Photochemistry of the Phototoxic Drug Lomefloxacin: Paths Observed in the Presence of Amines or NaOH and from the Methyl Ester

An exploratory and mechanistic study of the defluorination of an (aminofluorophenyl)oxazolidinone: S(N)1(Ar*) vs. S(R(+)N)1(Ar*) mechanism

The morpholinofluorophenyloxazolidinone 1 (the antibacterial drug linezolid) is found to undergo reductive defluorination upon irradiation in water (Phi 0.33), in some of the products accompanied by the simultaneous oxidative degradation of the morpholine side chain. In the presence of chloride, iodide and pyrrole, the fluorine is substituted by these groups (with pyrrole, in position 2). The defluorination is less efficient in methanol and mainly leads to reduction (Phi 0.053). These data can be accommodated through two different mechanisms, viz. either C-F bond heterolysis to give a phenyl cation [S(N)1(Ar*)], or ionization to give a radical cation [S(R(+)N)1(Ar*)]. Steady-state and time resolved data have been gathered for clarifying this issue. It is found that, indeed, ionization of 1 is efficient and proceeds from the singlet, but leads to no irreversible change. On the contrary, triplet (3)1 (lifetime 0.5 micros in MeOH, <0.1 micros in water) fragments and gives the corresponding triplet phenyl cation. The last intermediate explains well the observed hydrogen abstraction both inter- (from the solvent, when this is reducing) and intramolecularly (from the morpholine group), as well as addition to a charged anion or to a neutral pi nucleophile such as pyrrole. The rationalization is supported by the study of some related molecules. Thus, the only photochemical reaction from the non fluorinated analogue of linezolid (that ionizes just as 1) is an inefficient degradation of the morpholine chain (Phi 0.001), while a simple model such as N-(2-fluorophenyl)morpholine undergoes photosolvolysis in water and is not trapped by pyrrole.

Fluoroquinolones in soil—risks and challenges

Fluoroquinolones (FQs) are among the most important antibacterial agents used in human and veterinary medicine. Because of the growing practice of adding manure and sewage sludge to agricultural fields these drugs end up in soils, where they can accumulate and have adverse effects on organisms. This paper presents an overview of recent developments in the determination of FQs in solid environmental matrices and describes the risks and challenges (persistence, fate, effects, and remediation) which result from their presence in soil.