1. Department of Organic Chemistry, Faculty of Science, University of Yaoundé I, P.O. Box 812, Yaoundé, Cameroon
2. Department of Chemistry, Organic and Bioorganic Chemistry, Bielefeld University, P.O. Box 100131, 33501 Bielefeld, Germany
3. Department of Chemistry, Higher Teachers' Training College, University of Maroua, P.O. Box 55, Maroua, Cameroon
4. Inorganic and Structural Chemistry, Department of Chemistry, Bielefeld University, Bielefeld, Germany