A convenient [2+2] cycloaddition–cycloreversion reaction for the synthesis of 1,1-dicyanobuta-1,3-diene-scaffolded peptides as new imaging chromophores

We report on the chemoselective coupling between colorless peptide fragments functionalized with a mutually reactive electron-rich Nα-(4-ethynylphenyl)-Nα-(methyl)-glycyl- and an electron-deficient [4-(2,2-dicyanovinyl)]benzoyl moiety. The resulting donor-substituted 1,1-dicyanobuta-1,3-dienes represent a new class of orange-red colored (λmax = 450–500 nm, with molar extinction coefficients (ε) above 5,000 mol−1 dm3 cm−1) peptide-based imaging chromophores.