Photocontrolled Retro-Ene Reaction for Triazolinedione Release from Indole Adducts.

TAD-indole adducts are valuable surrogates for triazolinediones (TADs) in bioconjugation and polymer chemistry, yet current retro-ene activation is constrained by elevated temperatures (>40 °C) or mechanical force with bulky side chains. We introduce a photoactivated retro-ene strategy liberating TADs under mild conditions. Upon excitation, adducts undergo efficient retro-ene release, with liberated TADs trapped by thiols (urazoles) or dienes (Diels-Alder adducts). The wavelength is tunable via the C2 indole substituent: the 2-phenyl adduct requires 370 nm, whereas 2-phenanthryl or 2-pyrenyl analogues respond to 370-420 nm or 370-460 nm, enabling precise photocontrol. Remarkably, release proceeds even at -78 °C. Combining this method with our direct synthesis from indoles and urazoles establishes a recyclable system among these components, promising for recyclable polymers. Additionally, efficient ligation with a tyrosine derivative opens avenues for peptide and protein labeling.