Efficient Syntheses of the Keto-carotenoids Canthaxanthin, Astaxanthin, and Astacene

Three keto-carotenoids were prepared by the oxidation of the stable C40 trisulfone 6, which has been used as the key compound in our β-carotene synthesis. The first allylic oxidation to the unsaturated ketone and the second oxidation to the α-hydroxyketone produced the C40 trisulfones 7 and 10, respectively. The Ramberg−Bäcklund reaction of the oxidized C40 trisulfone was efficiently effected by the use of a mild base, NaOMe, in the presence of CCl4 as a halogenating agent to give the C40 disulfones 8 and 11. Base-promoted dehydrosulfonation reaction of the disulfone compounds produced the fully conjugated polyenes of canthaxanthin (1), astaxanthin (2), and astacene (3).