Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit Aldehyden

Experiments concerning the photochemical condensation of 3-phenyl-2H-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 (1b and 1c) 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)

Photochemistry of some heterocyclic systems

Whereas 2H-benztriazoles are photochemically stable, 1H-benztriazoles yield biradicals after spliting of nitrogen. These biradicals attack aromatic substrates such as anisole, toluene, fluorobenzene or benzonitrile preferentially in the o- and p-positions with formation of the corresponding o-aminobiphenyls (cf. schemes 2-5). With a deficiency of aromatic substrate, attack at the o-position is strongly favored (cf. Figures 1, 2). Thereafter the very complex photochemistry of 3,5-diaryl-2-isoxazolines will be discussed (cf. schemes 7-9). This led to the discovery of the photochemical cycloaddition between 3-phenyl-2H-azirines and aldehydes, with formation of 3-oxazolines (cf. schemes 10, 11). The nitrile methylene ylides produced photochemically from azirines react, mainly in good yield, with substrates which contain cumulative (cf. schemes 15, 16) or activated (cf. schemes 12,13) double bonds. Noteworthy, is the photochemical incorporation of carbon dioxide, leading to the little known isoazlactones (scheme 15). After a short discussion of the photochemistry of the sydnones (cf. schemes 19-23), which has been only briefly investigated so far, the photolysis of indazoles, benzisoxazoles, and anthranils in strongly acid solution will be discussed (cf. schemes 24-27). These photochemical reactions give rise to aryl azenium ions or aryl oxenium ions, which react with the solvent SH preferentially in the position para to the nitrogen or oxygen, respectively