Carotenoids and related compounds. Part XXIX. Stereochemistry and synthesis of the allenic end group. Absolute configuration of zeaxanthin

Abstract

Three of the four racemates of the allenic ketone, 4-(2,4-dihydroxy-2,6,6-trimethylcyclohexylidene)but-3-en-2-one, have been synthesised and their configurations determined. Except for optical activity, one of these racemates is identical with the allenic ketone from the grasshopper Romalea microptera, and its mono-(secondary) acetate with a degradation product of fucoxanthin. The latter has been converted into an optically active allenic ketone identical with that from grasshoppers. The absolute configuration of the product was established by X-ray crystallographic analysis of its p-bromobenzoate.

These results reveal that zeaxanthin (3,3′-dihydroxy-β-carotene) has the 3R,3′R-configuration.