Synthese von optisch aktiven, natürlichen Carotioiden und strukturell verwandten Naturprodukten. X. Synthese von (3 R ,3′ S , 5′ R )‐Capsanthin, (3 S ,5 R ,3′ S ,5′ R )‐Capsorubin, (3′ S ,5′ R )‐Kryptocapsin und einigen verwandten Verbindungen. Ein neuer Zugang zu optisch aktiven Fünfring‐Carotinoidbausteinen durch Hydroborierung

Synthesis of Optically Active Natural Carotenoids and Structurally Related Compounds. X. Synthesis of (3 R ,3′ S ,5′ R )‐Capsanthin, (3 S ,5 R ,3′ S ,5′ R )‐Capsorubin, (3′ S ,5′ R )‐Cryptocapsin, and Some Related Compounds. A New Approach to Optically Active, Five‐Membered‐Ring Carotenoid Building Units by Hydroboraton The synthesis of (3 R ,3′ S ,5′ R )‐capsanthin (1) , (3 S ,5 R ,3′ S ,5′ R )‐capsorubin (2) , and (3′ S ,5′ R )‐cryptocapsin (3) , found in the red paprika Capsicum annuum , is described using (+)‐camphor (7) as a readily available starting material. As the key reaction, the unsaturated acetal 16 is hydroborated with (+)‐diisopinocampheylborane to give the hydroxy ketone 12a in very high chemical and optical yield. A subsequent aldol condensation with 13 in THF/toluene gives 2 in high yield. The C 40 ‐compounds 1 and 3 are synthesized using the same type of condensation. The pigments 1–3 are transformed by an Oppenauer oxidation to (3 R ,5′ R )‐capsanthone (5) , (5 R ,5′ R )‐capsorubone (4) and (5′ R )‐cryptocapsone (6) , respectively.