Photorearrangement of Heterocycles

There are obviously no common mechanistic features for the photorearrangement of heterocycles in general. Yet three-membered rings349) and five-membered ring heterocycles 350) exhibit a quite particular behaviour. This topic has also been reviewed in a heterocyclic chemistry series 351). In this chapter only some selected reactions of preparative interest will be outlined. 

Oxonin has been obtained in the direct or sensitized irradiation of cyclo-octatetraeneoxide (3.33) 352). 

2-Diazepines are formed in almost quantitative yield in the irradiation of pyridinium ylides (3.35) 3540. 

3-dipolar intermediate obtained in the ring opening reaction of 3H-azirines via cleavage of the C—C bond has been trapped to give oxa-zolines 355) (3.36) and pyrrolines (3.37) 356). 

Electrocyclic ring opening of the aziridine in (3.38) affords aza[17]annu-lene 357 . 

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The mechanism of the related photorearrangement of heterocyclic N-oxides is still a matter for debate. Evidence that rearrangement of the phenanthridine N-oxides (48) to the N-substituted phenanthridones (49) and the dibenzo[d,f]-1,3-oxazepines (50) proceeds by way of biradical intermediates rather... 

One of the most rewarding and intensively investigated areas of research in heterocyclic chemistry has been the study of the photorearrangement of nitrones and heteroaromatic N-oxides. The subject was thoroughly reviewed in 1970.45 A more recent review concerned with the photochemistry of aromatic N-oxides has been published.46... 

Nitro compounds have also been reported to undergo photocyclizations. The intermediacy of an isoxazoline in the photorearrangement of o-nitro-benzaldehyde to o-nitrosobenzoic acid is now in doubt,318 but intramolecular hydrogen abstraction by an excited nitro group in nitrobenzene derivatives can result in the formation of heterocycles. 4-tm-Butyl-3-methoxy-2,6-dinitrotoluene (384) on irradiation in methanolic sodium hydroxide solution... 

One of the major areas of study in the photochemistry of heterocycles is the photorearrangement of five-membered heteroaromatic systems. Various mechanisms have been proposed to account for these transformations.123 A ring contraction-ring expansion pathway, via an intermediate cyclopropene, appears to be involved in the rearrangement of derivatives of furan. The isolation of an intermediate (140) of this type together with an allene (141) has been effected on irradiation of 2,4-di-tert-butylfuran (142), as shown in Scheme 9.124 Similar transformations have been reported in perfluorotri-... 

Pace, A., Buscemi, S. and Vivona, N. (2005) Heterocyclic rearrangements in constrained media. A Zeolite-directed photorearrangement of 1,2,4-oxadiazoles. Journal of Organic Chemistry, 70 (6), 2322-2324. 

Relatively few studies of photorearrangements in five- and six-membered nitrogen-containing heterocycles have been reported. Isoxazole-to-oxazole photorearrangements have been employed in the syntheses of 2-(2-aryl-5-oxazolyl)benzoates, and nitrogen-oxygen bond homolysis appears to be the first step (as shown in Scheme 5) in the photorearrangement of the fused... 

At the time of the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) (1984) little was known about compounds within the title categories. A single oxathiirane, produced by photorearrangement of a thioketone 5-oxide, had been characterized by its ultraviolet (UV) spectrum in 1976 <76JCS(P1)1404>. Dimethyldioxirane was mentioned only to the extent that its existence as an intermediate had been made plausible in 1980 <80JOC4758>. 

Professor Yates made important contributions to organic photochemistry, mechanistic organic chemistry, synthetic methods, and spectroscopy of organic compounds. His most important work in photochemistry was discovery of the photorearrangement of cyclic ketones to oxacarbenes. His studies of alpha-diazoketones led to syntheses of novel nitrogen and sulfur heterocycles. Among his many contributions to synthetic methodology was his discovery that some Oiels-Ajper reactions can be accelerated by Lewis acids. 

Photorearrangements of 5-membered heterocyclic aromatics are reviewed by Padwa. Thiophen and 2-phenylthiophen photorearrangements are the subject... 

Further studies of the photorearrangement of five-membered heterocycles have been described. The isoxazole-oxazole rearrangement has previously been shown... 

Padwa A, Carlsen PHJ, Tremper A (1978) Photochemical transformations of small-ring heterocyclic compounds. 99. 1,4-Substituent shifts in the photorearrangement of 2-hydroxymethyl-2H-azirine derivatives to N-vinyhmjnes. J Am Chem Soc 100 4481-4490... 

Ohkura, K., Noguchi, Y., and Seki, K., Acid-catalyzed photorearrangement of 1,3-dimethylcyclooc-tapyrimidine-2,4-dione into 10,12-diazatricyclo-[6.4.0.0 ]dodeca-3,7-diene-9,ll-diones, Heterocycles, 49, 59, 1998. 

Ohkura, K., Nishijima, K., Uchiyama, S., Sakushima A., and Seki K., Acid-catalyzed photorearrangement of the bond-switching isomer of a pentamethylcyclooctapyrimidine-2,4-dione. Heterocycles, 55, 1015, 2001. 

As in five-membered heterocycles, the introduction of a carbonyl group produces a dramatic effect on the photoreactivity. Pyran-2-ones undergo other electrocyclic processes in addition to those discussed in preceding sections.213 Photorearrangement, however, does not occur in the coumarin nucleus, although a photo-Fries rearrangement of 3-benzoyloxy-6,7-... 

Few new examples of photorearrangement in six-membered heterocycles have been reported. Irradiation of pentafluoropyridine in the gas phase affords the Dewar isomer with a half-life of 5 days at room temperature. Two transients, thought to be isomers of azafulvene, were detected on flash photolysis. The conversion of the lactam (73) into the seco-steroid (74) on irradiation in t-butyl alcohol is viewed as arising via an electrocyclic ring-opening process, followed by addition of solvent, as shown in Scheme 8. Spectroscopic evidence for the... 

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