Stereoselective Thermal Conversion of s-trans-Diallene into Dimethylenecyclobutene via s-cis-Diallene in the Crystalline State

A conrotatory [2+2] cyclization is the second step in the solid-state thermal reaction of s-trans-tetraaryldibromohexatetraenes 1 to cyclobutenes 4. Prior to the cyclization 1 rearranges into the cis conformer 3. Surprisingly the thermal rearrangement and the stereoselective cyclization occur readily in spite of the required motion of the sterically bulky substituents. R=Ph, p-MeC₆ H₄ .