Visible-light triggered quinone trimethyl locks are reported as a general design for long-wavelength photoremovable protecting groups for alcohols and amines. Intramolecular photoreduction unmasks a reactive phenol that undergoes fast lactonization to release alcohols and amines. Model substrates are released in quantitative yield along with well-defined, colorless hydroquinone byproducts. Substituent modifications of the quinone core allow absorption from 400 to 600 nm.

Mechanistic studies of a general reaction that decages a wide range of substrates on exposure to visible light are described. The reaction involves a photochemically initiated reduction of a quinone mediated by an appended thioether. After reduction, a trimethyl lock system incorporated into the quinone leads to thermal decaging. The reaction could be viewed as an electron-transfer initiated reduction of the quinone or as a hydrogen abstraction -Norrish Type II -reaction. Product analysis, kinetic isotope effects, stereochemical labeling, radical clock, and transient absorption studies support the electron transfer mechanism. The differing reactivities of the singlet and triplet states are determined, and the ways in which this process deviates from typical quinone photochemistry are discussed. The mechanism suggests strategies for extending the reaction to longer wavelengths that would be of interest for applications in chemical biology and in a therapeutic setting.

S2Materials and Methods. Unless otherwise stated, reactions were carried out in air-equilibrated solvents under ambient conditions. Commercially available reagents were used as received from Sigma Aldrich, AK Scientific, Alfa Aesar, or Acros Organics. Photolysis and UV-vis solvents were EMD Millipore (OmniSolv®) grade. When necessary, solvents were dried by elution through activated alumina or with molecular sieves where noted. Reactions were monitored by thin-layer chromatography with Sigma Aldrich silica gel coated plates with fluorescent indicator (0.25 mm).Silica gel chromatography procedures were as described by Still et al. (W. C. Still, M. Kahn, A.