1. Centre for Synthesis and Chemical Biology, School of Chemistry, Trinity College Dublin, Dublin 2, Ireland, Swiss Tropical Institute, Socinstrasse, 57, CH-4002 Basel, Switzerland, Department of Chemistry, Georgia State University, Atlanta, Georgia 30303-3083, Departamento de Parasitología, Facultad de Farmacia, Universidad Complutense de Madrid, Av. Complutense s/n, E-28040 Madrid, Spain, and Instituto de Química Médica, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain