Study On The Fragmentation Pathway And Relating Reaction Mechanism Of The Heterocycles By Atmospheric Pressure Ionization/Mass Spectrometry

The development of atmospheric pressure ionization, including ESI and APCI, greatly expanded the role of mass spectrometry in many applications. To solve a wide variety of structural identification problems is one of its advantages. The purpose of this dissertation is to address the fragmentation mechanisms of some novel heterocycles, and apply on the reaction mechanism study.First, the protonated chalcone and its derivatives with different functional groups have been studied by APCI and tandem mass spectrometry. The proposed fragmentation pathways have been elucidated by FTICRMS/MS. Moreover, the fragmentation patterns were obviously affected by substituted position as shown with breakdown curves, which shows advantage to identify and characterize the constitutional isomers.In the third chapter, the complete fragmentation mechanisms of 2-amino-4H-1, 3-oxazines and octahydro-2H-pyrano[2, 3-d]pyrimidine-2ones have been investigated under ESI and tandem mass spectrometry and identified by high-resolution mass spectrometry and deuterium-labeled experiments. The two type compounds gave rise to fragment ions via RDA reaction, but they have different fragment patterns. Besides, each type of compounds presented a novel decomposed pathway, respectively, which was not favorable under electron impact ionization in the previous studies. 2-amino-4H-1, 3-oxazines revealed obviously eliminations of CH₂N₂ and CHNO molecules in the initial fragmentations. On the other hand, octahydro-2H-pyrano[2, 3-d]pyrimidine-2ones showed losses of urea and 3, 4-Dihydro-2H-pyran directly.Chapter 4 described the fragmentation mechanisms of the three types compounds, 2-Amino-4H-1, 3-thiazines, 2-Amino-5, 6-dihydro-4H-1, 3-thiazines, and 3, 4-di-hydropyrimidines, respectively. Although they have similar framework, they presented different and characteristic fragmentation pathways. Moreover, the effect of the substituents on the fragmentation progress was demonstrated experimentally with Hammett constants, which provided a quantitative description of the trend loss of substituted phenyl group. Once again, breakdown curves were applied to distinguish isomers from their MS³ mass spectra.Finally, the combination of HPLC-ESI/MS and MSⁿ has been used to probe out important reaction intermediates which are labile and difficult to isolate and identify by the other techniques. This work, on the three-component reaction, showed the advantage of HPLC-ESI/MSⁿ techniques to isolate and characterize the key intermediate, N-acylimine, with short lifetime labile in the reaction. On the other hand, the technical development was used to characterize and identify side-products with great selectivity and sensitivity. It is believed that the application of modem mass spectrometry techniques is likely to find broad use in studying the mechanisms of organic and bioorganic in the coming future.