Maleimide (thiol reactive) functionalized trans-cyclooctene derivative for incorporation of the cyclooctene moiety into thiol containing compounds or biomolecules. Trans-cyclooctenes are useful in strain-promoted copper-free click chemistry cycloaddition reactions with 1; 2; 4; 5-tetrazines. This cyclooctene will react with tetrazine functionalized compounds or biomolecules without the need for a catalyst to result in a stable covalent linkage. The 4+2 inverse electron demand Diels-Alder cycloaddition between trans-cyclooctene and tetrazines is the fastest biologically compatible ligation technology reported and has had many applications in biological labeling and imaging. The PEG spacer allows for increased water solubility; less aggregation and an increased distance between the thiol to be modified and the reactive alkene.
