1. Department of Organic Chemistry, Faculty of Agricultural and Applied Biological Sciences, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium, TIBOTEC, Gen. De Wittelaan L11B3, B-2800 Mechelen, Belgium, Botany Department, University of Nairobi, P.O. Box 30197, Nairobi, Kenya, and TRAMEDA, P.O. Box 66514, Nairobi, Kenya