Dienolether condensations–a powerful tool in carotenoid synthesis
The catalytic dienolether condensation reactions, discovered by Nazarov and Makin in 1958, are of special interest in the carotenoid ®eld for chain extension by ®ve carbon atoms. In this paper, new developments in the synthesis of apoesters and apocarotenals using vinyl ketene acetals and dienolethers, respectively, as C5-building blocks are discussed. New `Wittig-free' routes to astaxanthin and canthaxanthin applying these Lewis acid-catalysed dienolether condensation reactions are described.