Irradiation rearrangement, photochemical

Both pyrazine iV-oxides and 2,3-dihydropyrazines rearrange photochemically to imidazoles. Irradiation of the 2,5-disubstituted oxides (187) gives mixtures of the imidazole products (188, 189), probably through the intermediacy of oxaziridine intermediates (Scheme 109). 

Vinylcyclopropenes have been found to rearrange photochemically to cyclopen-tadienes. Thus, irradiation of l,2,3-triphenyl-3-vinylcyclopropene (233) in t-BuOH using a 450 W medium pressure lamp and a Pyrex filter for 3.5 h afforded 1,2,3-triphenylcyclopen-tadiene (234) in 87% yield (equation 153) . 

Four isomeric cycloheptatrienes are formed by irradiation of diazoacetic ester in benzene, but of these only (573) gives an adduct (574) with Oj generated by microwave discharge. This rearranges photochemically to the bis-epoxide (575). ... 

Photochemical Reactions. Further work has been reported on thermal, photochemical, and acid-catalysed rearrangement of aP-epoxy-ketones in particular, epoxyketone (70) rearranges photochemically to the spiro-diketone (71) which has also been found as a thermal rearrangement product of (70) in contrast to an earlier report. Dispiro-1,3-cyclobutanedione (72) was converted into spiro[7,6]tetradecan-2-one by irradiation in the presence of oxygen in methanol, treatment of the resultant hydroperoxide... 

Thus a solution of tridachiahydropyrone (9) was dissolved in CDCI3, degassed by purging with argon, and then irradiated. The photochemical sigmatropic rearrangement to form phototridachiahydropyrone (93) was monitored over time by analysis of the NMR spectrum of the reaction mixture. The relative intensities of the oleflnic proton signals of the two compounds were used in order to determine the ratio of 9 93 at various intervals. The results are depicted in Fig. 2.1. 

Benzisoxazoles.—The reaction of salicylaldehyde oxime with thionyl chloride in pyridine to yield 1,2-benzisoxazole (539 R = H) constitutes a new synthesis of this ring system.Thermolysis of the azido-ketone (540) affords the linearly annelated anthranil (541). ° The benzisoxazoles (539 R = H, Me, or Ph) rearrange photochemically to the benzoxazoles (543) via intermediate azirines (542). ° Irradiation of an aqueous solution of the benzisoxazolium salt (544) gives the phenolic ketone (545). ° ... 

A photochemical partial synthesis of aldosterone (19) made the hormone available on an industrial scale for the first time (114). Corticosterone acetate (51 acetate) is treated with nitrosyl chloride in pyridine at 20°C to yield the 11-nitrite (115). Irradiation of (115) leads to rearrangement with formation of the C g-oxime (116). Removal of the oxime residue with nitrous acid furnishes aldosterone (19) in excellent yield. 

Photoisomerization of perfluoro(4,5-diisopropylpyridazine) is postulated to proceed through Dewar diazabenzenes (25) and (26) to perfluoro(2,5-diisopropylpyrazine) (27), which is in equilibrium with the isomeric perfluoro(2,6-diisopropylpyrazine) (28) after prolonged irradiation in the liquid phase (Scheme 9) (75JCS(P1)1130). Benzo-fused pyridazines do not isomerize readily under photochemical conditions. An exception is perfluorocinnoline which rearranges to perfluoroquinazoline. 

Thiirenes are photochemically converted to thioketenes or to alkynylthiols (Scheme 22) (80PAC1623). 2-Acetyl-3-methylthiirene rearranges on irradiation at 310 nm to a mixture of acetylmethylthioketene and thioacetylmethylketene, identified by their IR spectra (80NJC703). 2,3-Diphenylthiirene 1-oxide at 130 °C is believed to be isomerized to monothiobenzil which is air-oxidized to benzil (Scheme 23) (79JA390). 

Photochemical cyclohex-2-enoneH> bicyclo (3.1.0) hexan-2-one rearrangement irradiation of testosterone acetate, 322 Photochemical rearrangements of cross-conjugated cyclohexadienones and their photoisomers... 

In 1966, a photochemical rearrangement by ultraviolet (UV) irradiation of hexafluorobenzene to hexafluorobicyclo[2 2 0]hexa-2,5-diene was achieved Since then, many reactions analogous to the valence tautomerism of benzene and bicy clo[2 2 0]hexadiene (1 e, Dewar benzene), as well as of fluonnated benzvalene and... 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

The rearrangement of phenyl- er -butyl nitrone to the isomeric oxazirane (9) occurred in 95% yield on irradiation in acetonitrile for 2 hr. Because 2-ieri-butyl-3-phenyloxazirane (9) can be reconverted into the more stable nitrone, the photochemical reaction involves the conversion of radiation energy into chemical energy. 

A photochemical variant, the so-called photo-Fries rearrangement, proceeds via intermediate formation of radical species. Upon irradiation the phenyl ester molecules (1) are promoted into an excited state 11. By homolytic bond cleavage the radical-pair 12 is formed that reacts to the semiquinone 13, which in turn tautomerizes to the p-acylphenol 3. The corresponding ort/zo-derivative is formed in an analogous way ... 

Direct irradiation of substituted 1,4-dihydro-1,4-epoxy naphthalenes gives small quantities of 3-benzoxepins 16 amongst other products.144 146,256 The primary product of this reaction is probably a 3-oxaquadricyclane derivative which can react to give the benzoxepin under thermal conditions. The formation of benzoylindene compounds as the main products from the photochemical rearrangement of l,4-diphenyl-l,4-dihydro-l,4-epoxynaphthalenes has been shown to proceed via the corresponding 3-benzoxepins.146 The irradiation of naphthalene 1,4-oxides in the presence of a photosensitizer did not lead to benzoxepin derivatives.144... 

Oxepin has also been converted photochemically to phenol in 74% yield. This reaction occurs under irradiation conditions by which benzene oxide is excited to a triplet state, e.g. by irradiation in acetone as solvent.207 A rare example for a nucleophilic catalysis of the aromatization of an oxepin/benzene oxide to a phenol has been reported for /err-butyl oxepin-4-carboxylate which undergoes a rearrangement reaction in the presence of trimethylamine to give a mixture of /m-butyl 3-hydroxybenzoate (94%) and 4-hydroxybenzoate (6%).243... 

Irradiation of a benzene solution of 7-benzyl-2,5-diphenyl-3,4,7-triazanorcara-2,4-diene (15) with a high-pressure mercury lamp (Pyrex filter) results in a photochemical walk rearrangement to give 4-benzyl-3,7-diphenyl-477-l, 2,4-triazepine (16) in 53 % yield, accompanied by A-benzyl-3,6-diphenylpyridazin-4-amine (6%), 1-benzyl-2,3-diphenylpyrrole (1%) and... 

The photochemical behavior of a number of substituted derivatives of thiochroman-4-one 1-oxides has been examined by Still and coworkers192-194. These authors also report that rearrangement to cyclic sulfenates, with subsequent reaction by homolysis of the S—O bond, appears to be a particularly favorable process. For example, ultraviolet irradiation of a solution of 8-methylthiochroman-4-one 1-oxide (133) in benzene for 24h afforded a single crystalline product which was assigned the disulfide structure 134 (equation 54). More recently, Kobayashi and Mutai195 have also suggested a sulfoxide-sulfenate rearrangement for the photochemical conversion of 2,5-diphenyl-l,4-dithiin 1-oxide (135) to the 1,3-dithiole derivatives 136 and 137 (equation 55). 

Another example of an intramolecular photochemical rearrangement occurs when 1,1,2-trimethylsilene is irradiated at 250 nm, isomerizing to vinyldimethylsilane as the result of a 1,3-H shift.26... 

The effect of substituents and ring size on this photochemical rearrangement was studied by Paquette and Meehan.(43) Irradiation of the following ketones resulted in steady state mixtures of starting ketone and photoketone whose ratios depended upon structure ... 

The reverse process has also been examined. 2-Phenyloxazole is converted in a similar fashion to 3-phenyl-2//-azirine-2-carbaldehyde on irradiation in benzene or cyclohexane.128 Further rearrangement to the corresponding isoxazole can be effected thermally but not photochemically. A competing pathway leading to the formation of 4-phenyloxazole has also been observed and is thought to involve a bicyclic intermediate arising by 2,5-bonding. 

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