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

Abstract

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.