Photochemische Synthese von 4-Phenyl-3-oxazolin-5-onen und deren thermische Dimerisierung

Irradiation of 3-phenyl-2H-azirines affords reactive benzonitrile-methylide intermediates, which can be trapped by carbon dioxide to yield 4-phenyl-3-oxazolin-5-ones (Scheme 1). The first section of the paper deals with the experimental description of this reaction, which already has been preliminarily communicated. Upon irradiation the 3-oxazolin-5-ones undergo photoextrusion of carbon dioxide to reform the corresponding benzonitrile-methylides, which can be trapped by dimethyl acetylenedicarboxylate. In the second section of the paper, reactions of 2-methyl-4-phenyl-3-oxazolin-5-one (3c) are described. Upon heating to 130°, this compound is partially converted to 2-methyl-4-phenyl-2-oxazolin-5-one (azlactone 4e). Prolonged heating of 3c affords the dimer 7 (Scheme 3) as well as the imidazole derivative 9 (Scheme 4). Compound 7 is related to the ‘Rügheimer compound’ C18H14N2O4, formed from hippuric acid methylester. The structure of 7 was determined by X-ray crystallography and this supports the formula assigned earlier to the ‘Rügheimer compound’ (12) and related pyrrolidin-2,4-diones. The possible mechanism of the thermal formation of 7, which is also base catalysed, is represented in Scheme 3, the one for the formation of the imidazol 9 in Scheme 4. Under the influence of oxygen the 2,4-diphenyl-3-oxazolin-5-one (3b) undergoes a dehydro-dimerization to yield compound 16 (Scheme 7). Section three contains the structure elucidation of compound 16 and a mechanistic proposal for the formation of the pyrazine 19 upon thermolysis of 2,4-diphenyl-2-oxazolin-5-one (4b, Scheme 7)

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