Synthesis of cytostatic tetradecacyclic pyrazines and a novel reduction-oxidation sequence for spiroketal opening in sapogenins

Aiming towards spiroketal-modified artificial cephalostatin molecules, two orthogonal approaches were investigated. First, the introduction of 17-O-fu nctionality into hecogenin derivatives with a closed spiroketal moiety was accomplished by different remote-oxidation procedures. These allowed the sy nthesis of tetradecacyclic artificial cephalostatin molecules with improved tumor-inhibiting properties. Second, a novel reduction-oxidation pathway f or spiroketal opening in sapogenins was discovered, which should provide th e basis for a broad access towards spiroketal-modified building blocks for cephalostatins.