Fast and Bioorthogonal Release of Isocyanates in Living Cells from Iminosydnones and Cycloalkynes

Bioorthogonal click-and-release reactions are powerful tools for chemical biology, allowing, for example, the selective release of drugs in biological media, including inside animals. Here, we developed two new families of iminosydnone mesoionic reactants that allow a bioorthogonal release of electrophilic species under physiological conditions. Their synthesis and reactivities as dipoles in cycloaddition reactions with strained alkynes have been studied in detail. Whereas the impact of the pH on the reaction kinetics was demonstrated experimentally, theoretical calculations suggest that the newly designed dipoles display reduced resonance stabilization energies compared to previously described iminosydnones, explaining their higher reactivity. These mesoionic compounds react smoothly with cycloalkynes under physiological, copper-free reaction conditions to form a click pyrazole product together with a released alkyl- or aryl-isocyanate. With rate constants up to 1000 M^-1 s^-1, this click-and-release reaction is among the fastest described to date and represents the first bioorthogonal process allowing the release of isocyanate electrophiles inside living cells, offering interesting perspectives in chemical biology.

A mechanochemical approach to the synthesis of sydnones and derivatives

Sydnones are heterocyclic compounds which display important biological activities, including their abilities to react in 1,3-dipolar additions for applications in the development of new prodrugs. Capitalizing on our preliminary work on the mechanosynthesis of sydnones, an extension of this work to two related families of molecules, diarylsydnones and iminosydnones is reported. A ball-milling approach towards the synthesis of diaryl sydnones was developed, a necessary step for the synthesis of potential sydnone-based ligands of metal complexes. A mechanochemistry-based synthesis of iminosydnones was optimized, including the preparation of active pharmaceutical ingredients (API) related to feprosidnine, linsidomine, mesocarb and molsidomine. This work demonstrated that the ball-milling procedures were efficient and time saving through avoiding purification steps, and reduced the use of organic solvents.

Click and Bio-Orthogonal Reactions with Mesoionic Compounds

Click and bio-orthogonal 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 dipole partners. Mesoionics are a very old family of molecules, but their use as reactants for click and bio-orthogonal chemistry is quite recent. The facility to derivatize these dipoles and to tune their reactivity toward 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.

Access to N-Carbonyl Derivatives of Iminosydnones by Carbonylimidazolium Activation

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.

Oxidative addition of carbon dioxide into mesoionics

This work examines the prospect of making stable mesoionic compounds of the type mesomeric betaine R+-CO2- from direct oxidative additions of carbon dioxide to suitably-delocalized singlet carbene moieties, with bold objectives of carbon sequestration and overall energy storage. A set of possible candidates for such mesoionic compounds is theoretically explored through DFT calculations, inspecting coupling paths, thermodynamic and kinetic stabilities, and geometric and electronic structural features. Among others, the addressed cationic parts include aromatic rings in their broader sense, phenalene systems, and odd linear polyenic chains. Various structurally-close neutral alternatives such as oxiranones or carbene-acid forms are also considered. In the linear polyenic chain family, there is stark contrast between 4N + 1 and 4N - 1 lengths, with ensuing substantial consequences for stabilities and structures. Amino substitutions can favor mesoionic arrangements through their cation-stabilizing π-donor properties, further supported by possible strong intramolecular hydrogen bonds, but they can also contribute to weaken their kinetic stability through the existence of stable neutral imino alternatives. All in all, mesoionics including tropylium, phenalene, or 4N + 1 odd polyene frames as cationic parts could be reasonable targets.

Esterase-activated chromane–sydnonimine prodrug hybrids

The preparation and characterization of two vitamin E analogs-sydnonimine conjugates, delta-tocopheryloxycarbonyl-3-morpholinosydnonimine (2) and troloxoxycarbonyl-3-morpholinosydnonimine (3), in which the hydroxyl group of the tocopheryl moieties is linked via an enzymatically cleavable urethane group to the sydnone moiety is described. In the presence of porcine liver esterase, these tocopheryl-sydnonimine conjugates generated the expected antioxidant moieties, i.e., delta-tocopherol or Trolox, and were found to convert oxyhemoglobin to methemoglobin at 37 degrees C in 50 mM phosphate buffer at pH 7.4, thus providing evidence for nitric oxide release. Their potency as antioxidants was indirectly studied by associating the two products of the hydrolysis, SIN-1, and delta-tocopherol or Trolox. Our findings suggest that unlike the other members of the sydnonimine family these chromane-sydnonimine derivatives do not act as peroxynitrite donors, and require enzymatic bioactivation before nitric oxide or nitroxyl anion (NO(-)) can be released.