Photolysis of nitrone

To illustrate the synthetic use of photochemical rearrangement, the photolysis of nitrones (249) leading to the formation of bicyclic lactams (250) is an example (Scheme 2.88) (459). 

The photolysis of nitrones has been used as a method of preparing oxaziridines. A table of fairly stable oxaziridines prepared from the photolysis of the corresponding nitrones has been included in an extensive review. The photochemical method, though limited in its scope, often serves as an alternative to the peroxidation of imines. The synthetic usefulness of this method is demonstrated by the conversion of nitrones 7 and 9 into oxaziridines 8 and 10 in yields of 78% and 74%, respectively. (Apparently, the phototoxicity of benzodiazepine A(-oxides, e.g., 7, 9, is due to this isomerization. ) Similarly nitrones 11a, h, c yielded oxaziridines 12a, b, c (25-30%). ... 

Dipolar cycloadditkm reactions of nitrones to olefins, 46, 1,3-Dipolar cycloadditions with 3-phenylsydnone, 45, 98 Dispiro[5 1 5 l]tetradecane-7,14-dione, photolysis to cyclohexylidene-cyclohexane, 47, 34 preparation from cyclohexanecarbonyl chlonde and triethylamine, 47,34 Displacement of bromine from 1-bromo-2-fluoroheptane to give 2-fluoro-heptyl acetate, 46, 37... 

Oxaziridines, 6-oxa-l,4-diazabicyclo[3.1.0]hex-3-enes (245) and their 4-oxides (246), obtained in the photolysis of 4H -imidazole-3-oxides (223) and 1,3-dioxides (219), underwent a quick thermal isomerization to the starting nitrones. Further... 

Photolysis of nitro-steroids 225 yields the aci-nitronate at 254 nm131. This in turn gives various products, among them are ketone 226 and hydroxamic acid 227 (equation 105) which could be formed from the intermediate anions of the Af-hydroxyoxaziridines, with a possible participation of gem-hydroxynitroso transient (or its anion see Scheme 10). For comparison, IV-butyl spiro-oxaziridine 228 in ethanol is photolysed at 254 nm (equation 106) to give 7V-butyl lactam 229 (50%) and the ketone 230 (25%). The former process is a well-known photoprocess of oxaziridine131. 

Extensive work has been carried out on the photolysis of aromatic (V-oxides (B-71MI20500, 74HC(i4-S2)i). The early work has been reviewed in comparison with the photochemistry of azoxy compounds and nitrones (70CR231), but the following discussion is based on the classification used by Streith and Bellamy in their review (76H(4)1391>. Two competing... 

Enmein was converted to 20-hydroxykaur-6-en-15a-pyranylether (382), which was oxidized with chromium trioxide in pyridine to afford the aldehyde 383. The latter was converted with hydroxylamine to the oxime 384. The nitrone 385 was prepared by treatment of 384 with bromine azide. Photolysis of 385 gave the desired compound 381 in 46% yield. This intermediate possesses several useful functionalities (e.g., carbinolamine ether linkage), which may be of interest for synthesis of C20-diterpenoid alkaloids after minor changes in this scheme. 

An improved synthesis7 of aldosterone acetate lb was based on the readily available dienone 30. Conversion to the nitrite and photolysis affords the oxime 31 with no attack on C19. On warming in iso-propyl alcohol this oxime 31, cyclized smoothly to the nitrone 32 with the loss of water. This nitrone is at the right oxidation level to rearrange to the 21-acetoxyimine. The overall yield of nitrone from 30 was 55%. 

Flash photolysis of misonidazole, metronidazole, and nitrobenzothiazoles has been carried out in [1369-1371], Laser flash-photolysis (355 nm) allows to determine relatively stable anion-radicals of misonidazole and metronidazole in aqueous solutions [1370], Solvated electrons have been formed at harder irradiation, the result of which interaction with nitroimidazole molecules is generation of their radical anions [1372], The authors [1372] have also found that fluorescence intensity of metronidazole is about 20 times more than that of misonidazole in same conditions. Photochromic properties of benzothiazole derivatives containing nitro and methyl groups in the ortho positions with respect to each other were studied by flash photolysis [1371], The application of the thermodynamic approach to predict the kinetic stability of formed nitronic acids is limited owing to specific intramolecular interactions. The lifetime of photoinduced nitronic acid anions tends to increase with rise in the chemical shift of the methyl protons. The rate constants photoinduced nitronic acids and their anions increase as the CH3C-CN02 bond becomes longer [1371],... 

A number of optically active oxaziridines have been synthesized by the photolysis of the corresponding nitrones in (+) or (—)2,2,2-trifluoro-l-phenylethanol/ fluorotrichloromethane as solvents (Table 6). ... 

During the last 10 years many additional oxaziridines have been isolated after photolysis of nonaromatic nitrones. The three-membered ring isomers of heteroaromatic A-oxides proved to be too reactive for direct detection but... 

The photochemical behavior of the isoxazolidine ring is not extensively studied. Depending upon the experimental conditions, 5-nitroxazolidines undergo photolysis to yield either 4-isoxazolines by the elimination of nitrones or to P lactam derivatives, by ring opening followed by reclosure... 

The formation of the hydroxamic acids (213) by cyclization of the nitroso-aldehydes (212), which are intermediates in the photolysis of 17/3-yl nitrites, was shown not to involve electronically excited species. Further studies relevant to the photochemistry of veratr-13(17)-enin-ll/8-yl nitrites (214) have been reported photolysis of the etiojerva-5,13(17)-dien-ll/S-yl nitrite (215) gave the parent alcohol, the nitrone (216), and the iV-oxide (217). Although this reaction is more complex than that reported for the 12a-analogue (214), the... 

The photoreactions (X >435 nm in dichloromethane) of tetranitromethane (261) with styrenes (260) to yield nitro-trinitromethyl adducts (269), diastereo-meric oxazolidines (270), nitro ketones (271) and nitronic esters (272) may be rationalised in terms of the interlinked pathways in Scheme 3. SET yields nitrogen dioxide (262), the styrene radical cation (263) and the trinitromethanide ion (264). Addition of nitrogen dioxide (262) to the styrene (260) initiates a radical chain process. Benzylic radical (266) is oxidised by tetranitromethane (261) to the benzylic cation (265). Nitro-trinitromethyl adducts (269) result from coupling of the cation (265) with the anion (264). A competing pathway involves reaction of radical cation (263) with anion (264). C clisation of the resulting radical (268) yields the aminoxyl (267), from which loss of nitrogen dioxide yields the nitronic ester (272), whereas coupling of (267) with the radical (266) forms oxazolidine diastereomers (270). The nitroketones (271) may arise, at least in part, from secondary photolysis of the oxazolidines (270). ... 

Photolysis of the CT complex of benzofuran and tetranitromethane at 435 nm in CH2CI2 gives the epimeric pairs of adducts (60 - 63), the nitronic ester (64), and smaller amounts of dimeric products. There is, however, a marked solvent effect. In acetonitrile similar photolysis gives the same group of products with the addition of epimeric dinitro (65) and hydroxynitro (66) adducts, together with 3-nitrobenzofuran while in l,l,l,3,3,3-hexafluoropropan-2-oI mainly dimeric... 

Esters.—Photolysis of the 1 l/ -nitrite (671) in the c-nor-D-homo series affords the nitrone (672), through the reaction sequence illustrated. Use of N-labelled nitrite has revealed that the NO group suffers isotopic scrambling in an inter-molecular process. ... 

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