1. Laboratory of Drug Discovery Research and Development, Developmental Therapeutics Program, Division of Cancer Treatment, Diagnosis and Centers, National Cancer Institute, Frederick, Maryland 21702−1201, and Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg, Germany, and Prins Leopold Instituut voor Tropische Geneeskunde, Nationalestraat 155, B-2000 Antwerpen, Belgium