1. Department of Chemistry, M/C 0212, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, Missouri Botanical Garden, P.O. Box 299, St. Louis, Missouri 63166-0299, and B.P. 3391, Antananarivo, Madagascar, Centre National d'Application des Recherches Pharmaceutiques, B.P. 702, Antananarivo 101, Madagascar, and Eisai Research Institute, 4 Corporate Drive, Andover, Massachusetts 01810