Isoxazoles isoxazole—oxazole photoisomerization

Photolysis of the pyrazole-4-carboxyaldehyde 169 has been found to afford imidazole 172. This reaction is analogous to the isoxazole-oxazole photoisomerization discussed above. It is most likely that a 1,5-electrocyclic ring closure of iminonitrile imine 171 is responsible for imidazole formation. ... 

Isoxazole-oxazole photoisomerization was studied by irradiation of matrix-isolated 3,5-dimethylisoxazole (18) at 222 nm. 2-Acetyl-3-methyl-2H-azirine (20) was obtained, likely through an acetyl vinyl nitrene intermediate 19 as primary photoproduct, while upon longer time UV irradiation, two additional photoproducts were identified as acetyl nitrile ylide 21 and 2,5-dimethyloxazole (22) (13JOC10657). Analogously, 3,5-diphenylisoxazole and 2-benzoyl-3-phenyl-2f/-azirine behaved as precursors to triplet vinyl nitrene (of type 19) through laser flash photolysis (13JOC11349). Reductive heterocycle—heterocycle transformations of (2-nitrophenyl)isoxazole precursors, such as 23 and 26, afforded 4-amino quinolines of type 24, quinolin-4(lfJ)-ones 25, and 3-acylindoles 27. Che-moselective heterocyclizations were observed from 3,4-,4,5-, and 3,4-bis(2-nitrophenyl)isoxazoles (13OL2062). 

Photoisomerization of pyrazoles, isoxazoles and isothiazoles into imidazoles, oxazoles and thiazoles, respectively, is described in Section 3.4.1.2.4. 

On the other hand, the photoisomerization of oxazoles to isoxazoles involves not only the interchange of the 2 and 3 positions, but that of the 2 and 4 positions. To explain the latter reaction, the intervention of a Dewar compound was proposed (Scheme 13).41 The walk of the oxygen atom is characteristic of this mechanism. A quantum mechanical calculation suggested that the O—C-2 bond was weak (for the azirin) and the bond order between C-2 and C-5 was high (for the Dewar intermediate) in the excited state. 

Another rearrangement occurs in the photoisomerization of isoxazoles to oxazoles, which is interpreted in terms of a 1,5-electrocyclization (cf. p 129) involving an isolable 3-acylazirine 25 as an intermediate ... 

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