Nitrone photolysis

During the chemical interconversion of gibberelin-A 5 and enmein, Somei and Okamoto (181) investigated a nitrone photolysis reaction which gave the veatchine-type compound 381. 

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... 

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). 

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... 

Simple nonconjugated nitrones have a strong absorption maximum in the UV at around 230 nm. Acyclic nitrones are, in general, photo-chemically cyclized to the corresponding oxaziridines.148 The cyclic nitrone, 5,5-dimethyl-l-pyrroline 1-oxide (164) is similarly converted into the oxaziridine (165) by photolysis in ethanol/cyclohexane solution,146 and several 2-substituted 1-pyrroline 1-oxides were subsequently found to undergo the same cyclization in cyclohexane solution.147... 

Although hexamethyl-2,3-dihydropyrazine 1,4-dioxide (166) does not exhibit typical nitrone properties, oxaziridines are nevertheless formed on photolysis with light of wavelength greater than 300 nm, only the monooxaziridine (167) is formed, whereas with light of wavelength down to 200 nm, cis- (168) and trans- (169) dioxaziridines are obtained.148... 

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 azomethine ylid 53, generated thermally from the aziridine 52, underwent cycloaddition in near quantitative yield. With DMAD, only one product is possible with EP the sole product was 54, and EPP gave 54 and 55 in 80 20 ratio. This latter result indicates that steric effects are important with the phenylpropiolic ester.52 Nitrones (57) add to benzylideneacenaphthenone (56) forming the spiroisoxazolidines (58), which lose aldehyde fragments on heating or photolysis to give the... 

Under photo-stimulation, isoindolyloxyl radical (5) abstracts primary, secondary, or tertiary hydrogens from unactivated hydrocarbons including cyclohexane, isobutane, or n-butane (Scheme l).23 The nitroxide (5) traps the resultant carbon-centred radical (R ) and so afford the A -aI koxyisoindo les (6). Blank photolysis experiments with no added hydrocarbon have shown some unprecedented / -fragmentation of (5) to afford the nitrone (7). A number of C60 nitroxide derivatives have been synthesized and characterized by ESR spectroscopy which show features common to nitroxide radicals.24 Reaction of nitroxide and thionitroxide radicals with thiyl radicals have been observed, from which sulfinyl, sulfonyl, and sulfonyloxy radicals were generated.25 The diisopropyl nitroxide radical was generated in the reaction of lithium diisopropylamide with a-fluoroacetate esters.26... 

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],... 

Direct conversion of diol 395 to cytisine 31a was accomplished by a one-pot procedure. Oxidative cleavage to give the dicarboxylic acid, followed by treatment with aqueous ammonia and catalytic reduction provided racemic cytisine 31a in good yield <20000L4205>. Photolysis in benzene at 254 nm of the nitrone 396 afforded smooth rearrangement to 17-oxosparteine 195 likely through the intermediacy of an oxaziridine derivative (Scheme 80) <2002OL2577>. 

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%). ... 

The photochemical isomerization reaction has been shown" to be stereospecific following the disrotatory photochemically allowed cyclization path. Thus the stereochemistry of the nitrone is preserved in the oxaziridine. However, since some nitrones undergo trans-cis isomerization under the conditions of photolysis, mixtures of oxaziridines often result. 

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