N-Oxides photolysis

There are various photochemical transformations of pyridazines, their corresponding benzo analogs, N-oxides and N-imides. Gas-phase photolysis of pyridazine affords nitrogen and vinylacetylene as the main products. Perfluoropyridazine gives first perfluoropyrazine, which isomerizes slowly into perfluoropyrimidine. 

Isoxazolines bromination, 6, 78 chemical potentials, 6, 7 melting and boiling points, 6, 9 oxidation, 6, 37 photolysis, 6, 37-38 N-oxides... 

Pyridazine N-ethoxycarbonylimide photolysis, 3, 13 Pyridazine, 4-gJycosyloxy-rearrangement, 3, 15 Pyridazine, halo-applications, 3, 56 Pyridazine, hexahydro-, 3, 40 photoelectron spectra, 2, 20-21 Pyridazine, hydrazino-reductive cleavage, 3, 34 synthesis, 3, 35 Pyridazine, hydroxy-acidity, 3, 4 Pyridazine, 3-hydroxyl-oxide... 

Photolysis of a variety of substances including hydrogen peroxide, phthalimide hydroperoxides (104), N-hydroxypyridinethiones (105), pyrimido[5,4-g]pteridinetetrone N-oxides (106), and N-arylalkyl-Af-phenylhydroxylamines (107) has been reported to generate hydroxyl radical in aqueous solutions. ... 

Acyl nitroso compounds react with 1, 3-dienes as N-O heterodienophiles to produce cycloadducts, which have found use in the total synthesis of a number of nitrogen-containing natural products [21]. The cycloadducts of acyl nitroso compounds and 9,10-dimethylanthracene (4, Scheme 7.3) undergo thermal decomposition through retro-Diels-Alder reactions to produce acyl nitroso compounds under non-oxidative conditions and at relatively mild temperatures (40-100°C) [11-14]. Decomposition of these compounds provides a particularly clean method for the formation of acyl nitroso compounds. Photolysis or thermolysis of 3, 5-diphenyl-l, 2, 4-oxadiazole-4-oxide (5) generates the aromatic acyl nitroso compound (6) and ben-zonitrile (Scheme 7.3) [22, 23]. Other reactions that generate acyl nitroso compounds include the treatment of 5 with a nitrile oxide [24], the addition of N-methyl morpholine N-oxide to nitrile oxides and the decomposition of N, O-diacylated or alkylated N-hydroxyarylsulfonamides [25-29]. 

Photolysis of benzofurazan N-oxide 292 in chloroform generates the nitroxyl radical 393153 (equation 141), which is formed due to hydrogen abstraction from the solvent... 

Cyano- and 10-chloro-acridine A-oxides yield 1,2-oxazepines on photolysis in benzene (Scheme 237). When the 10-substituent is methyl the unstable 1,2-oxazepine undergoes electrocyclization to a benzisoxazoline (320). Secondary photoproducts may be formed by direct irradiation of N- oxides or by rearrangement of the primary photoproduct. 

Hydroxypyridines have been obtained by direct photolysis of several N- oxides, but only one example exists in the quinoline series. Reaction involves the intermediacy of... 

Direct proof of an oxaziridine intermediate was achieved in photolysis experiments in an organic glass at 77 K (80JA5643). Oxaziridine (75), formed by photolysis of N-oxide (74) and evidenced by UV spectroscopy under the above conditions, decomposed at higher temperature to form the imino ether (76) by N—O bond cleavage and C -> O migration of an aryl group. 

Photolysis of the N-oxides 92 in dichloromethane results in rearrangement to the isomers 95 via the bicycles 93 (detectable by H-NMR at - 78°C) and 94 (Scheme 34).108126 in refluxing methanol, 95 is still the main product, but at - 55°C the intermediate 93 is attacked by the solvent to give an open-chain oxime.127... 

The expected pyridone 286, isolated as a by-product in the above reaction, was also obtained by photolysis of the N-oxide in benzene.88... 

Photolysis of pyridazine N-oxide and alkylated pyridazine iV-oxides results in deoxygenation. When this is carried out in the presence of aromatic or methylated aromatic solvents or cyclohexane, the corresponding phenols, hydroxymethyl derivatives or cyclohexanol are formed in addition to pyridazines. In the presence of cyclohexene, cyclohexene oxide and cyclohexanone are generated. 

Certain aromatic N-oxides undergo rearrangement by a process equivalent to that observed in nitrones to give the corresponding lactam. This is illustrated by the conversion of 2,5-dimethylpyrazine 1-oxide (179) into the lactam (180) by photolysis in aqueous solution.150... 

The first example of direct epoxidation of an aromatic double bond was reported by Jerina et al.63 The reaction involves photolysis of aromatic N-oxides to give 45 in 1% yield. The method does not have preparative significance. 

Alternatively, N-hyroxypyridine-2-thione, a versatile antibacterial and antifungal agent commercially available as Omadine, has been used. As expected, in the presence of DNA its photolysis causes SSBs (via OH), but after its photolytic consumption continuing photolysis at 350 nm caused the photooxidation of G in DNA and in dGuo (Adam et al. 1999). Apparently this effect is induced by one of its photolytic products that are largely the N,AT-dioxide, the mono-N-oxide, the disulfide and the sulfonic acid (Adam et al. 1999). 

---