Thionated coumarins and quinolones in the light triggered release of a model amino acid: synthesis and photolysis studies

Model amino acid ester conjugates bearing heterocycles with a thiocarbonyl group (thiocoumarins and thioquinolones) were prepared by a thionation reaction of the corresponding carbonyl precursors, with the aim of obtaining conjugates with higher photosensitivity at longer wavelengths. Photolysis studies were carried out under irradiation at different wavelengths in a photochemical reactor (250, 300, 350 and 419 nm), and it was found that the ester bond between the heterocycle and the model amino acid in the thionated analogues cleaved readily with shorter irradiation times than those of the corresponding precursors. Moreover, fast photolysis at 419 nm for thioquinolone ester conjugates (within minutes) was particularly interesting and the fact that some of the reported heterocycles displayed very different irradiation times, may provide the possibility for selective removal of one group in the presence of another photocleavable group. The (quinolin-2-thione-4-yl) methyl group may be considered as a new and very effective addition to the family of photocleavable protecting groups for carboxylic acids.