1. Laboratory of Drug Discovery Research and Development, Developmental Therapeutics Program, Division of Cancer Treatment and Diagnosis, National Cancer Institute, Frederick, Maryland 21702-1201, Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, Maryland 20892-0805, Department of Medicinal Chemistry, University of Utah, Salt Lake City, Utah 84112, and School of Chemistry, The University of Melbourne, Parksville, Victoria 3052, Australia