Copper-free Click Reaction Reagents

　Among the “click chemistry” proposed by K. B. Sharpless in 2001, the Huisgen cyclization using azides and terminal alkynes has been widely used in various reseach fields such as chemical biology and material science due to its excellent chemoselectivity.¹⁾ Generally, the Huisgen cyclization requires some copper salts to promote the reactions. However, use of the copper salt-mediated click reactions for in vivo applications is often highly restricted owing to the toxicity of active oxygen in vivo generated by the copper salt. Therefore, the development of advanced click reactions without using any copper salt has been investigated.
　In the research fields using copper-free click reactions, a number of studies aimed for in vivo applications such as living cells and biological experiments on animals have been reported. To be suitable for such purposes, molecular design of alkyne derivatives has been performed mainly to improve two chemical properties of second order reactions, rate constant and lipophilicity (log P).
　In 2004, C. R. Bertozzi et al. have reported the copper-free click reaction using highly-strained cyclooctyl groups as reactants. These cyclooctyl compounds satisfy the above mentioned two requests and are widely used as a molecular tool to reveal the metabolic systems.²⁾ Recently, they have been used to construct the microenvironment of hydrogels supporting the basis of regenerative medicine research. Because of that, Bertozzi et al. have suggested general and accessible experimental procedures for researchers who don’t do major synthetic organic chemistry, and their practical examples can be referred to in the research article.³⁾
　Furthermore, van Delft has developed more effective cyclooctyl compounds usable for in vivo bioorthogonal and 3D imaging.⁴⁾ For instance, DBCO-amine [A2763] and BCN-amine [B4062] are suitable molecules for the copper-free click reaction in biological research because they have low lipophilicity and high reactivity. Especially, DBCO-amine is used for PEGylation of proteins and nuclear imaging by positron emission tomography (PET) using a fluorine-18 [¹⁸F] radioisotope. As stated above, DBCO-amine and BCN-amine are expected to be powerful versatile tools for the copper-free click reaction because they can also be used for surface reforming of solid materials and chemical modification of organic molecules in material science research as well as live imaging and microarray analysis in life science research.