Reactions of keten acetals. Part VI. Total syntheses of the anthraquinones (±)-nalgiovensin, (±)-isorhodoptilometrin, and (±)-rhodoptilometrin

The three title compounds have been synthesized and compared with the natural products. Emodin trimethyl ether(11) was brominated and converted into the propenyl derivative (15). The latter could then be transformed into the 3-(2-hydroxypropyl) compound (19) either by oxymercuration–demercuration or by epoxidation and reduction. (±)-Nalgiovensin [1,8-dihydroxy-3-(2-hydroxypropyl)-6-methoxyanthraquinone](24) was then obtained by partial demethylation. The completely demethylate(±)-isorhodoptilometrin (25) was prepared analogously from the propenylanthraquinone (15) after demethylation and acetylation. For the synthesis of (±)-rhodoptilometrin [1,3,8-trihydroxy-6-(1-hydroxypropyl)anthraquinone](34), the propenyl derivative (15) was hydrogenated, de-methylated, and brominated; substitution of the bromine by acetate and saponification gave the desired product. Conditions for the preparation of the starting material (11) have been further investigated.