[4.4.1]Propellatetraene and Related Propellanes

Reaction of dichlorovinylene carbonate with 1,6‐methano[10]annulene (2) followed by hydrolysis of the intermediate adduct afforded α‐diketone 3. Photolysis of 3 at liquid nitrogen temperature yielded the title compound, 1. The activation parameters for the thermal isomerization 1 → 2 were ΔH+ = 5.1 kcal/mole and ΔS−= −19 eu (100 K). The structure of 3 was established by X‐ray analysis. This analysis and MM2 calculations on a number of related compounds provide additional examples of the so‐called “Klammer” effect in propellanes.