Novel Methods for Efficient Conjugation of Two Azide Molecules

“Click reaction”, epitomized by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), is an emerging method for conjugating molecules efficiently in broad fields, including materials chemistry and chemical biology. A copper-free variant, strain-promoted azide-alkyne cycloaddition (SPAAC), using functionalized cyclooctyne derivatives that spontaneously react with azides, have enabled the chemical modification of azido-incorporated biomolecules in cultured cells and in living animals, greatly expanding the utility of click chemistry. A variety of cyclooctyne derivatives bearing functional moieties, such as fluorescent or biotinyl groups, have been developed and some of them have become commercially available. However, on-demand synthesis is required for those with a new functionality, which is not always easy because of the high reactivity of the strained alkyne moiety. To conjugate two azide molecules easily, we have developed strain-promoted “double-click” reaction using Sondheimer diyne that bears two strained alkyne moieties. This novel convergent method capable of conjugating three molecules spontaneously has allowed for facile modification of an azido-biomolecule with a small reporter azido-molecule. We have also developed a transient protection method of cyclooctynes from cycloaddition with an azide via 1：1 complexation with a cationic copper(I) salt. Protection of a cyclooctyne bearing a terminal alkyne with a copper salt enabled the selective copper-catalyzed click conjugation with an azide at the terminal alkyne moiety, facilitating easy preparation of cyclooctyne derivatives.