In order to clarify the initial stage of the photoexcitation of the rigid Ph–C=N–O system, the photochemistry of several 2-isoxazolines possessing fused cycloalkane ring has been investigated. Several photoproducts such as 2H-azirine, cyclic enamine aldehydes, bicyclic oxazolines were isolated, and a generalized pattern of the photoreaction which is initiated by the N–O bond fission is proposed.
-In connection with photochemistry of the nonconstra1ned Ph-C=N-0 system, those of severa1 2-isoxazo1ine derivatives containing cyc1obutene, cyc1obutane, cyclopropane and cyc1ohexane rings, have been investigated as the mode1s of the rigid and constrained Ph-C=N-0 system. In most cases, such 2-isoxazo1ines, upon irradiation, underwent the nitrogenoxygen bond fission as the primary process, and produced a variety of products depending on the structures of the starting materia1s. For instance, the irradiation of 4~ary1-2-oxa-3-azabicyc1o[3.2.0]hepta-3,6-dienes afforded 2-ary1-1,3-oxazepines, sometimes accompanied by pyrro1e a1dehydes, whereas 2-isoxazo1ines fused with sma11 ring yie1ded ß-aminoa1dehydes, oxazo1ines, azirine a1dehydes, and sma11 fragments such as benzonitri1e. The photochemica1 reaction mechanismsof severa1 2-isoxazo1ines have been studied using 1ow-temperature technique. In addition, the pyro1ytic reactions of 4-pheny1-2-oxa-3-azabicyc1o[3.2.0]heptadiene and 1,3-oxazepines as we11 as their va1ence isomers have been examined to compare with their photochemica1 behavior. It was found that in pyro1ysis of bicyc1o[3.2.0]heptadiene derivatives containing the C=N-0 or N=C-0 gnoup, the cyc1obutene ring fission occurs prior to the nitrogen-oxygen or carbon-oxygen bond c1eavage. Reaction mechanisms for the deep-seated rearrangements of the 1,3-oxazepines and of 2-oxa-3-azabicyclo[3.2.0]hepta-3,6-dienes to pyrro1e a1dehydes were discussed. The resu1ts indicate the major differences in the chemica1 behaviors of the photo1yti-ca11y and therma11y excited states for those heterocyc1es.
980, Japan) Sumvnary Irradiation of 2-phenyl-1,3-oxazepine (I) with gives no bicyclic valence is0mer.l It has also been pointed 300 or 350 nm light afforded 3-phenyl-2-oxa-4-azabicyclo-out that tetra-and penta-aryl-l,3-oxazepines are quite [3,2,0]hepta-3,6-diene (11), whereas irradiation of (11) photostable.2 We have now found, contrary t o our earlier with 250 nm light caused ring opening to give (I). report, that (I) can be transformed photochemically into the bicyclic isomer (11), and that (11), in turn, reverts to (I).To our knowledge, this is the first example of photochemical valence isomerization between monocyclic hetero-epines and the corresponding bicyclic isomers.WE have already reported that irradiation of the least substituted 1 , 3-oxazepine which is known, compound (I),
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