Bioorthogonal Reactions Utilizing Nitrones as Versatile Dipoles in Cycloaddition Reactions

Bioorthogonal chemical reactions have emerged as convenient and rapid methods for incorporating unnatural functionality into living systems. Different prototype reactions have been optimized for use in biological settings. Optimization of 3 + 2 dipolar cycloadditions involving nitrones has resulted in highly efficient reaction conditions for bioorthogonal chemistry. Through substitution at the nitrone carbon or nitrogen atom, stereoelectronic tuning of the reactivity of the dipole has assisted in optimizing reactivity. Nitrones have been shown to react rapidly with cyclooctynes with bimolecular rate constants approaching k₂ = 10² M^-1 s^-1, which are among the fastest bioorthogonal reactions reported (McKay et al. Org. Biomol. Chem. 2012, 10, 3066-3070). Nitrones have also been shown to react with trans-cyclooctenes (TCO) in strain-promoted TCO-nitrone cycloadditions reactions. Copper catalyzed reactions involving alkynes and nitrones have also been optimized for applications in biology. This review provides a comprehensive accounting of the different bioorthogonal reactions that have been developed using nitrones as versatile reactants, and provides some recent examples of applications for probing biological systems.