1. Institute for Organic and Biomolecular Chemistry, University of Göttingen, Tammannstrasse 2, D-37077 Göttingen, Germany
2. Department of Organic Chemistry, Faculty of Science, University of Yaoundé I, P.O. Box 812, Yaoundé, Cameroon
3. Department of Chemistry, Higher Teachers’ Training College, University of Yaoundé I, P.O. Box 47, Yaoundé, Cameroon