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

Electron impact ionization (EI), chemical ionization (CI), electrospray ion ization (ESI) and tandem mass spectrometry (MS/MS) were used to investigate a number of relatively large and structurally related new heterocycles suc h 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 un dergo 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 i s 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 reacti ons while the hexahydroquinolines form a very stable even-electron pyridium ion derived from the loss of the C4 substituent. Copyright (C) 2001 John W iley & Sons, Ltd.