Zur Photochemie von 3,5‐Diaryl‐2‐isoxazolinen. 30. Mitteilung über Photoreaktionen

Irradiation of 3,5‐diphenyl‐ or 3‐(p‐tolyl)‐5‐phenyl‐2‐isoxazoline (12 and 13, respectively) in benzene with a high‐pressure mercury lamp yields 4,5‐diphenyl‐ or 4‐(p‐tolyl)‐5‐phenyl‐3‐oxazoline (17 and 19, respectively) and the β‐amino‐chalcones 18 or 20 in addition to benzaldehyde, benzonitrile and p‐tolunitrile, respectively (scheme 6 and ‘Anmerkg.’ p. 2600). The 3‐oxazolines 17 and 19 are formed by route a (scheme 8) via 3‐phenyl‐ or 3‐(p‐tolyl)‐2H‐azirine (23, R = H and CH3, respectively) and their photochemically rearranged successors, the nitrile methylides 24, as intermediates. The discovery of this reaction has served as a basis for the quickly developing photochemistry of 3‐aryl‐2H‐azirines [2] [24]. Photolysis of the 2‐isoxazoline 13 in methanol leads to the formation of a mixture of syn/anti‐p‐tolyl trans‐styryl ketoximes (syn/anti, trans‐30) and anti, cis‐30, 2‐(p‐tolyl)‐quinoline (29), the 4‐hydroxymethylated derivative 32 of the latter (in small amounts), besides the β‐aminochalcone 20, benzaldehyde, p‐tolualdehyde and p‐tolunitrile (scheme 9). It could be shown that the stereoisomeric ketoximes 30 are photochemically interconvertible (scheme 12) and that at least one mechanism of formation of 2‐(p‐tolyl)‐quinoline (29) is the photo‐induced cyclisation of p‐tolyl‐cis‐styryl ketoximes (cis‐30) (scheme 13). A tentative mechanism for the formation of p‐tolual‐dehyde is given in scheme 10; the crucial step is the protonation of p‐tolunitrile methylide (24, R = CH3) by methanol at the nitrile carbon atom, after which hydrolysis yields the aldehyde.