TCO-NHS Ester is an amine-reactive building block used for the modification of amine-containing molecules in organic media. This product is supplied as a single axial (minor, rel-(1S-2S-5E-pS)) diastereomer. This specially formulated crystalline provides easy handling and an extended shelf life. Axial TCO derivatives demonstrate much higher reactivity in the inverse electron demand cycloaddition (IEDDA) reaction compared to equatorial TCO derivatives. An increase in reactivity of up to 10-fold was reported compared to equatorial TCO derivatives. Furthermore, the axial TCO derivatives undergo cis-trans deactivation at faster rate compared to equatorial TCO derivatives.