Bioorthogonal reactions are widely used for the chemical modification of biomolecules. The application of vinylboronic acids (VBAs) as non‐strained, synthetically accessible and water‐soluble reaction partners in a bioorthogonal inverse electron‐demand Diels–Alder (iEDDA) reaction with 3,6‐dipyridyl‐s‐tetrazines is described. Depending on the substituents, VBA derivatives give second‐order rate constants up to 27 m −1 s−1 in aqueous environments at room temperature, which is suitable for biological labeling applications. The VBAs are shown to be biocompatible, non‐toxic, and highly stable in aqueous media and cell lysate. Furthermore, VBAs can be used orthogonally to the strain‐promoted alkyne–azide cycloaddition for protein modification, making them attractive complements to the bioorthogonal molecular toolbox.

Bioorthogonal reactions are widely used for the chemical modification of biomolecules.T he application of vinylboronic acids (VBAs) as non-strained, synthetically accessible and water-soluble reaction partners in ab ioorthogonal inverse electron-demand Diels-Alder (iEDDA) reaction with 3,6-dipyridyl-s-tetrazines is described. Depending on the substituents,VBA derivatives give second-order rate constants up to 27 m À1 s À1 in aqueous environments at room temperature, which is suitable for biological labeling applications.T he VBAs are shown to be biocompatible,n on-toxic, and highly stable in aqueous media and cell lysate.F urthermore,V BAs can be used orthogonally to the strain-promoted alkyne-azide cycloaddition for protein modification, making them attractive complements to the bioorthogonal molecular toolbox.Inthe last decades,t he development of selective reactions between two reactants that are unaffected by any of the naturally occurring biological functionalities has been amajor research area in chemical biology.[1] These bioorthogonal reactions ( Figure 1A)m ake it possible to chemically modify biomolecules in their native cellular environment and gain ab etter understanding of their role in as pecific biological system or process.The bioorthogonal reaction should be high yielding and rapid, and the reactants and product(s) should be soluble and stable in aqueous media and non-toxic to the biological system. Theu se of small reactants is preferred to minimize steric interactions with the biomolecule or to facilitate incorporation by the endogenous cellular machinery.TheCarboni-Lindsey (CL) reaction between an electronpoor tetrazine ( Figure 1B, I)a nd an alkene has gained considerable attention for use in bioorthogonal applications. [2] As linear and unmodified alkenes ( Figure 1B, VI)s howed only poor reaction rates with tetrazines, [3] most research has been directed to the development and use of strained alkenes such as trans-cyclooctene (TCO), [4] norbornene, [5] cyclopropene, [6] or N-acylazetine [7] ( Figure 1B, II-V). In addition to strain, the rate of the CL reaction can be significantly enhanced by the introduction of electron-donating substituents on the alkene bond.[8] This aspect, however,h as been poorly investigated in relation to bioorthogonal applications.Vinylboronic acids ( Figure 1B, VII)a re an interesting class of compounds with unique electronic properties,w hich are due to their vacant p-orbital. As aresult of the inductive effect that is caused by the electronegativity difference of boron and carbon, the electronic deficiencyofboron and the electron-donating oxygens attached to boron, boronic acid is considered to be aw eak electron-donor.[9] Furthermore, boronic acids are mild organic Lewis acids,w hich in basic aqueous media are in equilibrium with their boronate anion ( Figure 1B, VIII), which is as trong electron-donor. [9] Although Diels-Alder reactions are known to proceed faster in aqueous media, the CL reactions of vinylboronic acids and alkylboronic acids are re...