Entartete und strukturelle thermische umlagerung von 7-methylenbicyclo[4.2.0]oct-2-en

Besides a structural isomerization to $\text{2a}$ the title compound $\text{1a}$ undergoes a faster automerization as shown by D-labelling; the change in the ratio of rearrangements of $\text{1a}$ to ring contracted $\text{1b}$ points to conformational effects in the reorganizations of $\text{1a}$.