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 simil arly whether the substituent at C-2 is phenyl or 3/4-pyridyl. If, howe ver, it is methyl, 2-pyridyl or 1-piperidyl, the balance of fragmentat ions is different. All the studied compounds are stable and give an in tense molecular ion peak A great difference exists between the fragmen tation patterns of the piperidino compounds and those of the more arom atized pyrido compounds. The loss of hydrogen aromatizes the piperidin o derivatives to some extent, especially the 2-(2-pyridyl)-substituted compounds, forcing them towards a more planar structure. In 2-(2-pyri dyl) derivatives an intramolecular hydrogen bond between the 3N-H and the 2-pyridyl nitrogen atoms strengthens the effect. Deuterated analog s were used to clarify hydrogen rearrangements and to confirm ion stru ctures. Semiempirical AM1 calculations were carried out on 70 tautomer ic 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.