Characterization of 4-Aryl-5-ethoxycarbonyl-6-methyl-3, 4-dihydropyrimidin-2 (1H) -thiones by El-TOF Mass Spectrometry1

The electron impact (EI) time-of-flight mass spectra (TOFMS) of the title compounds (thioxo-Biginelli compounds) were studied to establish their fragmentation pathways. With a TOF instrument of high resolution, the exact mass for each fragment was determined. These data were used to infer the molecular formulas and the ionic elemental compositions for all the compounds through interpretation by software. By further applying chemical intuition, majority of the fragmentation ions were fully assigned. All the compounds give strong signals (average 76%) for their molecular ions in the EI spectra. Two kinds of characteristic fragmentation pathways from the molecular ion were observed. One is related to the loss of the ester group, forming a resonance stabilized ion with a moderate abundance (average 40%). The other concerns the loss of the aromatic-ring radical, forming another resonance stabilized ion at m/z 199 with a high abundance (average 79%), from which further important fragmentations including the formation of an ion at m/z 171 (average 24%) by the loss of C2 H2 (via the McLafferty rearrangement), and an ion at m/z 153 (average 10%) by the elimination of a water molecule preceed. In addition, ion [M - Et˙]+ with a high abundance (average 62%) is a characteristic for the Biginelli compounds with an ethoxycarbonyl group. Differences and similarities among the fragmentations observed from the EI-TOFMS of oxo-Biginelli compounds are discussed.