S

H2O or CHCI3

S-i2 (218) Scheme 60

l-Hydroxy-2-thiopyridine, the antibacterial pyrithione (217), dimerises to the disulfide di TV-oxide (218) upon irradiation in chloroform or aqueous buffer; further irradiation causes N-

deoxygenation to (219) (Scheme 60).207 The antibacteric trimethoprim (220) undergoes oxidation at the benzyl position (e. g. 221) as well as hydrolysis of the amino groups in the pyrimidine ring (e.g. 222) and of the 4'-methoxy function (some of the products are shown in Scheme 61).208

Scheme 61

The antibacterial nalidixic acid (223, Scheme 62) is decarboxylated by irradiation in 0.1 M sodium hydroxide solution and yields the corresponding naphthyridinone (224) and

The antibacterial nalidixic acid (223, Scheme 62) is decarboxylated by irradiation in 0.1 M sodium hydroxide solution and yields the corresponding naphthyridinone (224) and

Nalidixic acid has been the precursor of the largely used fluoroquinolone antibiotics. These are weak oxygen sensitisers211 and are photolabile to a degree which changes greatly with the structure. Two different paths have been observed. The first one involves oxidative degradation of the amine side chain, as it is the case with ofloxacin (227) which gives products (228) (Scheme 63)212 and ciprofloxacin.213 The latter path involves defluoruration,214 and is the dominating process when an 8-fluoro substituent is present, as is the case for the highly reactive lomefloxacin (229, Scheme 64) from which products (230)-(233) have been isolated.215

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