1. a Department of Organic Chemistry, Faculty of Science , University of Yaounde I , P.O. Box 812, Yaounde, Cameroon
2. b Department of Chemistry , University of Paderborn , Warburger Straße 100, 33098, Paderborn, Germany
3. c Department of Chemistry, Higher Teachers' Training College , University of Yaounde I , Yaounde, BP 47, Cameroon
4. d Institute of Microbiology, Technical University of Braunschweig , Braunschweig, D-38106, Germany