Overcrowded molecules. Part IX. Fatigue-free photochromic systems involving (E)-2-Isopropylidene-3-(mesitylmethylene)succinic anhydride and N-phenylimide

(E)-2-Isopropylidene-3-(mesitylmethylene)succinic anhydride and N-phenylimide undergo reversible photochemical conrotatory ring closure reactions to form 1,8a-dihydronaphthalene intermediates, which do not experience ring opening up to 160°. This is presumably because of steric interactions between the 1 - and 8a-methyl substituents which must occur in the allowed disrotatory mode. Neither a methyl shift nor elimination of ethane occurs at this temperature; these thermally stable photochromic systems thus do not exhibit irreversible side reactions.