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-4H-chromenes

Abstract

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‐4H‐chromenes. In the EI spectra the hexahydroquinolines undergo the loss of the substituent attached at the C4 position, while the 4H‐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 4H‐chromenes. The ESI‐MS/MS spectra reveal results similar to the EI and CI spectra, since the 4H‐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.