1. National Center for Natural Products Research, Research Institute of Pharmaceutical Sciences, and the Departments of Pharmacognosy, Medicinal Chemistry and Pharmacology, School of Pharmacy, The University of Mississippi, University, Mississippi 38677, and Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, and Veterinary Resources Program, Office of the Director, National Institutes of Health, Bethesda, Maryland 20892