TCO OH, formally known as (E)-cyclooct-4-enol, is a strained trans-cyclooctene derivative featuring a terminal hydroxyl group. This compound is widely used in bioorthogonal chemistry, particularly in click reactions with tetrazines, due to its rapid kinetics and high selectivity.

The strained alkene moiety of TCO OH enables rapid and selective conjugation, making it ideal for applications requiring fast labeling or controlled release. The hydroxyl functionality allows for further derivatization, enabling the synthesis of a wide range of functionalized TCO derivatives.