Hetero-Diels-Alder and CuAAC Click Reactions for Fluorine-18 Labeling of Peptides: Automation and Comparative Study of the Two Methods.

Radiolabeled peptides are valuable tools for diagnosis or therapies; they are often radiofluorinated using an indirect approach based on an F-18 prosthetic group. Herein, we are reporting our results on the F-18 radiolabeling of three peptides using two different methods based on click reactions. The first one used the well-known CuAAC reaction, and the second one is based on our recently reported hetero-Diels-Alder (HDA) using a dithioesters (thia-Diels-Alder) reaction. Both methods have been automated, and the 18F-peptides were obtained in similar yields and synthesis time (37-39% decay corrected yields by both methods in 120-140 min). However, to obtain similar yields, the CuAAC needs a large amount of copper along with many additives, while the HDA is a catalyst and metal-free reaction necessitating only an appropriate ratio of water/ethanol. The HDA can therefore be considered as a minimalist method offering easy access to fluorine-18 labeled peptides and making it a valuable additional tool for the indirect and site-specific labeling of peptides or biomolecules.

Rapid and Highly Selective Fluorescent Labeling of Peptides via a Thia-Diels-Alder Cycloaddition: Application to Apelin.

Herein, we describe a catalyst-free thia-Diels-Alder cycloaddition for the chemoselective labeling of fully deprotected phosphonodithioester-peptides in solution with fluorophores functionalized with an exocyclic diene. The reaction was optimized on the model tripeptide 1 containing a lysine residue, which enabled its rapid and straightforward labeling with three different fluorophores (fluorescein, lissamine rhodamine B, and squaraine) in very mild conditions (H2O/iPrOH, 37 °C, 1 h). The reaction was then successfully applied to the chemoselective labeling of fully deprotected apelin-13 with squaraine dye. The resulting fluorescent ligand 18 exhibited a high affinity (0.17 ± 0.03 nM) for apelinR. It enabled the development of time-resolved FRET-based competition assays for high-throughput screening and drug discovery. Thanks to its fluorogenic properties, ligand 18 was also successfully involved in the live-cell optical imaging of apelinR in no-wash conditions.

Synthesis of Medium-Sized Heterocycles by Transition-Metal-Catalyzed Intramolecular Cyclization.

Medium-sized heterocycles (with 8 to 11 atoms) constitute important structural components of several biologically active natural compounds and represent promising scaffolds in medicinal chemistry. However, they are under-represented in the screening of chemical libraries as a consequence of being difficult to access. In particular, methods involving intramolecular bond formation are challenging due to unfavorable enthalpic and entropic factors, such as transannular interactions and conformational constraints. The present review focuses on the synthesis of medium-sized heterocycles by transition-metal-catalyzed intramolecular cyclization, which despite its drawbacks remains a straightforward and attractive synthesis strategy. The obtained heterocycles differ in their nature, number of heteroatoms, and ring size. The methods are classified according to the metal used (palladium, copper, gold, silver), then subdivided according to the type of bond formed, namely carbon-carbon or carbon-heteroatom.

Synthesis of Sulfur-Containing Exo-Bicyclic Dienes and Their Diels-Alder Reactions To Access Thiacycle-Fused Polycyclic Systems.

The stereocontrolled synthesis of unprecedented sulfur-containing exo-bicyclic 1,3-dienes is reported through a palladium-catalyzed reductive cyclization of sulfur-linked 2-bromoenynes. The fused bicyclic structure provides a better stability to the thiacyclic diene compared to the simple 3,4-dimethylenetetrahydrothiophene. Their reactivity toward several dienophiles has been investigated, and various original thiacycle-fused polycyclic systems have been obtained with high or total diastereoselectivity. Moreover, they are the first exo-bicyclic dienes used in Diels-Alder reactions. The relative configurations of four cycloadducts have been unambiguously assigned by X-ray crystallographic analysis. Mechanistic details of the cycloadditions have been examined by computational means.

Synthesis of Benzo[c]silole Derivatives Bearing a Tetrasubstituted Exocyclic C=C Double Bond by Palladium-Catalyzed Domino Reactions.

The synthesis of diversely substituted 2,3-dihydro-benzo[c]siloles through an unprecedented palladium-catalyzed domino sequence is reported, involving a cyclocarbopalladation of an internal silylalkyne. This reaction proceeds with complete stereoselectivity to lead to a fully substituted exocyclic C=C double bond. Notably, the overall domino sequence appears to be crucial to obtain the desired cyclic vinylsilanes.

Recent advances in the chemistry of organic thiocyanates.

Organic thiocyanates are important synthetic intermediates to access valuable sulfur-containing compounds. In this review the different methods for their preparation and their synthetic applications will be presented. The literature of the last 15 years will be covered, highlighting selected recent advances in the chemistry of this class of compounds. We hope to offer chemists the tools to have a good grasp of this singular functionality and open the door to further progress in this chemistry.

The IMS Library: from IN-Stock to Virtual.

A chemical library is a key element in the early stages of pharmaceutical research. Its design encompasses various factors, such as diversity, size, ease of synthesis, aimed at increasing the likelihood of success in drug discovery. This article explores the collaborative efforts of computational and synthetic chemists in tailoring chemical libraries for cost-effective and resource-efficient use, particularly in the context of academic research projects. It proposes chemoinformatics methodologies that address two pivotal questions: first, crafting a diverse panel of under 1000 compounds from an existing pool through synthetic efforts, leveraging the expertise of organic chemists; and second, expanding pharmacophoric diversity within this panel by creating a highly accessible virtual chemical library. Chemoinformatics tools were developed to analyse initial panel of about 10,000 compounds into two tailored libraries: eIMS and vIMS. The eIMS Library comprises 578 diverse in-stock compounds ready for screening. Its virtual counterpart, vIMS, features novel compounds guided by chemists, ensuring synthetic accessibility. vIMS offers a broader array of binding motifs and improved drug-like characteristics achieved through the addition of diverse functional groups to eIMS scaffolds followed by filtering of reactive or unusual structures. The uniqueness of vIMS is emphasized through a comparison with commercial suppliers' virtual chemical space.

Hetero-Diels-Alder click reaction of dithioesters for a catalyst-free indirect 18F-radiolabelling of peptides.

The HDA reaction of dithioesters was developed as a new click-reaction compatible with the indirect 18F-labelling of peptides. It involves dithioester-peptides and a radiofluorinated diene as a novel prosthetic group. The method was applied to a PSMA-ligand for the in vivo detection of LNCap tumors in xenografted mice.

Synthesis of 3-amino-thiochromanes from 4-benzyl 2-thiazolines, via an unprecedented intramolecular electrophilic aromatic substitution.

A one-pot synthesis of various N-substituted 3-amino-thiochromanes from 4-benzyl-2-methyl thiazoline via a thiazolinium salt is described. The obtained 3-amino-thiochromanes are enantiopure, as their precursors derive from chiral 2-aminoalcohols. The reaction involves the formation of a disulfide, which subsequently takes part in an unprecedented intramolecular electrophilic aromatic substitution.

Three cheers for nitrogen: aza-DKPs, the aza analogues of 2,5-diketopiperazines.

Nitrogen-containing heterocycles represent a major source of pharmacological probes and drug candidates. To extend their molecular diversity and their potential biological activities, it is of importance to design and synthesize new N-heterocyclic scaffolds. Therefore, aza-diketopiperazines (aza-DKPs), the aza analogues of well-known 2,5-diketopiperazines (DKPs), emerged as a promising new scaffold. Although the first synthesis of an aza-DKP dates from 1951, significant developments have been made during the last decade. This feature article summarizes the different synthetic strategies to access and functionalise aza-DKPs. Their biological properties and potential applications in medicinal chemistry and drug discovery are discussed as well.

Tandem Double-Cross-Coupling/Hydrothiolation Reaction of 2-Sulfenyl Benzimidazoles with Boronic Acids.

A new tandem palladium-catalyzed reaction involving a Suzuki-Miyaura coupling, a desulfenylative coupling, and a hydrothiolation of a triple bond is reported. Notably, the desulfenylative coupling occurs without copper(I) assistance and the hydrothiolation is totally regioselective and stereoselective. The overall process results in the double incorporation of the boronic acid and the reincorporation of the sulfenyl moiety into the product structure. Starting from 2-(bromobenzylsulfenyl)-1-propargyl benzimidazoles, the transformation led to variously substituted benzimidazoles bearing a stereodefined alkenyl sulfide.

On the actual EPR detection of thioacyl radicals.

The direct EPR detection of thioacyl radicals has been reported only once, while thioacyl nitroxides remain an elusive species. We failed to detect the thioacyl radicals from two thioaldehydes and from phosphoryldithioformates but have obtained EPR evidence that thioacyl radicals react with 2-methyl-2-nitrosopropane to give thiocarbonyloxyaminyls rather than thioacyl nitroxides. The results of DFT calculations support this unexpected reactivity of thioacyls, while making questionable their previous EPR identification.

Chemo- and enantioselective sulfoxidation of bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) into chiral BEDT-TTF-sulfoxide.

Selective sulfoxidation of BEDT-TTF (bis(ethylenedithio)-tetrathiafulvalene) with enantiopure (camphoryl-sulfonyl)oxaziridine derivatives provided the inner monosulfoxide, as demonstrated using single crystal X-ray analysis, with an enantiomeric excess of 44% (up to 74% after recrystallization).

Pyridinedithioesters as heterodienophiles: application to the synthesis of aprikalim.

[reaction: see text] Pyridinedithioesters can be used as efficient heterodienophiles when activated by complexation with BF(3), by protonation, or by oxidation of the nitrogen atom of the pyridine moiety. The hetero-Diels-Alder reaction using 3-pyridinedithioester as a heterodienophile was the key step in a new synthesis of Aprikalim in racemic form. The methodology can be reliably extended to prepare new analogues of Aprikalim.

Aerobic Copper-Mediated Domino Three-Component Approach to 2-Aminobenzothiazole Derivatives.

An unprecedented three-component reaction involving a 2,2'-diaminodiaryl disulfide, copper cyanide, and an electrophile is described. This transformation is based on an oxidative copper-mediated S-cyanation as a key step and involves a cyanation/cyclization/acylation domino sequence enabling a rapid and efficient synthesis of diversely substituted 2-aminobenzothiazole derivatives. Notably, this reaction proceeds via an original mechanism involving an intermolecular migration of the acyl group.

Synthesis of new 2-C-(2,3:5,6-di-O-isopropylidene)-beta-D-mannofuranosyldithioacetate derivatives.

Various new C-glycosides have been synthesized through the thioacylation reaction of different amines by a 2-C-mannofuranosyldithioacetate. Amino acids, di- and polyamines (putresceine, spermine, spermidine) and one amino alcohol were in particular used to generate glycothiopeptidic or precursors of "bola" structures.

Synthesis and properties of thiazoline based ionic liquids derived from the chiral pool.

A novel class of chiral ionic liquids derived from amino alcohols is prepared in multi-gram scale. Their potential in chiral recognition is shown in a preliminary example with racemic Mosher's acid salt.

5-Endo-Trig Radical Cyclizations of Bromomethyldimethylsilyl Diisopropylpropargylic Ethers. A Highly Diastereoselective Access to Functionalized Cyclopentanes.

An efficient radical sequence involving a 5-exo-dig, a diastereoselective 1,5-H transfer, and a rarely observed in an all-carbon system 5-endo-trig cyclization allows the construction of cyclopentyl derivatives 2 bearing four controlled stereogenic centers from diisopropyl precursors 1. Olefins 3 were also isolated as minor side products. The effect of the acetylenic substituent Y has been investigated, and the scope and the limitations of the cascade have been delineated.

Cyclocarbopalladation/cross-coupling cascade reactions in sulfide series: access to sulfur heterocycles.

Cyclocarbopalladation/cross-coupling cascade reactions were applied for the first time in a sulfur series and represent an efficient route to access sulfur heterocycles. Stille or Suzuki-Miyaura cross-coupling was successfully used as the final reaction. The products are original benzothiolane and isothiochromane scaffolds with a stereodefined tetrasubstituted exocyclic double bond. To illustrate the application of this method to the synthesis of bioactive molecules, a sulfur analogue of the anticancer agent tamoxifen was prepared as a potential selective estrogen-receptor modulator.