1. Department of Organic Chemistry, Faculty of Science, University of Yaoundé I, Yaoundé, Cameroon
2. Faculty of Chemistry, Philipps-Universität Marburg, Marburg, Germany
3. Department of Chemistry, Higher Training College, University of Yaoundé I, Yaoundé, Cameroon
4. Institut für pharmazeutische Chemie, Philipps-Universität Marburg, Marburg, Germany
5. Department Chemie, Universität Fribourg, Fribourg, Switzerland