N

so3h

^conhJ ps n

CH2OAc COOH

Penems are quite labile. One derivative of this group (197) has been shown to undergo retrocyclisation to give the thiazole (198) and 3-hydroxybutyric acid (detected as the methyl ester, 199) by irradiation in solution. In the solid state a different reaction takes place, viz intramolecular hydrogen abstraction followed by addition of oxygen to a radicalic centre to finally yield compound (200) (see Scheme 56).197

2.6.4 Antibacterials: Heterocyclic Drugs. Among five-membered heterocyclics, one should mention metronidazole (201, R=CH2CH2OH) and related antibacterials,198"200 which show a typical nitro group photoreaction. This is initiated by nitro - nitrite rearrangement (see Scheme 1, b) to give (202) followed by shift of the NO group to the vicinal position. Hydrolytic ring opening - ring closure from oxime (203) finally leads to l,2,4-oxadiazol-3-carboxamide (204) (Scheme 57). These in turn undergo photoaccelerated hydrolysis to oxaldiamides (205). This rearragement is also one of the processes occurring upon radiolysis and does not involve the radical anion of the substrate.200' 201 The photochemistry of metronidazole is changed in the presence of citrate.202

Scheme 57

The urinary antibacterial nitrofurantoin (206) is cleaved to nitrofurancarboxyaldehyde (207) upon UV irradiation (Scheme 58).203 Among nitrofuran antiseptics for topical use, nitrofiirazone (208) undergoes rapid syn-anti isomerisation to give (209) and fragmentation of the N-N bond leading to the azine (210).204 Furazolidone (211) is cleaved and hydrolysed to aldehyde (207) (Scheme 58).203

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