Palladium‐Catalyzed S‐Allylation of Vinyl Carbinols – Total Syntheses of Carotenoids by Julia‐Kocienski Olefination

Abstract

Palladium catalyzed <i>S</i>‐allylation of vinyl carbinols from ionones (or conjugated carbonyls) with 2‐mercapto‐1,3‐benzothiazole (BT‐SH) provides versatile C<sub>15</sub> dienyl benzothiazolyl (BT) sulfides. This Pd(dppe)Cl<sub>2</sub>‐catalyzed <i>S</i>‐allylation is highly selective for free allylic alcohols in the presence of allylic acetates, resulting in 3‐ or 4‐acetoxy C<sub>15</sub> dienyl BT‐sulfides suitable for xanthophyll synthesis. The corresponding C<sub>15</sub> dienyl BT‐sulfones undergo Julia‐Kocienski olefination reactions with C<sub>10</sub> 2,7‐dimethyl‐2,4,6‐octatrienedial under mild conditions to produce carotenoids natural products. Efficient synthesis is achieved for a range of carotenoids including xanthophylls such as iso‐zeaxanthin, zeaxanthin, and lutein as well as carotenes like β‐carotene, ε‐carotene, and 9'‐<i>Z</i>‐phenyl‐carotene.