Technische Verfahren zur Synthese von Carotinoiden und verwandten Verbindungen aus Oxo-isophoron. I. Modifizierung der Kienzle-Mayer-Synthese von (3S 3′S)-Astaxanthin

Technical Procedures for the Synthesis of Carotenoids and Related Compounds from 6-Oxo-isophorone. I. Modification of the Kienzle-Mayer-Synthesis of (3S, 3′S)-Astaxanthin An efficient synthesis of (3S, 3′S)-astaxanthin ( 1a ) in high yield and optical purity starting from (4R, 6R)-4-hydroxy-2,2,6-trimethylcyclohexanone ( 4 ) is reported. The absolute configuration of 1a , previously derived from ORD. data, has been confirmed by X-ray analysis of 5 , a derivative of 6-oxo-isophorone ( 2 ). The key features of the improved synthesis are the two-step conversion of 4 to the key intermediate (4S)-2,6,6-trimethyl-4-hydroxy-2-cyclohexen-1-one ( 14 ), a new method for the partial reduction of the sterically hindered triple bond of (S)-6-hydroxy-3-(5-hydroxy-3-methyl-3-penten-1-ynyl)-2,4,4-trimethyl-2-cyclohexen-1-one ( 32 ), and Wittig olefination of the dialdehyde 1,6-dimethyl-1,3,5-octatrienedial ( 38 ) using phosphonium salt 37 with a free hydroxyl group.