Overcrowded molecules. Part XI. A doubly ‘forbidden’ symmetry-allowed pericyclic reaction: the thermal rearrangement of (E)-2-benzylidene-(Z)-1-mesityl(phenyl)methyleneindane into (Z)-2-benzyl-1-mesityl(phenyl)methyleneindene

Abstract

(E)-2-Benzylidene-(Z)-1-mesityl(phenyl)methyleneindane cyclises by a disrotatory mode at 180 °C to give cis-9a,10-dihydro-5,10-diphenyl-6,8,9a-trimethyl-11H-benzo[b]fluorene, which, at this temperature, undergoes two ‘forbidden’ processes in concert, namely conrotatory ring opening and a suprafacial [1,7] hydrogen shift, to yield (Z)-2-benzyl-1-mesityl(phenyl)methyleneindene.