Fragmentation Study of 2-Amino-4H-1,3-oxazines and Octahydro-4-phenyl-2H-pyrano[2,3-d]pyrimidin-2-ones: Identification of Heterocyclic Six-membered Ring in Three-component Reactions

The fragmentation behavior of ten 2-amino-4H-1,3-oxazines and seven octahydro-4-phenyl-2H-pyrano[2,3-d]-pyrimidin-2-ones was investigated by electrospray ionization tandem mass spectrometry (ESI-MSⁿ). When the precursor ions were protonated oxazines, two predominant fragmentation pathways were observed with elimination of CH₂N₂ and HCNO, respectively. The product ion [M+H−60]⁺ could be observed from both pathways. While in the type of pyrimidin-2-ones, only direct loss of 60 Da (urea) could be observed. The fragmentation pathways were investigated by high-resolution mass spectrometry and deuterium-labeling experiments, and the characteristic fragmentations presented here could be convenient for future identification of resembled derivatives, especially in high throughput screening combinatorial chemistry.