1. Institut für Organische Chemie and Biochemie II, Technische Universität Muenchen, Garching, Germany, Laboratory of Pathology and Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, and Laboratory of Medicinal Chemistry, Center for Cancer Research, National Cancer Institute at Frederick, National Institutes of Health, Frederick, Maryland 21702