Photochemische Cycloadditionen von 3‐phenyl‐2 h ‐azirinen mit Aldehyden. 31. Mitteilung über Photoreaktionen

Experiments concerning the photochemical condensation of 3‐phenyl‐2 H ‐azirines 1 with aliphatic and aromatic aldehydes to 3‐oxazolines 4 are fully described ( cf . scheme 1 ). Photochemically nitrile methylides of type 2 are first formed, which then very quickly react thermally with the aldehydes in a regiospecific manner to give the 3‐oxazolines 4 . Azirines monosubstituted in position 2 (l b and 1 c ) give mixtures of cis , trans ‐oxazoline isomers, in which the cis isomer predominates. The stereoselectivity of the cycloaddition reaction can be rationalized by a simple model (scheme 10). The stereoisomeric 3‐oxazolines 4 are distinguishable in the NMR. spectra by the large homoallylic coupling constants between the H atoms on C(2) and C(5).