First Total Synthesis of the Bioactive Anthraquinone Kwanzoquinone C and Related Natural Products by a Diels–Alder Approach

The first total synthesis of the novel anticancer agent kwanzoquinone C ( 1 ), an anthraquinone glycoside which has been isolated from extracts of the roots of orange daylilies(Kaempfer) ( Hemerocallis fulva ) is reported. The strategy involves the preparation of the tetrasubstituted 1,3‐butadiene 6 by an electrocyclic conrotatory ring opening of the cyclobutanone rac ‐ 11 , which is obtained by a [2+2] cycloaddition.Diels–Alder reaction of 6 with the juglone derivative 5 gives the bis‐benzyl‐protected anthraquinone 4 after aromatization. Finally, glycosidation of the phenolic hydroxy group using 2,3,4,6‐tetra‐ O ‐acetyl‐α‐ D ‐glucopyranosyl trichloroacetimidate followed by the cleavage of the benzyl ethers and the solvolysis of the acetate groups yields the natural product 1 . A related anthraquinone natural product 2 with antioxidative properties was obtained by using the 1,3‐butadiene 17 and the juglone derivative 16 as substrates. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)