Photochemistry of triptycene-1,4-quinone

Photolysis of 9,10-dihydro-9,10[1’,2’]benzenoanthracene-1’,4’-quinone C20H12O2, in oxygenated acetone gives the novel photoproduct 9,10-dihydro-9,10-ethanoanthracen-11-one-12-spiro-2’-cyclopent-4’-ene-1’ ,3’-dione, C20H12O3. The reactant quinone molecule has ideal mm symmetry and lies on a crystallographic mirror plane in Pnma; the photoproduct molecule has ideal m symmetry, which is not utilized in packing in Pna2(1). This product is formed only in the presence of oxygen and a mechanism for its formation is proposed. Unreactivity in attempted solid-state photolysis can be rationalized in terms of crystal packing.