Abstract The photo-decolouration of o-fuchsone is caused by [8+ 2] cycloaddition of singlet oxygen to the quinonoid trienone π-system. The arising 1, 2, 3-trioxanaphthalene undergoes further fragmentations and recyclizations to give 1, 3-benzodioxol. The course of this reaction was investigated using fuchsones I-IV as substrates and hydrogen peroxide and tert- butyl hydroperoxide as oxidants. The thermolabile epoxy ketone XV was identified as the ...