Reactions of keten acetals. Part 7. Total syntheses of the tetramethyl ethers of the 1-acyl-2,4,5,7-tetrahydroxyanthraquinones rhodolamprometrin and rhodocomatulin
Abstract
Acylketen acetals or (better) their trimethylsilyl enol ethers condense regiospecifically with 2-halogeno-1,4-naphthoquinones. Reactions of 1,1-dimethoxy-3-trimethylsilyloxyhexa-1,3-diene and -octa-1,3-diene with 2-chloro-6,8-dimethoxy-1,4-naphthoquinone gave the corresponding 1-alkyl-2,4,5,7-tetramethoxyanthraquinones. The latter underwent spontaneous photochemical oxidation at the benzylic position and yielded the title compounds. The method has been adapted to provide a simple synthesis of questin.