The electron ionization time-of-flight (TOF) mass spectra of a series of 5-ethoxycarbonyl-4-substituted-6-methyl-3,4-dihydropyrimidin-2(1H)-ones were studied to establish their fragmentation processes. Using the high resolution capabilities of the TOF instrument, exact masses for each fragment were determined. These data were used to infer molecular formulas and elemental compositions for all molecular ions and fragments through software interpretation and according to the established fragmentation rules the majority of ions were fully assigned. Two main fragmentation routes can be found in this work. First of them, for all the title compounds, includes the formation of three cations, by loss of R1 from the position 4, C2H4 (via a McLafferty rearrangement) from the ester group and H2O via a cyclic-six-membered transition state. The second route, for 4-aromatic compounds, consists of the formation of a cation by loss of EtCO2. Several additional fragmentations for individual compounds are proposed.%研究了一系列5-乙氧羰基-4-取代-6-甲基-3,4-二氢嘧啶-2(1H)-酮的电子轰击飞行时间质谱,建立了此类化合物的裂解方式和分子离子峰的强度规律.利用飞行时间质谱的中分辨能力,测得了各碎片离子的精确质量,这些数据经OpenLynx软件导出所有分子离子和碎片离子的元素组成.根据质谱裂解规律,主要质谱离子得到了归属.两类主要裂解途径为:对所有标题化合物,通过失去4位上的R1,酯基上的C2H4(麦氏重排)和H2O(通过环状六员过渡态)形成三种阳离子;对4-芳基化合物,通过失去EtCO2形成另一类阳离子.此外,还提出了个别化合物的一些重要裂解行为.
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