Photochemistry of some fluoroquinolones: effect of pH and chloride ion

A 6-fluoroquinolone (norfloxacin) and the naphthyridine analogue enoxacin give the corresponding 6-hydroxy derivatives by irradiation in water at pH 7.2 and, with lower efficiency, at pH 4.5 and 10. At pH 1 no defluorination takes place and the piperazinyl side chain is degraded. The 6,8-difluoro derivative lomefloxacin is defluorinated selectively from position 8 over the entire pH range considered (pH 1 to 10). The intermediate cation in position 8 does not add water and rather undergoes insertion into the β-CH bond of the neighboring N-ethyl group. The cation adds chloride, however. The structure–photoreactivity relationship for fluoroquinolones and the relation with the known phototoxicity of these compounds are commented upon.