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 (3 S , 3′ S )‐Astaxanthin An efficient synthesis of (3 S , 3′ S )‐astaxanthin ( 1a ) in high yield and optical purity starting from (4 R , 6 R )‐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 (4 S )‐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.