1. Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0212, SmithKline Beecham Pharmaceuticals, 709 Swedeland Road, King of Prussia, Pennsylvania 19406-0939, Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901, and Natural Products Branch, National Cancer Institute, Frederick Cancer Research and Development Center, P.O. Box B, Frederick, Maryland 21701-1013