Comparison by mass spectrometry of ring cleavage in barbituric acid and 5-fluorobarbituric acid derivatives

The mass spectrometric behaviour of barbituric acid and 1,3-dimethylbarbituric acid was compared with that of the corresponding 5,5-difluoro derivatives and of some 5-fluoro-5-alkyl derivatives in order to study the influence of fluorine in the fragmentation processes. This investigation, performed using both electron impact ionization and positive- and negative-ion fast atom bombardment, evidences well the role of fluorine in barbiturate ring bond cleavages.