The emerging inverse electron demand Diels–Alder (IEDDA) reaction stands out from other bioorthogonal reactions by virtue of its unmatchable kinetics, excellent orthogonality and biocompatibility. With the recent discovery of novel dienophiles and optimal tetrazine coupling partners, attention has now been turned to the use of IEDDA approaches in basic biology, imaging and therapeutics. Here we review this bioorthogonal reaction and its promising applications for live cell and animal studies. We first discuss the key factors that contribute to the fast IEDDA kinetics and describe the most recent advances in the synthesis of tetrazine and dienophile coupling partners. Both coupling partners have been incorporated into proteins for tracking and imaging by use of fluorogenic tetrazines that become strongly fluorescent upon reaction. Selected notable examples of such applications are presented. The exceptional fast kinetics of this catalyst-free reaction, even using low concentrations of coupling partners, make it amenable for in vivo radiolabelling using pretargeting methodologies, which are also discussed. Finally, IEDDA reactions have recently found use in bioorthogonal decaging to activate proteins or drugs in gain-of-function strategies. We conclude by showing applications of the IEDDA reaction in the construction of biomaterials that are used for drug delivery and multimodal imaging, among others. The use and utility of the IEDDA reaction is interdisciplinary and promises to revolutionize chemical biology, radiochemistry and materials science.

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Inverse electron demand Diels–Alder reactions in chemical biology

Diels-Alder reactions of azadienes

Disease mechanisms are increasingly being resolved at the molecular level. Biomedical success at this scale creates synthetic opportunities for combining specifically designed orthogonal reactions in applications such as imaging, diagnostics, and therapy. For practical reasons, it would be helpful if bioorthogonal coupling reactions proceeded with extremely rapid kinetics (k > 103 M–1 s–1) and high specificity. Improving kinetics would minimize both the time and amount of labeling agent required to maintain high coupling yields. In this Account, we discuss our recent efforts to design extremely rapid bioorthogonal coupling reactions between tetrazines and strained alkenes.These selective reactions were first used to covalently couple conjugated tetrazine near-infrared-emitting fluorophores to dienophile-modifed extracellular proteins on living cancer cells. Confocal fluorescence microscopy demonstrated efficient and selective labeling, and control experiments showed minimal background fluorescence. Multis...

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Biomedical applications of tetrazine cycloadditions.

Inverse electron demand Diels–Alder reactions (iEDDA) between 1,2,4,5-tetrazines and olefins have emerged into a state-of-the art concept for the conjugation of biomolecules Now, this reaction is also increasingly being applied in polymer science and materials science The orthogonality of this exciting reaction to other well-established click chemistry schemes, its high reaction speed and its biocompatibility are key features of iEDDA making it a powerful alternative to existing ligation chemistries The intention of this tutorial review is to introduce the reader to the fundamentals of inverse electron demand Diels–Alder additions and to answer the question whether iEDDA chemistry is living up to the criteria for a “click” reaction and can serve as a basis for future applications in post-synthetic modification of materials

https://pubs.rsc.org/en/content/articlepdf/2013/cs/c3cs60049a

R. A. Carboni

Papers

Reactions of Tetrazines with Unsaturated Compounds. A New Synthesis of Pyridazines