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

Abstract

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 ‘Rugheimer 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 ‘Rugheimer 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).