Oxepins photochemical rearrangement

Photochemical rearrangement of 1, 4, and 64 has been examined in dichloromethane by steady irradiation and nanosecond transient spectroscopy. The ring enlargement of 1 to 292 and 4 to 293, respectively, has been postulated to occur via the singlet excited state. Phenolic derivatives 294 and 295 are formed from 1 and 4, respectively, via their triplet state. Product sensitization is proposed for the conversion of 1 and 4 to 294 and 295, respectively.167 Van Duuren et al. have found that on UV irradiation (254 nm) in methylene chloride, 4 rearranges to oxepin 293,4-hydroxypyrene, pyrene, and other unidentified products.168... 

An electrocyclic photochemical rearrangement of oxepins to cyclobutene ring-containing valence isomers has also been reported. Thus 2-oxabicyclo-[3.2.0]hepta-3,6-diene has been isolated upon photoisomerization of 1. Similarly, when 1-benzoxepin 100 was irradiated, the valence tautomer3,4-benz-2-oxabicyclo[3.2.0]-hepta-3,6-diene was formed. ... 

Photochemical rearrangement of chloroanil in the presence of cycloheptatriene or 1,3-cyclo-hexadiene gives the respective oxepin (38) in 70-95% yield (Equation (15)), while in the presence of... 

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... 

The formation of the oxepin is reasonably explained by an electrocyclic ring opening of rearranged epoxide 299 in a thermal reaction. As mentioned above, two routes to 299 are possible. If the rearrangement is concerted, a 1,5-sigmatropic reaction with inversion of the reaction center (oxygen) in 299 is photochemically allowed. It is possible to separate a nonconcerted process... 

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