1. Department of Chemistry, Higher Teachers’ Training College, University of Yaoundé 1, P.O. Box 47, Yaoundé, Cameroon, Chemistry Department, Organic and Bioorganic Chemistry, Bielefeld University, P.O. Box 100131, 33501 Bielefeld, Germany, Department of Organic Chemistry, University of Yaoundé 1, P.O. Box 812, Yaoundé, Cameroon, and Chemistry Department, Inorganic and Structural Chemistry, Bielefeld University, P.O. Box 100131, 33501 Bielefeld, Germany