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)‐2 H ‐azirine ( 23 , R = H and CH 3 , 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‐2 H ‐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 = CH 3 ) by methanol at the nitrile carbon atom, after which hydrolysis yields the aldehyde.