Technische Verfahren zur Synthese von Carotinoiden und verwandten Verbindungen aus 6‐Oxo‐isophoron. III. Ein neues Konzept für die Synthese der enantiomeren Astaxanthine

Technical Procedures for the Synthesis of Carotenoids and Related Compounds from 6‐Oxo‐isophorone. III. A New Concept for the Synthesis of the Enantiomeric Astaxanthins A new and efficient concept for the total synthesis of (3 S , 3' S )‐ and (3 R , 3' R )‐astaxanthin ( 1a and 1c , resp.) in high overall yield and up to 99,2% enantiomeric purity is described. Key intermediates are the ( S )‐ and ( R )‐acetals 10 and 17 , respectively (Scheme 2) . These chiral building blocks were synthesized via three different routes: a) functionalization of the enantiomeric 3‐hydroxy‐6‐oxo‐isophorons 4 ) 2 and 11 , respectively (Scheme 2) ; b) optical resolution of 3,4‐dihydroxy‐compound 4 ) 19 (Scheme 3) , and c) fermentative reductions of 6‐oxo‐isophorone derivatives (Schemes 4 and 5) . ‐ The absolute configurations of the two intermediates 12 and 13 (Scheme 2) have been confirmed by X‐ray analysis. ‐ The final steps leading to the enantiomeric astaxanthins are identical with those described for optically inactive astaxanthin [1].