Abstract
Similarly to aldehydes [6] ketones form 3-oxazolines via cycloaddition to the benzonitrile-methylides 2 that arise photochemically from the 3-phenyl-2H-azirines 1. With various ketones benzonitrile-isopropylide (2a) gives cycloaddition products in very good preparative yields (scheme 1). Benzonitrile-ethylide (2c) and benzonitrile-benzylide (2b) [8] react, however, sluggishly with ketones, smooth cycloaddition being observed in their case only with ‘activated’ ketones (2,2,2-trifluoroacetophenone, 1,1,1-trifluoro-2-propanone). With 1a acetonylacetone 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 group in acylcyanides and alpha-keto-esters, giving with the former 5-cyano-3-oxazolines and with the latter 5-ethoxycarbonyl-3-oxazolines (schemes 6 and 7). Beta-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, trifluoroacetamide 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 cycloaddition reaction (scheme 10). The cycloaddition in this case proceeds with the aldehyde group inversed as compared to the related intermolecular benzonitrile-methylide addition to aldehydes.
Claus, Peter