Carotenoids and related compounds. Part XXXII. Synthesis of astaxanthin, phoenicoxanthin, hydroxyechinenone, and the corresponding diosphenols
Autoxidation of 4-oxo-carotenoids in the presence of potassium t-butoxide gives the corresponding diosphenols (3,4-didehydro-3-hydroxy-4-ones). These on reduction with potassium borohydride give the 3,4-dihydroxycompounds which, on carefully controlled oxidation, yield the 3-hydroxy-4-ones.
Astaxanthin (3,3′-dihydroxy-ββ-carotene-4,4′-dione), phoenicoxanthin (3-hydroxy-β,β-carotene-4,4′-dione), and hydroxyechinenone (3-hydroxy-β,β-caroten-4-one) have been synthesised in this way from canthaxanthin (β,β-carotene-4,4′-dione), echinenone (β,β-caroten-4-one), and their 15,15′-acetylenic analogues.