Irradiation of the dihydrooxepine monoester (+)‐4 gives the bridged bicyclobutane (+)‐5 in 77% yield. On the other hand, the corresponding diester 1 yields the cyclobutene 2. Possible explanations for this differing behaviour are discussed.

Synthesis of Medium and Large Rings. Part 44. Synthesis of Optically Active Bridged Bicyclobutane Derivatives. -The bicyclobutane derivative (II) is prepared on a gram scale. -(TOCHTERMANN, W.; PANITZSCH, T.; PESCHANEL, M.; WOLFF, C.; PETERS, E.-M.; PETERS, K.; VON SCHNERING, H. G.; Liebigs Ann.