1. Cancer Research Institute and Department of Chemistry and Biochemistry, Arizona State University, P.O. Box 872404, Tempe, Arizona 85287-2404, Osaka University of Pharmaceutical Sciences, 4-20-1, Nasahara, Takatsuki, Osaka 569-1094, Japan, and Natural Products Branch, Developmental Therapeutics Program, Division of Cancer Treatment & Diagnosis, National Cancer Institute, P.O. Box B, Frederick, Maryland 21702-1201