sentence: Those so-called strain-promoted click reactions can be carried out between cyclooctyne derivatives and azides ( strain-promoted azide-alkyne cycloaddition , SPAAC ) [ 7 – 13 ] or tetrazines ( tetrazine-trans-cyclooctyne ( TTCO ) ligation ) [ 14 – 17 ] as well as between norbornene derivatives and tetrazines [ 18 ] .