1. Institut für Pharmazeutische Chemie, Philipps-Universität Marburg, Marbacher Weg 8, Marburg D-35032, Germany
2. Biomolecular NMR, Bavarian NMR Center, Technical University of Munich, Lichtenbergstraße 4, Garching D-85747, Germany
3. Institute of Structural Biology, Helmholtz Zentrum München, Ingolstädter Landstr. 1, Neuherberg 85764, Germany
4. Laboratory of Organic Chemistry, ETH Zürich, Vladimir-Prelog-Weg 3, HCI, Zürich CH-8093, Switzerland