Three routes to polyene 0-diketones are described.Condensation of 8'-apo-P-carotenal (4) and of 8,8 '-diapo-carotene-8,8'-dial (6) with butylamine in the presence of tri-isobutyl borate gave the Schiff's bases (5) and ( 7 ) respectively. These reacted with the boric oxide complex of acetylacetone t o give the polyene B-diketones (8) and ( 9).Reaction of the di-lithium salt of 3-methylpent-I -en-4-yn-3-01 (32) with pivalic anhydride (1 1 ) yielded the acetylenic ketone (34)' which on hydration furnished the P-diketone (36). This was converted into the Wittig salt (39) which condensed with the trienedial (44) t o give the polyene pdiketones ( 45) and ( 46).Polyene carboxylic esters were found t o condense with methyl ketones, in the presence of lithamide, t o give P-diketones in high yield. Thus methyl 8'-apo-~-carotenoate (59)' for which a synthesis from ( 44) is reported, condensed with the trimethylsilyl derivative of the optically active hydroxy-ketone (64) t o give, after hydrolysis of the protecting group, trikentriorhodin (1). Comparison of the c.d. properties of the synthetic compound, w i t h those reported for the natural pigment, indicate that the latter has the 3S, 5R configuration.

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The stereochemistry of mytiloxanthin, an acidic pigment of Mytilus edulis, is shown to be as depicted in formula (I).