8 Sydnone-Based Cycloadditions in Click Chemistry

The 1,3-dipolar cycloaddition of sydnones (1,2,3-oxadiazolium-5-olates) with dipolarophiles, such as alkynes, has recently emerged as a versatile click reaction, with applications ranging from the mild and regioselective preparation of polysubstituted pyrazoles for drug discovery to the metal-free bioorthogonal ligation of biomacromolecules in living cells. This chapter reviews the importance of metal catalysis for controlling the regioselectivity of the copper-mediated reaction (CuSAC), as well as the development of fluorogenic probes, the click and release strategy, and photo-triggered ligations based on strain-promoted sydnone–alkyne cycloadditions (SPSAC).