Transition metal catalyzed reactions in organic synthesis, part VI. Cobalt mediated intramolecular [2+2+2]-cycloaddition of enediynes towards linear annelated polycycles

The cobalt mediated [2+2+2]-cycloaddition of enediynes 13 and 18 affords linear annelated polycycles such as the hexahydro naphthalene 14 and the decahydro anthracene 19. This latter compound can be transformed into the enantiomerically pure anthracene derivative 21 in only two reaction steps. 1,9,10-Trihydroxy octahydro anthracene (21) represents the ABC-framework of many anthracycline antibiotics such as the antitumor active daunomycine.