Chemo- and stereoselective synthesis of fluorinated enamides from ynamides in HF/pyridine: second-generation approach to potent ureas bioisosteres

Transition-Metal-Free Hydrodefluoroamination of Trifluoromethyl Enones for the Synthesis of α-Fluoroenamides

A three-component strategy was developed to enable hydrodefluoroamination of β-trifluoromethyl enones by selectively activating two C(sp3)-F bonds in the trifluoromethyl group. The method involved a sequence of carbonyl reduction, hydrodefluorination, and defluoroamination under transition-metal-free conditions. Synthetically useful (E)-stereospecific α-fluoroenamides were obtained in good yields with diverse functional group tolerance, which could be easily transformed into valuable organofluorides and heterocycles. The carbonyl auxiliary exerts both electronic and steric impacts on the CF3-alkenes, allowing for controllable and selective defluorination.

Determination of the Hammett Acidity of HF/Base Reagents

Harnessing the acidity of HF/base reagents is of paramount importance to improve the efficiency and selectivity of fluorination reactions. Yet, no general method has been reported to evaluate their acidic properties, and experimental designs are still relying on a trial-and-error approach. We report a new method based on 19F NMR spectroscopy which allows highly sensitive measures and short-time analyses. Advantageously, the basic properties of the indicators can be determined upstream by DFT calculations, affording a simple yet robust semiempirical approach. In particular, the indicators used in this study were rationally designed to fit on the conceptually appealing and commonly used Hammett scale. This method has been applied to commercially available and recently developed HF/base reagents.

Regio- and Stereocontrolled Hydrohalogenation of Ynamides with N-Halosuccinimides (NXS) as the Halogen Source: Synthesis of (E)-α-Haloenamides

Presented herein is a facile stereoselective construction of synthetically versatile chiral and achiral (E)-α-haloenamides under mild conditions utilizing N-halosuccinimides and diphenylphosphine oxide. This reaction is metal-free, mild, efficient, very rapid, and practical and highlights the synthetic versatility of ynamides. The reaction has a broad substrate scope; both chiral and achiral ynamides have been transformed into the corresponding (E)-α-haloenamides within a very short period of time without compromising selectivity or complexity.

Hydrofluorination of Alkynes: From (E) to (Z)

The hydrofluorination of alkynes is an efficient synthetic route to monofluoroalkenes or difluoroalkanes. Both fluorinated motifs have found applications in medicinal chemistry and beyond. This review explores the recent advances in the hydrofluorination of diverse alkynes through various activation methods, from classical coinage metal catalysis to metal-free conditions. The range of alkynes goes from the simplest unactivated alkynes to activated ones (ynones and derivatives, ynamides, alkynyl sulfides and sulfones as much as haloalkynes). Regio- and stereoselective methods exists, but there is still room for improvement depending on the type of alkyne.

Au(I)-Catalyzed Regioselective Hydrofluorination of Propargylamines Using Aqueous HF

Gold-catalyzed regioselective hydrofluorination of alkynes using aqueous HF has been achieved by employing an amide directing group. For both aryl- and alkyl-substituted propargylamines, the fluorination occurred at the site distal to the amino group.

Chemo- and regio-divergent access to fluorinated 1-alkyl and 1-acyl triazenes from alkynyl triazenes

The 1,1,2,2-tetrafluoroethylene unit is a prevalent pattern in bioactive molecules and functional materials. Despite being in principle a straightforward strategy to access this motif, the direct tetrafluorination of alkynes involves...

gem-Heteroatom-Substituted Fluoroalkenes as Mimics of Amide Derivatives or Phosphates: A Comprehensive Review

gem-Heteroatom-substituted fluoroalkenes have received little attention despite their great potential in medicinal chemistry or in fine chemistry. Indeed, due to the electronic and steric similarity between the fluoroalkene moiety and the amide bond as well as the high strength of the carbon-fluorine bond, these gem-heteroatom-substituted fluoroalkenes could be envisioned as stable mimics of various important organic functions, such as phosphates, carbamates, S-thiocarbamates and ureas. We present herein an overview describing the syntheses over the last decade of heteroatom-substituted fluoroalkenes in geminal position. This review will be divided into several sections covering each the common following heteroatom: oxygen-, nitrogen-, sulfur-, phosphorus-, boron- and silicon-substituted fluoroalkenes.

Recent advances in urea- and thiourea-containing compounds: focus on innovative approaches in medicinal chemistry and organic synthesis

Urea and thiourea represent privileged structures in medicinal chemistry. Indeed, these moieties constitute a common framework of a variety of drugs and bioactive compounds endowed with a broad range of therapeutic and pharmacological properties. Herein, we provide an overview of the state-of-the-art of urea and thiourea-containing pharmaceuticals. We also review the diverse approaches pursued for (thio)urea bioisosteric replacements in medicinal chemistry applications. Finally, representative examples of recent advances in the synthesis of urea- and thiourea-based compounds by enabling chemical tools are discussed.

Intermolecular Addition of Carbon-Centered Radicals to Ynamides. A Regio- and Stereoselective Route to Persubstituted α-Iodo-enamides

Rather surprisingly, C-C bond formation through "intermolecular" radical addition to internal ynamides has never been reported. Actually, ynamides are excellent acceptors for "electrophilic" carbon-centered radicals. These processes enable the introduction of functionalized alkyl chains at Cβ, groups that have not yet been introduced via the addition of organometallics. Radical carboiodination affords persubstituted α-iodo-enamides in moderate to high yield. The addition is totally stereoselective. Theoretical support to the mechanism and the scope and limitation of the reaction are discussed.

Synthesis of fluorinated amphoteric organoborons via iodofluorination of alkynyl and alkenyl MIDA boronates

The iodofluorination of alkynyl and alkenyl MIDA boronates led to the synthesis of fluorinated organoborons bearing a valuable C–I bond.

Efficient and Divergent Synthesis of α-Halogenated Amides and Esters by Double Electrophilic Activation of Ynamides

An efficient and modular entry to α-halogenated amides and esters is reported. This reaction is based on an underestimated double electrophilic activation of ynamides sequentially involving highly reactive activated keteniminium and iminium ions. Upon simple reaction with HCl and an electrophilic halogenation reagent in the presence of water or an alcohol, a broad range of ynamides can be transformed, in a highly divergent manner, to α-halo amides and esters with high efficiency and under mild conditions.

Silver-Mediated Decarboxylative Fluorodiiodination of Alkynoic Acids: Synthesis of Regio- and Stereoselective Fluoroalkenes

A variety of arylalkynoic acids reacted with 1,3-diiodo-5,5-dimethylhydantoin and HF·pyridine in the presence of AgOAc to provide the corresponding 1-fluoro-2,2-diiodovinylarenes in good yields and high regioselectivity. In addition, Pd-catalyzed cross-coupling reaction of 1-fluoro-2,2-diiodovinylarenes afforded diaryl coupling products in the Suzuki reaction and monoaryl coupling products with high stereoselectivity in the Hiyama reaction. It was found that C-F-activated borylation of fluoroalkenes using Pd catalyst afforded the vinylboranes with good yields.

Synthesis of α-Fluorinated Imides via Direct Fluorohydroxylation of Ynamides

A practical synthesis of α-fluorinated imides via the catalyst-free fluorohydroxylation of ynamides is developed. The reaction employs commercially available Selectfluor (F-TEDA-BF₄) and H₂O as the fluorine and hydroxyl sources, respectively. A broad range of aryl- or alkyl-substituted ynamides were well applicable to the reaction with good functional group tolerance under simple and mild reaction conditions. The synthetic utility of the α-fluoroimide products was demonstrated by several value-added transformations. Preliminary mechanistic studies were conducted.

Fluoroenesulphonamides: N-sulphonylurea isosteres showing nanomolar selective cancer-related transmembrane human carbonic anhydrase inhibition

After hydrofluorination of ynesulphonamides in superacid or in the presence of hydrofluoric acid/base reagents, a series of α-fluoroenamides has been synthesised and tested for the inhibition of carbonic anhydrase (CA, EC 4.2.1.1) isoforms. This study reveals a new, highly selective family of cancer-related transmembrane human (h) CA IX/XII inhibitors. These original fluorinated ureido isosters do not inhibit the widespread cytosolic isoforms hCA I and II and selectively inhibit the transmembrane cancer-related hCA IX and XII, offering interesting new leads for future studies.

A Highly Regio- and Stereoselective Syntheses of α-Halo Enamides, Vinyl Thioethers, and Vinyl Ethers with Aqueous Hydrogen Halide in Two-Phase Systems

A metal-free regio- and stereoselective method is achieved for the preparation of ( E)-configured α-halo enamides, vinyl thioethers, and vinyl ethers using aqueous HX (X = F, Cl, Br, I), which features high functional group compatibility and regio- and stereoselectivity, mild conditions, high efficiency, and rapid transformation. Additionally, the isomers could be yielded readily from the ( E)-configured α-halo enamides via photocatalysis or under Sonogashira coupling conditions.

(Radio)fluoroclick Reaction Enabled by a Hydrogen-Bonding Cluster

We have developed a widely applicable nucleophilic (radio)fluoroclick reaction of ynamides with readily available and easy-to-handle KF(18 F). The reactions exhibited high functional-group tolerance and needed only an ambient atmosphere. This 18 F addition to C-C unsaturated bonds proceeded with extraordinarily high radiochemical yields.

Cationic polycyclization of ynamides: building up molecular complexity

Polycyclization reactions are among the most efficient synthetic tools for the synthesis of complex, polycyclic molecules in a single operation from simple starting materials. We report in this manuscript a full account on the discovery and development of a novel cationic polycyclization from readily available ynamides. Simple activation of these building blocks under acidic conditions enables the generation of highly reactive activated keteniminium ions, which triggers an unprecedented cationic polycyclization yielding highly substituted polycyclic nitrogen heterocycles possessing up to seven fused cycles and three contiguous stereocenters.

Fluorine and Fluorinated Motifs in the Design and Application of Bioisosteres for Drug Design

The electronic properties and relatively small size of fluorine endow it with considerable versatility as a bioisostere and it has found application as a substitute for lone pairs of electrons, the hydrogen atom, and the methyl group while also acting as a functional mimetic of the carbonyl, carbinol, and nitrile moieties. In this context, fluorine substitution can influence the potency, conformation, metabolism, membrane permeability, and P-gp recognition of a molecule and temper inhibition of the hERG channel by basic amines. However, as a consequence of the unique properties of fluorine, it features prominently in the design of higher order structural metaphors that are more esoteric in their conception and which reflect a more sophisticated molecular construction that broadens biological mimesis. In this Perspective, applications of fluorine in the construction of bioisosteric elements designed to enhance the in vitro and in vivo properties of a molecule are summarized.

Hydrogen-Bonding-Assisted Brønsted Acid and Gold Catalysis: Access to Both (E)- and (Z)-1,2-Haloalkenes via Hydrochlorination of Haloalkynes

We have developed an efficient synthesis of both (Z)- and (E)-chlorohaloalkenes via hydrochlorination of haloalkynes, based on two distinct hydrogen-bond-network-assisted catalytic systems: Brønsted acid catalysis and gold catalysis. Both systems offer high stereoselectivity, good chemical yields, and diverse functional group tolerance.

(IPr)CuF-catalyzed α-site regiocontrolled trans-hydrofluorination of ynamides

With Et3N·3HF as the fluorination reagent, (IPr)CuF-catalyzed α-site regiocontrolled trans-hydrofluorination of ynamides has been achieved, affording (Z)-α-fluoroenamides in moderate to excellent yields. It was interesting to note that the regioselectivity of the reaction is reversed to that observed in the (Ph3P)3CuF-catalyzed hydrofluorination of ynamides. Additionally, a variety of different ynamides including oxazolidinonyl-, imidazolyl-, and N-sulfonyl ynamides were suitable for the reaction system and the subsequent oxidation of the fluorinated products enables a convenient synthesis to α-fluoroimides.

Metal-free, Regio-, and Stereo-Controlled Hydrochlorination and Hydrobromination of Ynones and Ynamides

We developed an atom-economical and metal-free method for the regio- and stereo-selective hydrohalogenation of ynones and ynamides using easy to handle DMPU/HX (X = Br or Cl) reagents. The reaction operates under mild conditions and a range of functional groups is well tolerated. We propose that the hydrohalogenation of ynones gives the anti-addition products via a concerted multimolecular Ad_E3 mechanism and that the hydrohalogenation of ynamides produces the syn-addition products via a cationic keteniminium intermediate.

Ynamide Preactivation Allows a Regio- and Stereoselective Synthesis of α,β-Disubstituted Enamides

A novel ynamide preactivation strategy enables the use of otherwise incompatible reagents and allows preparation of α,β-disubstituted enamides with high regio- and stereoselectivity. Mechanistic analysis reveals the intermediacy of a triflate-bound intermediate as a solution-stable, effective keteniminium reservoir, whilst still allowing subsequent addition of organometallic reagents.

Highly regio- and stereoselective trans-iodofluorination of ynamides enabling the synthesis of (E)-α-fluoro-β-iodoenamides

A highly regio- and stereoselective trans-iodofluorination reaction of ynamides with NIS and Et₃N·3HF has been achieved, affording (E)-α-fluoro-β-iodoenamides in moderate to good yields. The reaction proceeds under mild reaction conditions and exhibits good functional group compatibility.

Bioactive Natural Product and Superacid Chemistry for Lead Compound Identification: A Case Study of Selective hCA III and L-Type Ca2+ Current Inhibitors for Hypotensive Agent Discovery

Dodoneine (Ddn) is one of the active compounds identified from Agelanthusdodoneifolius, which is a medicinal plant used in African pharmacopeia and traditional medicine for the treatment of hypertension. In the context of a scientific program aiming at discovering new hypotensive agents through the original combination of natural product discovery and superacid chemistry diversification, and after evidencing dodoneine's vasorelaxant effect on rat aorta, superacid modifications allowed us to generate original analogues which showed selective human carbonic anhydrase III (hCA III) and L-type Ca²⁺ current inhibition. These derivatives can now be considered as new lead compounds for vasorelaxant therapeutics targeting these two proteins.

Brønsted acid-catalyzed α-halogenation of ynamides from halogenated solvents and pyridine-N-oxides

The keteniminium ions generated from the protonation of ynamides formed reversible adducts with counter anions and pyridine-N-oxides as well as halogenated solvents.

Synthesis of α-haloenamides via zinc halide mediated direct addition of benzhydryl halides to ynamides

A regio- and stereoselective synthesis of highly substituted α-haloenamides was described via the zinc halide mediated direct addition of benzhydryl halides to ynamides under mild conditions. The products α-haloenamides were further transformed into multisubstituted enamides via Suzuki and Sonogashira cross-coupling reactions.

Triphenylphosphine promoted regio and stereoselective α-halogenation of ynamides

Metal free α-halogenation of ynamides at room temperature.