Hydroxyl modified cyclooctene derivative. 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.