1. Department of Biotechnology, University of Kaiserslautern, Paul-Erlich-Strasse 23, D-67663, Kaiserslautern, Germany, Institute of Biotechnology and Drug Research, IBWF, Erwin Schoedinger 56, D-67663, Kaiserslautern, Germany, Chemistry Department, Kenyatta University, P.O. Box 43844, Nairobi, Kenya, and Department of Organic Chemistry 2, Lund University, P.O. Box 124, S-221 00 Lund, Sweden