A Novel 1,3-Sigmatropic Rearrangement in an Iminium Ion Generated in Collision-induced Decomposition during Electrospray Ionization High Resolution Multi-stage Mass Spectrometry

Rationale Rapid structural elucidation of unknown pharmaceutical impurities is critical throughout the whole life cycle of drug development. It can be achieved by using multi-stage mass spectrometric analysis, in conjunction with the understanding of fragmentation mechanisms of the ions generated by electrospray ionization. Methods Liquid chromatography-UV/photo diode array detection - high resolution multi-stage mass spectrometric analysis (HiRes MS n ) was used to elucidate the structure of an impurity of silodosin. The structure was confirmed by 1D and 2D NMR spectroscopy. Results The impurity is identified as N -cyanomethylsilodosin, a solution degradant of silodosin. A novel 1,3-sigmatropic rearrangement was observed during one of its MS 3 fragmentation pathways. A mechanism is proposed for the 1,3-sigmatropic rearrangement, which is supported by the MS n behavior of a 13 C-isotope labelled derivative of N -cyanomethylsilodosin. Conclusion Elucidation of such an intra-molecular rearrangement mechanism not only enriches our understanding of the gas phase fragmentation chemistry, but also should facilitate future rapid structural characterization of unknown pharmaceutical impurities and metabolites using multi-stage mass spectrometric analysis.