Copper-Free Click Reaction Sequence: A Chemoselective Layer-by-Layer Approach.

An additive-free chemoselective ligation of dual clickable building blocks is demonstrated. The challenge of balancing reactivity and stability was achieved by employing a small, electron-deficient tetrazine bearing an azido group and an enol ether functionalized cyclooctyne. The chemoselective sequence of strain-promoted azide-alkyne cycloaddition (SPAAC) and inverse-electron-demand Diels-Alder (IEDDA) reaction is demonstrated with a cholic acid derived triazide as a molecular surface model for layer-by-layer synthesis.