Noncommunicating photoreaction paths in some pregna-1,4-diene-3,20-diones

The photochemistry of three pregna-1,4-diene-3,20-diones bearing a hydroxy or alkoxy group at C(17) (4-6) has been examined. Irradiation at 254 or 366 nm, where absorption by the cross-conjugated ketone moiety in ring A is predominant or exclusive, causes the 'lumiketone' rearrangement of this chromophore in low to medium quantum yield (Phi(r) 0.05 to 0.31). On the contrary, irradiation at 310 nm, where the isolated ketone at C(20) absorbs a large portion of light causes Norrish-I fragmentation of that chromophore with a higher Phi(r) (0.11-0.83). This leads to end-products arising from the conversion of the C(17) alkyl radical, in a way depending on the structure and the medium (reduction by hydrogen donating solvent, addition of oxygen when present). No intramolecular T-T energy transfer between the separated chromophores occurs. The 'lumiketone' rearrangement occurs independently from the irradiation wavelength (Phi(r) 0.06-0.18) with the strictly related androsta-1,4-dien-3-one 8 lacking the C(20) ketone function.

Multifaceted photoreactivity of 6-fluoro-7-aminoquinolones from the lowest excited states in aqueous media: a study by nanosecond and picosecond spectroscopic techniques

Nanosecond and picosecond absorption and emission spectroscopic techniques were applied to the investigation of the reactivity from the lowest excited states of some 6-fluoro-7-piperazino-4-quinolone-3-carboxylic acids (FQs) in aqueous media at neutral pH, in the absence and presence of different sodium salts. Following the detection of various transients, we proposed a mechanism for the cleavage of the carbon-fluorine bond that proceeded through different reaction pathways, dependent on the molecular structure and the characteristics of the medium. The drug lomefloxacin (LOM), a 6,8-difluoroquinolone derivative, underwent heterolytic cleavage of the C8-F bond from the excited singlet state. With the 6-monofluoroquinolone norfloxacin (NOR) and the corresponding 1,8-naphthyridinone enoxacin (ENX), the lowest singlet state was not significantly reactive and an important deactivation channel was intersystem crossing (ISC) to the triplet manifold. The lowest triplet state underwent cleavage of the C6-F bond through a solvent mediated process possibly via a cyclohexadienyl anionic adduct. In the presence of sulfite or phosphate buffer a novel defluorination mechanism, induced by electron transfer from the inorganic anions to the FQ triplet state, was observed. The correlation between the transients observed and the final photoproducts in the different media was elucidated.

Photoinduced C-F Bond Cleavage in Some Fluorinated 7-Amino-4-quinolone-3-carboxylic Acids

The photochemistry of some fluorinated 7-amino-4-quinolone-3-carboxylic acids used in therapy as antibacterials and known to be phototoxic has been investigated in water. All of them undergo heterolytic defluorination, and this appears to be a path for the generation of aryl cations in solution. 6-Fluoro derivatives such as norfloxacin (Phi(dec) = 0.06) and enoxacin (Phi(dec) = 0.13) give the corresponding phenols. Insertion of an electron-donating substituent makes defluorination inefficient; thus, ofloxacin, an 8-alkoxy derivative, is found to be rather photostable (Phi(dec) = 0.001) and reacts in part via a process different from defluorination (degradation of the N-alkyl side chain). With a 6,8-difluoro derivative, lomefloxacin, the reaction is more efficient (Phi = 0.55) and selective for position 8. Contrary to the previous cases, the aryl cation undergoes insertion in the neighboring N-ethyl group rather than solvent addition (a carbene-like chemistry). With all of the above fluoroquinolones an intensive triplet-triplet absorption is detected and is quenched by sulfite (k(q) = (1-5) x 10(8) M(-)(1) s(-)(1)). Under this condition, reductive defluorination via the radical anion takes place. The relation of the above chemistry to the phototoxicity of these drugs is commented upon briefly.

Structure and medium-dependent photodecomposition of fluoroquinolone antibiotics

The photochemical reactivity of four fluoroquinolone antibiotics is examined. For norfloxacin (NOR), enoxacin (ENX) and lomefloxacin (LOM), the only process occurring is defluorination (from position 6 for the first two drugs, from position 8 for the last one). The quantum efficiency is both structure and medium dependent (phi close to 0.5 both in water and in 0.1 M phosphate buffer for LOM; 0.01 for ENX and 0.004 for NOR in buffer, but more than an order of magnitude higher in neat water). Ofloxacin (OFL) is less light sensitive (phi 0.001) and undergoes, in part, reactions different from defluorination. The photoreaction involves heterolytic C-F bond fragmentation and its efficiency is determined by the internal charge-transfer character of the excited state (increasing in the series OFL < NOR < ENX < LOM according to the electronegativity of the substituent in position 8) and by the stabilization of the resulting aryl cation (larger for the 8-cation than for the 6-cation). The relevance of these data for the rationalization of the known phototoxicity of these drugs is discussed.