1. Department of Chemistry, Faculty of Science , University of Dschang , P.O. Box 67, Dschang , Cameroon
2. Institut für Pharmazie (Pharmazeutische Biologie) , Freie Universität Berlin, Königin-Luise-Str. 2-4, D-14195 Berlin , Germany
3. Department of Natural Products Chemistry, Graduate School of Pharmaceutical Sciences , Kyushu University , Fukuoka 812-8582, Japan
4. Department of Chemistry, Faculty of Science , University of Dschang , P.O. Box 67, Dschang , Cameroon , Tel.: +237 675004826