Unexpected photoreactions of some 7-amino-6-fluoroquinolones in phosphate buffer

The products of irradiation of some 6-fluoro-7-piperazino-4-quinolone-3-carboxylic acids in phosphate buffer arise from a combination of reductive defluorination of the heteroaryl ring and oxidative fragmentation of the piperazine side chain. This unusual reaction contrasts with the fluorine atom substitution observed in neat water. The results of steady-state and time-resolved experiments are consistent with initiation of the process by electron-transfer quenching of the triplet state of these heterocycles by the phosphate anion. For one of the compounds, a transient band (lambda (max) = 670 nm), previously attributed to the defluorinated cation, must now be reassigned to the radical anion. This intermediate undergoes inefficient reductive defluorination of the ring. In the process, a phosphate radical anion is expected to be formed and to abstract a hydrogen atom from the piperazine group, leading to degradation of the latter.