1. Department of Chemistry, Faculty of Science, University of Douala, Douala, Cameroon
2. Department of Organic Chemistry, Faculty of Science, University of Yaoundé 1, Yaounde, Cameroon
3. Department of Chemistry, Higher Teacher Training College, University of Yaoundé 1 1, Yaounde, Cameroon
4. Department of Bioorganic Chemistry, Leibniz Institute of Plant Biochemistry, Halle (Saale), Germany
5. Organic and Bioorganic Chemistry, Department of Chemistry, Bielefeld University, Bielefeld, Germany