Photochemische Synthese von 4‐Phenyl‐3‐oxazolin‐5‐onen und deren thermische Dimerisierung. 45. Mitteilung über Photoreaktionen

Abstract

Photochemical Synthesis of 4‐Phenyl‐3‐oxazolin‐5‐ones and their Thermal Dimerization. Irradiation of 3‐phenyl‐2 H ‐azirines affords reactive benzonitrilemethylide 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’ C 18 H 14 N 2 O 4 , 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 dehydrodimerization 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‐on ( 4b , Scheme 7 ).