N

Scheme 50

Another important group of chemicals pertaining to this category is that of sunscreens. These are supposed to absorb UV light and degrade it to heat avoiding skin photosensitisation. The most largely used derivative is />-aminobenzoic acid (PABA, 175).174 However, most of these compounds do show some photoreactivity and this requires careful evaluation before their use as active ingredients in sunscreens is accepted. The subject is discussed by J. M. Allen in this book (p. 171)

H^cooh

2.6 Chemotherapeutic Agents

2.6.1 Antibacterials: Sulfa Drugs. The photochemistry of sulfa drugs has been extensively investigated. The main process with JV-substituted sulfanylamide derivatives (176) is cleavage of the S-N and C-S bonds, with extrusion of S02 and formation of aniline (177) and of the appropriate amine (178) (Scheme 51), although the yield of the photoproducts changes greatly with the structure.175

OMe

Scheme 51

Radicals are produced during the photolysis.176 With N-[2-(5-methylisoxazolyl)] sulfanylamide, sulfamethoxazole (179), rearrangement to the corresponding 2-(5-methyloxazolyl) derivative (180) also takes place (Scheme 52).177 Sulfacetamide (181) has been the subject of a photophysical study, and the triplet has been characterised;178 the main process by irradiation in water is deacetylation to sulfanilamide (182) (Scheme 53).179 As for the last compound, this undergoes oxidation of the amino group to give the azo (183) and the nitro (184) derivative when irradiated in water;180 in ethanol oxidation of the solvent to acetaldehyde (revealed by the formation of 2-methyl-quinoline-6-sulfonamide, 185) accompanies the above process (Scheme 53).181 Irradiation of the secondary amine sulfadimetoxine (176, R=2,6-dimethoxy-4-pyrimidinyl) in methanol causes methylation of the amino group.182

2.6.2 Antibacterials and Antivirals: Aromatic Derivatives. Irradiation of aqueous solutions of tetracycline (186) under oxygen causes homolytic deamination followed by oxygen addition of the resulting radical to finally yield quinone (187) (Scheme 54).183' 184 It has been shown that both singlet oxygen and superoxide anion are formed during irradiation of this and related compounds.185 On the other hand, different photoprocesses occur with some substituted tetracyclines. Thus, the 7-chloro derivative of (186), Chlortetracycline, undergoes homolytic dechlorination when irradiated in aqueous buffer at pH 7.4, while no such reaction occurs in demeclocycline, the 7-chloro derivative lacking the 6-methyl group.186

S02NHAc (181)

h2o orEtOH

H2N02S

h2o orEtOH

so2nh2

H2N02S

(185) (inEtOH) Me so2nh2

Scheme 53

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