An overview of the retro‐Diels‐Alder reaction in semiunsaturated heterocyclic rings: mass spectra of new substituted 1,4,5,6,7,8‐hexahydroquinolines and their oxo‐analogues 5,6,7,8‐tetrahydro‐4 H ‐chromenes

Electron impact ionization (EI), chemical ionization (CI), electrospray ionization (ESI) and tandem mass spectrometry (MS/MS) were used to investigate a number of relatively large and structurally related new heterocycles such as substituted 1,4,5,6,7,8‐hexahydroquinolines and their oxa‐analogues 5,6,7,8‐tetrahydro‐4 H ‐chromenes. In the EI spectra the hexahydroquinolines undergo the loss of the substituent attached at the C4 position, while the 4 H ‐chromenes undergo a retro‐Diels‐Alder reaction (RDA) after elimination of the C4 substituent. Under chemical ionization conditions the RDA reaction is observed only for the 4 H ‐chromenes. The ESI‐MS/MS spectra reveal results similar to the EI and CI spectra, since the 4 H ‐chromenes undergo RDA reactions while the hexahydroquinolines form a very stable even‐electron pyridium ion derived from the loss of the C4 substituent. Copyright © 2001 John Wiley & Sons, Ltd.