On treatment with manganese dioxide, carotenoid diosphenols are converted into 2-nor- analogues with a cyclopentenedione end group. Thus astacene (3, 3′-dihydroxy-2, 3, 2′, 3′-tetradehydro-β, β-carotene-4, 4′-dione) yields the blue pigment violerythrin (2, 2′- dinor-β, β-carotene-3, 4, 3′, 4′-tetraone).