is capable of coupling diverse substrates, including pharmaceutical intermediates, to rapidly form complex ketones.

The synthetic versatility of the Tiffeneau–Demjanov chemistry in homologation tactics

Abstract

The Tiffeneau–Demjanov rearrangement can be regarded as an interesting alternative to the more common semi-pinacol transposition. It is usually employed for ring extension but, under specific conditions, it can also be used for ring contraction. Compared to other techniques, such as the Demjanov rearrangement or homologations with diazo compounds, the Tiffeneau–Demjanov pathway presents attractive features including high yielding and selective processes. Ring enlargements follow very strict and simple rules, such as the movement of the less substituted carbon and retention of the configuration. The rearrangement process is mainly affected by steric factors, due to presence of neighbouring groups, rather than electronic ones. The ring contraction may be achieved positioning the amine within the ring, thus achieving a high level of control. Unfortunately, applications of the reaction in modern homologation chemistry are rare; therefore, the aim of the review is re-proposing to the synthetic community the versatility of this venerable reaction and thus, spurring its employment for tackling challenging homologations processes.

Graphic abstract

Direct and Chemoselective Synthesis of Tertiary Difluoroketones via Weinreb Amide Homologation with a CHF2-Carbene Equivalent

The homologation of Weinreb amides into difluoromethylketones with a formal nucleophilic CHF₂ transfer agent is reported. Activating TMSCHF₂ with potassium tert-amylate enables a convenient access to the difluorinated homologation reagent, which adds to the acylating partners. The high chemoselectivity showcased in the presence of variously multifunctionalized Weinreb amides, jointly with uniformly high yields, enables the strategy of general applicability without requiring any stabilization element for the putative carbanion.