1. Laboratory of Biological Chemistry, Department of Chemistry, McGill University, Otto Maass Chemistry Building # 230, Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada, Institute of Parasitology, McGill University, McDonald Campus, Ste-Anne de Bellevue, Quebec H9X 3V9, Canada, Kenyan Institute of Medicinal Research of Nairobi, P.O. Box 54840, Nairobi, Kenya, and Department of Organic Chemistry, University of Yaounde I, P.O. Box 812, Yaounde, Cameroon