1. Contribution from the Department of Biomolecular Chemistry, Leibniz Institute for Natural Product Research and Infection Biology, Hans-Knöll-Institute, Beutenbergstrasse 11a, 07745 Jena, Germany, Kekulé-Institute for Organic Chemistry and Biochemistry, University of Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany, and Friedrich-Schiller-University, Jena, Germany