Electrospray-Ionization Mass Spectrometry. Part III. Acid-Catalyzed Isomerization of N,N′-Bis[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines by the Zip Reaction

The electrospray tandem mass spectra (ESI-MS/MS) of the three N,N′-bis(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines 1–3 displayed the same fragment-ion signals. These isomers could not be differentiated by ESI-MS/MS, since their fragmentation patterns are similar. (E,E)-N-(3-¹⁵N]Aminopropyl)-3,3′-bis(4- hydroxyphenyl)-N,N′-(butane-1,4-diyl)bisprop-2-enamide] (¹⁵N(1)])-( 1 ) was synthesized in order to get further information about the fragmentation mechanisms. The comparison of the ESI-MS/MS of 1 and ¹⁵N(1)]- 1 revealed a transamidation, the Zip reaction, under mass-spectral conditions of the 1 + H]⁺ ions. Because of this reaction, the three isomers 1–3 could not be distinguished.