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‐[ 15 N]Aminopropyl)‐3,3′‐bis(4‐ hydroxyphenyl)‐ N,N′ ‐(butane‐1,4‐diyl)bis[prop‐2‐enamide] ([ 15 N(1)])‐( 1 ) was synthesized in order to get further information about the fragmentation mechanisms. The comparison of the ESI‐MS/MS of 1 and [ 15 N(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.