SYNTHESIS OF (6R,ALL-E)-NEOXANTHIN AND RELATED ALLENIC CAROTENOIDS

We present the first synthesis of enantiomerically pure neoxanthin (1) by a Wittig-Homer condensation between the ylide from the novel diethyl 12'-apo-15,15'-didehydroviolaxanthin-12'-phosphonate (35) and the allenic C-15-aldehyde 31 (Scheme 4) via the crystalline 15,15'-didehydroneoxanthin (36: 70% yield). After partial hydrogenation of the triple bond of 36 and isomerisation of the (15Z)-intermediate 37, neoxanthin (1) was obtained in good yield. Similar syntheses gave (15Z,9' Z)-neoxanthin (45; Scheme 5) and (9Z)-15,15'-didehydroneoxanthin (47; Scheme 6). Comparison of the physical data of synthetic 1 with those of a freshly isolated sample of neoxanthin from the flowers of Trollius europaeus confirmed their identity. The unusually low melting point of 1 is caused by a very easy thermal isomerisation into a mixture of the neochromes 4 and 5 (Scheme 1). Such a thermal rearrangement is not observed with 15,15'-didehydroneoxanthin (36). To explain this, we assume a zwitterionic excited state of the allenic group that induces the rearrangement of the violaxanthin end group into the furanoid epoxide (Scheme 7).