Photodecarboxylation and photohydration of pyrimidine derivatives

Thymine-1-acetic acid is shown to undergo photodecarboxylation while uracil-1-acetic acid is subject to photohydration and photodecarboxylation. In contrast, no photodecarboxylation was observed for thymine-1-propionic acid and uracil-1-propionic acid. With the esters and amides of these acids, only photohydration occurred. In no instance was lactonization or azalactone formation detected in these photoreactions. Irradiation of these compounds in the dry solid state resulted in little change; however, in frozen aqueous solutions cyclobutyl dimers were isolated without decarboxylation. Detailed mechanistic study revealed that photohydration and photodecarboxylation of these pyrimidines probably are simultaneous processes which may occur from the vibrationally excited levels of the ground state.