Synthese von (6 R , all‐ E )‐Neoxanthin und verwandten Allen‐Carotinoiden

Synthesis of (6 R , all‐ E )‐Neoxanthin and Related Allenic Carotenoids We present the first synthesis of enantiomerically pure neoxanthin ( 1 ) by a Wittig ‐ Horner 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 (15 Z )‐intermediate 37 , neoxanthin ( 1 ) was obtained in good yield. Similar syntheses gave (15 Z , 9′ Z )‐neoxanthin ( 45 ; Scheme 5 ) and (9 Z )‐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 ).