Pyrolysis of Bicyclo[n.2.2] Bridgehead Acetates with a Bridgehead Double Bond

Pyrolysis of bicyclo[n.2.2] bridgehead acetates 2a-c (n = 4-6) with a bridgehead double bond has been undertaken with a view to synthesizing bicyclo[n.2.2] bridgehead dienes by elimination of acetic acid. It is proved that bicyclo[6.2.2] bridgehead dienes 3 and 4 can be synthesized successfully by the pyrolysis of 2c. The symmetric bridgehead diene 3, which is a dihydro derivative of [6]paracyclophane, shows remarkable properties due to the presence of two distorted double bonds in close proximity. In the case of bicyclo[5.2.2] system 2b, pyrolysis gives 2,8-dimethylenebicyclo[5.2.0]nonane (10) as a sole product probably derived from [3,3] sigmatropic rearrangement of unstable bridgehead diene intermediate 12. Moreover, in the case of bicyclo[4.2.2] system 2a, pyrolysis affords trienes 13 and 14 as the major products which are formed by retro-ene reaction of 2a followed by elimination of acetic acid rather than through bicyclo[4.2.2] bridgehead diene intermediates such as 17 and 18, presumably because of the strain inherent in bicyclo[4.2.2]dec-1(8)-ene skeleton.