Stereospecificity of “Retro-Diels-Alder” Fragmentation under Electron Impact

“Retro-Diels-Alder” fragmentation under electron impact and two analogous processes which are accompanied by hydrogen migrations are highly stereospecific in three systems of diketones. The resulting ions a, b, and c are very abundant in the case of cis isomers 3, 7, and 9, but of negligible abundance in trans diketones 4, 8, and 10. The high degree of stereospecificity suggests that no rearrangement of the molecular ions to common structures precedes these fragmentation processes, and that two C,C bonds are cleaved concurrently in the course of retro-Diels-Alder fragmentation and the analogous processes in the examined systems.