Synthese von optisch aktiven Carotinoiden mit 3,5,6‐Trihydroxy‐5‐6‐dihydro‐β‐Endgruppen

Syntheses of Optically Active Carotenoids with 3,5,6‐Trihydroxy‐5,6‐dihydro β‐End Groups For the specification of the relative and absolute configuration in carotenoids with 3,5,6‐trihydroxy‐5‐6‐dihydro β‐end groups, several ionone derivatives and carotenoids bearing this end group were synthesized. Acid‐catalyzed hydrolysis of (3S,5S,6R)– acetoxy‐5,6‐epoxy‐5,6‐dihydro‐β‐ionone (7) and of its (3S,5R,6S)‐isomer (13) gave the diols 8 and 15, respectively, with exclusive inversion at c(5) (Scheme 2). Compared to this, mild acid hydrolysis of caroten‐5‐6‐expoxides in the presence of H2O resulted in the formation of 5,6‐diols with either inversion or retention of the configuration at C(6) (Scheme 3). Spectroscopic data allowed us to distinguish the relative configurations (3R*,5S*,6S*) (see A), (3R*,5R*,6R*) (see B), (3R*,5S*,6R*) (see C), and (3R*,5R*,6S*) (see D), of the 3,5,6‐trihydroxy‐5‐6‐dihydro β‐end groups. Syntheses of the optically active carotene‐hexols 20 and 21 and comparison with published data led to a revision of the structure of mectrazanthin (now formulated as 20), heteroxanthin (now formulated as 28), and further carotenoids with 3,5,6‐trihydroxy end groups.