1. University of Heidelberg, Department of Organic Chemistry, INF 270, D-69120 Heidelberg, Germany, Helmholtz Centre for Infection Research (HZI), Medicinal Chemistry, Inhoffenstrasse 7, D-38124 Braunschweig, Germany, Helmholtz Centre for Infection Research (HZI), Microbial Drugs and Saarland University, Pharmaceutical Biotechnology, P.O. Box 151150, D-66041 Saarbrücken, Germany