Carotenoids and related compounds. Part 40. Synthesis of trikentriorhodin and other β-diketones

Abstract

Three routes to polyene β-diketones are described.Condensation of 8′-apo-β-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 to give the polyene β-diketones (8) and (9).Reaction of the di-lithium salt of 3-methylpent-1-en-4-yn-3-ol (32) with pivalic anhydride (11) yielded the acetylenic ketone (34), which on hydration furnished the β-diketone (36). This was converted into the Wittig salt (39) which condensed with the trienedial (44) to give the polyene β-diketones (45) and (46).Polyene carboxylic esters were found to condense with methyl ketones, in the presence of lithamide, to give β-diketones in high yield. Thus methyl 8′-apo-χ-carotenoate (59), for which a synthesis from (44) is reported, condensed with the trimethylsilyl derivative of the optically active hydroxy-ketone (64) to give, after hydrolysis of the protecting group, trikentriorhodin (1). Comparison of the c.d. properties of the synthetic compound, with those reported for the natural pigment, indicate that the latter has the 3S,5R configuration.