Studies in mass spectrometry—XV: Retro-diels-alder fragmentation and double hydrogen migration in some polycyclic diketones

In contrast to adducts I of bi-1-cycloalken-1-yls and p-benzoquinone, their reduction products II do not exhibit a double hydrogen migration from δ positions accompanying a ‘retro-Diels-Alder’ type fragmentation. An ordinary retro-Diels-Alder fragmentation was found to take place, with charge retention in the diene portion of the molecule. A double hydrogen migration has been detected in II leading to m/e 112 ion c₂, which differed, however, from that in I in charge retention and in the origin of the migrating hydrogen atoms. Adducts III of di-1-cycloalken-1-yls and naphthoquinone behave similarly to II. They exhibit relatively low abundance ions a, however, due to a double hydrogen migration from δ positions, similarly to I. The origin of the migrating hydrogen atoms have been determined by deuterium labelling. Mechanistic suggestions are presented to explain the observed facts.