1. Institut für Organische Chemie der Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany, Department of Medicinal Chemistry, Danish University of Pharmaceutical Sciences, Universitetsparken 2, DK-2100 Copenhagen, Denmark, Department of Botany, University of Dar es Salaam, P.O. Box 35060, Dar es Salaam, Tanzania, and Swiss Tropical Institute, Socinstrasse 57, CH-4002 Basel, Switzerland