Total syntheses of 2,2′-epi-cytoskyrin A, rugulosin, and the alleged structure of rugulin

The total syntheses of 2,2'-epi-cytoskyrin A, rugulosin, and the alleged structure of rugulin are described. These naturally occurring bisanthraquinones and their relatives are characterized by novel molecular architectures at the core, at which lies a more or less complete, cage-like structural motif termed skyrane. The strategies developed for their total synthesis feature a cascade sequence called the cytoskyrin cascade and deliver these molecules in short order and in a stereoselective manner.