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Herein, we describe a methodology for iminosydnone chlorination and we demonstrate the high beneficial effect of this modification on the reactivity of these mesoionic dipoles in strain-promoted cycloaddition reactions. Exploiting their reaction with cyclooctynes, we used these new iminosydnones for bioorthogonal release of amide, urea and sulfonamide containing drugs. Notably, drugs containing a terminal amide function were released for the first time with good kinetic constants.

Herein, we report a metal-free synthesis of cyclic amidines, oxazines, and an oxazinone under mild conditions by electrophilic amide activation. This strategy features an unusual Umpolung cyclization mode and enables the smooth union of α-aryl amides and diverse alkylazides, effectively rerouting our previously reported α-amination transform.

A new methodology for N-exocyclic functionalization of iminosydnones was developed involving the addition of a large variety of nucleophiles on carbonyl-imidazolium-activated iminosydnones. This practical and highly versatile method provided access to new classes of iminosydnones and opened a straightforward synthetic route to prepare iminosydnone-based prodrugs.

A direct C-C coupling process that merges Michael acceptors and Eschenmoser's salt is presented. Although reminiscent of the Morita-Baylis-Hillman reaction, this process requires no Lewis base catalyst. The underlying mechanism was unveiled by a combination of kinetic, isotopic labelling experiments as well as computational investigations, which showcased the critical role of HFIP as a superior mediator for proton-transfer events as well as the decisive role of the halide counterion.

Reported here is the reactivity of mesoionic 1,3-dithiolium-4-olates towards strained alkynes, leading to thiophene cycloaddition products. In the process, the potential of these dipoles towards orthogonal reaction with azides, allowing efficient double ligation reactions, was discovered. A versatile process to access benzo[c]thiophenes, in an unprecedented divergent fashion, was developed and provides a new entry to unconventional polyaromatic thiophenes.

We report the synthesis and use of sydnone-based profluorophores as tools for imaging applications. These new probes display exquisite reactivity towards strain promoted cycloaddition reactions with cycloalkynes allowing fast, efficient and selective labeling in biological media. Styryl-pyridinium sydnone probes were found particularly interesting for click reactions to proceed selectively inside cells.

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., A direct γ‐oxidation of β,γ‐unsaturated amides is reported. This regioselective reaction with TEMPO proceeds under mild conditions, tolerant of other carbonyl functionality. A rare example of regioselective amide activation of unsaturated substrates; it results in products carrying oxyaminated substitution ripe for remarkably selective radical cyclisation reactions.

International audience; β-Lactams, the cornerstone of antibiotherapy, inhibit multiple and partially redundant targets referred to as transpeptidases or penicillin-binding proteins. These enzymes catalyze the essential cross-linking step of the polymerization of cell wall peptidoglycan. The understanding of the mechanisms of action of β-lactams and of resistance to these drugs requires the development of reliable methods to characterize their targets. Here, we describe an activity-based purification method of β-lactam targets based on click and release chemistry. We synthesized alkyne-carbapenems with suitable properties with respect to the kinetics of acylation of a model target, the Ldtfm L,D-transpeptidase, the stability of the resulting acylenzyme, and the reactivity of the alkyne for the cycloaddition of an azido probe containing a biotin moiety for affinity purification and a bioorthogonal cleavable linker. The probe provided access to the fluorescent target in a single click and release step.

International audience; Click and bioorthogonal reactions are dominated by cycloaddition reactions in general and 1,3-dipolar cycloadditions in particular. Among the dipoles routinely used for click chemistry, azides, nitrones, isonitriles and nitrile oxides are the most popular. This review is focused on the emerging click chemistry that uses mesoionic compounds as dipoles partners. Mesoionics are a very old family of molecules but their use as reactants for click and bioorthogonal chemistry is quite recent. The facility to derivatize these dipoles and to tune their reactivity towards cycloaddition reactions makes mesoionics an attractive opportunity for future click chemistry development. In addition, some compounds from this family are able to undergo click and release reactions, finding interesting applications in cells, as well as in animals. This review covers the synthetic access to main mesoionics, their reaction with dipolarophiles and recent applications in chemical biology and heterocycle synthesis.

A new methodology for

Controlled protein functionalization holds great promise for a wide variety of applications. However, despite intensive research, the stoichiometry of the functionalization reaction remains difficult to control due to the inherent stochasticity of the conjugation process. Classical approaches that exploit peculiar structural features of specific protein substrates, or introduce reactive handles via mutagenesis, are by essence limited in scope or require substantial protein reengineering. We herein present equimolar native chemical tagging (ENACT), which precisely controls the stoichiometry of inherently random conjugation reactions by combining iterative low-conversion chemical modification, process automation, and bioorthogonal trans-tagging. We discuss the broad applicability of this conjugation process to a variety of protein substrates and payloads., Controlled protein functionalization holds great promise for a wide variety of applications.

International audience; The chemistry and reactivity of mesoionic compounds represent a chemical space largely unexploited. Using a MS-based screening approach, the reactivity of mesoionics toward terminal alkynes under copper catalysis has been explored. This approach led to the discovery of new reactions allowing chemoselective ligation of the two reaction partners and, simultaneously, the release of a fragment molecule. The reactivity of aza-iminosydnones allowing the selective formation of 1,2,3 triazoles or hydrazineylidene-propanal derivatives from alkynes was revealed. This copper-catalyzed reaction was found to be under the control of copper-ligand and alkyne structures. The scope of the reactions and mechanistic investigations are presented.

Emerging applications in the field of chemical biology are currently limited by the lack of bioorthogonal reactions allowing both removal and linkage of chemical entities on complex biomolecules. We recently discovered a novel reaction between iminosydnones and strained alkynes leading to two products resulting from ligation and fragmentation of iminosydnones under physiological conditions. We now report the synthesis of a panel of substituted iminosydnones and the structure reactivity relationship between these compounds and strained alkyne partners. This study identified the most relevant substituents, which allow to increase the rate of the transformation and to develop a bifunctional cleavable linker with improved kinetics.

We report the discovery of a new bioorthogonal click-and-release reaction involving iminosydnones and strained alkynes. This transformation leads to two products resulting from the ligation and fragmentation of iminosydnones under physiological conditions. Optimized iminosydnones were successfully used to design innovative cleavable linkers for protein modification, thus opening up new areas in the fields of drug release and target-fishing applications. This click-and-release technology offers the possibility of exchanging tags on proteins for functionalized cyclooctynes under mild and bioorthogonal conditions.

International audience; Copper-catalyzed and copper-free sydnone-alkyne cycloaddition reactions have emerged as complementary click tools for chemical biology but their use in bioorthogonal labeling is still in its infancy. Herein, combinations of alkynes and coumarin-sydnones were screened for their ability to generate pyrazole products displaying strong fluoroscence enhancement compared to reactants. One sydnone was identified as a particularly suitable new turn-on probe for protein labeling. Clickable fluorogenic probes are important chemical tools to label, visualize and study biomolecules. 1 They are typically designed with a profluorophore bearing a clickable function that both suppresses the fluorescence and allows the ligation to the corresponding click partner. 2 The structural changes accompanying the click reaction lead to unquenching of the profluorophore and result in the enhancement of the fluorescence. Many of these probes are based on the Cu(I)-catalyzed azide-alkyne [3+2] cycloaddition reaction (CuAAC), which can be considered as the archetype of click chemistry , and its copper free version using strained cycloalkynes. 3 Over the last decade, a series of azides 4 and alkynes 5 profluorophores have been developed and successfully used for biological imaging. Inspired by this remarkable amount of work, we wondered if sydnones might be used to expand the toolbox of clickable fluoro-genic probes. Our group identified sydnones as an interesting dipole partner for the Cu-catalyzed cycloaddition reaction with terminal alkynes, leading to 1,4-pyrazoles. This reaction was called CuSAC for Cu-catalyzed Sydnone Alkyne Cycloaddition. 6 Following this work, the copper free version (SPSAC, Strained Promoted Sydnone Alkyne Cycloaddition) was developed by the group of J. W. Chin, 7 our group 8 and by the group of J. M. Murphy. 9 Later, we showed that sydnones can be tuned to provide highly reactive clickable reagents for both SPSAC and CuSAC reactions, displaying higher kinetic properties than reactions involving azides. 10 Very recently, sydnone-based luminophores were developed and used for the labeling of proteins and living cells using SPSAC. 11 An example of fluorogenic photoligation reaction using diarylsydnones was also recently disclosed and applied to protein labeling. 12 In this contribution, we designed a series of sydnone-coumarins and evaluated their ability to behave as effective fluorogenic clickable turn-on probes, both in CuSAC and SPSAC reactions. The enhancement of the fluorescence results from the suppression of internal quenching and the structural modifications triggered by the transformations (Scheme 1). Coumarins were chosen as profluorophore candidates because they are stable, small in size and routinely used for molecular imaging. Substitutions in position 3-and 7-of coumarin dyes are well known to have a strong impact on their fluorescence properties. We reasoned that the mesoionic ring could be installed on these positions and synthetized a series of coumarin derivatives , as depicted in Scheme 2. Though the synthesis of coumarins is well established, and the insertion of a variety of heterocycles has been reported, sydnone-coumarin scaffolds are still unexplored. 13 7-Sydnone-coumarins 1 and 2 were obtained from commercially available 7-amino-coumarins through a standard reductive amination step in presence of glyoxylic acid and sodium cyanoborohydride as a reducing agent. The glycine intermediates were nitrosylated in presence of tert-butyl nitrite, and subsequent cyclodehydration Scheme 1 Schematic principle of turn-on sydnones probes.