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.