1. Laboratory for Organic Chemistry, Swiss Federal Institute of Technology (ETH) Zürich, HCI F 315, Wolfgang-Pauli-Str. 10, CH-8093 Zürich, Switzerland
2. Present address: Institute for Organic and Biomolecular Chemistry, Georg-August-Universität Göttingen, Tammannstr. 2, D-37077 Göttingen, Germany
3. German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, D-69120 Heidelberg, Germany