Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit Ketonen,Acylcyaniden und Ketoestern. 33. Mitteilung über Photoreaktionen

Similarly to aldehydes [6] ketones form 3-oxazolines via cyclo-addition to the benzonitrile-methylides 2 that arise photochemically from the 3-phenyl-2H-azirines 1 . With various ketones benzonitrile-isopropylide ( 2a ) gives cyclo-addition products in very good preparative yields (scheme 1). Benzonitrile-ethylide ( 2c ) and benzonitrile-benzylide ( 2b ) [8] react, however, sluggishly with ketones, smooth cyclo-addition being observed in their case only with «activated» ketones (2,2,2-trifluoro-acetophenone, 1,1,1-trifluoro-2-propanone). With 1a acetonyl-acetone forms the bis-adduct 12 While the azirine 1a reacts with cyclohexanone to yield essentially only the spiro-(3-oxazoline) 13 , it gives with cyclopentanone, depending on the reaction conditions, either the spiro-(3-oxazoline) 14 or the butenyl-3-oxazoline 15 (scheme 3). The formation of 15 has to be preceded by the photochemical formation of 4-pentenal from the ketone. Norcamphor and camphor react in a similar way (schemes 4 and 5). The azirines 1a–c react smoothly with the keto groups in acylcyanides and α-keto-esters, giving with the former 5-cyano-3-oxazolines and with the latter 5-ethoxycarbonyl-3-oxazolines (schemes 6 and 7). β-Keto-esters (acetoacetic ester) form with the dipole arising from 1a the expected addition product 31 and, via the protonated dipole d (scheme 8), finally the benzylidene-acetoacetic ester. Analogous results are obtained with malonodinitrile, trifluoro-acetamide and other weak acids such as alcohols [29] [30] (scheme 9). The light-induced rearrangement of the bicyclic isoxazoline 37 into the oxazoline 38 is visualized as an intramolecular cyclo-addition reaction (scheme 10). The cyclo-addition in this case proceeds with the aldehyde group inversed as compared to the related intermolecular benzonitrile–methylide addition to aldehydes.