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The structural complexity of molecules isolated from biological sources has always served as an inspiration for organic chemists. Since the first synthesis of a natural product, urea, chemists have been challenged to prepare exact copies of natural structures in the laboratory. As a result, a broad repertoire of synthetic transformations has been developed over the years. It is now feasible to synthesize organic molecules of enormous complexity, and also molecules with less structural complexity but prodigious societal impact, such as nylon, TNT, polystyrene, statins, estradiol, XTC, and many more. Unfortunately, only a few chemical transformations are so mild and precise that they can be used to selectively modify biochemical structures, such as proteins or nucleic acids; these are the so-called bioconjugation strategies. Even more challenging is to apply a chemical reaction on or in living cells or whole organisms; these are the so-called bioorthogonal reactions. These fields of research are of particular importance because they not only pose a worthy challenge for chemists but also offer unprecedented possibilities for studying biological systems, especially in areas in which traditional biochemistry and molecular biology tools fall short. Recent years have seen tremendous growth in the chemical biology toolbox. In particular, a rapidly increasing number of bioorthogonal reactions has been developed based on chemistry involving strained alkenes or strained alkynes. Such strained unsaturated systems have the unique ability to undergo (3 + 2) and (4 + 2) cycloadditions with a diverse set of complementary reaction partners. Accordingly, chemistry centered around strain-promoted cycloadditions has been exploited to precisely modify biopolymers, ranging from nucleic acids to proteins to glycans. In this Account, we describe progress in bioconjugation centered around cycloadditions of these strained unsaturated systems. Being among the first to recognize the utility of strain-promoted cycloadditions between alkenes and dipoles, we highlight our report in 2007 of the reaction of oxanobornadienes with azides, which occurs through a sequential cycloaddition and retro Diels-Alder reaction. We further consider the subsequent refinement of this reaction as a valuable tool in chemical biology. We also examine the development of the reaction of cyclooctyne, the smallest isolable cyclic alkyne, with a range of substrates. Owing to severe deformation of the triple bond from ideal linear geometry, the cyclooctynes show high reactivity toward dienes, 1,3-dipoles, and other molecular systems. In the search for bioorthogonal reactions, cycloadditions of cyclic alkenes and alkynes have now established themselves as powerful tools in reagent-free bioconjugations.

A nearly forgotten reaction discovered more than 60 years ago—the cycloaddition of a cyclic alkyne and an organic azide, leading to an aromatic triazole—enjoys a remarkable popularity. Originally discovered out of pure chemical curiosity, and dusted off early this century as an efficient and clean bioconjugation tool, the usefulness of cyclooctyne–azide cycloaddition is now adopted in a wide range of fields of chemical science and beyond. Its ease of operation, broad solvent compatibility, 100 % atom efficiency, and the high stability of the resulting triazole product, just to name a few aspects, have catapulted this so-called strain-promoted azide–alkyne cycloaddition (SPAAC) right into the top-shelf of the toolbox of chemical biologists, material scientists, biotechnologists, medicinal chemists, and more. In this chapter, a brief historic overview of cycloalkynes is provided first, along with the main synthetic strategies to prepare cycloalkynes and their chemical reactivities. Core aspects of the strain-promoted reaction of cycloalkynes with azides are covered, as well as tools to achieve further reaction acceleration by means of modulation of cycloalkyne structure, nature of azide, and choice of solvent.

Strain-promoted azide-alkyne cycloaddition (SPAAC) as a conjugation tool has found broad application in material sciences, chemical biology and even in vivo use. However, despite tremendous effort, SPAAC remains fairly slow (0.2-0.5 M À 1 s À 1 ) and efforts to increase reaction rates by tailoring of cyclooctyne structure have suffered from a poor trade-off between cyclooctyne reactivity and stability. We here wish to report tremendous acceleration of strain-promoted cycloaddition of an aliphatic cyclooctyne (bicyclo[6.1.0]non-4-yne, BCN) with electron-deficient aryl azides, with reaction rate constants reaching 2.0-2.9 M À 1 s À 1 . A remarkable difference in rate constants of aliphatic cyclooctynes versus benzoannulated cyclooctynes is noted, enabling a next level of orthogonality by a judicious choice of azidecyclooctyne combinations, which is inter alia applied in one-pot three-component protein labelling. The pivotal role of azide electronegativity is explained by density-functional theory calculations and electronic-structure analyses, which indicates an inverse electron-demand mechanism is operative with an aliphatic cyclooctyne.

Visualizing biomolecules by fluorescent tagging is a powerful method for studying their behaviour and function inside cells. We prepared and genetically encoded an unnatural amino acid (UAA) that features a bicyclononyne moiety. This UAA offered exceptional reactivity in strain-promoted azide-alkyne cycloadditions. Kinetic measurements revealed that the UAA reacted also remarkably fast in the inverse-electron-demand Diels-Alder cycloaddition with tetrazine-conjugated dyes. Genetic encoding of the new UAA inside mammalian cells and its subsequent selective labeling at low dye concentrations demonstrate the usefulness of the new amino acid for future imaging studies.

Quicker and slicker: An efficient metal‐free 1,3‐dipolar cycloaddition of dibenzocyclooctynes with nitrones proceeded with rate constants of up to 39 M−1 s−1, or up to 300 times faster than similar reactions with azides. This strategy is useful for the site‐specific N‐terminal modification of peptides and proteins (see scheme).

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A main challenge in the area of bioconjugation is to devise reactions that are both activatable and fast. Here, we introduce a temporally controlled reaction between cyclooctynes and 1,2-quinones, induced by facile oxidation of 1,2-catechols. This so-called strain-promoted oxidation-controlled cyclooctyne-1,2-quinone cycloaddition (SPOCQ) shows a remarkably high reaction rate when performed with bicyclononyne (BCN), outcompeting the well-known cycloaddition of azides and BCN by 3 orders of magnitude, thereby allowing a new level of orthogonality in protein conjugation.