Fragmentation – Fragmentations are controlled by stereochemistry

If a molecule is to undergo fragmentation/rearrangement, then orbitals have to be able to overlap. Consider the two quite different reactions of two quite similar compounds shown below. The starting materials vary only in their stereochemistry, but this small change has a massive effect on their ability to undergo fragmentation.

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‘cis’ isomer
‘trans‘ isomer

U. Burckhardt, C. A. Grob and H. R. Kiefer, Helv. Chim. Acta, 1967, 50, 231–244.