Photochemie von in 4‐Stellung substituierten 5‐Methyl‐3‐phenyl‐isoxazolen .44. Mitteilung über Photoreaktionen

Photochemistry of 4‐substituted 5‐Methyl‐3‐phenyl‐isoxazoles. 4‐Trideuterioacetyl‐5‐methyl‐3‐phenyl‐isoxazole ([CD 3 CO]‐ 27 ), upon irradiation with 254 nm light, was converted into a 1:1 mixture of oxazoles [CD 3 CO]‐ 35 and [CD 3 ]‐ 35 ( Scheme 13 ). This isomerization is accompagnied by a slower transformation of ([CD 3 CO]‐ 27 ) into [CD 3 ]‐ 27 . Irradiation of the isoxazole derivatives 28, 29, 30 and ( E )‐ 31 yielded only oxazoles 36, 37, 38 and ( E ), ( Z )‐ 39 ; no 4‐acetyl‐5‐alkoxy‐2‐phenyl‐oxazole, 2‐acetyl‐3‐methyl‐5‐phenyl‐pyrrole or 2‐acetyl‐4‐methoxycarbonyl‐3‐methyl‐5‐phenyl‐pyrrole, respectively, were formed ( Scheme 9 and 10 ). Similarly ( E )‐ 32 gave a mixture of ( E ), ( Z )‐ 40 only ( Scheme 11 ). Upon shorter irradiation, the intermediate 2 H ‐azirines ( E ), ( Z )‐ 41 could be isolated ( Scheme 11 ). Photochemical ( E )/( Z )‐isomerization of the 2‐(trifluoro‐ethoxycarbonyl)‐1‐methyl‐vinyl side chain in all the compounds 32, 40 and 41 is fast. At 230° the isoxazoles ( E )‐ and ( Z )‐ 32 are converted into oxazoles ( E ), ( Z )‐ 40 . The same compounds are also obtained by thermal isomerization of the 2 H ‐azirines ( E ), ( Z )‐ 41 . The most probable mechanism for the photochemical transformations of the isoxazoles, as exemplified in the case of the isoxazole 27 , is shown in Scheme 13 . A benzonitrile‐methylide intermediate is postulated for the photochemical conversion of the 2 H ‐azirines into oxazoles. 2 H ‐Azirines are also intermediates in the thermal isoxazole‐oxazole rearrangement. It is however not yet clear, if the thermal 2 H ‐azirine‐oxazole transformation involves the same transient species as the photochemical reaction. A mechanism for the photochemical isomerization of the 2 H ‐azirine 11 to the oxazole 15 is proposed ( Scheme 3 ).