1. Natural Products Laboratory, School of Pharmacy, University of North Carolina, Chapel Hill, North Carolina 27599, Natural Products Branch, Division of Cancer Treatment and Diagnosis, National Cancer Institute, Fairview Center, Frederick, Maryland 21702, Department of Chemistry, Paul M. Gross Chemical Laboratory, Duke University, Durham, North Carolina 27708, and Department of Biochemistry, Faculty of Science, Okayama University of Science, Ridai-cho, Okayama 700-0005, Japan