Electron impact mass spectrometric studies of 2-methyl, 2-phenyl, 2-(1-piperidyl), 2-(2/3/4-pyridyl), piperidino and pyrido[4,3-d]pyrimidin-4-ones

The title compounds were investigated to characterize their structure and fragmentation mechanisms by EI electron impact mass spectrometry (EI-MS) and collisionally activated decomposition. The results obtained on magnetic sector instruments show that the compounds fragment similarly whether the substituent at C-2 is phenyl or 3/4-pyridyl. If, however, it is methyl, 2-pyridyl or 1-piperidyl, the balance of fragmentations is different. All the studied compounds are stable and give an intense molecular ion peak A great difference exists between the fragmentation patterns of the piperidino compounds and those of the more aromatized pyrido compounds. The loss of hydrogen aromatizes the piperidino derivatives to some extent, especially the 2-(2-pyridyl)-substituted compounds, forcing them towards a more planar structure. In 2-(2-pyridyl) derivatives an intramolecular hydrogen bond between the 3N-H and the 2-pyridyl nitrogen atoms strengthens the effect. Deuterated analogs were used to clarify hydrogen rearrangements and to confirm ion structures. Semiempirical AM1 calculations were carried out on 70 tautomeric model structures. The results are not in contrast to the MS results and they support, e.g., the proposed intramolecular hydrogen bonding between the 3N-H and the 2-(2-pyridyl) nitrogen. (C) 1998 John Wiley & Sons, Ltd.