Dipeptide 3, tetrapeptide 4 and pentapeptide 5, containing adamantylphthalimide and tyrosine, were synthesized and their photochemical reactivity investigated. Upon excitation to the triplet excited state, 3 does not give any photoproduct, although the photoinduced electron transfer (PET) should take place based on the thermodynamic properties. Tetrapeptide 4 and pentapeptide 5 are photochemically reactive, undergoing decomposition upon excitation. The lack of anticipated photodecarboxylation reactivity is explained by PET between the tyrosine and the phthalimide. However, deprotonation of the phenoxyl radical-cation giving phenoxyl radicals or back electron transfer giving starting material are probably faster than intrastrand single electron transfer which would lead to carboxyl radical and decarboxylation. The results indicate the importance of fine-tuning the molecular structure to attain the desired photoreactivity by the right choice of the reactants redox potential, as well as their acid/base properties.
Graphical Abstract