1. Department of Organic Chemistry, University of Yaounde I, P.O. Box 812 Yaounde, Cameroon, Laboratoire de Chimie des Substances Naturelles U.R.A. 401, Museum National d'Histoire Naturelle, 63, Rue Buffon, 75005 Paris Cedex 05, France, and Department of Chemistry and Biochemistry, Rand Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa