1. a Department of Organic Chemistry, Faculty of Science , Yaounde University I , PO Box 812, Yaounde, Cameroon
2. b Department of Chemistry , University of Paderborn , Warburger Straße 100, 33098 Paderborn, Germany
3. c Institute of Microbiology , Technical University of Braunschweig , D-38106 Braunschweig, Germany