Cycloalkene budding: mass spectrometric studies of competitive and dual cycloalkene extrusion reactions from doubly unsaturated aldehyde N,N-dimethylhydrazones

Mass spectral fragmentation pathways of four doubly unsaturated aldehyde N, N-dimethylhydrazones were investigated using EI-MS and tandem mass spectrom etry (MS/MS) under electron ionization and collisionally activated decompos ition (CAD) conditions. Cyclopentene extrusion was found to be slightly fav ored over cyclohexene loss in a hydrazone capable of losing either cycloalk ene, Evidence for the regeneration of a chain-shortened iminium radical cat ion as a result of cycloalkene extrusion was provided by studying substrate s capable of undergoing successive cycloalkene budding sequences. EI-MS of these compounds shows sequential loss of both cyclopentene and cyclohexene, in accord with expectations for a cascade mechanism. Although these MS/MS experimental results are also compatible with alternative mechanisms which would entail the simultaneous loss of both neutral cycloalkenes or of a mac rocyclic diene, a rapid cascade of cycloalkene budding accounts best for th e experimental observations. Copyright (C) 2000 John Wiley & Sons, Ltd.