1. Department of Chemistry, Higher Teacher Training College, University of Yaoundé 1, P.O.
Box 47, Yaoundé, Cameroon
2. Institute of Organic and Biomolecular Chemistry, University of Göttingen, Tammannstrasse 2, D-37077 Göttingen, Germany
3. Department of Organic Chemistry, Faculty of Science, University of Yaoundé 1, P.O. Box 812, Yaoundé, Cameroon
4. School of Pharmaceutical Sciences, University of Geneva, University of Lausanne, Rue Michel-Servet 1, CH-1211 Genève 4, Switzerland
5. Institute of Inorganic and Structural Chemistry, Heinrich-Heine University Düsseldorf, Universitätsstrasse 1, D-40225 Düsseldorf, Germany