Photoinduced, strain-promoted cycloadditions of trans-cycloheptenones and azides

An unprecedented photoinduced azide-alkene cycloaddition has been developed, the key elements of which include the photoisomerization of cis-cycloheptenone to trans-cycloheptenone, strain-promoted 1,3-dipolar cycloaddition of trans-cycloheptenone with azide, and aerobic oxidative aromatization of the resulting 1,2,3-triazoline. The newly developed photoclick reactions display several appealing features including high overall efficiency, broad substrate scope, environmentally benign conditions, and operational simplicity, which provide an enabling tool for accessing structurally diverse 1,2,3-triazoles. In addition, the photoisomerization-cycloaddition strategy could also be extended to the 1,3-dipolar cycloadditions of trans-cyclooctenone with azide andtrans-cycloheptenone with nitrile oxide. More appealingly, the title reaction has been successfully applied as a bioconjugation tool for in vitro protein labeling.